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| United States Patent |
5,760,935
|
|
Sabry
, et al.
|
June 2, 1998
|
Optical communications network
Abstract
An optical communications system comprises an inner core network, a
plurality of outer networks coupled to the core, and local networks
coupled to the outer networks whereby to provide access to terminals
coupled to the network. Information transported between terminals across
the network is carried via pixels in a discrete communications space
defined by time and wavelength co-ordinates.
| Inventors:
|
Sabry; Martin (London, GB);
Midwinter; John (London, GB)
|
| Assignee:
|
Northern Telecom Limited (Montreal, CA)
|
| Appl. No.:
|
591543 |
| Filed:
|
January 26, 1996 |
| PCT Filed:
|
September 12, 1994
|
| PCT NO:
|
PCT/GB94/01976
|
| 371 Date:
|
January 26, 1996
|
| 102(e) Date:
|
January 26, 1996
|
| PCT PUB.NO.:
|
WO95/08247 |
| PCT PUB. Date:
|
March 23, 1995 |
Foreign Application Priority Data
| Current U.S. Class: |
398/75; 370/404; 398/1; 398/59; 398/168 |
| Intern'l Class: |
H04J 004/00; H04J 014/00 |
| Field of Search: |
359/123,118-119,117,168
370/404,405,406,408
|
References Cited [Referenced By]
U.S. Patent Documents
Other References
Sabry et al., "A Modular and Scalable Transparent Optical Network", ECOC
'93, Sep. 12-16, 1993, Proceedings, vol. 3. pp. 97-100.
|
Primary Examiner: Negash; Kinfe-Michael
Attorney, Agent or Firm: Lee, Mann, Smith, McWilliams, Sweeney & Ohlson
Claims
We claim:
1. An optical communications network, including an inner core network
having a plurality of nodes, a plurality of outer networks coupled to at
least some of said nodes, and local distribution networks each coupled to
a said outer network and each providing access to a plurality of
terminals, wherein information transported via the network between
terminals is carried in optical signals comprising elements of a two
dimensional discrete communications space extending throughout the network
between said terminals, each said element in said discrete communications
space being uniquely defined by respective time and wavelength
co-ordinates.
2. An optical network as claimed in claim 1, wherein each said element
corresponds to an optical synchronous transport module channel.
3. An optical network as claimed in claim 1, wherein switching is effected
by space switching of elements.
4. An optical network as claimed in claims 1, wherein said inner core
network comprises a mesh.
5. An optical network as claimed in claim 4, wherein each said outer
network comprises a ring.
6. An optical network as claimed in claim 5, wherein said ring is
unidirectional.
7. An optical network as claimed in claim 4 wherein each said local network
comprises a passive optical network (PON).
8. An optical network as claimed in claim 5, wherein capacity within the
inner core and the outer network rings is allocated in subframes, and
wherein each said ring corresponds in circumference to an integral number
of subframes.
9. A method of transporting information from a transmitter terminal to a
receiver terminal in an optical communications network having an inner
core network having a plurality of nodes, a plurality outer networks
coupled to at least some of said nodes, and local distribution networks
each coupled to a said outer network and each providing access to a
plurality of terminals, the method comprising converting the information
at the transmitter terminal to corresponding optical signals comprising
elements of a two dimensional discrete communications space, each said
element of said discrete communications space being defined uniquely by
respective time and wavelength co-ordinates, transporting the optical
signals comprising said elements via the network between said terminals,
and recovering said information from the optical signals received at said
receiver terminal.
Description
This invention relates to communications networks and in particular to
optical communications networks.
BACKGROUND OF THE INVENTION
Optical fibre transmission is a widely used technique in the communications
field. The technique has the major advantages of low attenuation and high
bandwidth in comparison with electronic transmission techniques. However,
in communications network applications, the use of fibre has not extended
significantly beyond trunk transmission. Thus, local transmission and
switching are performed electrically. This requirement for both electrical
and optical communications introduces two significant limitations. Firstly
there is the need to provide electronic/optical and optical/electronic
interfaces between the two transmission media. Secondly, the bandwidth
restrictions of the electrical part of the network present the full
potential bandwidth available in the optical spectrum from being utilised.
