175 lines
10 KiB
Plaintext
175 lines
10 KiB
Plaintext
Comprehensive Look at Switching Systems
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by Terminus
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This file was originally posted on MetroNet: 'The Intelligent Phreak's Choice'
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@ 301-944-3023 * 24 hours
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Switching Generations
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The whole concept of dial or automatic switching is the basis of telephone
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communications today.When one of the millions of telephone users lift a hand-
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set,switching equipment located in the c.o. will:
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o Locate and identify the calling line
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o Give the signal to proceed ( dial tone ).
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o Determine how and where to get access to it.
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o Locate and test the numerous transmission paths leading to it.
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o Select and link up the most appropriate combination of these paths.
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o Then,if it is not in use, ring the called destination number.
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A serving vehicle has carried out the switching function.Since 1891,when the
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Strowger automatic system was publicized as the 'girl-less,wait-less tel.',
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switching has taken three big steps,most of them during the last 20 years.
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There are three main switching generations:
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1. Step by Step,direct or progressive
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2. Crossbar
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3. Common Control
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The last divides into electronic and computer based solutions.The step by step
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system was first born before the beginning of this century.It was the basis of
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the first private branch exchange (PBX).With the introduction of the step by
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step PBX,'number,please' manual switchboards rapidly disappeared.Intraoffice
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conversations were established by direct dialing.
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The first generation switching systems were also called POTS (plain old
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telephone systems),and many businesses requiring dial switching still use
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them.Such systems can be expanded indefinitely as long as space can be pro-
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vided for the bulky frames and switches they require.In step by step switch-
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ing,a call progresses one step at a time as the telephone user dials each
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successive digit of the destination number.The system is also called direct
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control because each switching function is directly controlled by the pulses
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from the dialing telephone.The switch train is composed of:
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o the line finder
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o the selector
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o the connector
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The dial pulses from the calling telephone directly control the switches that
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establish the desired connection.This is simple,economical and a complete
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modular solution except that it is totally obsolete.This is due to the high
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cost of maintenance,switching delays,no place to put the huge monsters,and
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also due to the inherent noise (electrical) will cause problems with data
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being sent through the lines,from MetroNet to your modem..haha (had to toss it
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in seeing that I am writing this phile..)
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Common Control employs logic circuitry.Address digits generated by the dialing
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instrument are stored,translated,and flexibly used for switching within the
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system,or for establishing a connection with the outside network.The switching
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equipment stores the entire number,then the operation starts.
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Crossbar has been a milestone preceeding the electronic common control tech-
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nology,and many consider it as the first phase of common control.The crossbar
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switch is much smaller than the step switch.When they were introduced,crossbar
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systems provided many helpful features (haha),but they have been overtaken by
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electronics.Five key components make up this system:
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o The Marker is the portion of the switch through which all calls must pass.It
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identifies a line requesting service and assigns it to an originating reg-
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ister.
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o The Originating Registers record the number dialed.
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o The Register/Scanner is a dual circuit in electronic crossbar systems which
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may perform the marker and originating register functions.
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o The Matrix consists of a set of horizontal and vertical bars.To close a set
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of cross-points,the horizontal bar moves first.The point at which these two
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meet establishes the connection.
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o The Sender,or trunk interface unit is the equipment used to process calls
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from the PBX to the serving central office.
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Common Control equipment makes it possible to adopt flexible numbering plans
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to meet a carrier's requirements and,in the case of private interconnect
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systems to meet many specific user applications.
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Common Control equipment also premits new features to be readily adapted;they
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need only be applied to the circuits.Even when we talk about crossbar technol-
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ogy,at the central office switching level the sender transmits the called num-
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ber to the different types of distant,central office equipment.All told,I
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speak of a specialization of functions which largely revamped the way switch-
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ing was working with the step-by-step system.
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As in the 1970's,however,rapid technological advances in computer design
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channeled into telephone switching have changed the system further.
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ELECTRONIC SWITCHING SYSTEMS: The Greatest Threat and the Greatest Tool!
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Electronic Switching Systems (ESS's) offer the greatest potential for both
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voice and data communications,together with the capability for an almost
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bewildering array of internal service features.An ESS consists of:
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o A computer
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o Memory or Storage
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o Programming Capability
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o An extremely rapid switching component
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The principal advantages of the stored program are that it allows the system
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to expand it's capability to perform self-diagnostic checking and automatic
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reporting of malfunctions,enables the technicians to perform many system
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changes,and permits inputting new requirements through tele-typewriter term-
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inals rather than by manually re-wiring various switch point connections.
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Note: See Cosmos Tutorials...
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A computer-based common control switching equipment implies two distinct type
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of units:
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o Control
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o Switching
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The Common Control recieves,stores,and interprets dial pulses,and then selects
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an available path through the Switching hardware to complete a connection.
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Paralell processing on common control equipment for only a portion of a call
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is very important.Once the connection for a call between two telephones has
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been made,common control releases and can complete more calls while the two
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parties are talking.
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Effecient high speed common control equipment can complete many calling con-
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nections during the time of the average phone call.Thus it saves alot of time
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and money for A.T.T..This is the prime difference between common and progress-
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ive step by step control switching systems.
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Furthermore,the switching network can be directed for many lines by one common
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group of control devices.The control unit is the brain of this switching sys-
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tem;it can typically complete it's function for a single call in a small
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fraction of a second,allowing it to service many stations and lines.
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An important element of the various ESS offerings is the computer's ability
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to perform a wide variety of traffic analysis and telephone related account-
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ing functions.. { this is the killer,people.. }.For these very reasons,elec-
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tronic switching systems have found an expanding market in PBX at different
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levels of sophistication.
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ESS is not the latest in developments,and electronic switching is still per-
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formed by electromechanical devices.Improvements come one at a time.First,
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wire relays provide the logic required for supervision and control over the
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system.Sometime later,computer based software is used for control and super-
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vision.
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Essential to the performance of switching functions through solid state
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circuitry is that no moving parts are employed,but the basic concepts do not
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necessarily change.Many of the features incorporated into ESS originated in
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the crossbar era.Let's take a look at them one by one:
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Station Transfer: sees to it that the user can transfer an incoming call from
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outside the office to any telephone within the crossbar
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system.This eliminates the use of a switchboard operator
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for call transfer.Consult and Hold assures that an incoming
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call can be held while the person dials another number to
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secure information for the caller.Through an Add-On Confer-
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ence a third person may be dialed so that the outside caller
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,the call recipient,and the third person may conduct a three
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way conversation.
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Camp On: an interesting feature! If the user's line is busy,the operator can
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'camp' an incoming call onto the station line.When the user hangs up,
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their telephone rings,and the waiting call is connected.When the call
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is camped,the user hears a beep indicating that a call is waiting.
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Night Answer: provides that after the switchboard closes,audible signals
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announce calls arriving on central office trunks.Any person
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working after hours can dial a code,pick up the incoming call
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and transfer it to the person sought.With selective toll
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restriction,a particular station is permitted to make local
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calls but the equipment rejects any long distance dialing.This
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applies to a small PBX system rather that C.O. type hardware.
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All cross bar systems offer these features;however,for lack of training,a
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large percentage of phreaks are not aware of them.
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Many more sophisticated capabilities are available with switching systems that
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are computer-run.The computer is programmed to produce almost any feature de-
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signed by the user.Such facilities are contained n the central switch,not in
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the telephone instrument.The user commands the computer by dialing specified
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codes related to each available function.In the more expensive electronic PBX
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systems a function key is pressed to energize a certain attribute.
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