Roaming can be understood as a possibility for a mobile subscriber to obtain service from a PLMN different from his Home PLMN. For roaming, it needs 2 operators involved to sign National & International roaming agreements, various operators allow mobile subscribers to access GPRS/UMTS network continuously & freely in all regions under network coverage. GPRS Subscription Data Information (GPRS-SDI) is stored in HLR, when a user roams to a visiting PLMN (VPLMN) it is the SGSN in the VPLMN which controls the access to the Network by verifying user identity based on the GPRS-SDI in the Home HLR.
There are 2 basic roaming scenarios
1) MS Connects via VSGSN & HGGSN (Home Routed)- Most Common
2) MS Connects via VSGSN & VGGSN (Local Break Out) –Not Common due to charging issue.
Another piece of information I need to share is that there are 2 types of modes of inter-connection btn the operators in GPRS roaming.
1) From GRX Network, GRX is roaming inter-connection provider, From the GRX, the operators can easily connect each other for international roaming. GRX providers include BICS,Comfone etc
2) Direct Connection, it is very easy to understand, two operators directly interconnect each other’s PS Core Networks from Private VPN or Public internet.
#PerfectionIsANorm!
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