For example, the complete radio and microwave spectrum has a bandwidth of
about 300 GHz, whereas the potential bandwidth available in the optical
spectrum in the typical transmission wavelength range of 1500 to 1600 nm
is of the order of 4000 to 6700 GHz.
Furthermore, in a communications network, it is necessary to provide a
number of levels of multiplexing, the highest level being used for trunk
transmission. This necessitates the provision of multiplexing and
demultiplexing equipment. Although the recent introduce of synchronous
systems (SDH) to replace the present plesiochronous systems (PDH) has
resulted in a reduction of the volume of multiplexing/demultiplexing
equipment required, this equipment still represents a significant
proportion of the overall system cost.
It is an object of the invention to minimise or to overcome those
disadvantages.
SUMMARY OF THE INVENTION
According to the invention there is provided an optical communications
network, including an inner core network having a plurality of nodes,
outer networks coupled to at least some of said nodes, and local
distribution networks each coupled to a said outer network and each
providing access to a plurality of terminals, wherein information
transported via the network between terminals is carried in elements of a
two dimensional discrete communications space, each said element being
defined by respective time and wavelength co-ordinates.
The communications space extends throughout the network
The network architecture is designed to be integrated with, and eventually
replace, the current network in incremental steps from the top down using
a common set of components throughout. In general this will ensure that
maximum benefit can be derived by spreading costs amongst users at the
higher network levels, and reductions in component costs, due to mass
production of standardised modules at the subsequent lower levels.
In the first stage of evolution we envisage existing transmission links
within an inner core network being upgraded to use Dense WDM transmission
but with each carrier operating within the Synchronous Digital Hierarchy
(SDH) to standard interfaces. Later these interfaces would be replaced by
transparent wavelength switches and thereby facilitate transmission
between rings entirely in the optical domain. Ultimately, individual
customers could be serviced directly through passive optical networks
(PONs) optically connected to the rings.
In a high capacity network carrying traffic multiplexed from many users,
one would not expect the mean traffic flow along routes to fluctuate by
large amounts in short times. Accordingly, we propose that the traffic
carrying ability in the inner and outer core networks (mesh and rings)
should be allocated on a slowly time varying basis (.about.hours),
initially in blocks of one optical carrier e.g. at the synchronous
transport module standard STM-4. The choice of STM-4 may seem low, at a
time when much interest centres on transmitting at STM-16 or higher rates,
however, we have two reasons for proposing it. Firstly, a nation-wide
transparent optical network with transmission at 1550 nm over distances up
to the order of 1000 km is most likely to be limited to a data rate less
than 1 GBit/s per carrier due to material dispersion, as much of the dark
fibre installed and the fibre already used is optimised for transmission
around 1300 nm. Secondly, in order to minimise the amount of time domain
processing on the multiplexed signals we are concerned to identify a
traffic capacity building block appropriate as the minimum unit of
capacity to be allocated between any two core nodes in the network.
However, as the network is extended to include rings and the
opto-electronic boundary is brought closer to the customers, we can
introduce a limited degree of time division (albeit without time shifting
buffers at nodes) in order to further sub-divide the communication space.
Recognising that telecommunications is based around an 8 kHz sampling
cycle corresponding to a 125 .mu.s frame duration, we propose dividing it
into 16 sub-frames (SF), each of about 7.8 .mu.s. Thus the minimum
capacity block would be reduced to about 38 MBit/s. By using more
sophisticated technology the basic building block units may for example be
designed around the higher rate STM-16. It will be appreciated that the
technique is in no way limited to any particular rate. As the
time/wavelength communications space extends throughout the network. This
obviates the need for any multiplexing or demultiplexing facilities thus
providing a significant cost saving in comparison with a conventional
network.
