Section: 4.3.7.2 Load balancing between MMEs
Content: The MME Load Balancing functionality permits UEs that are entering into an MME Pool Area to be directed to an appropriate MME in a manner that achieves load balancing between MMEs. This is achieved by setting a Weight Factor for each MME, such that the probability of the eNodeB selecting an MME is proportional to its Weight Factor. The Weight Factor is typically set according to the capacity of an MME node relative to other MME nodes. The Weight Factor is sent from the MME to the eNodeB via S1-AP messages (see TS 36.413[ Evolved Universal Terrestrial Radio Access Network (E-UTRAN); S1 Application Protocol (S1AP) ]  [36]). If a HeNB GW is deployed, the Weight Factor is sent from the MME to the HeNB GW. NOTE 1: An operator may decide to change the Weight Factor after the establishment of S1-MME connectivity as a result of changes in the MME capacities. E.g., a newly installed MME may be given a very much higher Weight Factor for an initial period of time making it faster to increase its load. NOTE 2: It is intended that the Weight Factor is NOT changed frequently. e.g. in a mature network, changes on a monthly basis could be anticipated, e.g. due to the addition of RAN or CN nodes. In some networks, the eNodeB may be configured to select specific MME for UEs configured for low access priority with a different load balance to that used for MME selection for other UEs. NOTE 3: The eNodeB can determine whether or not the "UE is configured for low access priority" from information received in the RRC establishment or RRC resume signalling. When DCNs are used, load balancing by eNodeB is only performed between MMEs that belong to the same DCN within the same MME pool area, i.e. MMEs with the same PLMN and MMEGI value. When an MME serves multiple DCNs and one DCN is supported by multiple MMEs, in order to achieve load balancing across the MMEs of the same MME pool area supporting the same DCN, each DCN supported by this MME may have its own Weight Factor (Weight Factor per DCN). The Weight Factor per DCN is set according to the capacity of an MME node for a specific DCN relative to other MME nodes' capacity for that DCN within the same MME pool area. The eNodeB is provided with per DCN Weight Factors, if any, by the connected MMEs at the set-up of the S1 connection. The DCN Load Balancing functionality permits UEs that are entering into a pool area or being re-directed to an appropriate DCN to be distributed in a manner that achieves load balancing between the CN nodes of the same DCN. The eNodeB may be configured to select MME(s) from a specific CN for UEs configured for low access priority only for the case that no other information and configuration is available for selecting an MME from a specific DCN.