MME Pooling and S-GW Service Area

MME Pool Area

MME pool area is a collection of MMEs configured to serve any common part of a radio network. It is termed as MME pool service area. The pool service area can consists of several Tracking areas (TAs) served by eNodeBs in an LTE radio network. All eNodeBs serving the pool service area are connected to all the MMEs in the MME pool.

The MMEs in a pool share the load from all the UEs in the pool service area. As long as the UE remains in the pool service area, it is normally attached to the same MME. If an MME becomes unavailable, the eNodeB reroutes the signaling from the UE to another MME in the MME pool.

Pool Service Area

A pool service area is generally larger than a service area served by a single MME. The advantage is it results in fewer Inter-MME Tracking Area Update (TAU) procedure and reduced signalling to other core network elements such as Home subscriber server (HSS) and serving gateway (S-GW).

MME Pool Areas

To increase the pool capacity, additional MMEs can be added to the pool. The additional MMEs must be connected to all the eNodeBs in the pool service area. The eNodeBs in the pool service area then take the new MMEs into account in their traffic distribution procedure. The changed MME configuration initiates an MME Configuration update procedure towards each connected eNodeB.

When the UE leaves a pool service area and performs an Attach or a TAU to an MME outside the pool service area, the new MME fetches the UE context from the old MME.

Serving Gateway Service Area and Pooling 

A Serving Gateway (S-GW) area is defined as an area where the UE is served without having to change the S-GWs.  An S-GW service area is served by one or more SGWs in parallel. SGW service areas are collection of complete TAs. SGWs service areas may overlap each other.

When a UE is attached to the network , it is assigned to one of the MMEs that belong to the MME pool associated with the eNodeB through which the UE is attaching , the MME then selects an S-GW in the S-GW pool. No change of MME or S-GW is required while the UE moves around among eNodeBs belonging to the same MME or S-GW pool. As long as packet data network gateway (PGW) does not changes, the IP address of the UE does not changes, even if S-GW changes.

The whole procedure of serving gateway service area and pooling is shown detailed on the free hand diagram below.

MME/S-GW relocation occurs primarily only when moving between MME Pool Areas / S-GW service area

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