The network is not having only one GGSN, it may have a few GGSNs. Also, it is having a lot of APNs. Some of those APNs exist on all GGSNs, and some exist only on one of the GGSNs. Therefore, GGSN selection mechanism have to exist, in order for the SGSN to be able to select the proper GGSN.

Here, we will learn how the SGSN selects the proper GGSN.

The SGSN selects the GGSN based on the requested APN. In addition, the SGSN may take into consideration the location of the MS while selecting the GGSN. This is done using the DNSs.

In GPRS network, we have Internal DNSs 'iDNS', and External DNSs 'eDNS'.

GGSN selection based on the APN

The SGSN selects the GGSN based on the APN that the MS should connect to. The SGSN will resolve the GGSN IP address that it should use for this APN. Therefore, the SGSN will ask the iDNS. Then the iDNS may need to ask the eDNS as shown blow,

iDNS usage

• when the request comes to the SGSN from the MS to create a PDP context, the SGSN will check the name of the APN.
• Then the SGSN will ask the iDNS about this APN name , to which GGSN IP address the SGSN should forward this request to ?.
• The PS core engineer should configure on the iDNS all the names of the APNs in the network, mapped to the GGSN IP addresses for each APN.
• Taking into consideration that, the PS core engineer may configure more than one GGSN for the same APN.

eDNS usage

• In case the iDNS didn’t find a record for the APN, this may indicate that, this APN doesn’t exist in this network. And it may exist in an outside network.
• This may happen if an 'Inbound roaming' user is requesting to connect to the APN of his home network.
• Therefore, the iDNS will ask the eDNS regarding this APN.
• The eDNS connects to a network called the GRX, which contains all the eDNSs of all networks in the world.
• Therefore, if this APN exist in an external network, the eDNS will able to resolve its GGSN IP address, and then replies the iDNS.
• Then, the iDNS replies the SGSN with the GGSN IP address that should be used for this APN.

GGSN selection based on the Location

• The GGSN selection also may take into consideration the location of the MS inside the network, which is the LAC/RAC the MS is currently under.
• In this case, the PS core engineer should add the LAC/RAC along with the name of the APN in the records exist in the iDNS.

Summary

• The GGSN selection is based on the APN the MS is requesting, in addition to the LAC/RAC the MS is currently under.
• The iDNS will tell the SGSN which GGSN IP address shall be used.