BS 8521:2009 pdf download – Specification for dual-tone multi-frequency (DTMF) signalling protocol for social alarm systems 5 Communication 5.1 Session initiation Following completion of call set‑up signalling, the local unit and controller shall send an enquiry tone to the line to indicate that automatic equipment is using the line. When the ARC detects ringing current, it shall loop the line and shall send a data request tone to the local unit and controller. If no data request tone is detected by the local unit and controller, it shall disconnect from the network. A received tone shall be interpreted as a data request tone by the local unit and controller only if its duration is not less than 500 ms. When the local unit and controller detects the data request tone, it shall cease to send the enquiry tone and shall wait until the end of the data request tone; after a delay of (80 ± 5) ms the local unit and controller shall then send an alarm transmission message to the ARC. If no acknowledge tone or start of data transmission is received by the ARC within (600 ± 30) ms of the end of a data request tone, the ARC shall send a further data request tone. This shall be repeated up to four times, after which the ARC shall disconnect from the network if no acknowledge or start of data transmission is received by the ARC within (600 ± 30) ms of the end of the fourth data request tone. If an acknowledge tone or start of data transmission is received by the ARC within (600 ± 30) ms of the end of a data request tone, the ARC shall cease to send the data request tone and shall wait to receive a valid data transmission. If a valid data transmission is not received by the ARC within 25 s from the end of the transmission of the originating data request tone, it shall disconnect from the network. If no acknowledge tone is received by the local unit and controller within (600 ± 30) ms of the end of an alarm transmission message, the local unit and controller shall send a further alarm transmission message. This shall be repeated up to four times, after which the local unit and controller shall disconnect from the network if no acknowledge tone is received by the local unit and controller within...
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