Cisco Cisco ASR 5000
SGSN Pooling
▀ Feature Description
▄ SGSN Administration Guide, StarOS Release 18
Feature Description
An SGSN pool is a collection of SGSNs configured to serve a common geographical area for a radio network. This
common part is referred to as the SGSN pool service. SGSN Pooling is also referred to as Iu/Gb flex support based on if
the access is 3G or GPRS respectively.
common part is referred to as the SGSN pool service. SGSN Pooling is also referred to as Iu/Gb flex support based on if
the access is 3G or GPRS respectively.
An SGSN pool provides a flexible and resource-efficient architecture with built-in network redundancy for the
GPRS/UMTS packet core network. Each BSC/RNC has the ability to connect to all SGSNs in the pool. If any SGSN
becomes unavailable, any terminal attached to that SGSN will be automatically re-routed to another SGSN in the pool
by the BSC/RNC. This implies that the SGSN pool provides network level redundancy. SGSN failure is discovered by
the BSCs/RNCs and the uplink traffic from the terminal is routed to another SGSN in the pool. The substituting SGSN
orders the terminal to re-attach and re-activate any PDP contexts. Therefore service availability is maintained. Please
note that all SGSNs in a pool are required to have the same capacity, feature sets and scalability and hence the same
vendor, failing which might lead to varying subscriber experience across SGSNs.
GPRS/UMTS packet core network. Each BSC/RNC has the ability to connect to all SGSNs in the pool. If any SGSN
becomes unavailable, any terminal attached to that SGSN will be automatically re-routed to another SGSN in the pool
by the BSC/RNC. This implies that the SGSN pool provides network level redundancy. SGSN failure is discovered by
the BSCs/RNCs and the uplink traffic from the terminal is routed to another SGSN in the pool. The substituting SGSN
orders the terminal to re-attach and re-activate any PDP contexts. Therefore service availability is maintained. Please
note that all SGSNs in a pool are required to have the same capacity, feature sets and scalability and hence the same
vendor, failing which might lead to varying subscriber experience across SGSNs.
In a pooled network, Inter-SGSN routing area updates (RAUs) are avoided and this provides a faster response time,
compared to non-pooled networks. With SGSN pool for GPRS/UMTS, Inter-SGSN RAU is replaced by Intra-SGSN
RAU, for terminals moving within the pool area. Intra-SGSN RAU provides reduced interruption time for data transfer,
compared to Inter-SGSN RAU. Furthermore, due to the fewer Inter-SGSN RAUs, there is less signalling generated on
the Gr interface.
compared to non-pooled networks. With SGSN pool for GPRS/UMTS, Inter-SGSN RAU is replaced by Intra-SGSN
RAU, for terminals moving within the pool area. Intra-SGSN RAU provides reduced interruption time for data transfer,
compared to Inter-SGSN RAU. Furthermore, due to the fewer Inter-SGSN RAUs, there is less signalling generated on
the Gr interface.
When an UE connects to an SGSN in the pool, by Attach or Inter-SGSN RAU (ISRAU) procedures, the UE is allocated
a Packet Temporary Mobile Subscriber Identity (P-TMSI) containing a Network Resource Identifier (NRI) identifying
the SGSN. The BSC/RNC then identifies the SGSN from the NRI, and routes the user data to the correct SGSN. Load-
sharing between the SGSN pool members is thus based on the NRI routing algorithm in the BSC/RNC. UEs that have
not yet been assigned a P-TMSI, and MSs without matching NRI, are distributed among the pool members by the
BSC/RNC according to the traffic distribution procedure. Once a UE has been allocated a P-TMSI, it stays connected to
the same SGSN as long as it remains in the pool area. This period can be quite long, since MSs normally keep the P-
TMSI even after power off.
a Packet Temporary Mobile Subscriber Identity (P-TMSI) containing a Network Resource Identifier (NRI) identifying
the SGSN. The BSC/RNC then identifies the SGSN from the NRI, and routes the user data to the correct SGSN. Load-
sharing between the SGSN pool members is thus based on the NRI routing algorithm in the BSC/RNC. UEs that have
not yet been assigned a P-TMSI, and MSs without matching NRI, are distributed among the pool members by the
BSC/RNC according to the traffic distribution procedure. Once a UE has been allocated a P-TMSI, it stays connected to
the same SGSN as long as it remains in the pool area. This period can be quite long, since MSs normally keep the P-
TMSI even after power off.
A Basic Pool Structure
A basic SGSN pool structure is depicted in the diagram below: