The IBM 8750 Business Communications System is a voice and data switching system, suitable for medium to large numbers of
extensions, used on customer premises, and usually connected to public and private communication networks.
The role of an 8750 Business Communications System is essentially to connect many different types of line. A wide variety of
lines is supported, allowing an 8750 to be integrated into an existing network, and to function as an establishment’s
In addition to the basic functions of a Private Automatic Branch Exchange, the 8750 can be the gateway to public Integrated
Services Digital Networks (ISDN), as well as to IBM SNA networks.
The 8750 is the successor to the IBM 1750 and 3750 Switching Systems. Migration from and coexistence with 1750/3 750 is assured
by the compatibility of their network design.
Countries and product releases
The IBM 8750 Business Communications System can operate in the following countries where it meets the PTT requirements for
connection to the public telephone network:
Austria, Belgium, France, Germany, Italy, Luxemburg, UK.
Introduction of the 8750 into additional countries is dependent upon PTT regulations, homologation and other factors.
The 8750 is made available in two releases:
- Release 1: from 91 to 1400 extensions depending on configuration
- Release 2: from 91 to 3000 extensions depending on configuration.
In addition to greater capacity, release 2 improvements include:
- Digital trunks in Belgium, Italy and UK
- Radio paging
- Advanced networking based on a 2 Mbps digital link (Common Channel Signalling)
- Main/Satellite in 1750/3 750 networks
- Network Management workstation support
- Data: Third-party connect
- System availability in Luxemburg.
The upgrade of release 1 software to release 2 is provided free to all customers, and is planned to cause minimum disturbance
to the telephone service.
8750 switching is based on Stored Program Control and Time Division Multiplexing. The digitisation of the information conforms
with the European CCITT recommendations: 8 bits, 8kHz, A-law. Information, speech and data, is transmitted through the system over
a bus with a bandwidth of 295 Megabits/second, understood to be the fastest in the industry.
An 8750 can support a total of over 1000 simultaneous voice conversations or data sessions, in various mixtures of
conversations and sessions.
The modular line interface cards each house up to 16 extensions. The microprocessor of these cards allows adaptation to the
various PTT parameters, providing unique flexibility.
In general, any card space in the system can be used for any function, thus eliminating rigid constraints in the mix of
extensions, trunks and other features.
The controller is based on the Motorola 68020 32-bit microcomputer. The operational program is loaded from a 3.5 inch diskette and
executed from a 30MB fixed disk. The controller can be duplicated.
NMOS or CMOS microcomputers are mounted on CMOS integrated circuits manufactured under the ‘surface mount’ process. The CEPT 2
Mbps interface is a compact single card using VLSI chip sets.
A system consists of a minimum of one cabinet and a maximum of five adjacent cabinets. Additional optional service cabinets are
All cabinets measure:
Depth: 82.6 cm (32 in.)
Width: 76.2cm (29 in.)
Height: 190.0 cm (75 in.).
Connection of the system to the customer Main Distribution Frame is provided through an Intermediate Distribution Frame (IDF)
and associated cabling. The IDF is an integral part of the IBM system, and represents the demarcation point between customer and
PTT equipment. In addition, the IDF provides the necessary safety barriers required by most PTTs.
The weight of each fully loaded cabinet does not exceed 415 Kg (915lb).
The noise level is below 75 dB in a multiple cabinet configuration.
An 8750 can support up to eight IBM 8755 Operator Consoles at release 1, and 16 at release 2. An IBM 8756 Braille Operator
Adapter can be connected to each 8755.
The IBM 8755 Operator Console is a desk-top unit for:
- Processing incoming and outgoing calls
- Monitoring system alarms.
Each console features:
- IBM PC keyboard design and style
- High reliability
- Low voltage power from the 8750.
The IBM 8756 Braille Operator Adapter consists of a microprocessor-based Braille pad, with a keyboard used for scrolling and
retrieval. It is connected to an IBM 8755 Operator Console via the V.24/V.28 interface. It is powered locally and has 8 hours
The basic system includes 91 to 96 extensions. 16 of these must be digital extensions. The remainder can be configured in any
combination of digital increments of 16, and analogue increments of 15.
Both rotary and multifrequency telephones can be attached. Long line extensions are supported.
Digital extensions are connected by ROLMlink
Suitable analogue DOD and DID trunks are available in each country. Meter pulses from a public exchange may be recorded if
metering cards are included in the configuration. These pulses may help determine the cost of calls.
