Hi Friends,
I hope the following ( a bit lengthy though ) explanation helps.
A class B phone on entering a CS dedicated mode sends a GPRS SUSPENSION REQUEST to BSC. Those of you having TEMS can see the layer 3 messages for a CS call. The BSC forwards this to the SGSN containing the TLLI and RAI and starts a timer called T3. When the SGSN acknowledges by sending SUSPEND ACK to BSC while T3 is running, the process is successfull.When the phone leaves the CS dedicated mode the BSC sends the RESUME message to SGSN containing the same TLLI and RAI and T4 is started. In case the SGSN acknowledges by sending RESUME ACK to BSC while T4 is running, then the channel release message to the MS contains “resumption of PS services successfully acknowledged” and there will not be any Routing Area Update. However if the channel release message does not contain this message then a Routing Area Update “will” be performed.
I have seen this in Ericsson and Siemens BSS. While in Ericsson the channel release message does contain the resumtion message and the Siemens does not. Thus the Routing Area Update occurs in my Siemens Area. While this only says “Why”, but I am still not sure “How” to solve it.
Phew !! That was a long one.