BRIEF DESCRIPTION OF THE DRAWINGS
An embodiment of the invention will now be described with reference to the
accompanying drawings, in which:
FIG. 1 is a schematic diagram of a communications network according to the
invention;
FIG. 2 illustrates the principle of communication within the network of
FIG. 1;
FIG. 3 illustrates the communication process at a core node of the network
of FIG. 1;
FIG. 4 shows the schematic construction of a core node for the network of
FIG. 1;
FIG. 5 shows the schematic construction of a ring node of the network of
FIG. 1;
FIG. 6 shows an interface between the network and a user terminal;
FIG. 7 shows a core network traffic module;
FIGS. 8a-8b illustrates the operation of a ring-passive optical network
interface; and
FIGS. 9, 10 and 11 illustrate mean success rate and throughput from the
network of FIG. 1;
DESCRIPTION OF PREFERRED EMBODIMENT
Referring to FIG. 1, the network provides communication between a plurality
of terminals 11 via access passive optical networks (PON's) 12 and a
network core comprising an inner mesh core 13 and outer core rings 14. The
inner core 13, which comprises the highest hierarchical level of the
network, is a highly interconnected mesh. Each of the core nodes 15
provides routing within the inner core and at least some core nodes also
provide access to respective outer core rings 14. The latter are provided
with ring nodes 16 which nodes access the passive optical networks 12 to
which the user terminals 11 are coupled. In the arrangement of FIG. 1,
traffic is carried end-to-end between terminals in a wholly optical manner
using time division and wavelength division multiplexing (WDM).
FIG. 2 illustrates the way in which information is transported in the
network of FIG. 1. The figure illustrates the concept of a communications
space within an optical fibre transmission path. Information is carried in
pixels or elements 21 in a two dimensional discrete communications space
20, each pixel or element being defined by respective wavelength
(.lambda.) and time (t) co-ordinates. The communications space is the same
in any fibre of the network whether that fibre is part of a trunk line or
is a user link to a subscriber terminal. The plane 20 in FIG. 2 represents
the communication space in a single fibre. As has already been pointed
out, it is possible to wavelength multiplex and one can therefore think of
the frequency dimension as divided into bands each representing a
transparent optical channel. In addition, time is also partitioned
analogous to conventional Time Division Multiplexing (TDM). Combined, the
two multiplexing techniques result in a matrix of pixels each of which may
be described by a unique (.lambda., T) reference within the fibre. Thus
each pixel within the communication space might correspond to one optical
STM (synchronous transport module) channel. A third dimension may be added
to accommodate the possibility that more than one fibre may exist between
any two points in a network. In contrast to the dimensions of the
time-channel plane, which are the same at any point in any optical
network, the extent of the fibre dimension may vary through a network.
Moreover, in order to maintain all optical transmission paths the only
type of operation that can be permitted when switching is to shift the
time-wavelength plane (or parts of it, say one pixel) within this
dimension, i.e., space switching.
A node in a mesh network will generally have several input and output
fibres connecting it to a number of different nodes. Given that nodes have
the ability to separate the individual wavelength slots on each of their
input fibres and reassemble them in a different configuration on their
output fibres, we can use the fact that the output fibres lead to
different nodes, to route wavelength channels through the network. If we
further assume that the amount of time required by the node to reconfigure
the mapping of wavelength slots is relatively small compared to the degree
of time division, each of the pixels in FIG. 2 may be thought of as a
"temporary" transparent data channel which can be used to make an
arbitrary point to point connection in the network. As we are proposing
all optical switching, the pixels must maintain their position within the
time-wavelength planes of the fibres, from the input to the output of a
node and indeed throughout the whole multinode network. FIG. 3 shows a
hypothetical all optical mapping for a node with 3 input and 3 output
fibres each filled (indicated by the shaded pixels) to half of the
theoretically available capacity. To perform switching entirely within the
optical domain, each pixel maintains its position within the
time-wavelength plane of the communications space. Accordingly the pixels
are mapped by shifting them along the fibre axis
The functionality of a core node is illustrated in a different manner in
FIG. 4. The node comprises a wavelength switch, without wavelength or time
shifting, which controls paths between core and ring inputs 41, 42 and
core and ring outputs 43, 44 respectively.
