the caller pages the person he wishes to contact, holds, and is connected Message capability No limit directly through the PABX when the person dials the 'meet me' number. Fig.45.1 shows the block diagram of a telephone coupled encoder which, via a telephone extension, decodes over dialled DTMF tones 45/6 Radio paging CLEAR TX KEY ANSWER OUTPUT INVERTER DRIVER OPTIONS DLC & COD PROTECTION INPUTS MODEM CONTROLLER RINGING INVERTER DRIVER EXCHANGE DETECTOR SPEECH ENCODER DIGITAL ENABLE ANSWER OUTPUT DECODER CIRCUIT TO TX LOW PASS INVERTER FILTER TERMINATION ANALOGUE LOW PASS TRANS ATTENUATOR SWITCH FILTER FORMER ANALOGUE OUTPUT LOW PASS TONE ENABLE TO TX FILTER GENERATORI Figure 45.1 Block diagram of a telephone coupled encoder to generate paging calls. Programmed software is stored in a 8K site systems. Such systems allow 2-way speech and 2-way signall- EPROM. Two isolation barriers are provided, one to connect to a ing in any combination provided that CTCSS (Continuous Tone PABX and the other for a dedicated line. All 16 DTMF tones are Controlled Squelch System) or selective calling are used in both decoded and encoded. Provision for different line levels has also to directions. A base receiver is required fitted with a matching CTCSS be made, e.g. -10dBm to -20dBm (600S) on the incoming tele- or selective calling squelch. phone line, and attenuated to -13dBm (6009) out to the private In the UK this system is licensed as local communications and wire. pairs of frequencies are allocated with the receivers in the 459MHz band, and transmitters in the 161MHz band. Typical specifications 45.5.5 Regulating authority requirements of a range of talkback pagers which are required for different European markets are shown in Table 45.3. In France talkback Duration of the speech part of paging messages is limited by many paging is at 445.5MHz with a channel spacing of 12.5kHz and maximum ERP of 50mW. European countries to 30 seconds only. In France this is limited to 10 seconds. While the usual compliance with and approval to