space, hyphen, opening and closing brackets and a spare symbol. be used within a zone, but where traffic is heavy simultaneous This format saves air time, but for full alphanumeric messages a full transmission must be employed. Since the transmitters are not 150 7-bit character set is used. locked together they run in a quasi synchronous mode, and therefore When using the numeric only format the pager address with the require higher stability with temperature and ageing (2 X 108 over message will have its function bits set to 00. When the 7 bits per -10°C to +50°C and 0.5 ppm /year) than a transmitter used for character format is used the pager address has its function bits set to sequential transmission. They must have a maximum line delay differential no greater than 250us and isochronous distortion no greater than 10%. 45.7.4 Wide area transmission It is possible in zone overlap areas or with time slotting, to receive the same paging call twice, albeit one of the calls is at a much lower signal strength level than the other. Many pagers have memory Transmitters of typically 100W/250W are used to cover wide area configured such that they reject the second identical call. paging zones, but there are many exceptions of much higher powers being used across the world depending on local terrain. The cover- age area for the system is generally divided into zones. These zones 45.7.4.1 Digital modulation vary considerably in size from 256 sq. km to 25,600 sq. km depend- ing on service that is being provided. The principle however is the To modulate the digital signal phase modulation is unsuitable, and same, in that the paging subscriber pays for the use of his pager in therefore Frequency Shift Keying (FSK) is used. For the particular chosen zones. characteristics of POCSAG and similar formats, non return to zero All paging calls are usually sent to a control centre where they are FSK is used. This means that the modulation frequency is either dispatched to the relevant zones transmitter controller. The paging +4.5kHz