How does paging procedure work for an UE for SMS service in a lte network ?

Question

There are two ways for tracking area update procedure to get triggered from UE to network . One is known as periodic and other one as non-periodic.

Answer 1

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:
1. SGs-NULL
2. LA-UPDATE-REQUEST
3. SGs-ASSOCIATED

Followings are defined states of SGs association at MME:
1. SGs-NULL
2. LA-UPDATE-REQUESTED
3. SGs-ASSOCIATED

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.

Answer 2

To add to the above, SMSs are piggybacked in NAS messages