09.603GPPGeneral Packet Radio Service (GPRS)GPRS Tunnelling Protocol GPT) across the Gn and Gp InterfaceRelease 1998TS
A possible way to support inter-PLMN roaming is discussed very briefly in this sub-section.
When an MS roams between two SGSNs within the same PLMN, the new SGSN finds the address to the old SGSN by the association old RA – old SGSN. Thus, each SGSN knows the address to every other SGSN in the PLMN.
When an MS roams from an SGSN to an SGSN in another PLMN, the new SGSN may not itself have access to the address to the old SGSN. Instead, the SGSN transforms the old RA information to a logical name of the form:
RACxxxx.LACyyyy.MNCzzzz.MCCwwww.GPRS; x,y,z and w shall be Hex coded digits.
The SGSN may then acquire the IP address of the old SGSN from a DNS server, using the logical address. Every PLMN should include one DNS server each. Note that these DNS servers are GPRS internal entities, unknown outside the GPRS system.
The above implies that at least MCC + MNC + RAC + LAC (= RAI) is sent as RA parameter over the radio when an MS roams to another RA.
If the new SGSN for any reason fails to obtain the address of the old SGSN, the same actions as when the corresponding event occurs within one PLMN are taken.
Introducing the DNS concept in GPRS gives a general possibility to use logical names instead of IP addresses when referring to e.g. GSNs, thus providing flexibility in addressing of PLMN nodes.
Another way to support seamless inter-PLMN roaming is to store the SGSN IP addresses in HLR and request them when necessary.