Abstract: A method of operating an availability service in a telecommunication system interconnecting a legacy telephone system with at least one presence system, said legacy telephone system including a plurality of terminals (T1 . . . T10), at least some (T1,T2) of which are terminals of subscribers to said availability service, said method including: (a) continuously monitoring each subscriber terminal (T1,T2) for a change of a call status of said subscriber terminal from the presence of a call to the absence of a call; (b) generating a first or “high availability” signal when said call status of said subscriber terminal changes from said presence of a call into said absence of a call; (c) monitoring an interval of time beginning with said first signal; (d) generating a second or “availability unknown” signal after a predetermined time period &tgr; within said interval; and (e) making said availability signals accessible to any interconnected presence system.

Abstract: In the Checkpoint Mode Protocol (CPM), checkpoint frames are transmitted at regular intervals for acknowledging received information frames and individually requesting retransmission of missing information frames. The disclosed invention reduces the flow of checkpoint frames in situations where their information is not required. This is achieved by either sending a Stop Checkpointing frame from sender to receiver in particular situations, or by inserting, in the case of duplex transmission (i.e. both stations are sending and receiving), Request-Reply bits into normal checkpoint frames for suppressing the flow of checkpoint frames when they are not necessary. In each case, provisions are made to resume the transmission of checkpoint frames as soon as they are required again.