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| United States Patent |
6,160,990
|
|
Kobayashi
, et al.
|
December 12, 2000
|
Cable network system with ingress noise suppressing function
Abstract
In a cable network system of this invention, GSUs are installed in almost
all subscriber residences. A G-STB or a G-MDM is set for a subscriber who
wants to receive bidirectional services. Alternatively, an NGA is attached
to an existing N-STB or N-MDM. The presence of an upward transmission
signal from an in-home device is monitored in the G-STB, the G-MDM, or the
NGA. While the upward transmission signal is detected, a transmission
indication signal is supplied to the GSU. Only while the transmission
indication signal is detected, the GSU sets a gate switch in an ON state.
On the other hand, while no transmission indication signal is detected,
the GSU keeps the gate switch in an OFF state.
| Inventors:
|
Kobayashi; Hiroshi (Tokyo, JP);
Hirai; Katsumi (Tokyo, JP);
Ibe; Hiroyuki (Yokohama, JP)
|
| Assignee:
|
Kabushiki Kaisha Toshiba (Kawasaki, JP)
|
| Appl. No.:
|
417017 |
| Filed:
|
October 12, 1999 |
Foreign Application Priority Data
| May 13, 1996[JP] | 8-117954 |
| Jul 23, 1996[JP] | 8-193491 |
| Jul 26, 1996[JP] | 8-197949 |
| Current U.S. Class: |
725/135; 725/1 |
| Intern'l Class: |
H04N 007/14; H04N 001/00 |
| Field of Search: |
348/12,13
455/5.1,4.2,3.1,6.1
|
References Cited [Referenced By]
U.S. Patent Documents
Primary Examiner: Hsia; Sherrie
Attorney, Agent or Firm: Oblon, Spivak, McClelland, Maier & Neustadt, P.C.
Parent Case Text
This application is a Continuation of application Ser. No. 08/717,296,
filed on Sep. 20, 1996 now abadoned.
Claims
What is claimed is:
1. A transmission path opening/closing device comprising:
transmission path opening/closing means provided in at least one
bidirectional transmission path of a cable network system for
bidirectionally transmitting signals between at least one central
equipment and at least one subscriber device via said bidirectional
transmission path,
said transmission path opening/closing means setting at least one upward
transmission path of said bidirectional transmission path in a conductive
state during a period when at least one upward signal transmitted from
said subscriber device to said central equipment passes through said
means, and setting said upward transmission path in a cutoff state during
a remaining period;
at lease one switch control section arranged corresponding to said
subscriber device, and
at least one gate switch section arranged in said bidirectional
transmission path;
said switch control section comprising at least one of means for detecting
a transmission period of the upward signal from said subscriber device,
and means for receiving notification information representing the
transmission period of the upward signal generated by said subscriber
device, and comprising means for generating a transmission indication
signal representing the transmission period on the basis of a detection
result of the transmission period or the received notification
information, and sending the transmission indication signal to said
bidirectional transmission path; and
said gate switch section comprising means for setting said upward
transmission path of said bidirectional transmission path in the
conductive state during the period when the upward signal transmitted from
said subscriber device passes through said gate switch section, and
setting said upward transmission path in the cutoff state during the
remaining period, in accordance with the transmission indication signal
sent from said switch control section;
wherein said switch control section sends a carrier having undergone
spectrum spreading in accordance with a predetermined spread code
sequence, as the transmission indication signal, and
said gate switch section reversely spreads and reproduces the transmission
indication signal, sent from said switch control section, in accordance
with the predetermined spread code sequence to open/close the upward
transmission path of said bidirectional transmission path in accordance
with the reproduced transmission indication signal.
2. A transmission path opening/closing device comprising:
transmission path opening/closing means provided in at least one
bidirectional transmission path of a cable network system for
bidirectionally transmitting signals between at least one central
equipment and at least one subscriber device via said bidirectional
transmission path,
said transmission path opening/closing means setting at least one upward
transmission path of said bidirectional transmission path in a conductive
state during a period when at least one upward signal transmitted from
said subscriber device to said central equipment passes through said
means, and setting said upward transmission path in a cutoff state during
a remaining period;
at lease one switch control section arranged corresponding to said
subscriber device;
at least one gate switch section arranged in said bidirectional
transmission path;
said switch control section comprising at least one of means for detecting
a transmission period of the upward signal from said subscriber device,
and means for receiving notification information representing the
transmission period of the upward signal generated by said subscriber
device, and comprising means for generating a transmission indication
signal representing the transmission period on the basis of a detection
result of the transmission period or the received notification
information, and sending the transmission indication signal to said
bidirectional transmission path; and
said gate switch section comprising means for setting said upward
transmission path of said bidirectional transmission path in the
conductive state during the period when the upward signal transmitted from
said subscriber device passes through said gate switch section, and
setting said upward transmission path in the cutoff state during the
remaining period, in accordance with the transmission indication signal
sent from said switch control section,
wherein said switch control section comprises means for sending, by using a
plurality of carriers, respective transmission indication signals having
undergone signal processing using a present parameter, and
said gate switch section comprises
means for receiving and reproducing the respective transmission indication
signals sent from said switch control section by using the plurality of
carriers,
means for setting said upward transmission path of said bidirectional
transmission path in the conductive state when all the transmission
indication signals with the plurality of carriers are reproduced, and
means for checking whether the plurality of reproduced transmission
indication signals having undergone the signal processing using the
parameter, and, if NO, restoring said upward transmission path of said
bidirectional transmission path to the cutoff state.
3. A transmission path opening/closing device comprising:
transmission path opening/closing means provided in at least one
bidirectional transmission path of a cable network system for
bidirectionally transmitting signals between at least one central
equipment and at least one subscriber device via said bidirectional
transmission path,
said transmission path opening/closing means setting at least one upward
transmission path of said bidirectional transmission path in a conductive
state during a period when at least one upward signal transmitted from
said subscriber device to said central equipment passes through said
means, and setting said upward transmission path in a cutoff state during
a remaining period;
at lease one switch control section arranged corresponding to said
subscriber device;
at least one gate switch section arranged in said bidirectional
transmission path;
said switch control section comprising at least one of means for detecting
a transmission period of the upward signal from said subscriber device,
and means for receiving notification information representing the
transmission period of the upward signal generated by said subscriber
device, and comprising means for generating a transmission indication
signal representing the transmission period on the basis of a detection
result of the transmission period or the received notification
information, and sending the transmission indication signal to said
bidirectional transmission path; and
said gate switch section comprising means for setting said upward
transmission path of said bidirectional transmission path in the
conductive state during the period when the upward signal transmitted from
said subscriber device passes through said gate switch section, and
setting said upward transmission path in the cutoff state during the
remaining period, in accordance with the transmission indication signal
sent from said switch control section,
wherein at least one of said switch control section and said gate switch
section comprises signal delay means for delaying the upward signal by a
time required for said gate switch section to set said upward transmission
path in the conductive state in accordance with the transmission
indication signal after said subscriber device starts to transmit the
upward signal,
wherein said signal delay means comprises
means for converting a first frequency of the upward signal into a second
frequency higher than the first frequency,
means for delaying the upward signal converted to having the second
frequency by the required time, and
means for returning the frequency of the delayed upward signal to the first
frequency.
4. A transmission path opening/closing device comprising:
transmission path opening/closing means provided in at least one
bidirectional transmission path of a cable network system for
bidirectionally transmitting signals between at least one central
equipment and at least one subscriber device via said bidirectional
transmission path,
said transmission path opening/closing means setting at least one upward
transmission path of said bidirectional transmission path in a conductive
state during a period when at least one upward signal transmitted from
said subscriber device to said central equipment passes through said
means, and setting said upward transmission path in a cutoff state during
a remaining period;
at lease one switch control section arranged corresponding to said
subscriber device;
at least one gate switch section arranged in said bidirectional
transmission path;
said switch control section comprising at least one of means for detecting
a transmission period of the upward signal from said subscriber device,
and means for receiving notification information representing the
transmission period of the upward signal generated by said subscriber
device, and comprising means for generating a transmission indication
signal representing the transmission period on the basis of a detection
result of the transmission period or the received notification
information, and sending the transmission indication signal to said
bidirectional transmission path; and
said gate switch section comprising means for setting said upward
transmission path of said bidirectional transmission path in the
conductive state during the period when the upward signal transmitted from
said subscriber device passes through said gate switch section, and
setting said upward transmission path in the cutoff state during the
remaining period, in accordance with the transmission indication signal
sent from said switch control section,
wherein at least one of said switch control section and said gate switch
section comprises signal delay means for delaying the upward signal by a
time required for said gate switch section to set said upward transmission
path in the conductive state in accordance with the transmission
indication signal after said subscriber device starts to transmit the
upward signal,
wherein said signal delay means comprises a plurality of delay elements for
dividing a band of the upward signal into a plurality of bends, and
delaying the upward signal for each band.
5. A cable network system for bidirectionally transmitting signals between
at least one central equipment and at least one subscriber device via at
least one bidirectional transmission path, comprising:
at least one transmission path opening/closing device provided in said
bidirectional transmission path to set at least one upward transmission
path of said bidirectional transmission path in a conductive state during
a period when at least one upward signal is transmitted from said
subscriber device to said central equipment and set said upward
transmission path in a cutoff state during a remaining period; and
at least one ingress noise monitoring/analysis device set in a portion of
said bidirectional transmission path, through which undesired signals
containing at least ingress noise pass,
said ingress noise monitoring/analysis device having
noise monitoring means for monitoring noise on said upward transmission
path which has passed through said transmission path opening/closing
device,
determination means for determining a degree of influence of the noise on a
transmission quality of said upward transmission path on the basis of
monitoring data obtained by said noise monitoring means,
coupling means to said central equipment for obtaining source information
of upward signals transmitted from said subscriber device to said central
equipment,
estimation means for extracting noise originating source information by
correlating the noise monitored by said noise monitoring means and the
source information of the upward signals received by said coupling means,
thereby estimating at least one noise originating source which has
transmitted the upward signals passed together with the noise through said
transmission path opening/closing device, and
notification means for notifying a determination result obtained by said
determination means and an estimation result obtained by said estimation
means to said central equipment.
6. A system according to claim 5, wherein said central equipment comprises
a central network management device and said notification means notifies
to said central network management device the determination result
obtained by said determination means and the estimation result obtained by
said estimation means.
7. A system according to claim 5, comprising a central network management
device connected by a communication line to said central equipment,
wherein said notification means notifies to said central network
management device the determination result obtained by said determination
means and the estimation result obtained by said estimation means through
said central equipment and said communication line.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention rates to a cable network system constituted in a tree
or star shape like, e.g., a bidirectional CATV (Cable Access TeleVision)
system and an HFC (Hybrid Fiber and Coaxial) system and, more particularly
to, a cable network system having a function of suppressing ingress noise
in an upward transmission path.
2. Description of the Related Art
In recent years, a trend of changing CATV systems providing mainly video
broadcast services into bidirectional systems and developing various
bidirectional transmission services has been activated. The bidirectional
transmission services include on-line services (to be referred to as PC
on-line services hereinafter) for data terminals such as personal
computers, real-time communications services for telephones, videophones,
and the like, and VOD (Video On Demand) services for rapidly providing
desired movie video and the like to users, as needed.
When the bidirectional services are to be realized in a cable network
system having a transmission path branched in a tree or star shape, like
the CATV system, measures against ingress noise are required.
More specifically, if there is a connector open terminal to which, e.g., no
in-home device is connected in each subscriber residence, or if a used
coaxial cable is insufficient in electromagnetic shielding characteristics
though the in-home device is connected, interference radio waves such as
shortwave broadcasts, and electromagnetic wave noise from electrical
motors, such as a vacuum cleaner, or motorcycles flow in via the connector
open terminal or the coaxial cable. FIGS. 27 and 28 show examples of data
obtained by experiments on inflow noise. FIG. 27 shows the spectrum
distribution of noise flowing from the connector open terminal. The inflow
noise was observed in a band of 40 MHz or less. FIG. 28 shows the spectrum
distribution of inflow noise in a state wherein a 5-cm lead line is
connected to a connector terminal, on the assumption that a used coaxial
cable is insufficient in electromagnetic shielding characteristics. More
typical inflow noise was observed in the band of 40 MHz or less. In
addition, FIG. 29 shows actual data obtained when a connector terminal was
terminated by a terminator. In this case, inflow noise was not
substantially detected.
If noise flows into respective subscriber residences in this manner, lots
of noise merge on an upward transmission path to increase the level and be
transmitted to a headend. This noise is generally called ingress noise.
