.11 Voice and data over one wire pair needs to balance cost against the functionality specified. In most onto the outgoing call handling software, the telephone or terminal systems, like any computer system, the software runs on a proces- handling software controls all the user features. sor(s) with an operating system handling the basic low level func- On a basic PABX telephone, with only DTMF signalling, access tions and then the application layer interacting with the operating to features is restricted to dialling special codes, which usually system. means that any call in progress needs to be put on hold before a All the call control software operates in real time and only periph- feature such as conference another party, or dial ahead to determine eral facilities, like call statistics and database administration, oper- if a colleague is free or busy. This limited signalling system and the ate in non real time mode. Control of a PABX is all centred round absence of visual feedback for confirmation of feature invocation, the software, which can be visualised as a number of concentric has led to the development of multifunctional terminals with dis- rings with the real time multitasking operating system in the centre plays and arrays of keys to enable easy feature activation. surrounded by the hardware drivers e.g. the Layers 1 and 2 functions Typical features on the modern PABX would be: of the OSI 7 layer model. The database for the system is then the application software. It is in the application software that all the logical functionality of the switch resides and this is the most 1. Discriminating ringing, where internal and external calls had a dynamic area of the software, which is required to evolve with the different ring cadence to alert the called party to the type of call users' needs. waiting to be answered. Access to the database is of prime importance to the PABX user, Intrusion on a call already set up, unless any of the attempted as this needs to be regularly updated to keep pace with the chang