Digital (CEPT 2Mbps) trunks supporting DOD and DID with Channel Associated Signalling in Belgium, France and Italy are available.
ISDN trunk with Common Channel Signalling is available in the UK (DASS 2).
In France, the CEPT 2Mbps trunk transmits data at 64kbps on dedicated channels (TRANSCOM on RTC64 service). This service can be
accessed from an extension via ROLMlink.
Tie line interfaces
Analogue tie lines are available to connect to other PBXs. 8750 can be connected to 1750 and 3750 ILS/INS networks.
Digital tie lines are available in two versions:
- Channel Associated Signalling to interlink 8750s using 1750/3750 in-band signalling (similar to ILS/INS)
- A pre-ISDN protocol (DPNSS) to interlink 8750s by Common Channel Signaling.
Digital public network interface
Primary access with the interim protocols as implemented by the PTTs is supported:
- In Belgium, France and Italy using Channel Associated Signaling
- In the UK: with DASS-2
- In France, the digital data transmission service, TRANSCOM (or RTC64), can be accessed from a ROLM1ink extension.
Additional equipment Based on the 19-inch rack dimension, interface panels with V.24 and V.36 sockets, fuse panels, Automatic
Emergency Switching (AES), and rack-mounted data communication modules, can be housed in Supplementary Service Cabinets.
Except for the Power Distribution Unit and rectifiers, all the above mentioned equipment can be housed in an SSC model 2.
Except for the Power Distribution Unit, rectifiers, batteries and Rack-Mounted Data Communication Module, all the
above-mentioned equipment can be housed in an SSC model 3.
The number of ports in one cabinet varies with the feature mix, and whether the controller is duplicated or not. The maximum
configuration of five cabinets provides 15 shelves.
Hardware circuit modularity
The hardware features of 8750 are mounted on cards, inserted into racks at installation time. The number of features mounted on a
card depends on their complexity: for example there are 16 digital extensions on one card. However, a CEPT trunk needs one card
per 2 Megabit link.
For each country except Italy, a single software package containing all available functions is offered. Italy has two software
packages; one is the standard package, while the other excludes Call Detail Recording and user intrusion. As a customer expands
from voice only, adding data, networking, and Automatic Call Distribution (ACD), the 8750 requires no additional software. Each
system is tailored by software tables to the customer’s numbering plan, Call Detail Recording requirements, and other
System Input/Output devices
The 8750 controller exchanges information locally with mass storage and, for system management, externally with an IBM Personal
Mass storage devices
A fixed disk with a capacity of 30 MB is used for:
- Automatic loading of programs and tables
- System log files
- The Call Detail Recording file.
A removable 3.5 inch diskette with a capacity of 1.44Mb is used for:
- Initialising the system
- The Call Detail Recording file
The system is initialised on-site by IBM. The operational program is merged with customer-specific system tables. These system
tables are prepared off-line using the 8750 table generation program on an IBM Personal System/2 model 50. The 8750 table
generation program is available to IBM staff for definition of the system and particular tables. It is also available in a limited
version to customers for definition of customer-specific data, such as extension and speed calling lists. In addition, both
versions can print ROLMphone templates.
System management is performed on an IBM Personal System/2 (PS/2) model 50 with at least 512Kb memory, a 20Mb fixed disk, one
1.44Mb 3.5 inch diskette, and an asynchronous V.24/V.28 communications adapter.
System management includes the following:
- Administration such as table updates, Call Detail Recording, traffic analysis, and ACD supervision
- Customer problem determination.
One Personal System/2 must be adjacent to each 8750, but further ones may be attached, each running their own applications.
When not used with 8750, the Personal System/2s can be used for other general purpose applications.
Supplementary Services Cabinet (SSC)
There are three models of SSC available. Two models have the same external dimensions as the 8750 and covers that match. These
models are equipped with 19-inch racks at the front and rear, to allow units (IBM or non-IBM) to be fitted to both sides. The
third model can be wall-mounted.
Model 1 may contain the following:
Power Distribution Unit (PDU)
This is required for DC-powered systems and is fitted with AC and DC contactors for Emergency Power Off (EPO).The PDU supplies AC
to the power outlets, to power the rectifiers, recorders and other equipment. The DC from the rectifiers is connected to a common
DC bus by a ‘plug-in’ cable. DC is distributed from the common bus to the fuses or circuit breakers to power up to five 8750
The 15 AC outlets serve to power the rectifiers, modem, DCMs, recorders, and other equipment mounted in the cabinet.