The ingress noise causes degradation of the transmission quality, and in
some cases causes the system to fail in transmission.
FIG. 30 shows an example of the interference percent availability of
ingress noise actually observed over several days in a bidirectional cable
network having 1500 subscribers. As is apparent from FIG. 30, the
availability of satisfying the C/I (Carrier to Interference) ratio of 10
dB per 1-MHz channel bandwidth was almost 100%. The availability of
satisfying the C/I ratio of 24 dB as a high-quality transmission
environment was 70 to 80% on average and was below 50% depending on the
band. It was found from remaining observation results that the
interference percent availability became lower as the channel bandwidth
was narrower and the number of subscribers was smaller.
The ingress noise can be roughly classified into three types: narrowband
coherent noise, broadband incoherent noise, and specific subscriber noise.
The narrowband coherent noise is electromagnetic waves having large
transmission power, such as shortwave broadcast waves and military radio
waves present in an upward frequency band (5 to 48 MHz in Japan, and 5 to
40 MHz in the United States). Although the bands are narrow, these
electromagnetic waves flow from the connector open terminals and the like
of almost all subscribers. If all signals which reach a headend are in
phase, the noise level observed at the headend equivalently increases by
2(logS [dB] for the average inflow noise per subscriber where S is the
number of accommodated subscribers. In fact, however, the noise level
increases by about 14 logS [dB] because signals have a difference in
propagation delay time therebetween.
The broadband incoherent noise is generated by strong electromagnetic waves
radiated in the atmosphere from the sparks of an electrical motor and a
gasoline engine, and discharge tubes and digital devices such as a
personal computer. Although the frequency band is broad (2 kHz to 100
MHz), the noise level decreases by 1/f as a frequency f becomes higher.
Noise flowing into the upward transmission path is not correlated with
other noise (incoherent). The noise can be considered as Gaussian noise.
For this reason, the noise level equivalently increases a. the headend by
10 logS [dB] for the average inflow noise per subscriber.
The specific subscriber noise is generated when a subscriber erroneously
connects an amateur radio device or a digital device such as a personal
computer to a cable, or when the subscriber intentionally sends an
interference signal to the cable. Since this noise directly flows into the
upward transmission path, the noise level may be kept high over a long
time.
In addition to the above-described three types of noise, there is harmonic
noise caused by signal distortion on the transmission path. The harmonic
noise is caused by a nonlinear effect generated by corrosion of a fitting
connector terminal on a trunk line cable. This noise can be prevented by
proper maintenance and management of cable industrial companies.
Inflow portions of the ingress noise can be classified into two portions: a
portion on a trunk system and a portion inside a subscriber residence. The
trunk system generally uses a coaxial cable excellent in electromagnetic
shielding characteristics. The ingress noise may flow from a loose
connector or an old or worn cable. However, such inflow of noise can be
prevented by proper maintenance and management of cable network system
industrial companies. To the contrary, no measure is provided with respect
to the noise flowing from the subscriber residence. The bidirectional
services must be performed in consideration of this noise.
FIG. 31 shows a comparison of the spectrum distribution of ingress noise
observed on an upward transmission and the assumed level of an upward data
signal. Strong noise estimated to be narrowband coherent noise was
observed around 6.5 MHz, 10 MHz, and 27 MHz, and noise estimated to be
broadband incoherent noise was observed at remaining frequencies. It is
supposed that a very-high-quality transmission path can be realized if
both the narrowband coherent noise and the broadband incoherent noise are
suppressed by more than 20 dB. However, this suppression cannot be
achieved, so that the various measures are proposed as follows.
(a) HFC (Hybrid Fiber and Coaxial) Architecture
The HFC architecture aims at a reduction in ingress noise level by
decreasing the number S of subscribers described above. The conventional
CATV system broadcasts television video from a headend to several ten
thousands of subscribers via only coaxial cables by using several tens of
bidirectional trunk amplifiers. This CATV system is subdivided into a
maximum of 500 home paths per subsystem by combining, e.g., optical fibers
and coaxial cables, as shown in FIG. 32. Note that the home paths
represent the number of homes to which cables are wired near the
residences or under the eaves and services are immediately provided if the
subscribers require them. The actual number of subscribers for the
services is 60% on average in the United States, i.e., 300 subscribers per
subsystem.
Referring to FIG. 32, a reception equipment for receiving television
broadcasts sent via communication satellites, and an information
transmission equipment for providing various bidirectional services, such
as servers, routers, and switching units are installed in a headend (H/E)
1. A plurality of distribution hubs (D/Hs) 2, . . . are connected to the
H/E 1 via exclusive optical fibers 3, . . . in a star shape. In the D/Hs
2, . . . , television broadcast signals and downward signals for various
bidirectional services which are sent from the H/E 1 are modulated and
then synthesized with each other. Thereafter, obtained signals are
converted into optical signals and transmitted to fiber nodes (F/Ns) 5, .
. . via optical fibers 4.
The F/Ns 5,. . . convert the optical signals sent from the D/Hs 2 into
electrical signals, and transmit them to subsystems 20 each having 500
home paths. In each subsystem 20, trunk line coaxial cables 6 (to be
referred to as trunk line cables hereinafter) are connected around the F/N
5 in a tree or star shape. Tap-offs 8 are arranged on these trunk line
cables 6 to branch the trunk line cables 6 into drop coaxial cables 9 (to
be referred to as drop cables hereinafter). The drop cables 9, . . . are
dropped in subscriber residences 10.
In the subscriber residences 10, as shown in FIG. 33, a television receiver
11 can be directly connected to an in-home splitter 18 to receive
television broadcasts, and a television receiver 13 can be connected to
the in-home splitter 18 via a set-top box (STB) 12 to receive the VOD
services. In addition, a personal computes 14, a videophone 15, or a
telephone 16 is connected via a modem to receive the PC on-line services
and the rial-time communications services. Bidirectional trunk amplifiers
7 for compensating the attenuation of signals are arranged at a plurality
of portions on the trunk line cables 6.
While the VOD services and the PC on-line services are received, upward
signals transmitted from the STB and the modem are sent to the F/N 5 via
the drop cables 9, the tap-offs 8, and the trunk line cables 6. The
signals are converted into optical signals by the F/N 5, and the optical
signals are sent to the D/H 2 via the optical fiber 4. The sent signals
are converted into electrical signals by the D/H 2 and then demodulated.
The resultant signals are sent to the H/E 1 and processed. Upward and
downward signals are transmitted on the bidirectional transmission path
using the trunk line cables 6 and the drop cables 9 in such a manner that
they pass through different frequency bands.
In general, the cable network system using the HFC architecture has a
service area of a maximum of about 50 km per headend. This cable network
system can accommodate a maximum of 300,000 home paths. The distribution
hubs are arranged at a maximum of about 15 portions. Therefore, one
distribution hub deals with a maximum of 20,000 home paths. A maximum of
40 subsystems are connected to the distribution hub. However, since
downward signals include analog video signals, a very expensive laser
diode having high lineality is required to convert electrical signals into
optical signals. For this reason, in many cases, identical signals are
transmitted in the downward direction to five subsystems as a unit, i.e.,
to every 2,500 home paths (the average number of subscribers: 1,500).
To the contrary, upward signals are basically operated for each subsystem
because of ingress noise suppression. However, to reduce the cost of a
demodulator for an upward signal set in the distribution hub, the upward
signals must be operated for 2 or more subsystems as a unit in fact.
FIG. 34 shows the degree of improvement by the HFC architecture when the
operation unit in a conventional system not using the HFC architecture is
set to 50,000 home paths. From FIG. 34, it was found that the HFC
architecture was an effective method to reduce the ingress noise. However,
as is apparent from the observation result (observation in 5 subsystems)
in FIG. 30, the influence of the ingress noise is still large even with
the HFC architecture in terms of the transmission quality. For this
reason, parallel use of the following various measures has conventionally
been examined.
(b) Frequency Agility
The frequency agility is a method of switching a specific frequency band to
another frequency band when the transmission quality is degraded in the
specific frequency band. This method is effective for avoiding the
influence of a strong interference wave caused by the narrowband coherent
noise or the specific subscriber noise. However, this method cannot
provide an essential solution such as the reduction of the ingress noise
itself.
(c) Low-Efficiency Modulation Scheme
The signal-to-noise ratio is low on the upward transmission path where the
ingress noise is present. For this reason, it is difficult to employ a
high-efficiency modulation scheme such as QAM in which signals are
symbolized and modulated in units of 4 or 6 bits. In fact, a modulation
scheme such as QPSK is employed at most.
(d) Error Correction/Retransmission
Error correction/retransmission is a method of estimating and correcting an
error portion when received data have a bit error due to the ingress noise
or the like, and if the error portion cannot be completely corrected,
requiring retransmission to a transmission source by a communication
protocol such as a TCP/IP (Transmission Control Protocol/Internet
Protocol). However, this method cannot provide an essential solution for
the ingress noise, either. In addition, the transmission efficiency
decreases due to an error correction code added to actual data or
retransmission.
(e) Band Reduction+Frequency Division Multiplex
Band Reduction+frequency division multiplex are a measure which pays
attention to a reduction in influence of the ingress noise by narrowing
the channel bandwidth, as described above, and tries to effectively use
frequency bands as much as possible by using bands free from interference
waves generated by, e.g., the narrowband coherent noise or the specific
subscriber noise. It is designed to ensure a desired transmission capacity
by determining a band for each channel and increasing the number of
frequency carriers, i.e., by applying a frequency division multiplex
scheme. However, this measure car,of provide an essential solution for the
ingress noise, either, similar to the above-described schemes.
(f) Bridger Switch
Bridger switches are arranged in units of trunk line cables branched from,
e.g., a fiber node. When strong ingress noise is observed, the bridger
switches are sequentially turned off to specify a trunk line cable to
which the generation source of the noise is connected, and to cut the
trunk line cable from the system. This scheme is effective for the
specific subscriber noise. However, services for all subscribers connected
to the cut trunk line cable are stopped. In addition, human-wave tactics
must be employed to search the subscriber as the generation source on the
specified trunk line cable. This may lead to a serious problem when the
bidirectional services get into stride in the future, and when the bridge
switches must be often operated due to the carelessness of subscribers.
(g) High-pass Filter
According to a method using high-pass filters, when the HFC architecture is
realized, high-pass filters which pass only signals in a downward
transmission band therethrough and cut off signals in an upward
transmission band are attached to all subscriber residences except for the
residences of subscribers who desire the bidirectional services. With this
arrangement, only the subscribers who desire the bidirectional services
can use the upward transmission band. This method is effective when the
number of subscribers who desire the bidirectional services is small.
However, as the number of subscribers who desire the bidirectional
services increases, the inflow amount of the ingress noise increases. If
the subscribers who desire the bidirectional services are 1% of all the
subscribers, the inflow in amount can be effectively reduced by 20 to 28
dB; if 20%, it is reduced by only 3 to 4 dB.
(h) CTU Scheme
In the CTU (Coaxial Termination Unit) scheme, services are divided on the
frequency band into services requiring a broadband for transmission in the
upward direction, such as the PC on-line services and the real-time
communications services, and services capable of using a narrow upward
transmission band, such as the VOD services. As for the former, as shown
in FIG. 33, a CTU 17 is arranged at a position before a subscriber
residence to terminate a cable, thereby preventing the inflow of noise. In
an actually proposed scheme, a frequency band of 10 to 40 MHz is assigned
to the former, and a band of 5 to 10 MHz is assigned to the latter, as
shown in FIG. 35. The CTU 17 incorporates a modem function for the PC
on-line services and the real-time communications services. The filter
characteristics and the like are designed not to flow noise to the band of
10 to 40 MHz from a subscriber residence.
As for the latter, the cable is directly connected to an STB or a
television receiver via an in-home splitter arranged in the subscriber
residence. For this reason, noise flows into the band of 5 to 10 MHz from
all subscribers. However, since the transmission rate is low, and the
transmission band is narrow, the influence of the ingress noise is
relatively weak.
In general, the expensive CTU 17 cannot be set for a subscriber who does
not desire the bidirectional services. For this reason, in the CTU scheme,
even if the CTUs 17 are set for 20% of the subscribers, the ingress noise
is reduced by only about 1 dB. For this reason, a remarkable effect cannot
be expected until the installation ratio of the CTU 17 increases to about
100%.
