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\hspace*{\fill}{\LARGE{\bf Telephony Seminar}}\hspace*{\fill}
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\hspace*{\fill}{\large{\bf 6.913}}\hspace*{\fill}
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\hspace*{\fill}{\large{\bf Final Report}}\hspace*{\fill}
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\hspace*{\fill}{\large{\bf Andr\'e DeHon}}\hspace*{\fill}
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This semester 6.913 followed the progression of basic telephone
technology from the invention of the telephone by professor Bell through
a number of the current electronic switching systems in use today.
The technology was surveyed both from the point of view of the individual
receivers and that of the switching systems.
Throughout the course, in addition to reviewing the electronic innovation
involved in each switching systems, the manner in which each system met the
needs of arbitration, control, and accounting was emphasized.
\par

The telephone had its origins in telegraph research.  The concept of
actually transmitting voice across wires in this manner began with
professor Alexander Bell.  While he was attempting to send multiple signals
down a telegraph line, he noted that he could get a tone at one end of a
line to actuate a resonate structure at the other.  With this he began
using various structures, such as diaphragms and aciduated water, to convert
acoustic signals into voltages which would then be transmitted across a
wire and decoded at a receiver.\par

With the advent of such a means of transmitting sound over distances,
people began to make use of the new technology.  Initially, a number of
``telephones'' were connected to each other via a common telegraph-like
wire thus providing a primitive {\it party line} for those who shared the
connection.  Such early systems were very ad hoc, and thus not
regulated.  Neither did they have any real means of switching, control, or
routing.  Eventually, as a first attempt to deal with the issue of
signalling over a phone system, a crank was added to wiggle the transducer
on a phone in a characteristic manner in order to both inform subscribers that
a call is being requested and to distinguish the intended recipient. \par

Soon, the crude coupling provided by directly connecting transducers 
was improved upon.  A second line was introduced to provide a common
point of reference for transmitted voltage signals.  The batteries within
each phone were decoupled using first capacitors, then transformers.  Early
on, a separate bell was added to indicate that a call was pending for a
particular locale.  The functions of the receiver and transmitter were split
into two distinct pieces of hardware.  Much improvement in the quality of
signal transmitted occurred with the introduction of the carbon
microphone as a common base for converting acoustic signals such as speech
into electrical currents.  Also the receiver and the transmitter were eventually
decoupled via transformers in order to minimize side tones and feedback.
\par

As the number of phones in user grew, more and more connectivity was
desired.  Originally, the desired party was indicated by the number of
rings in the ringing sequence.  Connecting everything point to point or
even as large {\it party-lines} was clearly unreasonable; additionally, as
the number of subscribers on a {\it party-line} increased, it both became
difficult to distinguish the destination specified and for an individual to
get a reasonable opportunity to use the phone when he wanted.  As a result, a
central switching 
scheme was devised to allow connections between local party lines and more
distant locations. \par

To meet this need, switching systems came into being around 1880.  The
first had electrical connections for each line the system serviced.  Then
it had a fixed number of selectors which could be used to route a call from
an originator to a destination.  The switching, routing control, and
signalling were virtually all supplied by a human attendant.  The attendant
would be signalled that a subscriber wished to route a call.  From there,
the attendant would attempt to satisfy the subscribers request by
signalling the destination addressee and routing the call when possible. 
This switching scheme proved to be quite limited in the number of calls that could
be connected at a given time.  \par

The next generation of switching was the
{\bf plug and jack} switch board.  This still relied on a human attendant
for most of the routing and arbitration, but provided a more sophisticated
set of hardware tools for the attendant to work with.  Each line terminated
in jack which represented the important parts of a given line (the tip and
sleeve).  Each line also had an additional plug that could be used to
connect a jack from another line to.  This allowed the arbitrary connection
of one line to another without limiting the number of calls that
could be routed simultaneously as severely as the earlier generation.  This
system provided the attendant with a direct way to test if the line was in use,
provide ringing request for attention to a subscriber, and obtain notification
that a call was terminated (via ring off).  Along with this generation,
switch boards such as this also incorporated a number of lines to other
switch boards so that connections could be made to subscribers outside of
the immediate population serviced by the given switchboard. \par

The next generation of switching systems introduced more automated
signalling and routing.  This was initiated with the {\bf step by step}
switching system.  Here the idea was to convey routing information along
with the call and provide a progressive automated switching.  Also by this
time the battery to provide power for the phone conversations was moved
from the individual hand-sets to the central switching office.  The step by
step system was then able to note a request for service from a local phone
set by checking the current drawn by the local set.  When a request for
service is detected, the switching system attempts to find a free line to
use for the call.  Once a line is found, the subscriber may ``dial'' the
desired destination.  The dial on the hand set produces a sequence of
pulses.  The step by step system uses each sequence to specify a routing
connection through the office.  This is done by {\bf stroger} switches
which use each pulse to advance a mechanical arm to a different pad to
route the connection.  In this manner, each ``digit'' in the number is
associated with a different piece of equipment.  The line finders
concentrate the incoming calls and respond to the service request.  The
incremental switching through {\bf stroger} style switches provides
distribution, routing, and expansion to connections with other phones in or
out of the office. On top of this, special offices called {\bf tandem
offices} were available to provide indirect routing from point to point.
If a given switchboard couldn't make a direct connection to a destination,
it would then route the call through a tandem office. \par