Switching between rings and core nodes, as well as from a ring to a core
and vice-versa are all required. However, as the capacity in both the core
and the rings is allocated in subframes, if M is the total number of
fibres entering (and leaving) each node, w the number of active wavelength
channels per fibre and F the number of sub-frames at the basic building
block rate of 38 MBit/s (or 155 MBit/s), then the node can be seen to be a
switch receiving M.about.w.about.F input channels and cross connecting
them to M-w-F output channels, irrespective of whether the individual
fibres are part of a ring or the mesh network. In other words, a single
type of core node may be provided, and it may be used either in
conjunction with rings or purely as part of the inner core network
switching fabric. As a consequence the rings should be integer multiples
of sub-frames in circumference as well as unidirectional, in order to
maintain both the timing within the inner core network and their
self-healing properties. In a typical arrangement, the total number M of
fibres may be 10, the number W of wavelength channels may be 50 and the
number F of subframes may be 16 giving a throughput of between 0.3 and
1.24 TBit/s made up of 8000 independently routed basic channels.
In order to synchronise the switching of subframes in the core network, all
sections of fibre in that network should be of a length corresponding to
integral multiples of the subframe interval. For example, an 8 kHz frame
rate divides into 16 subframes implies, on average, an additional 0.75 km
of fibre in each cable termination. Moreover, a general band of about 700
ns between data contained in consecutive subframes is desirable to allow
for differences in propagation delay caused by dispersion effects.
Although capacity may be allocated in blocks of 38 MBit/s (or 155 MBit/s)
within and between rings, if the opto-electronic border is to be brought
out to the individual customers, the network must realistically be capable
of providing as little capacity as 64 kBit/s channels. We therefore
propose that the sub-frames be composed of time slots of a duration
sufficient for the transmission of standard ATM (asynchronous transfer
mode) cells (about 680 ns at STM-4 or about 170 ns at STM-16) which may be
dropped into or extracted from a PON individually. As a result, it is
possible to transmit information from customer to customer entirely within
the optical domain in denominations ranging from an ATM cell upwards.
The ring-PON interface is shown in FIG. 5. Similar to the core node, it
accesses all the incoming wavelength channels. Several input and output
fibres may exist on the ring level. However, as all these fibre by
definition pass through the same sequence of nodes, no subframe to
subframe switching is required. The real purpose of these switches is to
establish access to one or more PONs from the ring. Consequently, the
routing which takes place is based on a sub-frame to ATM cell (C)
transformation. The nodes continuously examine optical pre-headers which
preceded every ATM cell (indicating the cells' final destination PON on
the ring) and then direct them individually, either to the appropriate
PON, or to the ring level output fibre for further transmission down the
line of PONs. Depending on the exact configuration of the ring-PON
interface the pre-header may be lost when the ATM cell is dropped into a
PON, but as no further routing decisions are required before the customer
terminal this is of no consequence. In the cases where a ring is made up
of several fibres, the node also ensures that packets injected from the
customers on a PON are directed to the correct ring fibre. Connections
between customers on different rings are then established by injecting ATM
cells into vacant slots in a sub-frame channel that connects the relevant
rings. Since the routing pattern of the sub-frames is predetermined at the
inner core level, the data channels i.e. the pixels within the fibre
communication space may be viewed as set of postal pigeon holes each
addressing a different destination ring. Consequently, the transmission of
information from source to destination is reduced to the insertion and
removal of ATM cells at the two involved ring-PON interfaces. Moreover, as
these are both strictly localised processes, maintaining precise timing is
required within the individual PONs.
Communication between customers on the same ring is achieved by allocating
additional sub-frame channels used for local traffic only. If the two
customers happen to be on the same PON or on PONs connected to the ring at
the same point, the ATM cells simply make one complete round trip before
they are dropped back into the PON. Alternatively, an additional level of
switching may be introduced, indicated by the dashed lines in FIG. 5 that
would allow ATM cells belonging to this category of traffic to be routed
directly within the ring-PON node. This would not require the introduction
of a new level of functionality, as the processing necessary is a subset
of that, which has already been specified for the other kinds of traffic
managed by this type of node.
It will be appreciated that electronic interfaces will be required at the
periphery of the network. In line with the development strategy outlined
above, the specification of these opto-electronic interfaces must remain
the same throughout the expansion of the network. As shown in FIG. 6 the
optics consist of two main components; a tuneable narrow line-width laser
TX and a photo-detector RX. A tuneable wavelength discriminatory device
may also be inserted into the receiving path if one desires the terminal
to be able to single out a unique wavelength channel if several appear on
the input fibre simultaneously. The two electronic modules are the control
circuitry CTRL required for the correct operation of the terminal and high
speed buffer memories. BUFR and BUFT are associated with the photodetector
and the laser respectively. The buffer memory BUFR ensures that when an
ATM cell destined for the terminal is detected by the optical receiver it
may be stored temporarily in order to allow the data to be read out at the
lower bit rate anticipated on the electronic side of the interface. In
this way the only high speed electronics (STM-4 or STM-16) used in the
network are the two 53 byte memory cells in each terminal.
