RAN Flexible Configuration (MSC Pool / RANflex)

Description

RANflex configuration is a core network architecture concept that decouples the relationship between radio access location areas and individual Mobile Switching Centres (MSCs). Instead of a strict one-to-one mapping where each location area is served by exactly one MSC, a RANflex configuration allows a single location area to be served by multiple MSCs operating in parallel. This is the GERAN-specific realisation of the broader MSC Pool concept standardised in 3GPP TS 23.236, often referred to as 'MSC in pool' or 'pooling'. The configuration is referenced in TS 43.068 and TS 43.069, which define the Voice Group Call Service (VGCS) and Voice Broadcast Service (VBS) over GERAN.

Architecturally, RANflex works by introducing a Network Resource Identifier (NRI) that is embedded within the Temporary Mobile Subscriber Identity (TMSI) allocated to a subscriber. When a mobile station performs a location update or originates a call, the Base Station Subsystem (BSS) examines the NRI contained in the subscriber identity and routes the request to the specific MSC within the pool that owns that NRI. This allows the BSS to distribute traffic across all MSCs in the pool while still ensuring that a given subscriber is consistently served by the same MSC for the duration of their registration. The pool itself is defined by a pool area, which is a collection of one or more location areas served by the same group of MSCs.

The primary benefit of RANflex is capacity and resilience. Because multiple MSCs share the traffic of a location area, the network can balance load across them, avoiding the situation where one MSC is overloaded while another is underutilised. If an MSC fails, subscribers registered on it can be reallocated to other MSCs in the pool, providing core network redundancy without requiring duplicate hardware for every MSC. This significantly improves service availability and reduces capital expenditure for operators.

In the specific context of group call services (VGCS/VBS), the RANflex configuration has an important limitation. A given location area is associated with a single group call serving MSC. This means that although multiple MSCs serve the location area for normal point-to-point traffic, only one of them is responsible for hosting the group call. Consequently, there is no support for group call redundancy: if the group call serving MSC fails, the group call service in that location area is disrupted, even though other MSCs in the pool remain operational for ordinary calls. This asymmetry is a deliberate design constraint documented in the specifications.

RANflex is a GERAN-era concept, but the underlying MSC Pool principle was later extended to UTRAN and E-UTRAN through the concept of the MME Pool and SGSN Pool, where similar NRI-based routing and pool-area concepts apply. Understanding RANflex is therefore foundational for understanding modern core network pooling and the evolution toward virtualised, distributed core networks in 5G.

Purpose & Motivation

RANflex configuration was created to overcome the rigid and inefficient one-MSC-per-location-area model that characterised early GSM networks. In that traditional model, each location area was hard-bound to a single MSC, which meant that capacity planning had to be done per MSC and per location area independently. An MSC serving a busy urban location area could become congested while a neighbouring MSC serving a rural area sat idle, and there was no mechanism to share the load. Furthermore, core network redundancy required expensive duplication: to protect against MSC failure, operators had to deploy standby MSCs or accept service outages.

The introduction of RANflex, based on the MSC Pool concept in TS 23.236, solved these problems by allowing multiple MSCs to serve the same location area in parallel. This enabled load sharing across MSCs, improved utilisation of installed capacity, and provided N+1 or N+M redundancy so that the failure of one MSC could be absorbed by the remaining MSCs in the pool. It also simplified network expansion: adding a new MSC to a pool increased capacity incrementally without re-planning location areas.

However, the design had to accommodate legacy group call services (VGCS/VBS), which rely on a single controlling MSC per group call per location area. The specification therefore explicitly states that in a RANflex configuration a given location area is associated with a single group call serving MSC, and that group call redundancy is not supported. This reflects the trade-off between the flexibility gained for point-to-point services and the constraints imposed by the group call architecture of the time.