97 lines
5.1 KiB
Plaintext
97 lines
5.1 KiB
Plaintext
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Understanding PBX Systems
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by Terminus
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This file was originally displayed on MetroNet: 'The Intelligent Phreak's
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Choice' @ 301-944-3023 * 24 hours.Other Systems,and Sysops may use this
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ile as long as they retain the full credits. 'Aww,Knock it off!' I say..
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Computer Based PBX
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To get a better understanding of what a PBX can do,here are a few basic
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fundamentals.The modern PBX is a combined Computer,Mass Storage Device,
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and of course a switching system that can:
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1 Produce itemized,automated billing procedures,to allow the
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identification and managQ ent of toll calls. hahaha
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2 Combine daytime voice grade communication circuits into
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wideband data channels for night time high speed data
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transfers.
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3 Handles Electronic Mail including office memos .
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4 Combine Voice channels into a wideband audio/visual
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conference circuit,with the ability to xfer and
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capture slides,flipcharts,pictures of any kind.
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Both the external and internal calling capacity of the PBX System must be
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carefully considered because many business operations run a very high ratio
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of internal station to station dialing and a low capacity system will not
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handle the requested traffic load.
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A critical factor is the number of trunks and the Central Office Facilities
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that are used for outside connections.Another is the number of junctions or
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links that make up the internal calling paths.
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To understand the services available on a typical computer run PBX it is
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necessary to introduce the subject of time division switching.In a time
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division switching network all connections are made via a single common bus
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called (of course) a 'time-division bus'.Every line trunk that requires a
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connection with another is provided with a port circuit.All port circuits
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have access to the time division bus through a time division switch.
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[When two ports require connection,their time division switches operate at
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a very high frequency (16,000 times per second).This technique,which is
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called 'speech sampling',allows many simultaneous connections over the same
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time division bus.Each connection is assigned a time interval,the 'time slot'
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,and the number of time slots identifies the number of simultaneous connect-
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ions among ports.]
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The next critical item is circuit PACKS.The system elements that we will be
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describing in future tutorials lines/trunks/switches,memory and control are
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contained on plug in circuit packs.Each line circuit pack contains a number
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of lines,in example,four.But the assagnment of station numbers to actual
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phone line circuits is flexible.
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The system memory is contained in circuit packs which provide the call
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processing functions.The circuit packs are held in small frames called
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'carriers'.Within each carrier,the circuit packs are plugged into positions:
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the 'slots'.Every circuit can be addressed by,say a five digit number which
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tells its location by carrier-slot-circuit.... starting to get the idea?
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There can be three types of carriers in a modern PBX system:
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o Line Carriers
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o Trunk Carriers
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o Control Carriers
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The line carriers contain station lines.In A.T.& T.'s "Dimension" model,for
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example,a total of 52 to 64 lines are provided.The trunk carriers contain
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slots for 16 trunk circuit packs.The control carrier includes processor,
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memory,control circuitry,data channels for attendant console control and
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traffic measurement outputs.
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PBX Systems will directly reflect the types of services offered at the C.O.
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o CCSA
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o CCIS
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o Picturephones (sooner than you think my phriends)
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Common Control Switching Arrangements ( CCSA ) permit any unrestricted tele-
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phone station to call any othet internal or exterNal system station by using
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the standard seven digit number.Alternate routing is a feature of CCSA service
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The interfacility,alternate routed calling paths are accomplished at the tele-
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phone company central office level,not at the PBX level.
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A system of interest to large scale telephone users is Common Channel Inter-
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office Signalling (CCIS).Typically,this technique employs common channels to
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carry all interfacility signalling instructions: dial pulses,on hook (idle),
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off hook (busy),and so on,between two switching centers. getting warm .
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CCIS replaces older methods of interoffice signalling such as 'in band' and
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'out of band' techniques. By the way,real phreaks are selling their boxes to
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idiots who still think the're worth alot...The former (in band) transmits
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signalling data within the normal conversation bandwidth.It's shortcoming is
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that false information may be transmitted due to unique tone or noise
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combinations set up in the talking path. this is the official reasoning.
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Out of Band signalling techniques placed the interoffice data in special
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channels,generally adjacent to and immediately above the voice path.To pre-
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serve interchannel integrity,out of band signalling requires very effecient
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filtering or greater 'band guard' seperating between channels.
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