Introduction and Overview

This was the first section in the System 75 article discussing the general idea of the PBX.

System 75:

Introduction and Overview


(Manuscript received July 11, 1984)

In 1980, a group of system designers at AT&T Information Systems Labatories (then a part of AT&T Bell Labatories) was asked to produce a new communications system for the intermediate-size business office (40 to 400 users), to complement a larger system already under development – System 85. The new system was named the System 75 office communication system. Its purpose was to meet the competitive challenge for high-function digital communication system whose integrated technology could address the evolving needs in office communications and automation. The proposal developed by our designers was based on an all-new hardware and software architecture.

In this intermediate-size range, the existing Feature Package of the Dimension PBX already had provided a challenging standard. Its more than 150 PBX features would have to be included in any new AT&T offering. Like the larger System 85, it would also provide integrated data switching capabilities, including 64-bp/s transparent switching; and simultaneous  voice/data transmission using the Digital Communications Protocol (DCP) that supports two 64-kb/s voice and data channels and one 8-kb/ s signaling channel at a single interface. With a significant fraction of customers requiring efficient multilocation service, it was recognized that the System 75 switch should also support multilocation networking services available in the current-generation Dimension systems and in System 85. These include Distributed Communication System (DCS) operation-permitting significant feature transparency between locations, a DS1 Interface to T1 facilities, Electronic Tandem Network support, and Centralized Attendant Service (CAS). AT&T Information Systems Architecture provided standards with regard to customer-system interactions, terminals, adjuncts and interfaces.

In mid-1983, an internal prototype with partial feature content underwent trials at AT&T Information Systems Laboratories. The first commercial customer received service early in 1984 and the product was publicly announced on April 26, 1984. The system’s initial features are listed in the Appendix A to this introduction.

Customer systems have come of age insofar as complexity and sophistication are concerned. Based on the technologies of microprocessors, modern software engineering, and VLSI, products such as System 85 and System 75 have a range and extent similar to the much larger switching systems designed for central office use. Since we felt that there was much of interest to be reported on their design, development, and project methodologies, early in 1984 we decided to produce this special issue of the AT&T Technical Journal on System 75. This collection of papers was therefore assembled to serve as an example of modern design in customer communication systems.

There are four major groupings of papers in this issue. The first two papers deal with the switch and control architecture- realized in hardware and firmware-and the physical architecture of the overall product. The next group of four papers describes those functional components realized in software: switch services (a generalization of what is traditionally known as “call processing”); system management, including database aspects and user interface; maintenance for all hardware and software elements; and the real-time operating system. The third group of three papers treats project methodology and software tools upon which the methodology and firmware/ software development rests. The first of these papers provides a project management overview and explains how designs from each development community are coordinated and integrated. The tools paper focuses on the software development methodology and tools environment. The test tool is described in a separate paper. Testing was a critical aspect of system development requiring a unique computer-based capability. The final topic deals with bringing the product to the people it serves-customers, and sales and service personnel-those who ultimately judge the acceptability of the product. The paper describes how the product is introduced to these users, how their reaction is measured, and how corrective measures are taken where necessary.

An important theme throughout these papers is the degree of overall design and development unity and coordination that was required and achieved. Tools, for example, were specified jointly by tools builders and end-user developers. Although an early version of the operating system had been built to support some exploratory call processing development prior to the start of this project, its evolution and optimizations reflected the needs discovered by those developing services for System 75. All maintenance design required significant support in hardware and device design, firmware (on port boards and in the common control) and, of course, in the software. Specification, design, integration, and testing spanned all project areas. System test, performed by a separate test group, depended on an intimate knowledge of software and software-firmware interfaces, and on quick turnaround from developers for fixes to permit testing to progress beyond the current trouble area. These are but a few examples of intra-project development coordination. The last paper of this issue illustrates the same broad coordination tasks involving all of AT&T Information Systems and its customers.


Only a few of the many people contributing to the design and development of System 75 are authors in this issue. To the other creative and dedicated individuals who had a part in its successful development, we are most thankful and appreciative. Although this group of papers stresses the development aspects of System 75, centered in one organization, the project received outstanding support and product contributions from other AT&T Information Systems and AT&T Bell Laboratories organizations. Specific individuals and the areas in which they led are: D. A. Keller and R. S. Breen (Systems Engineering); D. B. James (architectural consultant); R. S. Berryman (selected software tools); and C. O. Riddleberger (AT&T Bell Laboratories Power Systems).


System 75 Features

Standard System Features

Advanced private line termination

Automatic route selection

Code calling access

Direct department calling

Direct inward dialing

Direct outward dialing

Emergency transfer

Flexible numbering of stat ions

Foreign exchange central office access

Intercept treatment

Intercept with lockout

Listed directory number service

Loudspeaker paging

Modem pool

Multiple call appearances of extensions

Multiple listed directory numbers

Music-on-hold access

Off-premises stations

Outgoing facility management

Outgoing trunk queueing

Personal central office line

Primary extension

Recorded telephone dictation access

Remote access


   Code restriction

   Inward restriction

    Manual originating line

     Manual terminating line

     Miscellaneous trunk restriction

     Origination restriction

      Termination restriction

      Toll restriction

Serial calls

Simultaneous voice/data communication

Station busy indication

System Management



    System parameters

     Traffic measurements

Tandem tie trunk Switching

Terminal dialing

Through dialing

Tie trunk access

Touch-Tone calling

Touch-Tone sending

T0uch-Tone to dial pulse conversion

Uniforrm call distribution

WATS access

Standard Terminal Features

Abbreviated dialing

Personal, group and system lists

Automatic callback

Bridged call

Call answer from any station

Call coverage

   Caller response interval


    Coverage information display

    Temporary bridged appearance

Call forwarding-follow me

Call park

Call pickup

Call status indication

Call waiting

Called party identification (with display)

