The PBX Scene: December, 1984

Some Recent Events

Since the second edition of the BCR MANUAL OF PBXS came out in May, 1981, a number of new systems have come on the market. These include the AT&T Systems 75 and 85, the CXC Rose, the Ericsson MD110, the Executone Enterprise, the GTE Omni, the Hitachi Digital Communications Controller, the ITT 3100, the NEAX 2400, the Redcom, the Siemens Saturn, the Solid State Systems CEO and D-Tel, and the Ztel, to name a few. Older systems such as the Northern Telecom SL-1 and the Rolm have been upgraded to become almost completely new machines, while systems announced earlier, such as the Mitel SX-2000 and the Anderson Jacobsen IOX appear to be about ready to live up to earlier expectations. The great majority of systems not mentioned above have made steady progress and some have added ingenious new ideas and concepts.

However, some familiar systems are no longer with us. The Tele/Resources System 32, the IT&T TD-100 and 200, the Oki Discoveries, the Hitachi EX-10, the Anaconda 900D, the NEAX 22, and the Northern Telecom Pulse have all been replaced by newer systems. Others, such as the Rockwell-Wescom 580, the Datapoint ISX, and a few more, have simply departed the scene. Yet others, such as the Stromberg Carlson, have gone through complex ownership changes (General Dynamics to Plessey to United Technologies) which will, one must hope, give them the backing to survive and develop.

The only way for the BCR Manual to keep up with changes and progress in the field is appear in a subscription format, issuing new sections for systems as they come on the market, and updating pages as existing systems make improvements. However, the format of the second edition did not lend itself to convenient updating and, further, was in danger of being bypassed by new developments. Thus this third edition uses a completely revised format, expanded and expandable, along with a new line-by line description of each item included, to permit direct comparisons among systems. The narrative description, preceding each family's line-by-line data, highlights each system's special capabilities.

Ten Trends to Watch

Review of new material on PBXs shows a number of trends emerging. First, new PBXs coming on the market are digital and PCM in the T-carrier format, just at the time when several telephone companies are making the T-span-line (usually on fiber optics) a preferred way of implementing CO Trunks. As digital central offices (exemplified by the Northern Telecom DMS 100, the GTD-5 EAX, or the AT&T No. 5 ESS) are installed both to provide "equal access" to various carriers in a combined Class 4/5 mode and to interface PBXs for digital local connections, the importance of the right kind of digital takes on added significance.

A second, perhaps cosmetic trend, is the use of cabinets that stack one on top of another to provide a certain amount of system growth. ITT's System 3100, the Redcom and the NEAX 2400 use this approach, although it has been common for some time to add shelves in full sized cabinets to expand systems. A third trend is the use of data terminals (with full alphanumeric keyboards, soft function keys, etc., and CRT displays, sometimes in color) as consoles. Using a general purpose instrument that can be programmed to prompt the user through any operation, including those not yet invented, is obviously a step in the right direction.

The battle against "flash and feature code" operation has apparently been won. A fourth trend shows that all new PBXs coming on the market are offering proprietary telephones that can emulate 1A2 key systems and use additional buttons as a convenient means for activating other system features. The soft function keys, noted above in connection with consoles, are also becoming common on some telephone sets (the Mitel Superset 4, for instance), allowing a new level of flexibility in helping the user interface the system. Soft function prompts are common, but voice prompts are also being used, reflecting lowering costs in voice synthesizers.

Using alphanumeric displays, both on phones and CRTs, as a means of presenting messages, is a trend of its own. This fifth trend, in its simplest form, lets station users leave "canned" messages so that those calling them (the console and internal PBX extensions) can see that they are "out to lunch" or "in a meeting until 3 pm." With only a 12 button dialing pad, keying in non-standard messages with the three letters on each digit is possible but time-consuming. Where name is stored with the standard equipment-number to directory-number translation, it can be displayed to the called party on an internal call. CXC goes a little further; the console attendant can key in the outside caller's name so that the called party, even if busy on another call, can see who is calling. If he elects to call back later, the attendant keys in the caller's number and the call can be returned by simply pushing a "call" button when examining the list of stored messages.

The built-in directory, along with other stored information about each station user, is a sixth trend. All stored program systems have a look-up table to relate an extension number to the cabinet, shelf, card and position on the card (equipment number) where it is located. Adding the name associated with the directory number is a small thing, considering the present cost of memory, but highly useful. Software to call up the name-number relation and show it, either on a CRT console or an alphanumeric display, is only a little more complex.

