After getting immediate assignment through AGCH the MS switches to the assigned channel and activates it. By sending an SABM frame containing a „Service
Request Message“, it switches the SDCCH to the „Multiple Frame Acknowledge“mode, there by activating it. The Service Request message helps the network to recognize the service being requested by the MS. This information is used to decide
how to proceed.
Service request messages are:
· CM SERVICE REQUEST
· LOCATION UPDATING REQUEST
· IMSI DETACH
· PAGING RESPONSE
· CM REESTABLISHMENT REQUEST
Than the BTS send the acknowledgement that it has receive the service by means of UA frames.
RR SABM + MM CM SERVICE REQUEST
TCH, SAPI = 0
This is the first message that is sent after tuning to the
channel. The Mobile initiates a LAPm connection with the
BSC by sending a Set Asynchronous Balanced Mode
(SABM) message. The service request message meant
for the MSC is also sent in this message.
TCH, SAPI = 0
The BSS replies with Unnumbered Acknowledge (UA) to
complete the LAPm setup handshake
SABM has it starting point with X25 and ISDN.
GSM is built on ISDN therefore you will see LAP-D (LAP-B points
to X25).SABM(E) stands for Set asynchronous balanced mode extended.
It is used to establish a Link Layer. For example with ISDN
several INFO messages are exchanged before a SABME is sent by the
TE. ISDN Network will send a with UA.
HII, ANYONE CAN HELP ME TO GET KNOWN ABOUT THE WHOLE SIGNALLING ESTABLISHMENT BETWEEN THE MS AND NSS ACTUALLY WHICH SIGNAL IS REQUIRED WHAT TYPE OF CHANNEL IS REQUIRED FOR MOBILE ORIGINATING AND TERMINATING WHATEVER THE WHOLE PROCESS..
VERY THANK YOU..
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