There now follows an analysis of the communications network described
above. This analysis is given by way of example and demonstrates the
flexibility of the techniques described herein.
Analysis of the core network
As previously stated, the function of the core network is to establish high
capacity "transparent" data channels between geographically localised
communities of interest. We assume the traffic in the inner core can be
partitioned into R geographically localised communities, each of which can
be covered by a ring, and that all have similar statistical
characteristics in terms of local and inter community traffic flows. If
the distribution of traffic originating from a ring is plotted versus the
remaining rings, where these are ordered in sequence according to the
proportion of the traffic they receive, we further assume this to show a
substantially linear distribution with an order of magnitude difference
between the most and least popular communities of interest as illustrated
in the traffic module shown in FIG. 7. Consequently, if w.times.q.times.f
is the number of channels available for external traffic in a ring, where
w is the number of active wavelength channels per fibre, q is the number
of files and F is the number of subframes, then the value of this packet
must be maintained greater than or equal to (10/2).times.R. For example,
if there is a minimum of 2 fibres in the rings, each with an equivalent of
50 wavelengths, each partitioned onto 16 sub-frames dedicated to core
traffic, the network can accommodate a minimum of 50.times.2.times.16=320
rings. Within the context of a national UK network this would imply rings
with an average radius of roughly 30 km assuming a uniform population
density across the country. Alternatively, with 25.times.10.sup.6 network
subscribers, it implies .about.78,000 customers per ring.
Although we envisage that only part of the time-wavelength-fibre space will
be used for transmission at any one time, we also assumed that the
connection pattern in the core would be (albeit slowly) varying
dynamically. Consequently, there will be a finite probability that a new
channel along a given route can not be accommodated and therefore must be
denied if requested. If the network control algorithms do not possess any
"intelligence" and requests for new channels are made, as well as
allocated, utterly at random, the highest probability for blocking,
P.sub.B, arises when trying to make a connection along the longest route
where the transmission space, on all stages, is one channel short of being
utilised to its maximum permitted level. If f.sub.i denotes the maximum
fraction of the total number of wavelength slots allowed to be active in
each fibre, and q.sub.i the number of fibres in the i.sup.th cable along
the route, then P.sub.B is given by eqn. (1) where p is number of
wavelength slots tested.
P.sub.B =›1-(1-f.sub.l.sup.ql)x . . .
x(1-f.sub.i-l.sup.qi-l).times.(1-f.sup.qi)!.sup.p (1)
Assuming that the filling factor, f and the number of the fibres, q, is the
same throughout the whole of a route for a given connection and all vacant
channels are tested, eqn. (1) may be rewritten as eqn (2) below, where q,
l and f are defined as above and W is the number of channel slots in a
fibre.
P.sub.B =›1-(1-f.sup.q).sup.? !.sup.(1-f.q.W) (2)
For example, if there are 8 and 2 fibres per cable in the core and ring
networks respectively, with 100 wavelength slots and a 50% filling factor
in each fibre, then the probability for not making a connection between
two rings separated by 11 core nodes (longest path in 7.times.7
rectangular mesh) should be 5.times.10.sup.-28 on any particular chosen
route.
Analysis of the access network
In addition to the blocking which may take place within core network, there
exist an additional set of blocking probabilities within the access
network which arise from contention between ATM cells. FIG. 8 depicts a
conceptual view of the interaction between a ring consisting of two fibres
and a PON. The upper part of the figure shows a hypothetical switching
pattern for the ring-PON interface, depicted in a manner similar to that
previously presented for the core node, FIG. 3. The pixels resulting from
the original partition of the communication space, i.e. the basic core
communication channels are delimited by the bold lines. In accordance with
the proposed architecture, these have been further divided into smaller
areas within the time dimension to represent the individual ATM cells. As
was the case for the capacity available in the core network, these carry a
varying amount of traffic (indicated by the shaded areas) depending on the
offered load and the existing traffic patterns. As previously stated, a
ring-PON interface, represented in FIG. 5, does not perform any
SF.fwdarw.SF switching, nor does it facilitate ATM cell to ATM cell
switching between the ring input and output fibres.
