                   Comprehensive Look at Switching Systems

                                by Terminus

This file was originally posted on MetroNet: 'The Intelligent Phreak's Choice'

                           @ 301-944-3023 * 24 hours
Switching Generations

 The whole concept of dial or automatic switching is the basis of telephone
communications today.When one of the millions of telephone users lift a hand-
set,switching equipment located in the c.o. will:

o Locate and identify the calling line
o Give the signal to proceed ( dial tone ).
o Determine how and where to get access to it.
o Locate and test the numerous transmission paths leading to it.
o Select and link up the most appropriate combination of these paths.
o Then,if it is not in use, ring the called destination number.

A serving vehicle has carried out the switching function.Since 1891,when the
Strowger automatic system was publicized as the 'girl-less,wait-less tel.',
switching has taken three big steps,most of them during the last 20 years.

There are three main switching generations:

 1. Step by Step,direct or progressive
 2. Crossbar
 3. Common Control

The last divides into electronic and computer based solutions.The step by step
system was first born before the beginning of this century.It was the basis of
the first private branch exchange (PBX).With the introduction of the step by
step PBX,'number,please' manual switchboards rapidly disappeared.Intraoffice
conversations were established by direct dialing.
The first generation switching systems were also called POTS (plain old
telephone systems),and many businesses requiring dial switching still use
them.Such systems can be expanded indefinitely as long as space can be pro-
vided for the bulky frames and switches they require.In step by step switch-
ing,a call progresses one step at a time as the telephone user dials each
successive digit of the destination number.The system is also called direct
control because each switching function is directly controlled by the pulses
from the dialing telephone.The switch train is composed of:

      o the line finder
      o the selector
      o the connector

The dial pulses from the calling telephone directly control the switches that
establish the desired connection.This is simple,economical and a complete
modular solution except that it is totally obsolete.This is due to the high
cost of maintenance,switching delays,no place to put the huge monsters,and
also due to the inherent noise (electrical) will cause problems with data
being sent through the lines,from MetroNet to your modem..haha (had to toss it
in seeing that I am writing this phile..)

Common Control employs logic circuitry.Address digits generated by the dialing
instrument are stored,translated,and flexibly used for switching within the
system,or for establishing a connection with the outside network.The switching
equipment stores the entire number,then the operation starts.
Crossbar has been a milestone preceeding the electronic common control tech-
nology,and many consider it as the first phase of common control.The crossbar
switch is much smaller than the step switch.When they were introduced,crossbar
systems provided many helpful features (haha),but they have been overtaken by
electronics.Five key components make up this system:

o The Marker is the portion of the switch through which all calls must pass.It
  identifies a line requesting service and assigns it to an originating reg-
  ister.
o The Originating Registers record the number dialed.
o The Register/Scanner is a dual circuit in electronic crossbar systems which
  may perform the marker and originating register functions.
o The Matrix consists of a set of horizontal and vertical bars.To close a set
  of cross-points,the horizontal bar moves first.The point at which these two
  meet establishes the connection.
o The Sender,or trunk interface unit is the equipment used to process calls
  from the PBX to the serving central office.

Common Control equipment makes it possible to adopt flexible numbering plans
to meet a carrier's requirements and,in the case of private interconnect
systems to meet many specific user applications.
Common Control equipment also premits new features to be readily adapted;they
need only be applied to the circuits.Even when we talk about crossbar technol-
ogy,at the central office switching level the sender transmits the called num-
ber to the different types of distant,central office equipment.All told,I
speak of a specialization of functions which largely revamped the way switch-
ing was working with the step-by-step system.
As in the 1970's,however,rapid technological advances in computer design
channeled into telephone switching have changed the system further.

ELECTRONIC SWITCHING SYSTEMS: The Greatest Threat and the Greatest Tool!

Electronic Switching Systems (ESS's) offer the greatest potential for both
voice and data communications,together with the capability for an almost
bewildering array of internal service features.An ESS consists of:

             o A computer
             o Memory or Storage
             o Programming Capability
             o An extremely rapid switching component

The principal advantages of the stored program are that it allows the system
to expand it's capability to perform self-diagnostic checking and automatic
reporting of malfunctions,enables the technicians to perform many system
changes,and permits inputting new requirements through tele-typewriter term-
inals rather than by manually re-wiring various switch point connections.

Note: See Cosmos Tutorials...

A computer-based common control switching equipment implies two distinct type
of units:
                  o Control
                  o Switching

The Common Control recieves,stores,and interprets dial pulses,and then selects
an available path through the Switching hardware to complete a connection.
Paralell processing on common control equipment for only a portion of a call
is very important.Once the connection for a call between two telephones has
been made,common control releases and can complete more calls while the two
parties are talking.
Effecient high speed common control equipment can complete many calling con-
nections during the time of the average phone call.Thus it saves alot of time
and money for A.T.T..This is the prime difference between common and progress-
ive step by step control switching systems.
Furthermore,the switching network can be directed for many lines by one common
group of control devices.The control unit is the brain of this switching sys-
tem;it can typically complete it's function for a single call in a small
fraction of a second,allowing it to service many stations and lines.
An important element of the various ESS offerings is the computer's ability
to perform a wide variety of traffic analysis and telephone related account-
ing functions.. { this is the killer,people.. }.For these very reasons,elec-
tronic switching systems have found an expanding market in PBX at different
levels of sophistication.
ESS is not the latest in developments,and electronic switching is still per-
formed by electromechanical devices.Improvements come one at a time.First,
wire relays provide the logic required for supervision and control over the
system.Sometime later,computer based software is used for control and super-
vision.
Essential to the performance of switching functions through solid state
circuitry is that no moving parts are employed,but the basic concepts do not
necessarily change.Many of the features incorporated into ESS originated in
the crossbar era.Let's take a look at them one by one:

Station Transfer: sees to it that the user can transfer an incoming call from
                  outside the office to any telephone within the crossbar
                  system.This eliminates the use of a switchboard operator
                  for call transfer.Consult and Hold assures that an incoming
                  call can be held while the person dials another number to
                  secure information for the caller.Through an Add-On Confer-
                  ence a third person may be dialed so that the outside caller
                  ,the call recipient,and the third person may conduct a three
                  way conversation.
Camp On: an interesting feature! If the user's line is busy,the operator can
         'camp' an incoming call onto the station line.When the user hangs up,
         their telephone rings,and the waiting call is connected.When the call
         is camped,the user hears a beep indicating that a call is waiting.
Night Answer: provides that after the switchboard closes,audible signals
              announce calls arriving on central office trunks.Any person
              working after hours can dial a code,pick up the incoming call
              and transfer it to the person sought.With selective toll
              restriction,a particular station is permitted to make local
              calls but the equipment rejects any long distance dialing.This
              applies to a small PBX system rather that C.O. type hardware.

All cross bar systems offer these features;however,for lack of training,a
large percentage of phreaks are not aware of them.

Many more sophisticated capabilities are available with switching systems that
are computer-run.The computer is programmed to produce almost any feature de-
signed by the user.Such facilities are contained n the central switch,not in
the telephone instrument.The user commands the computer by dialing specified
codes related to each available function.In the more expensive electronic PBX
systems a function key is pressed to energize a certain attribute.

