along with the user's speech at may be performed automatically on a periodic basis under computer 100% system FM deviation, to control a tone decoder at the distant control, with the base console display being updated as required. receiver. The audio output of the receiver is bandwidth filtered to Alternatively the mobile unit may manually send a status update typically pass frequencies above 300Hz only, thus suppressing the in a short time slot by entering relevant digits on the personal unit received tone from the user. fascia, one such code normally being a request for a speech channel 46/10 PMR and Trunked Radio Systems modulation rate generally used is 1200 bits/sec, with a binary '1' corresponding to 1 cycle of 1200Hz, a binary '0' corresponding to 1.5 cycles of 1800Hz, with NRZ coding, as illustrated in Figure 46.9. The first bit of each message commences with either phase 0 degree or 180 degrees thus ensuring phase continuity (DTI, 1981c). Tone 1 Tone 2 Tone 3 Tone 4 Tone 5 Establishment Figure 46.8 Sequential tone signalling uses a tone sequence at commencement of transmission. A link establishment time period is required for simplex half duplex systems 1 Cycle 1.5 Cycles 1 Cycle or telephone interconnection followed by the required number being 1200Hz 1800Hz 1200Hz sent by sequential tone signalling. Table 46.3 tabulates internationally agreed tones from the EEA (Electronic Engineering Association), CCIR (Committee Consult- Figure 46.9 Fast frequency shift keying employs phase atif International radio Communication), ZVEI (Zurerein der Elec- continuity between digits Systems are generally capable of providing a data stream as Table 46.3 Sequential tone signalling frequencies shown, with a minimum transmission length of 96 bits. Trans- missions commence with a preamble of at least 16 bit reversals such that the receiver demodulator can acquire bit synchronisation. This Standard DZVEI CCITT preamble is normally sufficient for transmissions where the r.f. carrier is already present, although the preamble m