Further, a scheme using the CTU 17 in combination with the high-pass filter
can be considered. That is, the high-pass filter is attached for a
subscriber who does not desire the bidirectional services, and the CTU is
set for a subscriber who desires the bidirectional services, instead of
the high-pass filter. According to this scheme, the ingress noise does not
flow in the band of 10 to 40 MHz. In fact. however, the CTU scheme is not
practically used because of the following various problems.
More specifically, the first point is how to supply power to the CTU when
the CTU is attached outside a residence, e.g., under the eaves. In
telephone services using the cable system (to be described later), power
is supplied to a telephone modem at an AC voltage of about 100 V via a
drop cable due to the necessity of the supply of power to a telephone. It
is not economical or practical to supply power looking via the coaxial
cable ahead to the PC on-line services requiring higher speed operations
and new services to be provided in the future, in addition to the
telephone services. Therefore, power must be supplied from a commercial AC
power supply to the CTU. However, a work for a new power supply of the CTU
set outside the residence is undesirable because it complicates the work
and results in an increase in work cost.
The second point is how to perform an in-home wiring work for each service.
One of the features of the bidirectional cable system is to use a shared
medium, i.e., a single transmission medium for various purposes and
application purposes. To perform the in-home work for each new service is
to increase an economical burden on a subscriber, interrupting invitations
to the new services.
The influence of the ingress noise on the upward transmission path and the
prior art described above are described in detail in the following
references: [1] Cable Labs, "Two-Way Cable Television System
Characterization, Final Report", Apr. 12, 1995. [2] C. A. Eldering, et
al., "CATV Return Path Characterization for Reliable Communications", IEEE
Communication Magazine, pp. 62-69, August 1995.
As described above, various measures have been proposed. Some of these
measures are accompanied by enormous investment, like the HFC
architecture. Even if these measures are performed, the ingress noise
cannot be essentially removed. If there is provided a low-cost and
practical solution to sufficiently suppress the ingress noise, a great
effect will be attained.
SUMMARY OF THE INVENTION
It is an object of the present invention to effectively suppress upward
ingress noise by a low-cost and practical means, thereby allowing the
transmission of an upward signal under a high-quality transmission
environment.
To achieve the above object, in a cable network system of the present
invention, a transmission path opening/closing device is set in a
bidirectional transmission path. The transmission path opening/closing
device sets an upward transmission path of the bidirectional transmission
path in a conductive state during the pass period of an upward signal
transmitted from a subscriber device, and sets the upward transmission
path in a cutoff state during the remaining period.
According to the present invention, the upward transmission path is kept in
the cutoff state by the transmission path opening/closing device while the
subscriber device transmits no upward signal. For this reason, even if
noise flows into the upward transmission path due to an open connector
terminal in a subscriber residence or a used coaxial cable insufficient in
electromagnetic shielding characteristics, the inflow noise is cut off by
the transmission path opening/closing device not to flow out to the
upstream side of the bidirectional transmission path. Therefore, the
ingress noise level on the bidirectional transmission path is suppressed
low. Bidirectional transmission can be performed under a high-quality
transmission environment.
Note that the subscriber device represents a modem, an STB, and the like
for performing bidirectional communication with a central equipment or
signal transmission in only the upward direction. The subscriber devices
are classified as devices used inside subscriber residences and devices
used outside the residences, e.g., in a campus.
As the arrangement of the transmission path opening/closing device, the
following can be considered. That is, a switch control section and a gate
switch section are separately prepared. The switch control section is
incorporated in or added to an in-home device of a subscriber. The gate
switch section is arranged on the bidirectional transmission path outside
the subscriber residence. When the switch control section detects the
transmission period of the upward signal from the subscriber device or
receives the notification of the transmission period, the switch control
section generates a transmission indication signal representing the
transmission period and transmits it to the bidirectional transmission
path. The gate switch section opens/closes the upward transmission path of
the bidirectional transmission path in accordance with the transmission
indication signal sent from the switch control section.
With this arrangement, the gate switch section arranged on the
bidirectional transmission path outside the subscriber residence can be
reduced in size and cost.
As another arrangement of the transmission path opening/closing device, the
following can be considered. That is, the switch control section for
generating the transmission indication signal, and the gate switch section
for opening/closing the upward transmission path of the bidirectional
transmission path are accommodated in a common housing. This housing is
interposed and set in the bidirectional transmission path.
With this arrangement, an additional device incorporating the switch
control section is not prepared in the subscriber residence, and the
switch section need not be added to an existing device in the subscriber
residence. Therefore, a load on the subscriber can be reduced. In
addition, the transmission indication signal generated by the switch
control section need not be notified to the gate switch section via the
bidirectional transmission path. For this reason, the transmission
indication signal can be accurately notified without being affected by
noise and the like on the bidirectional transmission path. Therefore, the
operation reliability of transmission path open/close control can be kept
high.
As for the installation location of the transmission path opening/closing
device or the gate switch section on the bidirectional transmission path,
it may be incorporated in either an in-home splitter or a connection unit
for connecting the out-of-home and in-home transmission paths of the
bidirectional transmission path. Note that the connection unit includes a
surge suppresser set outside the residence, a unit set to guide an
underground cable to the ground, and the like.
The transmission path opening/closing device or the gate switch section may
be installed in a tap-off on the bidirectional transmission path. By
installing the transmission path opening/closing device or the gate switch
section in the tap-off, a subscriber cannot remove or remodel the
transmission path opening/closing device or the gate switch section.
When the transmission path opening/closing device and the gate switch
section is to be installed in the tap-off, they may be separately arranged
for a plurality of local lines or commonly arranged for the plurality of
local lines. When the transmission path opening/closing device or the gate
switch section is arranged for each local line, the influence of noise on
the transmission path opening/closing device or the gate switch section
can be reduced, thereby keeping the operation reliability high. To the
contrary, when the transmission path opening/closing device and the gate
switch section is commonly arranged for the plurality of local lines, the
influence of the ingress noise from each subscriber increases. However,
since only one transmission path opening/closing device is provided to a
plurality of subscribers, the installation number of transmission path
opening/closing devices can be advantageously decreased.
The transmission indication signal is sent upon modification in accordance
with a random data sequence as a notification means for the transmission
indication signal. Even if a plurality of transmission indication signals
collide with each other to cause interference, the degree can be
suppressed to reduce the malfunction of the gate switch section.
As the notification means for the transmission indication means, a spread
spectrum scheme and a CDMA scheme using this scheme may be applied. With
these schemes, the transmission indication signal has a high resistance to
noise and the like, and can also be protected from an interference signal.
At this time, high security can be ensured by managing a spread code
sequence as an encryption key or a secret key.
The transmission indication signals are sent from the switch control
section by using a plurality of carriers. The transmission indication
signals sent by using the plurality of carriers are received and
reproduced by the gate switch section. only when all the transmission
indication signals with the plurality of carriers are reproduced, the
upward transmission path of the bidirectional transmission path may be set
in the conductive state.
At that time, the transmission indication signals undergo signal processing
using preset parameters and are sent by using the plurality of carriers.
The gate switch section checks whether the received and reproduced
transmission indication signals with the plurality of carriers have
undergone the signal processing using the parameters. If NO, the upward
transmission path of the bidirectional transmission path may be restored
to the cutoff state. With this operation, when an interference signal is
sent due to noise, an intentional mischief, and the like, the resistance
to them can be further enhanced.
Further, a signal delay means is prepared. This signal delay means is for
delaying the upward signal by a time required for the gate switch section
to set the upward transmission path in the conductive state in accordance
with the transmission indication signal after the subscriber device starts
to transmit the upward signal. The signal delay means is arranged at least
one of the switch control section and the gate switch section. With this
arrangement, the conduction timing in the gate switch section can be
accurately synchronized with the pass timing of the upward signal. Noise
can be effectively prevented from flowing into the upward transmission
path, and the upward signal can be passed reliably.
As the arrangement of the signal delay means, the following can be
considered. That is, in the first arrangement, the frequency of the upward
signal is converted to a higher frequency. Then, the upward signal is
delayed by a required time via a delay element. The frequency of the
delayed upward signal is returned to the original frequency. With this
arrangement, the required delay amount can be obtained by one delay
element. In the second arrangement, a plurality of delay elements having
different pass bands are prepared. Upward signals are delayed in
accordance with their bands by combining these delay elements. With this
arrangement, the required delay amount can also be reliably obtained for
the broadband upward signals.
Power required for the operation of the gate switch section may be obtained
from the transmission indication signal output from at least one of the
switch control section and the subscriber device, from DC or AC power, or
a signal sent via a trunk line or a local line. A work related to the
power supply such as wiring of another supply line for the power supply
can be eliminated.
The switch control section may be incorporated in the subscriber device.
The switch control section detects the transmission period of the upward
signal from an in-home device main body, or sends the transmission
indication signal representing the transmission period upon reception of
the notification of the transmission period, thereby controlling the
ON/OFF operation of the gate switch section. With this arrangement, a new
subscriber need not separately purchase or install a subscriber device and
a switch control section.
An adaptor device accommodating the switch control section may be prepared,
added to the subscriber device, and used. With this arrangement, a new
subscriber device having a switch control function need not be prepared.
The effect of the present invention can be obtained by only adding the
adaptor device to the existing subscriber device.
According to another aspect of the present invention, a transmission path
opening/closing device is arranged in a bidirectional transmission path in
a cable network system for performing bidirectional transmission between a
central equipment and a subscriber devices via the bidirectional
transmission path constituted in a tree or star shape. At the same time,
an ingress noise monitoring/analysis device is arranged between the
transmission path opening/closing device on the bidirectional transmission
path and the central equipment. Noise on an upward transmission path which
has passed through the transmission path opening/closing device is
monitored in the ingress noise monitoring/analysis device. The degree of
influence of the noise on the transmission quality of the upward
transmission path is determined on the basis of the monitoring data. At
the same time, the subscriber of the originating source is estimated on
the basis of originating source information included in the upward signal
sent together with the noise. The determination result and the estimation
result are notified to the central equipment.
According to this aspect of the invention, if the ingress noise is not cut
off by the transmission path opening/closing device and flows out to the
upstream side of the bidirectional transmission path, the degree of
influence is determined by the ingress noise monitoring/analysis device.
At the same time, the subscriber of the originating source is estimated.
The determination data and the estimation data are notified to a network
management system installed in the central equipment to require system
maintenance/operation personnel to make an investigation and take a
required action. Therefore, even if a subscriber erroneously directly
connects, e.g., a personal computer to the transmission path, the
erroneous state can be quickly canceled without being left unchanged for a
long time.
In the ingress noise monitoring/analysis device, a noise monitoring means
is constituted by a plurality of filters having bandwidths equivalent to
measurement resolutions. Frequency bands as monitoring targets are
simultaneously monitored by these filters. With this arrangement,
intermittent noise which disappears in a very short time can be detected
reliably.
When the bidirectional transmission path as a monitoring target is branched
into a plurality of branch transmission paths, the noise monitoring means
of the ingress noise monitoring/analysis device sequentially monitors
noise on each of the plurality of branch transmission paths for a
predetermined time. If a branch transmission path of these branch
transmission paths on which the degree of influence of the noise exceeds a
predetermined level is found, the branch transmission path is monitored
more intensively than the remaining branch transmission paths. With this
operation, the branch transmission path on which the degree of influence
of the ingress noise is increasing can be intensively monitored, so that a
proper measure can be executed in an early stage.
According to still another aspect of the present invention, the
transmission path opening/closing device is set in the transmission path
of a one-way transmission system for only transmitting a signal from a
subscriber device to a central equipment. The transmission path
opening/closing device sets the upward transmission path of the
transmission path in the conductive state during the pass period of a
signal transmitted from the subscriber device, and sets the upward
transmission path in the cutoff state during the remaining period.
According to this aspect of the present invention, the influence of the
ingress noise can be reduced not only in the bidirectional transmission
system but also in a transmission system in which data obtained by the
inspection of a meter which represent the use amount of power, gas, water
supply, or the like by a subscriber are periodically transmitted to a
central equipment.
Additional objects and advantages of the invention will be set forth in the
description which follows, and in part will be obvious from the
description, or may be learned by practice of the invention. The objects
and advantages of the invention may be realized and obtained by means of
the instrumentalities and combinations particularly pointed out in the
appended claims.
BRIEF DESCRIPTION OF THE DRAWINGS
The accompanying drawings, which are incorporated in and constitute a part
of the specification, illustrate presently preferred embodiments of the
invention and, together with the general description given above and the
detailed description of the preferred embodiments given below, serve to
explain the principles of the invention.
