When an UE does combined i.e. EPS and non-EPS attache to network, it can receives SMS services from legacy CS network. VLR in CS network and MME in EPS may have SGs interface. VLR and MME both the nodes maintain SGs association states for each individual UE.
Followings are defined states of SGs association at VLR:
Followings are defined states of SGs association at MME:
For those UEs which are configured to use CS fallback & SMS services over SGs interface or SMS services only, VLR can initiate SGsAP-PAGING-REQUEST towards MME.