APPROVAL No.
An
Integrated Services Digital Exchange (ISDX) is a stored program control (SPC) type of
private automatic branch exchange (PABX), and provides efficient, reliable control of
fully integrated voice, text and data communications. It handles voice and data using a
variety of telephones, featurephones, terminals, workstations and other desktop units. It
can provide a link between word-processing, desktop and mainframe computing and other
essential services. It optimises the use of trunks giving to system managers more cost
control and planning information. Its modular design enables the system to be easily
upgraded to incorporate new hardware and software. The system allows evolution of private
networks, either on a single site, or as a sophisticated corporate network.The BT ISDX family comprises four sizes of exchange; large (ISDX-L), small (ISDX-S), mini (ISDX-MN) and an extended traffic version of the large exchange, (ISDX-T).
System overview
The ISDX system can be divided into two main parts:
- The hardware to connect users and to switch calls.
- The software/firmware to control the hardware and give the users access to a wide range of facilities and features.
The heart of the system is the switch matrix. This makes and breaks connections between:
- calling party and common services
- called party and common services
- calling party and called party
- a specific interface and maintenance access.
Either or both calling and called parties can be:
- a local analogue telephone
- a local digital telephone
- a local data service
- a digital trunk
- an analogue trunk
Switch control is effected by a connection table which gives the state of all switches within the switch matrix. The software cycles through all entries in the table at a fixed rate and issues instructions to each switch in the matrix.
Common services generate all tones and ringing cadences used in the system. Common servixes also includes conference control, recorded announcement and music-on-hold.
Call process control monitors the state of every interface in the system to initiate new instructions to switch control at the relevant stages of call establishment, connection and cleardown.
Extensions can be:
- conventional pulse-dialling telephones or feature-phones (rotary or pushbutton) using a decadic signalling type such as loop disconnect.
- pushbutton telephones or feature-phones which use a multi frequency type of signalling known as MF4. This system uses eight frequencies in two groups of four, and selects one frequency from each group to provide one of sixteen distinct codes.
- integrated Services Digital Terminal (ISDT). This is a limited range digital extension combining both voice and data in one instrument. An ISDT uses a digital signalling message structure. The signalling information is multiplexed into the 'D' channel of a 2B + D format and transmitted at 144 kbit/s in burst mode.
- a magneto telephone which uses a ring in, ring out type of signalling. A magneto extension cannot be connected to an ISDX-MN.
One or two local MMI terminals can be connected to a ISDX via V.24 ports. One port is known as the maintenance terminal for use by the maintenance authority, and the other known as the customer terminal for use by the operator or telecommunications manager. Access to the ISDX database can also be gained by remote MMI terminals or via a remote control centre (RCC), using an optional system feature known as the remote access system (RAS).
An MMI terminal can be a visual display unit (VDU) or a printer together with a keyboard. Commands are entered at the terminal using alphanumeric strings. A customer terminal is limited to the use of customer level commands only, by restrictions imposed by the system software at the customer port.
Operator consoles can be one of three types:
- two-piece console - CONSOLE mk1 (wood and stainless steel)
- three-piece console - CONSOLE mk2 (plastic)
- screen based console (SBC)
The main function of the operator console is to handle incoming and outgoing external calls, connecting these to internal extensions as required. The console has various facilities to make the operator's job easier. As an optional facility the console can be used in MMI mode whereby some of the extension and console facilities can be set up or altered, using a menu-driven structure. Programming using feature codes from the console as a special telephone, e.g. applying absent extension diversion to an extension, is not generally written to disk, nor listable by MMI.
The SBC is hosted on a personal computer (PC) with a customised keyboard. The SBC offers a far wider range of facilities than the two or three-piece console. These include:
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