| 摘要 |
In a duplex communications system with asynchronous exchange of messages between two stations, such as between a service switching point SSP and a service control point SCP in an Intelligent Network, each message sent from a station includes a sequentially assigned number indicated by a sending ID parameter SID and also includes a receiving parameter LPID indicating the number of the most recent message received from the other station. For example, the SCP sends a message with SID= 1*,LPID = 1 in response to message number 1 from the SSP. If the SSP sends a message SID=3,LPID=1* before receiving a message SID=2*,LPID=2 sent by the SCP in response to the previous message (SID=2,LPID=1*) sent by the SSP, the SSP can detect such message "crossing" (ie. the stations are effectively out of synchronisation) because the LPID =2 parameter indicates a response to message SID =2, rather than message SID =3, from the SSP. The SSP may then ignore message SID=2*,LPID=2 from the SCP and await the SCP's response SID=3*,LPID=3 to the SSP's most recent message (SID =3,LPID = 1*). Because of the sequential numbering, it can also be detected when a message has been lost during transmission (Fig.6).
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