While the {\bf step by step} automated much of the arbitration of call
routing, it had a number of draw backs.  The number of connections that
could be made was limited by the number of line finders and number switches for given
digits.  Also the system was very static.  The system was difficult to
reconfigure with changing usage patterns.  Also, it required physical
reconfigurations to change a telephone number. \par

The next generation, the {\bf panel office} sought to remedy some of these
deficiencies.  In the panel office, the serviced lines were arranged in a
switch network. The panel switch had a traveling vertical contact which
looked for active lines.  For the most part, the panel office still used
the sequences of pulses for specifying the desired user routing.  When the
digits were transmitted by the local hand set, a component known
as the {\it sender} would store the digits in a special relay, decode the stored
value, and translate this into the number of pulses necessary to set up the
connection in the switch matrix.\footnote{This operation relied on feedback
from the panel switch in order to position the connection accurately.}  In
this system all of the information necessary to connect the call was
resident in the machine rather than being distributed over several switches
as in the step by step scheme.  Additionally, this scheme provided more
direct means of connecting to {\bf trunks} which allowed connections to other
switching offices for requests for service to numbers not directly serviced
by the local switch.  This matrix scheme provided a much more uniform means
of connection without the necessity of dedicating multiple stroger switches
to each call.  Also it allowed for the more dynamic reconfiguration missing
in the step by step system. \par

The panel offices were eventually superseded by the {\bf crossbar offices}.
The crossbar theory was similar to that of the panel office, but it
provided a different means of implementation.  A line finder was used to
find connect a line requesting service to an open line in the cross bar
switching. A {\it Marker} would then then link the incoming lines via
several crossbar frames first to lines to the trunks, then to the
respective trunks out of the office or to the appropriate intra-office
connections.  Additionally, a crossbar frame services incoming calls from
outside of the office to connect these corresponding lines to the requested
local subscriber lines.  The crossbar office proved to be much more
maintainable than its predecessors.  Also, the crossbar office was designed
to provide more automatic accounting of system usage by subscribers.   The
crossbar office expands on the theme of reconfigurability for growth and
change like the panel office. \par

The next generation of switching was the {\bf electronic office}.  Here
solid-state electronic devices began to replace the electromechanical
equipment used in earlier systems.  Utilizing the more advanced technology,
this allowed more versatility, faster operation, easier maintenance, and
more services for the subscriber.  The electronic office provides
semipermanent programmable memory that defines the current configuration of
the system.  This includes the translation of a number to a
physical wire, what services a subscriber has, and other fixed information
about a given subscriber.  The office also has a dynamic memory to keep
track of current call status ({\it e.g.\ } which phones are busy/free,
dialing information, billing information).  Permanent memory ({\it e.g. }
magnetic tape) is used to actually store billing information for completed
calls.  The individual components are modular mechanical devices and easily
replaced when problems arise; the system diagnostics should be capable of
identifying faulty equipment which can be replaced as necessary to keep
equipment in operation.  While the technology is much more advanced with
the electronic office than the earlier offices, the switching scheme itself
is somewhat similar.  Lines into the office are polled for service
requests.  These are then linked through junctors to the appropriate
electronic routing to establish either a local talk path, or a path through
the trunks to remote switching to reach a location outside of the local
office.  In the electronic office, the operation of the entire switching
system is moderated by high speed computer which uses the semipermanent
memory as a pseudo-program and the various lines and switching as its
peripheral.
\par

Also of importance in the development of modern telephone technology is the
transmission of the call information over wires.  Conversations over
telephone wires suffer from attenuation, dispersion,\footnote{delay
distortion} and echoing.  The distortion and attenuation effects were
ameliorated by carefully loading the transmission line.  The loss in signal
strength over distance in long distance lines was first dealt with using
repeaters.  As opposed to telegraphs, phones required a second line so that
random noise in the line could be eliminated by observing only the
differential signal on the pair of lines.  Eventually, in order to
multiplex multiple conversations over a single line pair, modulation was
used as over long distance lines.  In this manner, a number of
conversations shared a line at different frequencies.  The multiplexed
scheme also reduced dispersion as the band width was smaller relative to the
transmission frequency.  \par

Recently, digital sampling has come to successful usage in telephone line
transmission.  Using sampling, the local office can sample the call at 8kHz.
Then it can use its local memory to store the sampled speech data and time
multiplex it over an outgoing long distance line with an additional bit of
routing information.  In this manner, multiple conversations can be
conveyed over a signal high band width data line digitally by sending
pieces of each conversation interleaved and reconstructing the digital
information into 
an audio signal at the other end.  This also has the advantage that the
digital information does not suffer attenuation, dispersion, or echoing as
the analog signal had.  At each step, the digital information can be
checked using some check-sum/parity scheme to see that it is correct and an
update requested if it is not.  \par

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