PXML No. 47

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 services 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 clear down.

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:
  • A directory package.  Allows a corporate directory to be constructed and revised by a supervisor.  The directory Is held on Winchester disk drive and can be downloaded to other SBCs on the same node over a local area network (LAN) or to an SBC on another node via a DPNSS link.  The directory can be used by an operator to dial a number by name.  Given the appropriate password an operator can print all or part of the current directory.
  • Graph source queues.  Variable height columns represent the number of calls waiting under a source button.
  • A blackboard to allow procedural information etc, to be sent simultaneously to all operators on the same LAN.
  • Emergency facility.  Allows recording and playback of a conversation involving the emergency services.
  • Password prompts.  Allows a supervisor to create passwords and user names for each operator on the system.

The two and three-piece operator consoles can also have an optional visually handicapped operator console attachment (VHOCA) connected to them.  This enables visually handicapped operators to cope with the high throughput of traffic normally presented at the console.

All peripheral equipment is connected to the ISDX via interfaces.

Signals from analogue extensions are restricted to a bandwidth of 300-3400 Hz before being sampled at an 8 kHz rate to produce a digital signal in 8-bit format using pulse code modulation (PCM).

Every interface port within the ISDX is connected to a time division multiplex (TDM) highway or bus.  Access to the bus Is segmented in time to provide 512 timeslots for an ISDX-L, S and MN and 1024 timeslots for the ISDX-T.  The timeslots cycle at an 8 kHz rate.  Control of connection to the TDM bus is performed by an area of random access memory (RAM) on the digital switch controller known as the connection table. The connection table holds two addresses for each timeslot.  These addresses define which interface ports are to be inter-connected on the TDM bus during that timeslot.

Tones, produced by the tone generators in the common equipment block, are digitally represented, allowing territory specific tones to be configured easily.

Trunk interfaces, which utilise common channel signalling (CCS) systems such as DPNSS, are polled by the delta channel controller (DCC) via the delta channel highway (DCH) using high-level data link control (HDLC) frames.  The link level (level 2) frames from the remote end are terminated at the interface, but the network level (level 3) messages are sent through to the central processing unit (CPU).

The ISDX-L and -T have two CPUs, whereas the ISDX-S and MN have one only.  For the two processor ISDX, one is actively controlling the system. The other CPU is a hot standby which continuously monitors the health of the active processor, ready to take over K required.  Automatic switchover takes place once every 24 hours, provided that there are no calls in progress.

ISDX Facility Codes - user guide

ISDX Brochure

IDX Brochure

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Last revised: December 22, 2021