FIG. 1 is a block diagram showing a cable network system according to the
first and fourth embodiments of the present invention;
FIG. 2 is a block diagram showing the arrangement of a gate switch unit
(GSU) provided to the system shown in FIG. 1;
FIG. 3 is a chart showing the filter characteristics of frequency
separation circuits 103 and 105 provided to the GSU shown in FIG. 2;
FIG. 4 is a timing chart used to explain the operation of the GSU shown in
FIG. 2;
FIG. 5 is a block diagram schematically showing the arrangement of a G-STB
or a G-MDM;
FIG. 6 is a timing chart used to explain the operation of the G-STB or the
G-MDM shown in FIG. 5;
FIG. 7 is a block diagram showing the arrangement of a non-associated modem
adaptor (NGA);
FIG. 8 is a timing chart used to explain the operation of the NGA shown in
FIG. 7;
FIG. 9 is a table showing ingress noise inhibiting effects obtained by
combining an HFC architecture and the GSU;
FIG. 10 is a circuit diagram showing a detailed configuration of the GSU;
FIG. 11A is graph showing the characteristics of gallium arsenide field
effect transistor of depletion type;
FIG. 11B is graph showing the characteristics of gallium arsenide field
effect transistor of enhancement type;
FIG. 12 is a circuit diagram showing another arrangement of the gate
switch;
FIG. 13 is a circuit diagram showing still another arrangement of the gate
switch;
FIG. 14 is a circuit diagram showing still another arrangement of the gate
switch;
FIG. 15A is a view showing a connection between a telephone modem and the
GSU;
FIG. 15B is a view showing another connection between the telephone modem
and the GSU;
FIG. 16 is a block diagram showing a detailed arrangement of a transmission
indication signal generation circuit;
FIG. 17 is a block diagram showing another detailed arrangement of the
transmission indication signal generation circuit;
FIG. 18 is a block diagram showing an arrangement of a transmission
indication signal detection circuit;
FIG. 19 is a block diagram showing another arrangement of the transmission
indication signal generation circuit;
FIG. 20 is a block diagram showing another arrangement of the transmission
indication signal detection circuit;
FIG. 21 is a block diagram showing an arrangement of a GSU attached tap-off
according to the second embodiment of the present invention;
FIG. 22 is a block diagram showing another arrangement of the GSU attached
tap-off according to the second embodiment of the present invention;
FIG. 23 is a block diagram showing the arrangement of a self-terminated
type gate switch unit (S-GSU) according to the third embodiment of the
present invention;
FIG. 24 is a timing chart used to explain the operation of the S-GSU shown
in FIG. 23;
FIG. 25 is a block diagram showing the arrangement of a distribution hub 2
provided with an ingress noise monitoring/analysis device 40 according to
the fourth embodiment of the present invention;
FIG. 26 is a block diagram showing the arrangement of the ingress noise
monitoring/analysis device shown in FIG. 25;
FIG. 27 is a graph showing the spectrum distribution of noise flowing from
a connector open terminal;
FIG. 28 is a graph showing the spectrum distribution of inflow noise when a
lead wire is connected to the connector open terminal;
FIG. 29 is a graph showing the spectrum distribution of inflow noise in a
state in which the connector terminal is terminated;
FIG. 30 is a graph showing an example of an interference percent
availability caused by ingress noise in a bidirectional cable network
system;
FIG. 31 is a graph showing comparison between the spectrum distribution of
ingress noise observed on an upward transmission path in a state having no
data transmission signal, and the assumed level of a data signal;
FIG. 32 is a view showing an arrangement of the cable network system using
the HFC architecture;
FIG. 33 is a view showing an arrangement of a subsystem in the system shown
in FIG. 32;
FIG. 34 is a table showing an example of the degree of suppression of
ingress noise caused by the HFC architecture; and
FIG. 35 is a graph showing an example of band division characteristics used
to explain a CTU scheme.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
(First Embodiment)
FIG. 1 is a block diagram showing a cable network system according to the
first embodiment of the present invention. The same reference numerals as
in FIG. 32 denote the same parts in FIG. 1 to describe the first
embodiment.
In the system of this embodiment, a gate switch unit (GSU) 30 is set below,
e.g., the eaves of each subscriber residence. A drop cable 9 is connected
to an in-home splitter 18 via the GSU 30. An in-home device desired by a
subscriber is connected to the in-home splitter 18. The in-home device
includes, e.g., television receivers 11 and 13, a personal computer (PC)
14, a videophone 15, and a digital telephone 16.
Of these in-home devices, the television receiver 13 for a subscriber who
desires the VOD services is provided with a set-top box (G-STB) 31
associated with the gate switch. If the television receiver 13 has only an
existing set-top box (N-STB) 33 not associated with the gate switch, a
non-associated modem adaptor (NGA) 34 is added to the N-STB 33.
Each of the personal computer 14, the videophone 15, and the digital
telephone 16 is provided with a modem (G-MDM) 32 associated with the gate
switch. If the device has only an existing modem (N-MDM) 35 not associated
with the gate switch, the non-associated modem adaptor (NGA) 34 is
provided in addition to the N-MDM 35.
The G-MDM 32 and the N-MDM 35 must be changed in accordance with the PC
on-line services and the telephone services. However, since these
functions are similar, and it can be expected that a common modem will be
used in the future, a case using the common modem will be described.
When the subscriber does not desire the bidirectional services, e.g., when
the subscriber receives only television broadcasting programs, the
television receiver 11 is connected directly to the in-home splitter 18
without mediacy of the G-STB 31 and the like.
For example, the gate switch unit (GSU) 30 is constituted as follows. FIG.
2 is a block diagram showing the arrangement of the GSU 30. The GSU 30
comprises first and second frequency band separation circuits 103 and 105,
a gate switch 109, and a transmission indication signal detection circuit
113.
The first frequency band separation circuit 103 comprises filters 103a and
103b for separating a frequency band H in the downward direction and a
frequency band L in the upward direction. The first frequency band
separation circuit 103 is connected to the drop cable 9 via a connector
102. The second frequency band separation circuit 105 comprises filters
105a, 105b, and 105c for separating the frequency band H in the downward
direction, the frequency band L in the upward direction, and a frequency
band C of a transmission indication signal, and is connected to the
in-home splitter 18 via a connector 106 and an in-home drop coaxial cable
107. FIG. 3 shows the filter characteristics of the first and second
frequency band separation circuits 103 and 105.
The gate switch 109 is interposed and set in an upward signal path between
the filter 105b of the second frequency band separation circuit 105 and
the filter 103b of the first frequency band separation circuit 103. The
ON/OFF state of the gate switch 109 is controlled by a switch control
signal output from the transmission indication signal detection circuit
113. FIG. 4 shows the timing relationship between the transmission
indication signal, the switch control signal, and an upward transmission
signal.
Referring to FIG. 4, when the transmission indication signal appears on the
in-home drop coaxial cable 107, the switch control signal output from the
transmission indication signal detection circuit 113 becomes active a time
t1 after this moment. The gate switch 109 is turned on upon reception of
this switch control signal. When the gate switch 109 is turned on, the
upward transmission signal appears on the in-home drop coaxial cable 107 a
time t2 after this moment. After passing through the filter 105b and the
gate switch 109, the upward transmission signal passes through the filter
103b and is sent from the connector 102 to the drop cable 9.
When the transmission indication signal is disabled, the switch control
signal becomes inactive a time t3 after this moment, thereby turning off
the gate switch 109. At this time, the transmission of the upward
transmission signal is completed before the transmission indication signal
is disabled. That is, the transmission of the upward transmission signal
is completed a time t4 before the moment at which the gate switch 109 is
turned off. For this reason, the upward transmission signal is prevented
from being interrupted upon turning off the gate switch 109.
The additional functions of the G-STB 31 and the G-MDM 32 will be described
below. FIG. 5 is a block diagram showing the arrangement of each circuit.
Each of the G-STB 31 and the G-MDM 32 comprises a transmission indication
signal generation circuit 161 and a frequency band separation circuit 153
in addition to a demodulation circuit 155 for demodulating a downward
signal and outputting the reception data, and a modulation circuit 158 for
modulating and transmitting transmission data.
The frequency band separation circuit 153 has filters 153a, 153b, and 153c
for separating the frequency band H in the downward direction, the
frequency band L in the upward direction, and the frequency band C of the
transmission indication signal, similar to the second frequency band
separation circuit 105 of the gate switch unit 30 described above. The
frequency band separation circuit 153 is connected to the in-home splitter
18 via a connector 152 and an in-home drop coaxial cable 19.
The transmission indication signal generation circuit 161 generates the
transmission indication signal upon reception of a transmission indication
control signal from a transmission data processing section side (not
shown). This transmission indication signal is sent to the in-home
splitter 18 via the filter 153c and the connector 152. FIG. 6 is a timing
chart for these signals. Referring to FIG. 6, the transmission data
processing section outputs the transmission indication control signal
prior to transmission of the upward transmission signal. Then, the
transmission indication signal generation circuit 161 generates the
transmission indication signal a time t5 after the moment at which the
transmission indication control signal becomes active. The transmission
data processing section supplies the upward transmission signal a time t6
after the generation start point of the transmission indication signal. On
the other hand, when the transmission of the upward transmission signal is
completed to make the transmission indication control signal inactive. the
transmission of the transmission indication signal is stopped a time t7
after this moment.
The non-associated modem adaptor (NGA) 34 connected to the existing STB
(N-STB) 33 or the modem (N-MDM) 35 not associated with the GSU 30 is
constituted as follows. FIG. 7 is a block diagram showing the arrangement
of the NGA 34. The non-associated modem adaptor 34 comprises first and
second frequency band separation circuits 205 and 203, a delay circuit
209, an amplifier 211, a transmission signal detection circuit 213, and a
transmission indication signal generation circuit 215.
The first frequency band separation circuit 205 has filters 205a and 205b
for separating the frequency band H in the downward direction and the
frequency band L in the upward direction. The first frequency band
separation circuit 205 is connected to the N-STB 33 or the N-MDM 35 via a
connector 206. The second frequency band separation circuit 203 has
filters 203a, 203b and 203c for separating the frequency band H in the
downward direction, the frequency band L in the upward direction, and the
frequency band C of the transmission indication signal. The second
frequency band separation circuit 203 is connected to the in-home splitter
18 via a connector 202. The first and second frequency band separation
circuits 205 and 203 have the same filter characteristics as those of the
first and second frequency band separation circuits 103 and 105 of the GSU
30 described above, i.e., the characteristics shown in FIG. 3.
The transmission signal detection circuit 213 detects the upward
transmission signal transmitted from the N-STB 33 or the N-MDM 35. The
transmission signal detection circuit 213 generates the transmission
indication control signal during the detection period of the upward
transmission signal and supplies it to the transmission indication signal
generation circuit 215. The transmission indication signal generation
circuit 215 generates the transmission indication signal in accordance
with the transmission indication control signal supplied from the
transmission signal detection circuit 213. The transmission indication
signal is sent to the in-home splitter 18 via the filter 203c of the
second frequency band separation circuit 203 and the connector 202.
FIG. 8 shows the timing relationship between the transmission indication
control signal, the transmission indication signal, and the upward
transmission signal. Referring to FIG. 8, when the upward transmission
signal is supplied from the N-STB 33 or the N-MDM 35, the transmission
indication control signal output from the transmission signal detection
circuit 213 becomes active a time t11 after this moment. Upon reception of
this transmission indication control signal, the transmission indication
signal generation circuit 215 starts to generate the transmission
indication signal a time t12 after this moment. The upward transmission
signal is input to the delay circuit 209. The delay circuit 209 outputs
the upward transmission signal by a delay time t14, which is the sum of
the time t11 required to detect the upward transmission signal, the time
t12 required to generate the transmission indication signal, and the
operation delay time t13 of the gate switch 109 of the GSU 30 described
above.
The delay amount t14 of the delay circuit 209 is about several to several
tens .mu.sec at most. Therefore, the delay circuit 209 can be realized by
a surface acoustic wave element, a glass delay element, or the like. Note
that, if the specific band (which is the ratio of the central frequency to
the pass bandwidth and is 1.55 for an upward band of 5 to 40 MHz) exceeds
1, it becomes difficult to realize the delay circuit 209 by one delay
element because of limitations on the characteristics of the delay
element.
As a solution for this problem, for example, the following methods can be
considered. That is, according to the first method, the frequency of the
upward transmission signal is converted into a higher frequency in the
delay circuit 209, and the obtained signal is input to the delay element.