In general there can exist three different types of contention within the
access network. When an ATM cell is injected into the network by a
terminal it will cause a collision if the given ATM slot already contains
another cell. Accordingly, the network control should keep track of which
of the slots in the various data channels in a ring are in use at any
given point in time and only make those that are empty available for "new"
connections. This implies that before a connection can be established
between two terminals, the one which initiates the request must signal the
network to determine the time and wavelength references of the appropriate
channels which are available as well as which ATM slots are unused within
them. Given that a ring consists of two or more fibres the control
algorithm must also verify that two terminals in the same PON are not
competing for identical ATM slots on different fibres as this will result
in the cells colliding within the PON. In such cases, only one of the
available slots may be allocated. The control issues associated with these
two types of collision may be administered by the local hardware which
oversees the dialogues between the PONs (terminals) and their rings. These
localised control centres would though not store sufficient information
about the whole network for a terminal to determine, at the time of
initiating a call, whether the receiving terminal is available or in fact
if the time-wavelength reference chosen for the transmission of ATM cells
will cause contention with already existing connections between other PONs
in other rings and the receiving PON. Thus, the third type of contention
arises from potential collisions in the receiving PON. Viewed from the
position of a ring receiving a number of calls (or request for calls),
however, this new set of administrative control issues are again
localised. We therefore suggest that this network architecture lends
itself to distributed control concentrated, for example, in each ring.
Communication between terminals would then only require the involvement of
the control centres directly affected, i.e. generally only two.
In order to further quantify the issues raised above, we present a general
analysis of the various levels of contention and blocking identified in
the access network. If x ATM cells are selected at random from W.f.q ATM
slots the probability that they are all destined for the same PON is given
by Px defined by eqn. (3).
##EQU1##
For example, if a ring has the dimensions used above and the ATM cells
transmitted in a certain slot are distributed between 500 PONs (which in
the context of a network with 25.times.10.sup.6 customers and 320 rings
implies .about.150 customers per P0N), the likelihood that a PON does not
receive an ATM cell is PO=0.819.
If terminals grab empty slots at complete random, the probability that a
given slot is duplicated in a PON is given by eqn. (4). Naturally, this is
impossible unless there are two or more fibres in a ring, i.e. for
q.gtoreq.2.
##EQU2##
In the case where receivers are not wavelength selective, it follows that
only one terminal per PON can receive an ATM cell per ATM slot.
Consequently, in order to fully use the allocated transmission capacity,
the number of PONs, N, in a ring must be greater than or equal to the
number of active wavelengths. N.gtoreq.W.f.q. Given that one or more
channels have been established between two rings the probability of making
an ATM connection is the product of a slot not being duplicated in the
transmitting PON, and being empty in the receiving PONs, eqn. (5), where p
is the number of empty ATM slots tested.
##EQU3##
For example, if two rings have 500 PONs and 2.times.50 active channels
each, and only one 38 MBit/s channel is connecting them, a single 64
kBit/s line may on average be established 96.7% of the time, as long as
the channel is operated at less than 99.6% of its total capacity.
Alternatively, if the same rings only had 100 or 50 PONs, the number of
customers per PON would increase to approximately 750 and 1500 and the
probability for making a given connection would be reduced to 59.8% or
24.8% respectively. It is understood that, if we measure hardware in terms
of the number of PONs (and hence size and number of ring-PON interfaces)
and performance in terms of the likelihood of being able to make a given
connection, we can generally trade one for the other. However, in order to
determine the overall blocking which takes place in a ring it is necessary
to take the traffic arriving from all the other rings in the whole network
into account. Eqn. (6), and FIG. 9 express the probability of being able
to allocate all the receiving capacity required (W.f.q.f.sub.R) without
contention (for N=500), given that the traffic pattern is totally random,
where f.sub.R is the fraction of the allocated capacity that is used. They
indicate that it is extremely unlikely that it will be possible to fully
allocate all the available capacity within a ring such that no contention
exists, but that this should continuously become more readily achievable
as the offered load is reduced.
