Pool Area

Description

A Pool Area is a concept in 3GPP core network architecture, introduced in Release 5, that defines a geographic area within which a Mobile Station (MS) may roam without the need to change its serving SGSN (Serving GPRS Support Node). A pool area is served by one or more SGSNs in parallel, meaning that multiple SGSNs share the responsibility for the MSs within that area. This is part of the Intra Domain Connection of RAN Nodes to Multiple CN Nodes (also known as Iu-Flex or Gb-Flex) feature, which allows a RAN node (e.g., BSC/RNC) to be connected to multiple core network nodes. The pool area concept is fundamental to achieving load balancing, redundancy, and improved scalability in the packet-switched core network.

The pool area is defined by a grouping of one or more Routing Areas (RAs) or Location Areas (LAs) that are served by a specific group of CN nodes. In the context of GPRS/UMTS, the pool area consists of one or more RAs. All the cells controlled by a Base Station Controller (BSC) belong to the same one or more pool areas. This ensures that when an MS moves within the pool area, it does not need to change its serving SGSN, even if it crosses RA boundaries. The MS remains registered with the same SGSN as long as it stays within the pool area. This reduces signaling load and improves service continuity.

The architecture of a pool area involves the following components: the pool area itself, which is a set of RAs; the SGSNs that serve the pool area; and the RAN nodes (BSCs/RNCs) that connect to these SGSNs. Each RAN node is configured with a list of SGSNs that serve the pool area. When an MS attaches or performs a routing area update, the RAN node selects an SGSN from the pool based on a load balancing algorithm (e.g., weighted round-robin). The selected SGSN becomes the serving SGSN for that MS. The MS is not aware of the pool area; it simply sees the network as usual. The pool area concept is transparent to the MS.

The benefits of pool areas include load sharing, redundancy, and reduced signaling. Load sharing is achieved by distributing MSs across multiple SGSNs, preventing any single SGSN from becoming overloaded. Redundancy is provided because if one SGSN fails, the RAN node can select another SGSN from the pool to serve the MSs, ensuring service continuity. Reduced signaling results from the fact that MSs can move within the pool area without changing SGSN, which minimizes location update signaling. This is particularly beneficial in areas with high mobility.

The pool area concept was initially defined in Release 5 for GPRS and UMTS networks. In later releases, the concept was extended to LTE and 5G, where similar concepts exist for MME (Mobility Management Entity) and AMF (Access and Mobility Management Function) pools. In LTE, the pool area is defined for MMEs and is known as an MME pool area. In 5G, the AMF pool area serves a similar purpose. The underlying principle remains the same: a group of CN nodes serving a set of tracking areas or registration areas, allowing UEs to move without changing the serving CN node.

The definition of pool area has evolved slightly over releases. In Release 5, it was defined as 'a grouping of one or more RA(s) that, from a RAN perspective, are served by a certain group of CN nodes, as defined for the Intra Domain Connection of RAN Nodes to Multiple CN Nodes.' In Release 8, the definition was changed to 'an area within which an MS may roam without need to change the serving SGSN. A pool area is served by one or more SGSNs in parallel. All the cells controlled by a BSC belong to the same one (or more) pool area(s).' This new definition is more descriptive and focuses on the MS perspective. The definition remained unchanged from Release 8 through Release 19, indicating the stability of the concept.

In summary, the Pool Area is a core network concept that enables multiple SGSNs to serve a group of RAs in parallel, providing load balancing, redundancy, and reduced signaling. It is a key enabler for scalable and resilient packet core networks.

Purpose & Motivation

The Pool Area concept was introduced to address the limitations of the traditional hierarchical core network architecture, where each RAN node was connected to a single CN node. This one-to-one connection created single points of failure and limited scalability. If a CN node failed, all subscribers served by that node would lose service. Additionally, as networks grew, the capacity of a single CN node could become a bottleneck. The Pool Area concept solves these problems by allowing multiple CN nodes to serve the same geographic area in parallel.

The primary motivation was to achieve load sharing and redundancy. By distributing subscribers across multiple SGSNs, operators can balance the load and prevent any single SGSN from being overwhelmed. This improves network performance and user experience. Redundancy is achieved because if one SGSN fails, the RAN node can redirect new subscribers to other SGSNs in the pool, and existing subscribers may be able to continue service if the failure is detected and recovery procedures are in place. This increases network availability and reliability.

Another motivation was to reduce signaling load. In a traditional network, when a mobile station moved between routing areas served by different SGSNs, it had to perform a routing area update and change its serving SGSN. This generated signaling traffic and could cause delays. With pool areas, the MS can move within the pool area without changing SGSN, as long as it stays within the RAs of the pool. This reduces the frequency of SGSN changes and the associated signaling, leading to more efficient network operation.

The Pool Area concept was standardized in Release 5 as part of the Intra Domain Connection of RAN Nodes to Multiple CN Nodes feature. It has since been adopted in LTE (MME pools) and 5G (AMF pools), demonstrating its enduring value. The concept continues to be relevant as networks evolve towards virtualization and cloud-native architectures, where load balancing and redundancy are even more critical.