Upon delay processing, the frequency of the upward transmission signal is
returned to the original frequency. According to the second method, the
delay circuit 209 is constituted by combining, e.g., two delay elements
for a band of 5 to 15 MHz and a band of 15 to 40 MHz. According to the
third method, e.g., the target frequency band of the NGA is limited to 20
to 40 MHz, and target existing services of the NGA are shifted to the band
of 20 to 40 MHz and operated.
The upward transmission signal output from the delay circuit 209 with a
delay in this manner is amplified by the amplifier 211 to compensate for
the attenuation amount of the signal in the delay circuit 209, and then
sent to the in-home splitter 18 via the filter 203b and the connector 202.
On the other hand, when the output of the upward transmission signal from
the N-STB 33 or the N-MDM 35 is stopped, the transmission indication
control signal goes to low level a time t15 after this moment. Upon
reception of the transmission indication control signal of low level, the
transmission indication signal becomes inactive a time t16 after this
moment. At this time, the time t15 is properly set in consideration of the
delay time t14 in the delay circuit 209. With this setting, the upward
transmission signal on the in-home drop coaxial cable 19 substantially
disappears a time t17 before the transmission indication signal becomes
inactive. The NGA 34 must be constituted such that the NGA 34 does not
malfunction due to noise, i.e., that the transmission signal detection
circuit 213 does not erroneously detect noise as the upward transmission
signal. For this purpose, the NGA 34 is directly coupled to the N-MDM 35
or the N-STB 33 without any connector open terminal therebetween.
As described above, in this embodiment, the GSUs 30 are installed in almost
all subscriber residences. In addition, the G-STB 31 and the G-MDM 32 are
set for a subscriber who wants to receive the bidirectional services.
Alternatively, the NGA 34 is attached to the existing N-STB 33 or N-MDM
35. The presence of the upward transmission signal from an in-home device
is checked by the G-STB 31 and G-MDM 32 or the NGA 34. While the upward
transmission signal is detected, the transmission indication signal is
supplied to the GSU 30. Only while the transmission indication signal is
output, the gate switch 109 in the GSU 30 is in the ON state to send the
upward transmission signal to the drop cable 9. To the contrary, while no
transmission indication signal is output, the gate switch 109 is kept in
the OFF state.
Even if, therefore, there is a connector open terminal to which no in-home
device is connected in the subscriber residence, the connection state of
the in-home device is bad, or a used coaxial cable is insufficient in
electromagnetic cutoff characteristics, noise flowing from the connector
open terminal or the coaxial cable is cut off by the gate switch 109 and
is not sent to the drop cable 9 except when the GSU 30 is in the ON state,
i.e., except during the transmission period of the upward transmission
signal. For this reason, the ingress noise level on the upward
transmission path of the system can be suppressed greatly.
For example, the noise removing ratio is set to 40 dB when the GSU 30 is in
the OFF state, and the maximum number of GSUs 30 simultaneously turned on
is 1% of the number of subscribers (the number of subscribers who
simultaneously perform communication at a given moment; almost equal to
the number of upward carriers). In this case, as shown in FIG. 9, the
narrowband coherent noise, the broadband incoherent noise, and the
specific subscriber noise can be suppressed to 28 dB, about 20 dB, and 40
dB, respectively, in addition to the above-described suppression effect of
the HFC architecture. A very-high-quality transmission environment can be
realized. In other words, the cable network system can be designed or
operated without considering the presence of ingress noise, thereby
achieving an enhanced effect.
More specifically, many of the above-described ingress noise measures,
which have been considered until now, are not required, and restriction
conditions can be less strict. For example, as for the HFC architecture,
the cable network system in the upward direction need not be operated for
every subsystem. If it is operated for 5 subsystems as a unit, similar to
the downward direction, the bidirectional services can be provided under a
high-quality transmission environment. In addition, not only the frequency
agility function but also band reduction+frequency division multiplex are
not required. To decrease the number of GSUs simultaneously turned on (the
number of subscribers who simultaneously perform communication), it is
desirable to employ a broadband time-division multiplex scheme. Further,
the error correction functions are also reduced, and the retransmission
number when errors cannot be corrected is also greatly reduced. The
transmission efficiency can be improved, and a high-efficiency modulation
scheme of 16 QAM, 64 QAM, or the like can be applied, which has been
difficult to apply in the upward direction.
In other words, a high-quality transmission environment can be provided
over the entire band of the upward transmission path by applyirg this
embodiment. For this reason, a bandwidth assignable to services can be
widened 2 to 5 times. The transmission ability of the upward transmission
path, which is conventionally limited to about 10 Mbps, can increase to
100 Mbps or more if a high-efficiency modulation system is applied. For
example, the upward operation unit is set to one subsystem (500 home
paths), the subscription ratio of the bidirectional services is 30%, and
the simultaneous use ratio is 30%. In this case, the transmission ability
of 2 Mbps or more can be ensured per subscriber. Even if the upward
operation unit is set for 5 subsystems, the transmission ability of 400
kbps or more can be ensured per subscriber. These transmission abilities
respectively correspond to an ISDN service H1 (1.544 Mbps; called T1 in
United States) and a service H0 (384 kbps) in a telephone system,
resulting in a great effect in the cable network system.
Note that cable industrial companies make investment in attachment of the
GSUs 30 to all the subscriber residences. However, if a plurality of
subscribers share one GSU 30, an equivalent effect can be obtained. For
example, two subscribers share one GSU 30. In this case, although the
narrowband coherent noise and the broadband incoherent noise are
respectively increased by about 3 to 4 dB, a transmission environment much
higher in quality than a conventional transmission environment can be
provided.
In the above description, the noise removing ratio is set to, e.g., 40 dB
when the GSU 30 is in the OFF state. Even if the noise removing ratio is
set to 20 dB or less with respect to the narrowband coherent noise or the
broadband incoherent noise, a sufficient noise suppression effect free
from any problem in practical use can be obtained. An increase in noise
suppression ratio means an improvement in resistance of the system to the
specific subscriber noise, which is supposed to be at high noise level.
Further, in the arrangement of this embodiment, all upward signals are
delayed by several to several tens .mu.sec. This is become transmission
paths are prolonged by about 1 km to several km at most, and does not pose
any problem in the operation of various bidirectional services.
When the GSU 30 is inserted between the drop cable 9 and the in-home
splitter 18 in the subscriber residence, its installation or design must
be considered carefully.
The first point is that a work required for an operator to enter the
subscriber residence must be avoided as much as possible. This is an
indispensable condition particularly in United States where people feel
anxious about the security aspect. Therefore, it is a prerequisite that
the GSU 30 is attached outside the residence, e.g., under the eaves.
The second point is that the GSU 30 must operate without any supply of
power from a commercial AC power supply. The reason is that an additional
work for the power supply is required to supply power from the commercial
AC power supply to the GSU 30 set, e.g., under the eaves, as described
above, resulting in an increase in work cost. It is possible to supply
power from the cable system side via a drop cable, like a cable telephone
system (to be described later). In a system not providing the services,
however, a tap-off, a splitter, a booster amplifier, or the like must be
exchanged with a corresponding one of the current through type. On the
other hand, according to another power supply method, power is supplied
from a G-MDM, a G-STB, or the like in the subscriber residence. In this
method, however, it must be taken into consideration that the G-MDMs or
the like are not installed in all the subscriber residences, and that the
installed G-MDMs or the like are not always powered on or turned on.
The third point is that the passing loss when an upward path is set in the
conductive state must be minimized, in addition to a reduction in passing
loss in the downward path. If the loss is large in the GSU 30, the signal
level distribution in the whole cable network system must be reconsidered.
It is important to reduce the loss to a degree (within 1 to 2 dB) posing
no problem in practical use under the condition of no external power
supply.
The fourth point is that the GSU 30 must be prevented from malfunctioning
due to noise flowing from a connector open terminal. It can be considered
as a type of malfunction that the GSU 30 is turned on with no normal
upward signal or fails to be turned on with a normal upward signal. In
addition, the GSU 30 is desirably provided with a protection (security)
means to prevent specific subscriber noise and particularly an
interference signal from a malicious subscriber from being easily
transmitted.
FIG. 10 shows a detailed circuit configuration of the GSU 30 in
consideration of the various problems. This circuit operates using the
transmission indication signal sent from the G-MDM or the like, as a power
supply. A high-pass filter (HPF) 310 is connected between a connector 301
connected to the drop cable 9 and a connector 305 connected to the in-home
splitter 18. The HPF 310 corresponds to the filters 103a and 105a shown in
FIG. 2, and is constituted by connecting capacitors and inductances 311 to
322 in a ladder shape.
A series circuit constituted by two bandpass filters (BPFs) 330 and 350 and
a gate switch 340 is connected between the connectors 301 and 305 so as to
be parallel to the HPF 310. The BPFs 330 and 350 correspond to the filters
105b and 103b shown in FIG. 2, respectively. Each BPF is constituted by
hybrid-connecting capacitors and inductors, as shown in FIG. 10.
The gate switch 340 corresponds to the gate switch 109 shown in FIG. 2, and
is constituted by field effect transistors 342, 343, and 346, and
resistors 341, 345, 390, and 391. The transistors 342 and 346 are of the
normally-ON type in which a transistor is in the ON state while no gate
voltage is applied. The transistor 343 is of the normally-OFF type in
which a transistor is in the OFF state while no gate voltage is applied.
More specifically, when no gate voltage is applied to the transistors 342,
343, and 346, the transistors 342 and 346 are in the ON state, whereas the
transistor 343 is in the OFF state. For this reason, noise flowing from
the in-home side via the connector 305 and the bandpass filter 330 is
terminated with a predetermined impedance defined by the resistor 341. As
a result, the inflow noise is prevented from flowing out to the drop cable
9 via the connector 301. That is, at this time, the GSU 30 can be
considered as if the GSU 30 was terminated with the predetermined
impedance defined by the resistor 345 when viewed from the drop cable 9
side. Note that the resistors 390 and 391 are for determining the source
and drain potentials of the transistors 342, 343, and 346, and have
resistance values larger than those of the resistors 341 and 345.
On the other hand, when the gate voltage is applied to the transistors 342
and 346 via a transmission indication signal detection circuit 370 to
reach a predetermined threshold value (negative voltage in the example of
FIG. 10; V.sub.G-), the transistors 342 and 346 are turned off, and
instead the transistor 343 is turned on. For this reason, after passing
through the connector 305 and the bandpass filter 330, the upward
transmission signal sent from the in-home splitter 18 side passes through
the transistor 343 and is sent to the drop cable 9 via the bandpass filter
350 and the connector 301.
The field effect transistors 341, 343, and 346 of this type can be realized
by gallium arsenide transistors of a depletion type (normally-ON type) or
an enhancement type (normally-OFF type). FIGS. 11A and 11B show the ON
characteristics of the gallium arsenide transistors of the depletion and
enhancement types, respectively. Each ON resistance can be set to less
than several ohms or a few ohms in accordance with design. Each transistor
can be provided with sufficient cutoff characteristics. Note that a
similar transistor can also be realized by a silicon transistor in
addition to the gallium arsenide transistor. Since the parasitic
capacitance between the source and drain is fractionally larger than that
of gallium arsenide transistors, a slight degradation of cutoff
characteristics occurs. To obtain the sufficient cutoff characteristics,
the resistance value in the ON state increases, resulting in an increase
in passing loss.
Therefore, by careful circuit or process design, the use of normally-ON
type and normally-OFF type silicon transistors, or a combination of
gallium arsenide transistors (normally-ON type) and silicon transistors
(normally-OFF type) is possible.
By applying field effect transistors of the normally-OFF type, any voltage
need not be applied at all to the respective transistors in a normal state
(the GSU is powered off or turned off). For this reason, only when the GSU
is turned on, a predetermined voltage is applied to the gates of the
respective transistors.
Furthermore, the transmission indication signal detection circuit 370 is
connected to the connector 305 on the in-home side via a bandpass filter
(BPF) 360. The bandpass filter 360 corresponds to the filter 105c shown in
FIG. 2, and is constituted by hybrid-connecting capacitors and inductors,
as shown in FIG. 10.
The transmission indication signal detection circuit 370 corresponds to the
transmission indication signal detection circuit 113 in FIG. 2, and is
constituted by a transformer 371, diodes 372 to 375 serving as rectifying
circuits, capacitors 376 and 377, resistors 378 and 379, and Zener diodes
380 and 381. The negative voltage V.sub.G- to be applied to the
transistors 342 and 346 and a positive voltage V.sub.G+ to be applied to
the transistor 343 are generated from a transmission indication signal
through these circuits.