##EQU4##
The average fraction .O slashed. of injected ATM cells which are
successfully transmitted, (evaluated as 1-P.sub.collision) versus the ring
traffic load can be expressed as the fraction of PONs receiving exactly
one wavelength with respect to the mean number of filled ATM slots, (mean
number of PONs receiving exactly one ATM cell per ATM
slot)/(W.f.q.f.sub.R), eqn (7).
##EQU5##
FIG. 10 plots .o slashed. and the gross throughput, given as the product of
the load and the success rate, versus f.sub.R. It reveals that rings with
the dimensions used in the above examples, even without wavelength
selective receivers, would have an 82% success (or 18% failure) rate, as
well as throughput, when fully loaded. In general, the throughput curves
calculated for the examples with 500 and 100 PONs per ring, do not exhibit
maxima. This implies that the throughput will increase continuously in
step with the load. In other words, we would not expect it to be necessary
to build safeguards into the network control to cope with congestion as is
currently proving to be necessary with conventional ATM. However, in the
example where the ring only had 50 PONs the throughput shows a shallow
maximum of about 0.5. This result suggests that when the number of PONs in
a ring is small compared to the address space desired in the inner core
(which is a function of the total number of rings and the respective
traffic distributions) care should be taken to limit the injected traffic
to the optimum level or the receivers in the terminals should be specified
as wavelength selective.
If terminals are given the ability to select the individual wavelength
channels appearing simultaneously on the input fibre, we can remove the
restriction of a single ATM cell per PON per ATM slot. In its place, it
will be necessary to prevent two ATM cells being transmitted in identical
ATM slots on different fibres (presupposes q.gtoreq.2), being dropped into
the same PON at the same time. Since the W.f active channels are selected
at random from W in each of q fibres the probability that a given
wavelength channel is used exactly j times is given as Qj.
Q.sub.j =f.sup.j ›1-f!.sup.qring.spsp.-j C.sup.qring (8)
The probability that this wavelength is chosen more than 1 time if x
wavelengths are chosen out of W.f.q is:
##EQU6##
Hence, the mean number of packets that collide in a PON per ATM slot is PQ.
##EQU7##
FIG. 11 plots the expected success rate and gross throughput for a ring
where all terminals have wavelength selective receivers, eqn. (11), versus
the offered load (where f in PQ is replaced by f.times.f..sub.R).
##EQU8##
A comparison between FIG. 10 & 11 reveals that this modification of the
customer terminals causes a marked improvement in all three examples. The
decision of whether to introduce the additional complexity and cost of
wavelength selectivity in each terminal will ultimately depend upon the
required performance and the relative dimensions of the inner and the
outer core, and the number of PONs in each ring.
The throughput graphs. eqn. (7 & 11), plotted in FIGS. 10 & 11 represent
the total fraction of traffic passing through the network and arriving
uncorrupted at its destination. Accordingly (1-Throughput) describes the
proportion of data which is lost or corrupted. Therefore, depending on the
protocol, this may be equated to either a Bit Error Rate (BER), or to the
proportion of the gross offered load which is retransmitted data. If the
basic protocol does not incorporate retransmission, the net throughput
will be equal to the gross throughput but with a Packet Error Rate imposed
by the protocol. In the case where the terminals are wavelength selective
this equates to a BER ranging from 5.times.10.sup.-3 to 5.times.10.sup.-4
for the three examples given. In comparison, if all corrupted packets are
retransmitted the control itself will not give rise to a BER, although the
net throughput will be reduced by a factor equal to the original success
rate, i.e. net throughput=offered load.times.success rate. For example, if
the overall transmission rate is 2.4 GBit/s (corresponding to STM.16) the
aggregate capacity per instruction at 100% loading is 2 MBit/s for
non-wavelength selective receivers and 3 MBit/s for wavelength selective
receivers. So, similarly to the hardware vs. performance trade-off
described above, we find that there exists a relationship between protocol
issues and the performance/characteristics of the network.
* * * * *
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