A social care alarm system 500 includes a client terminal unit 110 installed within a dwelling of a client, and a remote monitoring centre 200 coupled by a set of data carriers over one or more communication networks 300. The client unit communicates with the monitoring centre in a master-slave communications session (see fig.5) using a selected data carrier, which includes raising an alarm event and supporting a voice call. The client terminal unit is also able to send sessionless unsolicited short status messages (polling or heartbeat messages) at predetermined intervals to the monitoring centre using the selected data carrier without forming the master-slave communication session (see fig.7), in order to monitor viability of the selected data carrier with reduced bandwidth consumption. The client unit and monitoring centre may utilise a communication protocol which is operable over each of the set of data carriers, the protocol comprising a series of layers including an application layer (161,fig.3) and a transmission control layer (162,fig.3) which are independent of the set of data carriers, and a link layer packet (163,fig.3) appropriate to the selected data carrier.
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