Since the transmission indication signal sent from the G-STB 31, the G-MDM
32, or the NGA 34 passes through the in-home splitter 18 and the like, the
signal is attenuated by about several to 10 dB and input to the GSU 30. In
this arrangement, however, the transmission indication signal having
passed through the bandpass filter 360 with a central equipment frequency
of, e.g., 2 MHz is boosted by the transformer 371. The obtained signal is
converted into a DC signal by the diode rectifying circuits 372 to 375.
Further, predetermined negative and positive voltages are kept by the
Zener diodes 380 and 381.
The capacitors 376 and 377 and the resistors 378 and 379 give the gate
switch 340 a time constant. When the transmission indication signal is
sent, the diode rectifying circuits 372 to 375 have low resistances. For
this reason, the parasitic capacitances of the capacitors 376 and 377 and
the transistors 342, 343, and 346 are charged within a short time (e.g., 1
.mu.sec or less). When the transmission indication signal disappears, the
diode rectifying circuits 372 to 375 have high resistances. For this
reason, the capacitors 376 and 377 are gradually discharged in accordance
with the above time constant (e.g., several to several tens .mu.sec). With
this operation, the gate switch 340 can be reliably kept in the ON state
until the upward transmission signal passes through the gate switch 340,
as shown in FIG. 4.
An arrester 302 for preventing a surge voltage caused by thunder, and a
capacitor 303 are connected to the connector on the drop cable 9 side. The
capacitor 303 prevents AC power and DC power from being supplied to the
GSU 30 when the AC power is sometimes supplied to an in-home modem via the
drop cable 9, like the cable telephone services (to be described later).
The circuit having this arrangement can operate without any commercial
power supply or any supply of power via the drop cable. The passing loss
through the filters 310, 330, and 350 can be greatly reduced by selecting
element characteristics. The passing loss in the GSU 30 can be greatly
reduced to about 1 to 2 dB in both the upward and downward directions by
properly designing the field effect transistors 342, 343, and 346 for the
gate switch 340. In addition, the transmission indication signal detection
circuit 370 can be made less reactive with respect to electro-magnetic
noise flowing from a connector open terminal or the like by combining the
band limitation of the bandpass filter 360 and the field effect
transistors 342, 343, and 346 having predetermined threshold values, or by
adding a squelch circuit (not shown) having a predetermined threshold
value to the transmission indication signal detection circuit. In other
words, the transmission indication signal detection circuit 370 is
constituted to react to only the transmission indication signal having
predetermined power (voltage) and more within a predetermined frequency
band. With this arrangement, the operation is properly enhanced.
Assume that the GSU 30 is attached outside, e.g., under the eaves of the
subscriber residence. In this case, if the subscriber removes the GSU 30
without permission and directly connects the drop cable 9 to the in-home
splitter 18, or attaches another device such as a tap-off between the GSU
30 and the drop cable 9, the effect of the GSU 30 is degraded. To prevent
this degradation, for example, the connector 301 for connecting the drop
cable 9 is sealed upon connection of the drop cable 9, or the GSU 30 is
installed such that it cannot be removed without using a specific tool. To
prevent the GSU 30 itself from flowing noise to the upward transmission
pith, the housing of the GSU 30 is covered with a material having
electromagnetic shielding characteristics, such as a metal.
Another arrangement of the gate switch as a main constituent element of the
GSU 30 will now be described. In the above-described example, the ingress
noise is cut off by the enhancement type transistor 343 in a normal state
(the GSU 30 is in the OFF state). However, if noise larger than the
threshold value of the transistor 343 flows, the transistor 343 is turned
on to flow the noise from the GSU 30 to the drop cable.
FIG. 12 shows an example of a gate switch having a function of preventing
the outflow of the ingress noise. In a gate switch 580, resistors 581 and
586 and normally-ON type transistors 582 and 587 have the same functions
of the resistors 341 and 345 and the normally-ON type transistors 342 and
346, respectively. normally-OFF type transistors 583 and 584 and a
normally-ON type transistor 585 are combined to cut off excess ingress
noise. More specifically, the normally-OFF type transistor 583 is normally
in the OFF state. However, even if excess noise exceeds the threshold
value of the transistor 583 to turn on the transistor 583, the transistor
583 is short-circuited to the earth by the normally-ON type transistor 585
which is normally in the ON state. For this reason, such excess noise as
to turn on the normally-OFF type transistor 584 is not applied to the
transistor 584. The transistor 584 is kept in the OFF state.
On the other hand, when the voltages VG.sub.+ and V.sub.G- are applied
from the transmission indication signal detection circuit to the
transistors 583 and 584 and the transistors 582, 585, and 587,
respectively, the ON/OFF state of each transistor is inverted to allow the
upward transmission signal to pass through the gate switch 580. Resistors
588, 589, and 590 correspond to the above-described resistors 390 and 391,
and determine the source and drain potentials of the corresponding
transistors.
FIG. 13 shows the arrangement of another gate switch. A gate switch 700
uses PIN diodes 702 and 703 which are frequently used as RF switching
elements, instead of the transistors 583 and 584 of the normally-OFF type.
Referring to FIG. 13, even if no power is supplied from the G-MDM or the
like to the GSU 30, the PIN diodes 702 and 703 are normally in the OFF
state (the GSU 30 is in the OFF state). Even if noise larger than the
threshold voltage of the PIN diode is applied to the GSU 30, the PIN diode
703 is kept in the OFF state by the operation of a normally-ON type
transistor 716, similar to the example described above. As a result, noise
is prevented from flowing from the GSU 30.
On the other hand, when the GSU 30 is in the ON state, a switch 707 is
turned on by a signal sent from the transmission indication signal
detection circuit. A predetermined current flows through an inductance
705, the PIN diodes 702 and 703, an inductance 706, and a current source
708 to turn on the PIN diodes 702 and 703 in an RF manner. To the
contrary, a switch 722 is turned on to apply a negative potential to the
gates of transistors 713, 716, and 720 and to turn off the PIN diodes 702
and 703. Capacitors 710, 715, and 718 in FIG. 13 separate the PIN diode
system and the transistor systems in a DC manner. Resistors 714, 717, 721,
and 709 are resistors to normally apply a DC potential to the PIN diodes
and the transistors.
In this circuit, a current of about 10 mA must be supplied to the PIN
diodes, resulting in an increase in electric energy to be supplied from
the G-MDM or the like. However, DC or AC power may be supplied to the GSU
30 from the G-MDM or the like via the coaxial cable 19 and the splitter
18, in addition to the above-described transmission indication signal.
To prevent a current from a given modem from reversely flowing into another
modem when a plurality of modems or the like are simultaneously turned on,
a reverse flow prevention diode is inserted in the output stage of each
modem.
FIG. 14 shows the configuration of still another gate switch. A gate switch
600 is constituted by only depletion type transistors. Referring to FIG.
14, a negative potential is applied to the gate of a transistor 603 via
inverters 608 and 609 in a normal state (the GSU 30 is in the OFF state),
whereas a positive potential is applied to the respective gates of
transistors 604 and 605. To the contrary, when the GSU is in the ON state,
a positive potential is applied to the transistor 603, whereas a negative
potential is applied to the transistors 604 and 605. In this
configuration, all the transistors 604 and 605 can be of the depletion
type. Therefore, the process cost can be reduced, and a reliable operation
can be ensured. Note that slight power (about tens of microwatts) must be
constantly supplied to the GSU 30 via the drop cable or the like.
Note that any power supply means via the drop cable or any power supply
circuit corresponding to the supply of DC or AC power from the modem or
the like in the subscriber residences is not shown in FIG. 2, FIG. 10. Nor
are they described here since they are well known in the art.
A detailed example of the transmission indication signal generation circuit
will be described below. FIG. 16 is a block diagram showing the
arrangement of this circuit. Each of the transmission indication signal
generation circuits 161 and 215 comprises a QPSK modulator 401, a carrier
signal generator 402, and a BPF 403.
Upon reception of the transmission indication control signal, the carrier
signal generator 402 generates a carrier signal in accordance with this
signal, and supplies it to the QPSK modulator 401. The QPSK modulator 401
QPSK-modulates the carrier signal in accordance with a random data
sequence. The band of the modulated carrier signal is limited by the
bandpass filter (BPF) 403. Then, the resultant signal is output as the
transmission indication signal to the GSU 30 together with the upward
transmission signal.
The reason why the transmission indication signal is modulated in
accordance with the random data sequence is as follows. That is, assume
that a plurality of the G-STBs 31, the G-MDMs 32, or the NGAs 34 are
installed in a residence, and these devices simultaneously output
unmodulated transmission indication signals slightly different in
frequency. These signals collide with each other to generate a beat. At
this time, the levels of the colliding transmission indication signals may
extremely fall over a long time depending on differences in phase and
signal level between the colliding transmission indication signals. As a
result, the transmission indication signals seem as if they disappeared in
the GSU 30. In such a case, the gate switch 340 is turned off though the
transmission signals are passing therethrough. The transmission of the
transmission signals is undesirably stopped.
In the arrangement of this embodiment, however, the carrier signal
modulated in accordance with the random data sequence is used as the
transmission indication signal. For this reason, when a plurality of
transmission indication signals collide with each other, the time interval
at which the signal levels fall can be probabilistically shortened. As for
the fall of the signal levels, the malfunction of the GSU 30 can be
suppressed to a desired probability or less by setting the time constant
determined by the capacitor 376 and the resistor 378 to be sufficiently
larger than the time interval. For example, assume that the time constant
is set to 15 .mu.sec, and that the transmission rate of the random data
sequence is 1 Mbps, that the phase difference between undetectable
carriers is 20%, that the phase difference between undetectable symbols is
5%, and that the simultaneous transmission (collision occurrence)
probability is 1%. In this case, the malfunction probability of the GSU is
10.sup.-8 or less.
In the above example, the QPSK modulation is used. However, it is necessary
that only the difference in phase or signal level between colliding
signals changes randomly, and therefore another modulation scheme such as
FSK modulation can be employed, as a matter of course.
Another means for avoiding the collision of the transmission indication
signals is transmission control applied with a multiple access means such
as a CSMA (Carrier Sense Multiple Access) scheme. That is, in transmitting
the transmission indication signal, it is checked whether the transmission
indication signal is sent from another G-MDM 32 or the like onto the cable
19, by using the leakage of the transmission indication signal to another
G-MDM 32, G-STB 31, or NGA 34 via the in-home splitter 18. Only when no
other G-STB 31, G-MDM 32, or NGA 34 transmits the transmission indication
signal, the transmission indication signal is sent.
Another detailed example of each of the transmission indication signal
generation circuits 161 and 215 will be described. FIG. 17 is a block
diagram showing the arrangement of this circuit. Referring to FIG. 17, a
carrier signal is generated by a carrier signal generator 412 in
accordance with the input of the transmission indication control signal.
The carrier signal is first PSK- or FSK-modulated in accordance with the
random data sequence. The modulated output signal undergoes spread
spectrum modulation by a spread spectrum modulator 411 in accordance with
a predetermined spread code sequence, and is output. The band of the
spectrum-modulated output signal from the spread spectrum modulator 411 is
limited by a BPF 413. Then, the resultant signal is sent as the
transmission indication signal to the GSU 30.
On the other hand, a transmission indication signal detection circuit for
detecting the transmission indication signal having undergone the spread
spectrum modulation is constituted as follows. FIG. 18 is a block diagram
showing the arrangement of this circuit. Referring to FIG. 18, the
transmission indication signal sent from the G-STB 31, the G-MDM 32, or
the NGA 34 passes through a BPF 421 and is input to a power circuit 422
(e.g., the transmission indication signal detection circuit 370 shown in
FIG. 10). Then, the power circuit 422 is energized to start supplying
power to a gate switch control circuit 423 and a spread spectrum
demodulator 424. Immediately after the supply of power, a gate switch 425
is turned on, and a timer in the gate switch control circuit 423 starts
its counting operation.
If synchronization with the received transmission indication signal is
established within a predetermined time, the spread spectrum demodulator
424 stops the timer. For this reason, the gate switch 425 is kept in the
ON state. To the contrary, if the synchronization cannot be established
within the predetermined time, and if the received transmission indication
signal disappears to stop the supply of power, the timer has a time-out to
reset the gate switch 425 in the OFF state.
The above-described generation and detection schemes for the transmission
indication signal are realized by applying a so-called CDMA (Code Division
Multiple Access) scheme to the generation and detection of the
transmission indication signal. According to the CDMA scheme, even if
there are a plurality of signals spectrum-spread in accordance with
different spread code sequences or the same spread code sequence,
synchronization is established with one of these signals. After the
establishment, an operation is performed to keep this synchronization.