Trend seven carries the directory/inventory system a little further. The system must know which directory number(s) and features are operational on each set, and knowing the name of the principal is only a little more complex. However, all these items must be changed from time to time. The most important trend today is providing the communication manager with the ability to modify such information (Class of Service, Class of Restriction, etc.) as needed with a "user-friendly" menu-driven software system. In many systems, most moves, changes and alterations can be effected by the communication manager at a simple terminal in less time than it takes to write an order to a vendor. The savings come from not having to pay the vendor at $60 an hour to make simple changes, and then supervise and check the changes (several weeks later when they are actually carried out). By making the communication manager master in his or her own house, stored program control has finally begun to justify some of the hopes held for it over the past thirty years. AT&T has to be given the credit for pioneering the CAP (Customer Administration Panel) concept and arranging for customer personnel to be permitted to do work formerly reserved for telco personnel.

An eighth trend relates to station wiring. Designers today have two major approaches: single pair wiring with complex electronics, or two-pair wiring with somewhat simpler electronics. (Separate pairs for power may also be present.) In general, proprietary sets designed for new systems, and new sets designed for older systems, operate four-wire to the set for voice, and full duplex for data, with simultaneous voice-data transmission sharing the same wiring, set and line-card. Whether two-wire or four-wire is, of course, independent of the actual number of wires used.

The single pair approach uses time division ("ping-ponging"), frequency division as in short-haul carrier, or hybrids at set and line card to separate transmission directions. Two-pair wiring uses the obvious space division available with two pairs to separate transmission directions, without complex electronics. Another advantage is the possibility of running at higher frequencies without the digital signals on the pairs causing electromagnetic interference with other electronic systems. However, the simplicity and convenience of single pair wiring, particularly in terms of administration over the life of the system, is a powerful argument. Further, the ability to replace traditional 2500 sets with multi-line multi-featured electronic sets without changing the wiring is an obvious selling point for those who never liked the flash-and-feature-code approach. The ideal approach would be to use the same line card as well as the same wiring for single line, multi-line and more complex sets for both voice and data so that hardware changes could be avoided at the PBX when different sets are used. As yet, this refinement is quite rare.

A ninth trend, accelerating rapidly, is the addition of a distributed data base to distributed processing. Almost all new systems use many forms of distributed processing, sometimes by function, sometimes in system hierarchy, and sometimes in load sharing. But distributing processors by geography (one or more processors per cabinet) and giving each local group just enough data base to manage its own affairs and, upon finding it cannot complete a call, to use a data link to broadcast to other groups for help in completing the connection, seems to be an important variation. CXC, Ztel, Ericsson in the MD110 and Redcom, among others, use this approach. In all cases, the user interfaces the system as though the data base were monolithic, and the system itself sees to the distribution of information. The simplicity of a data base that is monolithic in reality, as with NEAX 2400 or the AT&T products may turn out to be better in the long run, but only time will tell.

The tenth trend is the line-group and group-selector approach used in all larger switches. Sometimes the group selector is a traditional space division switch, handling calls that originate and terminate on different line groups. The line groups use time division switching internally, and the group selector space switch is time-shared so that it makes a different connection in each time slot, and is not unduly large. This is, of course, the time-space-time pattern used in many large central office and toll switches.

But another approach, used by CXC and Ztel, puts circuit "rings" in the group-selector position. Here the group selector is not a switch located in a small area of space and accessed by each line group via "multiplexed links." Rather, it is a transmission medium that goes past all line groups, even if some are located a considerable distance away. Thus the rings act as both a transmission medium among line groups and as a switch for signals from one line group to another. If I am on node 1 and you are on node 3, the system may assign our conversation to time slot 48. My information enters the ring and goes past node 2 to node 3, where your line group picks it off and presents it to you. Similarly, your signal goes back into time slot 48, and rides around past nodes 4 and 5 back to me on node 1. Whether or not this approach has advantages over the conventional group selector is another question that only time can answer.

In passing, it should be observed that many manufacturers are calling their PBXs "fourth generation." The motivation seems to be the ability to use the terms LAN (Local Area Network) or Packet Switching in their advertising with a reasonably clear conscience. Moving data from one place to another is not very difficult these days, whether one uses a PBX or a LAN, a circuit switch or a packet switch. The real problem is the almost infinite series of speed, protocol and code conversions that must be produced to move data between non-compatible terminals. Such conversions are a function of the interface between the terminals and the transmission path and have nothing to do with moving he information through the path itself. A PBX can, in principle, provide access ports to do any kind of conversion that is needed, as can a packet network. Whether PBX designers will prove they know enough about data remains to be seen; however, a LAN will, if used, always be IN ADDITION TO the PBX for the foreseeable future. Indeed, because of the delay required for packet encoding of voice signals, there is some reason to believe that a packet-switched LAN may never be able to handle voice signals properly. In the meantime, three generations of PBX seem to be quite enough to make meaningful distinctions.

[ Top ] [ Next ] [ Table of Contents ]

Copyright 2005 Lee Goeller. All Rights Reserved.