Gs Interface between the SGSN and the MSC (VLR). Uses the BSSAP+ protocol. This
interface allows paging and station availability when it performs data transfer.
When the station is attached to the GPRS network, the SGSN keeps track of which
routing area (RA) the station is attached to. An RA is a part of a larger
location area (LA). When a station is paged this information is used to conserve
network resources. When the station performs a PDP context, the SGSN has the
exact BTS the station is using.