Signaling Top view [Under GSM]
Let us have a look at top view of how signaling proceeds.
1) For signaling, signaling channel is needed first. MS does this by sending request with reason (e.g. Location update, Mobile Originated Call etc.) through common (uplink) channel. As this channel is common, there is likelihood of collision of request and losing the request. And so MS need to retry after certain time. Remember that the request would have only minimal information as distance between MS and BTS is not known (refer "timing advance" in article 8)
2) Granting of signaling channel is also indicated through common (downlink) channel. Reference number given in request is used for distinguishing between request. MS wait on this channel. Decision of assigning the channel is done by BSC.
3) Once received, MS will activate Layer 2 LAPDm connection on granted signaling channel and send Layer 3 message. Layer 3 message need to contain IMSI (or TMSI) for identification. TMSI is temporary identification number given to MS for security purposes. TMSI is unique only in a VLR.
4) BSS will forward this message to MSC through BSSMAP message called "Complete Layer 3 information". Being a first message, it is always a BSSMAP message. Later in signaling steps, depending on whether RR or MM/CM, BSSMAP or DTAP messages will be used respectively.
5) MSC will inform about this access request to VLR. At this point, VLR may trigger "authentication" procedure.
6) Once authentication is successful, VLR may trigger "ciphering" (encrypting the later messages to avoid evesdropping). This is MM procedure.
7) After ciphering, VLR may trigger "TMSI reallocation" procedure. Regular TMSI reallocation provide more security. This again is MM procedure.
8) Depending on request made in Layer 3 message (step 3 above), signaling proceeds.
Though described for GSM, above steps are generic in nature and may be used in other wireless access systems.
We will continue with "Location Update" in next article.
References: GSM book by Mouly and Pautet, GSM Networks book by Heine, Signaling diagrams from 24.007/24.008 (R99).
© Copyright Samir Amberkar 2010
| MS Power On « | GSM Index | » Location Update - 1 |