Calling party identification (with display)

Common audible alerting



Data handlers from voice terminal

Dial access to attendant

Dialed number display (with display)

Distinctive alerting

Elapsed time display (with display)



Hot-line service



Integrated directory service

I-Use indication

Leave word calling

   Message retrieval (with display)

Manual signaling



Priority calling

Recall signaling

Repertory dialing

Station-to-station calling

Station-to-station-only calling

Time of day and date display


Standard Digital Modem Features

EIA RS-232C interface

Speeds up to 19,200 b/s

Synchronous or asynchronous operation (300 or 1200 b/s) for pooled modem operations

Full or half duplex operation

Keyboard dialing (ASCII)

Automatic answer

Automatic speed and mode detection

Automatic and manual self-test

Odd, even or no parity

Other data set like options (e.g. loss of carrier disconnect)

Standard Attendant Specific Features

Alternate console position(s)- up to 6

Attendant call waiting

Attendant direct extension selection with busy lamp field

Attendant display

   Class-of-service display

   Incoming call identification display

   Trunk identification display

Attendant lockout

Attendant transfer – all calls

Central attendant service

Direct trunk group selection

Night service

Release loop operation

Splitting – one way automatic

Splitting – auto manual

Straightforward outward completion

Trunk group busy/warning indicators to attendant

Trunk-to-trunk connections

Two party hold on console

Message Center Agent Features

Display shows:

    Called person’s name and telephone number

    Reason for call coverage (busy, now answer, send all calls, go to coverage)

   Messages for callers from called person (such as status information)

    Calling person’s name and telephone number (if internal call)

Can record messages for intended called person or multiple people (causes their automatic message waiting lamp to light) as requested by caller

Has access to directory service to provide caller additional information (like room number or supervision)

Peripheral Equipment


Single line voice terminals


     7101 A  with two fixed feature buttons

      7103  A   with four fixed feature buttons and 10 programmable feature buttons

Multiappearance Voice terminals

      7300 Series

          7303 with six fixed feature buttons and ten buttons each programmable to either activate features or as call appearances

     7305 Same as 7303 plus 24 programmable feature buttons

7400 Series

These terminals provide simultaneous voice and data transmission. Addition of a Digital Telephone Data Module to t he 7403D and 7405D models provides an RS232 interface, allowing the connection of data equipment (data terminals, etc.) to the voice terminal.


    7405D (also supports an optional 40-character numeric display and call coverage


Video Terminal

Data Terminal

  513 Business Communications Terminal (data only)

Voice/Data Terminals

    515 Buiness Communications Terminals Integrated With digital telephone

Data modules

These modules allow data equipment, such as terminals and computers, to be connected to System 75

Digital telephone Data Module

Provide an RS -232C interface for data equipment when used in conjunction with a digital terminal (see above)

Processor Data Module

Provides an RS -232C interface to a host computer or standalone terminal

Trunk data module

Provides an RS -232C interface to a private data line or Digital Data System® data service unit to a remote computer or  terminal

Attendant console

The attendant console includes a 40-character alphanumeric display, command keys, feature status indicators, alarm indicator and a direct extension selector with busy indicators. The console plugs into any standard telephone wall jack.

Applications processor equipment

  500 Business Communications Terminal

a data terminal with a video display and keyboard connected to an Applications Processor via hard-wired 56kilobit/second links.


   a family of printers offered to work with application processing services. The printers have varifying speeds, print quality and costs

   443 low-speed, draft quality matrix printer

  445 medium speed, draft-quality line printer

  460 medium speed, draft quality matrix printer

  450 low speed, reproduction-quality printer

Station Message Detail Reporting (SMDR)

  Processing Options

   COMSTORE II and Telseer Series

   Stores details of all calls made and does SMDR processsing using tarriff tables

  Local Storage Unit (LSU)

   Stores details of all calls made for MDR processing

   Applications Processor

 The Call Detail Recording and Reporting feature provides SMDR processing on the associate System.


   Local associate printer may be used to print formatted data.

System Access Terminal

   513 Business Communications Terminal

  Optional 470 printer


System 75 Specifications 

Switch Cabinet 

70″h X 32″w X 24″d


42 1/2″h x 32″w x 24:d


System Limits (First Release)

Time slots 512

Circuit switch 64 Kb/s

Calling Rate 1800/hr

Traffic Limit 7200 CCS

Stations 400

Data Modules 200

Trunks 200

Trunk Groups 50

Pooled Modems 32

Attendant Consoles 7

Cabling Limits

Analog   6000 ft.

Hybrid, MET  1000ft

Digital, Dta modules, 515 BCT 3400ft

513 BCT   5000ft

Thermal Output

Maximum 1250 watts (4250 BTU’s/hr)

Typical Average – 875 watts (3000 BTU’s/hr)

[Cabinet is equipped with forced air cooling]

Power Requirements: 115V 60 Hz 50A Dedicated unswitched outlet located within 10 feet. Approved grounding essential


Temperature – 40° – 110°F

Relative Humidity-10-95% up to 78° decreasing to 35% at 110°.

Well ventilated area free of corrosive gases and excessive dust or dirt.