ns of 30dB can be encountered due to Rayleigh code can have a user capacity of over 3000 this format could be fading effects, so this table is only a guide. adapted for speech and was suitable for even the largest on site systems. Table 45.2 Attenuation at different frequencies Table 45.1 A typical digital proprietary format 30MHz 150MHz 450MHz 850MHz Code capacity addresses 10 000 x 4 systems Urban Paging rate (call/sec) Max 13 c/s tone alerts preamble. Suburban 40ch. 0.38 c/s. Rural 80ch. 0.25 c/s Data rate 512bit/s At h.f., r.f. noise from electrical machinery, lighting, power lines Message rate 512bit/s and motor vehicles is prevalent. U.h.f. has marked benefits, in penetration of modern buildings and cellars. Some particularly r.f. Radio channel 25kHz noisy equipment used in hospitals e.g. diathermy units, are avoided Modulation 4.5kHz FM FSK for safety reasons by special frequency allocations for paging. Transmission mode Manchester or NRZ 45.5.4 Telephone coupling Multiple transmitter conditions Quasi-Sync or sequential Line transmission Data modems 1200bit/s For many users, with say 20 to 100 pager wearers, passing messages through a paging controller, who may well also be the switchboard Absolute line delay 0.25ms + 0.195ms from operator, is inconvenient and direct paging access by the caller's transmitter. Telegraph own telephone extension is desirable. A telephone coupled encoder distortion <10% may be installed via one of the PABX extensions to satisfy this Isochronous requirement. The units can have call transfer or absence registration Error correcting potential 2 errors in 12 bits preamble. programmed via the telephone for a limited number of pagers (e.g. 2 bits in 32 non-message 10). The system will be capable of sending tone, numeric messages, codewords. 1 bit in 32 followed by up to 120 seconds of speech, but this is limited to 30 message code words seconds speech transmission by regulations in the UK. Battery economy Yes. Variable unframed A 'meet me' facility is available via the PABX whereby 