5.8 Functionality for Intra Domain Connection of RAN Nodes to Multiple CN Nodes

23.0603GPPGeneral Packet Radio Service (GPRS)Release 17Service descriptionStage 2TS

The Intra Domain Connection of RAN Nodes to Multiple CN Nodes overcomes the strict hierarchy that restricts the connection of a RAN node to just one CN node, and hence also to one SGSN. This implies that a RAN node must be able to determine which of the SGSNs, covering the area where an MS is located, should receive the signalling and user traffic sent from an MS. To avoid unnecessary signalling in the core network, an MS that has attached to one SGSN, should generally continue to be served by this SGSN as long as the MS is in the radio coverage of the pool area, to which the SGSN is associated. The concept of pool area is a RAN based definition that comprises one or more RA(s) that, from a RAN perspective, are served by a certain group of CN nodes. This does not exclude that one or more of the SGSNs in this group serve RAs outside the pool area. This group of SGSNs is also referred to as an SGSN pool.

To enable the RAN node to determine which SGSN to select when forwarding messages from an MS, Intra Domain Connection of RAN Nodes to Multiple CN Nodes defines a routing mechanism (and other related functionality). Another routing mechanism (and other related functionality) is defined for the SGSNs that support the Intra Domain Connection of RAN Nodes to Multiple CN Nodes. The routing mechanism is required to find the correct old SGSN (from the multiple SGSNs that are associated with a pool area). When an MS roams out of the pool area and into the area of one or more SGSNs that do not know about the internal structure of the pool area where the MS roamed from, the new SGSN will send the Identification Request message or the SGSN Context Request message to an SGSN that is believed to be the old SGSN. This SGSN, which is associated with the same pool area as the actual old SGSN, resolves the ambiguity of multiple SGSNs in the pool area and determines the correct old SGSN from the P-TMSI (or the TLLI). The received message is then relayed to the correct old SGSN (unless it is itself the correct old SGSN). The routing mechanism in both the SGSNs and the RAN nodes utilises the fact that every SGSN that serves a pool area must have its own unique value range of the P-TMSI parameter within the pool area.

NOTE: Following idle mode mobility from E‑UTRAN to GERAN/UTRAN, the new SGSN needs to find the "correct old MME" rather than the "correct old SGSN". As specified in TS 23.401 [89], E‑UTRAN capable MSs process EPS IDs such that information in the RAI and P‑TMSI or TLLI information elements enable the new SGSN to reuse the existing mechanism for "finding the correct old SGSN" to instead "find the correct old MME".

The requirements on, and the detailed functionality needed to support, the Intra Domain Connection of RAN Nodes to Multiple CN Nodes are defined in TS 23.236 [73] and additional functionality and requirements related to interworking with E‑UTRAN are specified in TS 23.401 [89].