GSM Network Areas

Here is something very basic ....... I am planning to go step by step to provide an overview of GSM and 3G Core network. I don't know much about Access Stratum so not able to dicuss much about that.

How message routes .... MSC-SCCPR-HLR

Although it seems to be basic but Its good to know how message routing happen between GSM network components (MSC-SCCPR-HLR-MSC)

1. MSC changed IMSI to modified IMSI
2. MSC has configuration that which IMSI range is handled by which SCCP-R -- although now there are plans to have SCCP-R which has all the IMSI on it so now MSC will have only one SCCP-R (Big SCCPR)
3. SCCP-R strips off the modified part and search the HLR which is hosting this IMSI and sends message to it
4. HLR checks "Calling address" to send the response back.