Mobility management is an important function in maintaining cellular networks. The goal of mobility management is to track where cellular phones, or User Equipments (UEs), are located in order for mobile phone services to be provided to the various UEs included in any given network.
Cellular networks, for example Global System for Mobile Communication (GSM) or Universal Mobile Telecommunications System (UMTS), comprise radio networks featuring a number of base stations. Each base station covers a limited geographical area. Integrating the coverage of a number of base stations provides a cellular network with radio coverage over a much wider area. In an Evolved Universal mobile Telecommunications system Terrestrial Radio Access Network (E-UTRAN), an eNodeB or a group of these base stations may be referred to as covering a Tracking Area (TA).
A Tracking Area Update (TAU) procedure allows a UE to inform the cellular network when the UE moves from one TA to another. When an UE detects it has moved to a different TA and the UE is not registered for that TA, the UE will send a TAU request to the cellular network in order to get a Tracking Area Identity (TAI) list of TAs associated with the UE's current location. The UE then becomes registered for the TAs included in the TAI List.
If TAs are very large, there will be many UEs operating simultaneously, resulting in high paging traffic, as every paging request is broadcast to every base station within the same TA. This puts a strain on the bandwidth and power of the UE by requiring the UE to listen for broadcast messages too often. In contrast, if there are a large number of small TAs, the UE must contact the cellular network frequently once the UE moves from one TA to another.
The network node typically responsible for mobility management is the Mobility Management Entity (MME). The MME is a key control node typically responsible for idle mode UE tracking, paging procedures, bearer activation/deactivation processes, and the MME is also responsible for choosing a Serving Gateway (SGW) for a particular UE. MMES typically assign a TAI list to a UE during an TAU, where the TAU is performed in an operator specific manner.