Routeing Area Update

Description

The Routeing Area Update (RAU) is a fundamental mobility management procedure defined within the 3GPP packet-switched (PS) domain, primarily for GPRS and UMTS networks. It operates within the context of the GPRS Mobility Management (GMM) protocol. A Routeing Area (RA) is a logical area, typically comprising one or more Location Areas (LAs) used for circuit-switched services, defined for the purpose of tracking a User Equipment's (UE) location for packet data services. The core network element responsible for PS mobility management is the Serving GPRS Support Node (SGSN). The RAU procedure is triggered when a UE detects it has entered a new RA, based on the RA Identifier (RAI) broadcast by the network cells. It is also triggered periodically by a timer (periodic RAU) or upon UE power-up (RAU after power-on).

The procedure involves the UE sending a RAU REQUEST message to the new SGSN (if an inter-SGSN RAU) or to its current SGSN (if an intra-SGSN RAU). This message contains the UE's identity (e.g., P-TMSI) and its previous location information. In an inter-SGSN RAU, the new SGSN uses this information to contact the old SGSN to retrieve the UE's MM and PDP contexts, ensuring session continuity. The new SGSN then authenticates the UE and updates the UE's location with the Home Location Register (HLR), which in turn cancels the registration at the old SGSN. The SGSN then responds with a RAU ACCEPT message, assigning a new Packet-Temporary Mobile Subscriber Identity (P-TMSI) if necessary.

RAU is critical for maintaining an accurate location of the UE in the PS domain without requiring constant signaling. It allows the network to efficiently page the UE for incoming data packets only within its last known RA, rather than across the entire network. This reduces paging traffic and conserves radio resources. Furthermore, by grouping cells into RAs, the network balances the cost of location update signaling against the cost of paging. The procedure works in tandem with other states like IDLE, STANDBY, and READY, defining when location updates are required. Its successful completion ensures that the Gateway GPRS Support Node (GGSN) has the correct SGSN address to tunnel user data packets to the UE's current serving node.

Purpose & Motivation

RAU was created to address the need for efficient mobility management in packet-switched cellular networks, specifically GPRS and UMTS. Prior to GPRS, cellular mobility was primarily designed for circuit-switched voice, using Location Area Updates (LAU). However, packet data services have different traffic patterns—often bursty and intermittent—requiring a mobility management scheme optimized for data. The primary problem RAU solves is enabling the network to know a UE's approximate location to deliver downlink packets without requiring the UE to be constantly active on the radio, which would drain its battery.

Without RAU, the network would either lose track of the UE, causing data sessions to drop during movement, or would need to page the UE across an impractically large area for every incoming data packet, consuming excessive radio bandwidth and signaling resources. RAU introduces the concept of a Routeing Area, which can be larger than a Location Area, to reduce the frequency of updates compared to LAU for data-centric devices. This design acknowledges that data sessions can tolerate slightly less precise location knowledge than voice calls, trading off some localization granularity for significant gains in signaling efficiency and UE power savings. It forms the backbone of 'always-on' connectivity for mobile data, allowing a UE to be reachable for IP traffic while spending most of its time in a low-power, radio-conserving state.