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