Therefore, collision of the transmission indication signals with each
other becomes allowable by applying this CDMA scheme.
In addition, according to the CDMA scheme, the transmission indication
signal can be reliably discriminated from the specific subscriber noise,
or from broadband noise even when a subscriber erroneously connects a
digital device having a broad noise frequency band, such as a personal
computer, to the cable network. For this reason, the operation reliability
of the GSU 30 can be kept high.
When the transmission indication signal is generated and detected, the
above-described spread code sequence is managed as a privacy key. With
this management, the GSU 30 can be prevented from malfunctioning due to an
interference signal from a malicious subscriber. Further, if a high-level
security measure obtained by combining a public key scheme and a DES
scheme or the like is applied to the transmission of the transmission
indication signal, the transmission indication signal can be almost
perfectly protected against malicious interference.
The transmission indication signal generation circuit 161 or 215, and the
transmission indication signal detection circuit 113 can be respectively
constituted as follows. FIGS. 19 and 20 are block diagrams respectively
showing the arrangements of these circuits.
In these circuits, a plurality of carriers are transmitted as the
transmission indication signals. Only when a plurality of predetermined
carriers are detected in the GSU, the gate switch is turned on. With these
arrangements, the resistance to noise and a mischief can be enhanced.
Referring to FIG. 19, bit sequence generators 620a to 620n generate random
bit sequences or predetermined bit sequences. Encoders 621a to 621n encode
the same bit sequences into signal sequences always including clock signal
components, such as Manchester codes. The encoded signal sequences undergo
predetermined modulation such as phase modulation or frequency modulation
by corresponding modulators 622a to 622n. Thereafter, the modulated signal
sequences pass through corresponding level controllers 623a to 623n and
bandpass filters (BPFs) 624a to 624n, and are sequentially selected by
corresponding switches 625a to 625n and sent from an output terminal 640.
When the power supply of the transmission indication signal generation
circuit is turned on, the output signal level of each carrier is
initialized in accordance with the following procedure. That is, the
switches 625a to 625n are turned off in accordance with a command sent
from a CPU 632 so as not to output the transmission indication signal from
the G-MDM, the G-STB, or the NGA.
In this state, bit sequences are generated by the bit sequence generators
620a to 620n in accordance with a command sent from the CPU 632,
respectively. These bit sequences are encoded into Manchester codes by the
encoders 621a to 621n, and then phase- or frequency-modulated by the
modulators 622a to 622n, respectively. Carriers output from the modulators
622a to 622n pass through the corresponding level controllers 623a to 623n
and bandpass filters (BPFS) 624a to 624n to be input to a switch 626.
These carriers are sequentially selected by the switch 626 and input to a
wave detector 627. After wave detection, their wave detection signal
levels are digitized by an A/D converter 628, and the resultant data are
input to the CPU 632.
The CPU 632 calculates a difference between the wave detection signal level
of each carrier and a preset reference signal level. If this difference is
out of a predetermined error range, a gain control signal is output to
reduce this difference. The gain control signal is converted into an
analog voltage by a D/A converter 629. The converted signal is supplied to
a corresponding one of the level controllers 623a to 623n via a switch
630, thereby controlling the gain. Upon completion of the initial level
control for all the carriers, the bit sequence generators 620a to 620n and
the modulators 622a to 622n are set in a wait state.
When the CPU 632 receives the transmission indication control signal
requiring to output the transmission indication signal, via an interface
633 in this state, the CPU 632 supplies operation start instructions to
the respective circuits in the wait state. Therefore, the bit generators
620a to 620n and the modulators 622a to 622n which are in the wait state
are simultaneously start to operate. At the same time, the switches 625a
to 625n are also turned on, thereby sending the transmission indication
signals using a plurality of carriers.
Note that, in FIG. 19, signal paths from the CPU 632 and a clock/carrier
generator 631 to the bit sequence generators 620a to 620n, the encoders
621a to 621n, the modulators 622a to 622n, and the like are not shown to
avoid complexity.
The operation of the transmission indication signal detection circuit shown
in FIG. 20 will be described below. The plurality of carriers having sent
are separated by corresponding bandpass filters (BPFs) 651a to 651n, and
amplified by predetermined gains in corresponding amplifiers 652a to 652n.
Thereafter, the obtained carriers are wave-detected by corresponding wave
detectors 653a to 653n. Squelch circuits (or analog comparators) 654a to
654n check whether wave detection outputs from the wave detectors 653a to
653n have a predetermined signal level. The determination results are
input to an AND gate 655. Only when the all the carriers have the
predetermined signal level, the AND gate 655 generates a timer start
signal and supplied it to a timer circuit 661. Upon reception of the start
signal, the timer circuit 661 starts a counting operation, and outputs an
ON signal to the gate switch during this counting operation. Therefore,
the gate switch shifts to the ON state.
On the other hand, the outputs from the amplifiers 652a to 652n are also
guided to a mixer 656 to be mixed with each other. Assuming that the
central equipment frequencies of the respective carriers are fca, fcb,
fdb, . . . , an output from the mixer 656 includes a large number of
frequency components represented by
fca.+-.fcb.+-. . . . .+-.fda.+-.fdb.+-. . . .
The output from the mixer 656 is input to bandpass filters (BPFs) 657a to
657n. One or some frequency components are extracted from the large number
of frequency components by the BPFs 657a to 657n. The extracted
frequencies are input to an AND gate 660 via an amplifier (not shown),
wave detectors 658a to 658n and squelch circuits 659a to 659n. The input
frequencies are ANDed and input to the timer circuit 661.
The timer circuit 661 has a function of compensating the stable operation
of the GSU even if a plurality of transmission indication signals are
transmitted from a single residence, and the transmission level of the
transmission indication signal becomes unstable, as described above. That
is, the timer circuit 661 is energized by the signal output from the AND
gate 655 to turn on the gate switch, as described above. However, if no
output from the AND gate 660 is detected within a predetermined time, the
timer circuit 661 turns off the gate switch again. With this operation,
even if the GSU receives a plurality of carriers whose central equipment
frequencies coincide with each other due to a mischief or the like, the
gate switch of the GSU is not kept in the ON state unless the frequencies
of clock signals and parameters used in a modulation scheme or the like
coincide with each other. For this reason, a malicious subscriber cannot
continue to transmit an interference signal to the upward transmission
path.
As for the above-described signal level, various control modes can be
provided in accordance with control programs of the CPU 632. Assume that
the frequency of each carrier is set within an upward band (e.g., both the
ends of an upward band having poor group delay characteristics). In this
case, when the carrier frequency is always set lower by 10 dB than the
frequency of a data transmission signal so as to prevent troubles such as
cross modulation caused by collision of the transmission indication
signals with each other, the following control mode is effective. That is,
the data transmission signal is received via a terminal 641, and its
transmission level is detected to adjust the signal level of each carrier
so as to be lower by 10 dB than the transmission level. Alternatively, a
predetermined downward signal such as a pilot signal is received via the
terminal 641, and the attenuation amount in the network is calculated from
the received level. The transmission level of the data transmission signal
is estimated on the basis of the calculated value to adjust the signal
level of each carrier so as to be lower by 10 dB than the estimated level.
Furthermore, such a control mode can be employed in which an absolute
value is set to be lower than by 10 dB than the minimum level of the data
transmission level (e.g., 85 to 120 dBfV) of a modem.
When the detection circuit includes a circuit having a relatively large
power consumption, and the signal level of the transmission indication
signal is set lower by about 10 dB than the level of the upward
transmission signal, the transmission indication signal cannot supply all
the power required for operations in the GSU. In this case, DC or AC power
may be supplied from the G-MDM or the G-STB to the GSU, as described
above.
To allow telephone communication in a disaster such as an earthquake,
according to the services of public communication common carriers, DC
power is supplied from an office power supply to telephones via telephone
lines. To realize similar services on the cable network system, power must
be supplied to a telephone modem via the drop cable 9. In this case, power
must be supplied to both the telephone modem and the telephone, resulting
in an increase in required power. For example, AC power of about 100 V is
sometimes used. The telephone modem of this type is set outside the
residence in terms of a security measure for a subscriber. The AC power is
cut off by this telephone modem, and only DC power required for an
operation and a downward signal are introduced to an in-home device.
FIG. 15A shows an example of the connection between the telephone modem,
the GSU 30, and the like. Referring to FIG. 15A, the drop cable 9 also
serving as an AC power supply path is first dropped in the a telephone
modem 431. The telephone modem 431 comprises a DC power supply circuit. DC
power is generated by the DC power supply circuit on the basis of AC power
supplied via the drop cable 9. The DC power is supplied to an in-home
telephone via a twisted pair line 432. A GSU 433 is connected between the
telephone modem 431 and an in-home splitter 434. At this time, the AC
power is not applied to the GSU 433 because the AC power is cut off by the
telephone modem 431.
Note that the AC power is directly applied to the GSU attached to a
subscriber residence in which no telephone modem is installed. However,
since the AC power is cut off in the GSU, as described above, the AC power
is not applied to an in-home device not to degrade the security.
When power must be constantly supplied to the GSU 30 via the drop cable 9,
as described above, a current-trough type two-branch splitter is provided
to the end of the drop cable 9 to connect the telephone modem and the GSU
parallel to each other, as shown in FIG. 15B. With this arrangement, power
can be supplied to both the telephone modem and the GSU without applying a
high voltage on the drop cable 9 to an in-home device.
(Second Embodiment)
According to the second embodiment of the present invention, a GSU attached
tap-off as a unit obtained by incorporating a GSU in a tap-off main body
is provided to a trunk line cable. Noise flowing from a connector open
terminal of each subscriber residence is cut off by the GSU of the GSU
attached tap-off.
FIG. 21 shows the arrangement of the GSU attached tap-off according to this
embodiment. A GSU attached tap-off 470 is interposed and set in a trunk
line cable 471. The GSU attached tap-off 470 is constituted by a tap-off
472, and a plurality of GSUs 473a to 473m connected to respective branched
terminals. A G-MDM 32, a G-STB 31, and a NGA 34 in a subscriber residence
are connected to these GSUs 473a to 473m via corresponding drop cables
474a to 474m. The GSUs 473a to 473m open/close upward transmission paths
between the drop cables 474a to 474m and the tap-off 472 in accordance
with transmission indication signals sent from the G-MDM 32, the G-STB 31,
and the NGA 34 in the subscriber residence via the drop cables 474a to
474m.
With this arrangement, the GSU need not be installed in each subscriber
residence. For this reason, a subscriber cannot remove or remodel the GSU.
The reliability of the system can be further increased.
Note that the GSU attached tap-off can be modified as follows. FIG. 22
shows the arrangement. That is, a GSU attached tap-off 480 interposed in a
trunk line cable 481 comprises a tap-off 482 for branching one cable from
the trunk line cable 481, and a splitter 485 for branching this cable into
a plurality of drop cables 484a to 484m. A GSU 483 is interposed between
the tap-off 482 and the splitter 485. The GSU 483 opens/closes the upward
transmission path between the splitter 485 and the tap-off 482 in
accordance with transmission indication signals sent from a plurality of
subscriber residences via the corresponding drop cables 474a to 474m.
With this arrangement, noise flowing from a connector open terminal of each
subscriber residence is cut off by the GSU 483 not to flow into the trunk
line cable 481, as a matter of course. In addition, it becomes difficult
for a subscriber or the like to touch the GSU 483 with his/her hand by
accommodating the GSU 483 in the GSU attached tap-off 480, similar to the
arrangement shown in FIG. 21. Furthermore, one GSU is provided to a
plurality of subscribers. Therefore, investments on cable network system
industrial companies or subscribers can be reduced.
In the arrangement of FIG. 22, one 3SU 483 is provided to a plurality of
drop cables. When the GSU 483 is turned on, the amount of noise flowing
from the respective subscriber residences increases.
(Third Embodiment)
According to the third embodiment of the present invention, a
self-terminated type gate switch unit (S-GSU) as a unit obtained by
combining an NGA with a GSU is arranged to eliminate a GSU associated
modem, an STB, or an NGA from an in-home device.
FIG. 23 is a block diagram showing the arrangement of an S-GSU according to
this embodiment. An S-GSU 450 comprises first and second frequency band
separation circuits 452 and 453, a gate switch 457, a transmission signal
detection circuit 458, a delay circuit 455, and an amplifier 456.
The first and second frequency band separation circuits 452 and 453
comprise filters 452a and 452b and filters 453a and 453b for separating a
frequency band H in the downward direction and a frequency band L in the
upward direction, respectively. The first and second frequency band
separation circuits 452 and 453 are connected to connectors 451 and 454,
respectively.
