Reference - 3GPP TS 23.401 Section 4.3.7
As LTE is planned for larger deployment it is time to look at the load balancing and redundancy among the various core network nodes. I would like to present how a MME load balancing and redundancy can be achieved in this post. I will follow up this post with SGW and PGW pooling.
A MME in the network is identified by MME code and Group id. Each MME will have a weight factor configured that is conveyed to eNB during initial S1 setup. An eNB can talk to multiple MMEs in a pool. Based the weight factor eNB will decide which MME can be loaded with calls to what level. Assume that an eNB is communicating with 2 MMEs in a pool. MME-1 has weight of 100 and MME-2 has weight of 50. This means eNB, out of 3 UE attaches, will forward 2 attaches to MME-1 and 1 attach to MME-2. If in case MME-1 is down, then all calls will be routed to MME-2. This is one way of achieving load balancing and redundancy.
There is another way to achieve load rebalancing. This is the case when a MME feels overloaded, it can simply move the calls to other MME in a given pool. Assume above example. MME-1 has been overloaded and cannot handle any more calls. If MME-1 wants to free up some resources, than it release the S1 connections of UE towards eNB asking UE to perform a “load balancing TAU”. This is conveyed to UE by eNB in a RRC message. Once UE gets this message it shall send a Tracking area update message which is routed to MME-2 by eNB. This way MME-1 can gracefully move calls to other MME-2. It is assumed that MME-2 and MME-1 are connected over S10 interface as MME-2 should pull the UE context from MME-1.
If the MME is feeling overloaded, then it may also send Overload Start message to eNB asking it not to forward any calls to it. If necessary resources are available then MME may send Overload Stop indicating that it is ready to accept new calls.
Another way of achieving load balancing is using DNS at eNB. If UE is NOT connecting for the first time then it will populate GUMMEI to eNB in one of the RRC messages. Based on the GUMMEI eNB can perform a DNS query to obtain MME information and forward the UE messages to that particular MME. If the selected MME is not responding then eNB may forward the call to next available MME in the pool.
I believe this should help operators to plan their network.