Voice and data integration through modems Choosing the best method for intersite communications

by ALAN SIMMONS

H

igh rental costs of private wires and circuits for telex and data networks were the basic stimu-

lus for the development and use of systems which integrate voice and data. Any regular user of a private line

between two offices knows only too well the frustrations of being told that it is already being used ‘for a long

message’, and the cost of installing a second private line to relieve the congestion is often prohibitive.

Traditionally, companies have rented separate lines for speech and data links between their sites, but some years ago it was recognized that a system could be devised that would handle both on one circuit, and so speech-plus-data systems were inves-

tigated. The overall advantage of such a

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Abstract: Speech-plus-data systems offer financial advantages to companies with medium data trafficflow. Systems with data rates of 1200 - 2400 bitls are cheaper than high speed data channels and offer the possibility of transmitting speech as well as data. Lower data transmission systems, while not allowing speed of data ftransfer, are more flexible than the higher data rate systems. Examples of organizations using each type of system are dtscussed.

Kqzuords: data transmission, analogue s_vstems, modems.

Alan Simmons 1s sales engineering manager at Dowty Electronics, RFL Systems Division.

system depends very much on the level of traffic between the company sites. The communications manager

needs to identify the levels of speech and data traffic to determine what

type of system, if any, can be of economic benefit to the company. If the traffic between sites is low, then using the public network for both speech and telex data would be the economic solution. If, however, it is

very high, and large amounts of computer data are passing between sites, then separate high-speed data links between the computers will be necessary.

Speech-plus-data systems

In between these two extremes there

is a range of applications where speech-plus-data systems can provide a more economic solution. The first

equipment available allowed for, typi- cally, data transmission up to about 300 bit/s, or, alternatively, six 50 bit/s channels. This is only suitable for speech plus telex or slow-speed data applications, but can still provide a worthwhile saving where inter- company speech traffic and telex traf-

fic is significant, for example between a company’s factory and its head office, where intersite memos, accounts, orders and similar text have to be presented as hard copy.

Later developments in these sys- tems have allowed much higher bit/s rates, up to 2400, to be used in

conjunction with speech, thus enab- ling computer terminals and compu-

ters to talk to each other. In many cases, a data rate of 1200 bit/s or 2 400 bit/s can prove quite acceptable and is certainly a much cheaper solution

than the separate channels at higher speeds commonly used.

Where distances are greater than about 50 miles, and the requirements exist for speech and telegraph or data, then the use of either of these types of system can prove economic - the greater the distance the larger the

saving. The early design, with lower data

transmission, uses conventional fre-

quency shift keyed modems and low pass filters to implement a very simple system. The filter is used to divide the speech bandwidth (300-300 Hz in the UK) into two sections, the main part

of it being used for the speech and a small section at the top of the band being used for data. The system is

very flexible, allowing many options, as the cut-off point for the speech can be varied between 1800 Hz and 2700 Hz. The higher the cutoff point, the better the speech quality, but the smaller is the amount of data that can

be passed. Many applications find a cutoff at

2400 or 2 700 Hz quite satisfactory. Where speech quality is not so impor- tant, taking the cutoff down to 1800 Hz still provides intelligible speech, but with more bandwidth available for data.

~0126 no 10 december 1984 0011-684)3/84/100023-02$03.00 0 1984 Butterworth & Co Publishers) Ltd 23

The second type of system, allow- ing higher speed data, uses a more sophisticated arrangement to pass the speech and data together. A notch is taken out of the speech to allow the data to be transmitted nearer the middle of the band, where the line quality is better. Special modems are used in this system. For instance, in the RFL 6860 system, 1200 bit/s is transmitted in the band which is taken by a conventional 600 bit/s modem.

Additional processing is done to the speech to improve the quality, having taken out the central section. These systems, while giving higher speeds for data, do not have the flexibility of the first type of system. In both types of system, however, speech and data are full duplex. In addition, in both types there is a signalling channel to control the telephone signalling be- tween PABXs.

We now have systems which en- able speech plus 1200 bit/s, full duplex data to be carried simultane- ously over an office-to-office tieline. The two offices can exchange 1200 bit/s data 24 hours a day, while, at the same time, the tieline may be used for a PABX-to-PABX speech circuit. In this system, the voice circuit provides a quality similar to conventional low- pass circuits with bandwidths of 2 400 Hz, with the speech processor portion of the system using a voice cornpander to provide a low-noise circuit.

The data port at each office, which may be interfaced to a computer, VDU or other peripheral, is con- nected to the speech/data processor through a 25 pin D type connector. E & M signailing leads and speech lines are connected to the telephone ex- change. An extension at one exchange may then dial, through E & M signalling, to any extension on the remote exchange. The 1200 bit/s data port may also be used in a super- visory-control system for telemetry type application, for carrying alarm, status monito~ng, control, and other supervisor signals.

24

The system at Linread pk.

The lower speed systems provide similar facilities, with the data chan- nels usually used for telex or tele- metry signals. Both types of system have low enough data distortion to allow multiple links in tandem as well as radial configurations, thus, allow- ing great ~e~b~ity in network plan- ning.

UK organizations using low-speed systems

Among many wellknown users of the low speed system are the Navy, Army and Air Force Institute (NAAFI), Automobile Association (AA), Royal Automobile Club (RAC), Goodyear, Marley and the Morecambe Bay gas field. The NAAFI links its warehouse in Wolverhampton with offices in London, and the volume of traffic is such that two private lines are used, each carrying speech and one telex channel, and both used to near full capacity. In fact, maximum use is made of the lines, for while they are used for speech and telex data during the day, using the RFL 6850 system, they are switched at night to give a higher speed direct link between com- puters at the two sites, using separate 9600 bit/s modems.

The two motoring organizations (AA and RAC) use the lower speed system for linking their national net- work of offices with speech and telex. British Rail have used the system for speech and telex, and also in some cases for telemetry alarm monitoring of remote PABXs for engineering purposes. At Morecambe Bay, equip- ment is being supplied where the data portion of the channel is used pri-

marily for monitoring the complex radio communications network there.

Use of high-speed systems

A good example of the use being made of the higher speed system, with RFL 6860 processors, is at Linread plc, where the company’s head office in Birmingham is connected by a private telephone line to its factory at Peter- lee. This is used for normal speech traffic and also for transmitting data between the company’s sales order/ production control system computer at the offices and a VDU in the factory. The cost of the equipment is being recovered in about 18 months, and will save &4000 per year in rental charges.

The BBC also employs a higher speed system where the data channel is used for remote transmitter control and monitoring.

Future trends

The current systems probably repre- sent the end of development as far as analogue systems are concerned. But with British Telecom and others pro- ducing new generation digital sys- tems, it will not be long before the present terminals are replaqed by digital units, working on a 64 kbitis network. These will offer a higher data rate, well beyond the present 2400 bit/s limit, with at least as good quality speech as is provided by the analogue systems. 0

Dowty Electronin, RFL Systems Division, Marsbfieid, ~hip~~am, Wilts SNf4 SNN, UK.

data processing