The gate switch 457 is interposed and set in an upward signal path between
the second frequency band separation circuit 453 and the first frequency
band separation circuit 452. The ON/OFF state of the gate switch 457 is
controlled by a switch control signal output from the transmission signal
detection circuit 458. The transmission signal detection circuit 458
detects an upward transmission signal transmitted from an N-STB 33 or an
N-MDM 35. While the upward transmission signal is detected, the
transmission signal detection circuit 458 generates the switch control
signal and supplies it to the gate switch 457.
FIG. 24 shows the timing relationship between the switch control signal and
the upward transmission signal. Referring to FIG. 24, when the upward
transmission signal is sent from the N-STB 33 or the N-MDM 35, the switch
control signal output from the transmission signal detection circuit 458
becomes active a time t18 after this moment. Upon reception of the switch
control signal, the gate switch 457 is turned on almost simultaneously.
The upward transmission signal is delayed by the delay circuit 455 a time
t19 after this operation, and amplified by the amplifier 456. The
resultant signal starts to pass through the gate switch 457. On the other
hand, the switch control signal becomes inactive a time t20 after the
upward transmission signal substantially disappears, and the gate switch
457 is returned to the OFF state. The upward transmission signal has
already passed through the gate switch 457 a time t21 before this
operation.
As described above, according to this embodiment, the gate switch unit is
constituted to be of the self-terminated type. No additional function need
be provided to an in-home modem or STB. Therefore, an increase in cost due
to a new modem or STB can be suppressed. In addition, when existing or new
services are to be developed, the presence of the GSU need not be
considered.
To the contrary, the S-GSU 450 is expensive. The supply of power from a
modem or the like cannot be expected, so that power must be supplied from
a commercial AC power supply or the like in installation of the S-GSU 450.
An increase in work cost is inevitable.
(Fourth Embodiment)
In the system according to each of the first to third embodiments described
above, while an in-home device does not transmit an upward data signal,
the ingress noise is cut off by the GSU not to flow into the upward
transmission path as far as the GSU functions properly. However, since the
GSU is in the ON state in the transmission period of the upward data
signal, noise is sent to the upward transmission path together with the
upward data signal. A problem is posed when, for example, a subscriber
erroneously directly connects a data device such as a personal computer to
a connector terminal to flow strong noise to the upward transmission path.
In this case, the noise cannot be blocked by only the GSU.
The fourth embodiment of the present invention copes with such a situation.
An ingress noise monitoring/analysis device is provided to a distribution
hub located on the upstream of a bidirectional transmission path. The
ingress noise monitoring/analysis device observes the noise level included
in the upward signal sent via a fiber node and checks whether this noise
adversely affects the transmission quality. When it is determined that the
noise adversely affects the quality, a subscriber as the generation source
of the noise is estimated from a data transmission source subscriber
identification number. Information on the estimated subscriber is notified
to the network management system, together with the noise level
determination result. With this operation, maintenance personnel of the
system can investigate the noise generation source on the basis of the
information notified from the ingress noise monitoring/analysis device,
and can take a required action.
The fourth embodiment will be described in detail below with reference to
the accompanying drawings. The basic part of the system in this embodiment
is also the same as that described in the first embodiment, and the fourth
embodiment will be described with reference to FIG. 1.
Referring to FIG. 1, an ingress noise monitoring/analysis device 40 is
arranged in a distribution hub 2 located on the upstream of the
bidirectional transmission path. FIG. 25 is a block diagram showing the
arrangement of the distribution hub 2 provided with the ingress noise
monitoring/analysis device 40.
The distribution hub 2 comprises a plurality of central control units 501a
to 501n. The central control units 501a to 501n perform various kinds of
control and modulation/demodulation processing in a manner interlocked
with the above-described G-MDM 32 and G-STB 31. These control and
modulation/demodulation processing are required to provide the PC on-line
services and the VOD services. Downstream signals output from the central
control units 501a to 501n are synthesized with each other by a
multiplexer 502. The synthesized electric signal is converted into an
optical signal by an electrical-to-optical converter (E/O) 503. The
optical signal is branched into a plurality (k in FIG. 25) of systems by
an optical demultiplexer 504. The resultant signals are transmitted to
fiber nodes #a to #k.
On the other hand, the optical signals as upward (.ata signals sent via the
fiber nodes #a to #k are (converted into electrical signals by
optical-to-electric converters (O/E) 510a to 510k. The electric signals
are branched into n systems by demultiplexers 511a to 511k. The resultant
signals are input to the corresponding central control units 501a to 501n.
The central control units 501a to 501n send the received upward data
signals to a headend 1. At the same time, the central control units 501a
to 501n extract transmission source subscriber identification numbers from
header information added to the respective upward data signals and notify
the identification numbers to the ingress noise monitoring/analysis device
40.
The ingress noise monitoring/analysis device 40 is constituted as shown in
FIG. 26. More specifically, upward signals 521a to 521k sent via the fiber
nodes #a to #k are input to a spectrum analyzer 530 via a switch 533. The
spectrum analyzer 530 observes a spectrum distribution in an upward
frequency band by keeping the peak thereof for, e.g., every 5 minutes. The
observation data are input to an analysis device 531. The analysis device
531 receives the observation data together with the data transmission
source subscriber identification numbers output from the central control
units 501a to 501n during the observation period of 5 minutes. Analysis
processing in the analysis device 531 is schematically constituted by,
e.g., the following steps.
(1) Primary Process
The presence of a corresponding spectrum is checked on the basis of channel
information notified in advance from a network management system installed
in the headend, which information is assigned to each service in the
upward frequency band. If the corresponding spectrum is present, the
spectrum component is removed from the observation data. Note that the
channel information includes a central equipment frequency, an occupied
bandwidth, a signal level, and a spectrum distribution.
(2) Secondary Process
Noise components constantly and stably observed in past noise information
files accumulated in an auxiliary memory device 532 are removed from the
result of the primary process.
(3) Tertiary Process
Upon completion of the secondary process, noise components left without
being removed are classified into, e.g., the following three stages in
accordance with the intensities of the levels.
E (Emergency) level: noise level greatly affecting the transmission quality
W (Warning) level: middle warning noise level between the E level and an S
level (to be described below)
S (Safety) level: noise level negligible in terms of the transmission
quality
(4) Quartic Process
Noise of the E level and noise of the W level are collated with the past
noise information files accumulated in the auxiliary memory device 532.
Each past noise information file includes data representing the
characteristics of noise such as a frequency, a bandwidth, a noise level,
intermittency/continuity, a data transmission source subscriber number
obtained when the noise is observed. When similar noise is detected by
this collation, a coincidence count is incremented for the newly received
data transmission source subscriber identification number which coincides
with the data transmission source subscriber number accumulated in the
noise information file. If the coincidence count of the transmission
source subscriber identification number does not reach a predetermined
number, or the transmission source subscriber identification number fails
to be read as a data transmission source many times, the transmission
source subscriber identification number is deleted from the noise
information files. In addition, noise whose similarity cannot be detected
is registered as a new noise information file in the auxiliary memory
device 532. As for at least noise of the E level, the generation of this
noise is notified to the network management system to warn the
maintenance/operation person of the system.
(5) Quixotic Process
It is estimated that the generation source of the noise is or may be a
subscriber residence having the file, of the noise information files, in
which the coincidence count of the data transmission source subscriber
identification number reaches the predetermined standard. This estimation
is notified to the network management system to request the maintenance
person of the system to make investigation and take required action. The
ingress noise monitor ng/analysis device 40 constantly, repeatedly
performs the above processes sequentially for all the fiber nodes.
Even when, therefore, a subscriber erroneously directly connects a personal
computer or the like to the transmission path, strong noise having passed
through the GSU 30 in the ON state and transmitted to the upward
transmission path is monitored and analyzed by the ingress noise
monitoring/analysis device while data is being transmitted. The subscriber
as the transmission source is estimated. This estimation is notified to
the network management system. The maintenance/operation personnel of the
system can quickly take a proper action.
The description above is based on the assumption that the network
management system is located at the head end. Instead, the network
management system may be connected by a communication line to the head
end. In this case, the ingress noise monitoring/analysis device 40
supplies the estimation result to the network management system through
the head end and the communication line.
The above-described spectrum analyzer can be constituted by a filter group
having a bandwidth corresponding to the measurement resolution (e.g., 300
kHz). That is, the spectrum analyzer as a commercially available standard
measurement device scans a measurement frequency band with the resolution
set by a user in a software manner. Intermittent noise which disappears
within a very short time, or the like cannot always be detected. However,
such noise can be reliably detected by real-time observation using the
above-described filter group.
In the above description, the observation time of noise is set to, e.g., 5
minutes. However, when noise of E or W level is detected, the observation
time for a corresponding fiber node is shortened, or the observation
frequency for the corresponding fiber node is increased compared to the
remailing fiber nodes. Further, a fiber node where a bit error on an
upward data signal typically increases is observed concentratedly. With
this setting, a time required to estimate the subscriber of a noise
generation source can be shortened.
The present invention is not limited to the respective embodiments. For
example, the self-terminated type GSU described in the third embodiment
may be arranged instead of the GSUs 473a to 473m and 483 described in the
second embodiment.
When a surge suppresser unit is set under the eaves of a subscriber
residence, or a splitter is installed in the residence, the GSU may be
accommodated in the unit.
A bidirectional booster amplifier and a splitter may be inserted midway
along the drop cable 9 branched from a trunk line cable 6 to distribute
services to a plurality of subscribers. In this case, the bidirectional
booster amplifier may comprise the GSU function including a gate switch
function and the like, or the self-terminated type GSU function including
a gate switch function and a delay function for a transmission signal. The
bidirectional booster amplifier and the GSU share a frequency band
separation circuit, an amplifier, and the like, which are required for the
GSU function and the self-terminated type GSU function. As a result, the
GSU can be reduced in cost. Note that the amount of ingress noise
increases in this scheme because a plurality of subscribers share the GSU.
As a matter of course, the GSU may be inserted immediately after an
in-home splitter, i.e., the GSU may be inserted for each subscriber. In
this case, although the cost of the GSU cannot be greatly reduced, the
work cost can be reduced because the installation work for each subscriber
is eliminated.
The timing between the transmission indication signal, the switch control
signal, and the upward transmission signal can be arbitrarily set. For
example, even when the transmission indication signal disappears earlier
than the upward transmission signal, if the time constant of the
transmission indication detection circuit in the GSU is set large, the ON
state tan be kept until the upward transmission signal disappears. That
is, it is sufficient that the upward transmission path is kept open during
a period when the upward transmission signal passes through the GSU.
Various methods can be considered to realize this condition.
Various control schemes can be considered for a supply scheme of power to
the GSU by the transmission indication signal, measures against collision
of signals with each other, measures for noise discrimination, measures
against a malicious interferer.
Furthermore, the target cable network system in the respective embodiments
uses a so-called low-split scheme in which the frequency bandwidth in the
upward direction is as narrow as 5 MHz to 40 or 48 MHz. The present
invention can be applied to a mid-split or high-split scheme having a
broader band. The present invention can also be applied to a double cable
scheme in which coaxial cables are respectively assigned to upward and
downward transmission paths, an optical passive coupling system such as
FTTH, and an optical system in which a malicious subscriber may send an
interference signal to an optical shared transmission path via an optical
terminating unit (ONU: Optical Network Unit). In addition, each embodiment
exemplifies the system in which bidirectional transmission is performed
between the subscriber device and the central equipment via the
bidirectional transmission path. The present invention can also be applied
to a system in which data obtained by the inspection of a meter are
transmitted from a large number of subscribers to the central equipment,
like a collecting system for meter inspection data. Further, the present
invention can also be applied to a cable network system using either a
coaxial cable as a transmission medium and an optical fiber cable or a
twisted pair line.
The circuit configuration and installation location of the GSU, the
function and installation location of the ingress noise
monitoring/analysis device 40, and the like can be modified without
departing from the spirit and scope of the present invention.
The cable network system is not limited to a star-shaped one and a
tree-shaped one. Rather, it may be of any other type such as a ring-shaped
one. The system may have any structure in which ingress noise may be
generated.
Additional advantages and modifications will readily occur to those skilled
in the art. Therefore, the invention in its broader aspects is not limited
to the specific details, and representative devices shown and described
herein. Accordingly, various modifications may be made without departing
from the spirit or scope of the general inventive concept as defined by
the appended claims and their equivalents.
* * * * *
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