Test Jack Frame for Data (TJF-D)
The TJF-D provides a test point for all equipment installed in the cabinet, and consists of a 100-pair block-mounted to the side
of the 19-inch rack. The cables from modems and other data equipment are wired to the TJF-D to allow testing without removing
units from the rack.
Intermediate Distribution Frame (IDF)
The IDF is a 19-inch unit equipped with six 25-pair sockets, prewired to 150 pairs of connection and test points. The IDF panel
can be used as the PABX side of the Main Distribution Frame (MDF).
Main Distribution Frame (MDF)
The MDF is a 19-inch rack-mounted unit equipped with 200 pairs of connections.
The circuitry for controllers and lines offers a maximum of capacity for a minimum of space and power.
Non-IBM rotary and multifrequency telephones can be attached to the IBM 8750 System as long as they are approved by the
IBM has available a family of digital telephones, called ROLMphones.
A microprocessor in each ROLMphone digitises speech before transmission to the 8750. Certain ROLMphone models can have more than
one extension appearance, but all models need only a single twisted-pair line to the 8750.
Each model has a built-in loudspeaker and a handset. All have adjustable ringing, push
button dialling, and a message waiting indicator. Keys can be defined for line or feature access.
Four models are available, in increasing degree of sophistication:
- The ROLMphone 120 handles a single line.
- The ROLMphone 240 can have up to II line appearances. Versions are available as voice only, voice/data, and voice with a
- The ROLMphone 244, similar in appearance to the 240, is described separately below.
- The ROLMphone 400 can display the name and extension number of an internal caller, a called number, a call-back message, and
other information. This model can handle up to 29 line appearances all on a single line to the 8750.
An optional, built-in, Data Communication Module (DCM) is available for models 240 and 400. (See ROLMphone DCMs in the ‘Data
Two headset jacks are optionally available for models 240 and 400. Only the model 400, however, can have both a DCM and headset
jacks. The headset feature is commonly used for ACD (model 400H).
A ROLMphone 244 equipped with an enhanced DCM (asynchronous), interfaces with the communications port(s) of an IBM PC or Personal
System/2 and has been designed to complement the PC or Personal System/2. It allows the user to exchange commands and information
between the PC or Personal System/2 and the RP 244 using the “AT” (Hayes*) intelligent modem command set.
Applications such as Personal Telephone Management allow the user to:
Display information about the call being presented, such as:
- Calling extension number and name
- Message waiting indicator Time and date
- Dial from the PC keyboard
- In addition, asynchronous Data Communication is available simultaneously.
The following functions are available:
- Priority trunk queueing by trunk group
- Multi-group call queueing with overflow
- Up to 200 agents per group
- Up to 64 groups
- Sophisticated agent set, ROLMphone 400H
- Recorded Voice Announcement or music on hold
- Management reports; 12 reports are provided. Four reports are designed to be displayed on a Personal System/2, and eight
reports printed for management evaluation. These reports include information such as:
- Agent and ACD station performance Group summary report
- Complete ACD report Trunk usage (delayed/lost calls).
ACD software is provided in all 8750s.
IBM 8751 CBX Technical Characteristics
Depth of each cabinet 788 mm (31 in.), by 724 mm (28,5 in.) width, by 1900mm (75 in.) height. Covers add 38mm (1 in.) to depth and
Approximately 415 kilograms (915 lb) for a fully loaded cabinet.
45 kilograms per square meter, and 450 kilograms contact point load.
From 10 to 40.6 degrees Celsius
Air humidity 8 to 80 %
Altitude 0 to 2100 meters.
AC power through a Line Conditioning Module at the base of each cabinet. The 50 Hz input is rectified to 48 V DC output at 50
amps. Alternatively, DC power feeds the bus-bar directly. Maximum 1200 watts per cabinet.
The ROLMbus 295 provides the switching architecture. There are 2304 timeslots, based on an 8 KHz sampling rate. The clock rate is
18432 MHz. 2240 timeslots are used for voice and data. This allows over one thousand 64 Kbps simultaneous voice or data paths.
Taken from IBM 8750 technical publication 10/87