Computer Based PBX

• Produce itemized,automated billing procedures,to allow the identification and management of toll calls. {hahaha}
• Combine daytime voice grade communication circuits into wideband data channels for night time high speed data transfers.
• Handles Electronic Mail { including office memos }.
• Combine Voice channels into a wideband audio/visual conference circuit,with the ability to xfer and capture slides,flipcharts,pictures of any kind.

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 assignment 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:

• Line Carriers
• Trunk Carriers
• Control Carriers

PBX Systems will directly reflect the types of services offered at the C.O.

• CCSA
• CCIS
• Picturephones {sooner than you think my phriends}

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' seperation between channels.

Well,it is like 3:30 and I am too tired to go on....let's save the rest for part II,eh?