Pooling is mainly used to avoid network and node failures. In order to eliminate the risk,that one node failure may cause parts of the network to be out of service.
This is only possible because of many to many relation interfaces between eNodeBs and core network nodes, where each eNode B is associated with a set of MME and S-GW called MME and S-GW pool. The resulting network is non-hierarchical. Independent pooling MME and SGW are supported,it is however not possible to change an S-GW without involving the MME.
An operator may pool MMEs and S-GWs into one or several pools depending on organization , regulatory requirements , transport providers etc. The flexibility of the pooling concept makes it possible to enable partial sharing of networks i.e., to use only a part of the operator’s network as a shared network.
The individual pooled MMEs and S-GWs do not have to be located on the same physical site , but can be distributed in the network. All pools of a particular operator are assumed to be interconnected by means of an interface similar to S3/S4 / S10/S11 interface .
When a UE attaches 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 the eNodeBs belonging to the same MME or S-GW pool. If the UE moves out of the pools coverage it is reassigned to an MME or S-GW in the pool associated with the new eNode B.
The P-GW who performs charging , policy enforcement and UEs IP PoP is not changed when the S-GW is relocated. The main purpose of the S-GW is to act as a local mobility anchor and to buffer packets during LTE paging. In most case the SGW and PGW functionalities are performed by the same physical node. MME relocation may be more motivated since there may be limits on how many eNodeBs the MME is connected to.
Also keep in mind, partially overlapping pools are also supported. Overlapping pools may have some benefits since it makes it possible to avoid some of the negative effects of hard pool borders , however it comes with extra complexity.