THE benefits of radio-paging are frequently obscured by the success enjoyed by cellular telephones tn many countries across the world. However, paging offers major advantages, both to the operator and the user, in getting information to people who are out of reach of a telephone.

Recent developments in the United Kingdom in applying satellite technology to radiopaging provide improved performance and reduced costs, and have particular advantages where the telecommunications infrastructure is already overstretched.

The accelerating tempo of today's society makes ft increasingly important to be able to make contact with people on the move. Pagers provide a convenient and cost-effective solution to this problem in general.

In eastern Europe, where there is a shortage of telephones. pagers can provide a very valuable tool. Not only can they be used to contact individuals, but to provide an information update. In Britain, for example, message pagers are being used to provide up-to-the-minute stock exchange prices or a news service in a manner that is virtually unique. A "group call" is used to enable the same Information to be sent to a whole group of pagers. Group call has many other uses; for example, parttime firemen or lifeboat men can be "bleeped" when an emergency occurs.

Fundamental Difference

Pagers were originally developed as a means of getting medical staff at St Thomas's Hospital in London to contact the switchboard or their own department. Since then the concept has been developed to encompass both "private" and "public" systems, with a choice of numeric, voice, and alpha-numeric pagers that are much smaller than a packet of cigarettes and can be easily carried in a pocket or handbag or even clipped to one's belt. The fundamental difference between public and private systems is, obviously, that the farmer is operated as a public service with the pager units being rented or sold to members of the public while, with the latter, the system is operated for a closed community. Consequently, a public authority could have a system spanning an entire city or even an entire country with pagers carried by key members of staff to either receive messages on an individual basis or receive collectively what is, in effect, a broadcast message.

The operating principle is straightforward. A caller wishing to contact a particular pager telephones the operations centre where an operator takes the request and enters details of the request into the paging control system- The paging transmitter then broadcasts a unique code (address). to Identify the pager being called, followed by a message, which may be a simple alert, a telephone number, or even a quite detailed message of up to, say, 50 wferds. Every pager ignores messages except those uniquely addressed to it.

Landline Difficulties

As can be appreciated, a pager can only respond when it is within range of the transmitter. Consequently, in order to increase the service area, possibly to cover a whole country, a network of transmitters is needed. The normal method of sending the address and data message is via leased lines over the telephone network. This ts where technical problems arise and costs increase.

To achieve accurate, error-free paging, all ground-based transmitters need to be synchronised ("Simulcast") so that signal transmission to the pager is simultaneous. This is very difficult to achieve using landlines because of the automatic re-routing characteristics of the public switched network when there is a fault on the line. Providing the landlines to link all transmitters is expensive, and in some countries there may be diffi-