How NAS messages are transported between the UE and the MME.

Question

When the MME sends a DL NAS message is the connection between MME and UE held long enough just to receive the message ? What happens if there are back-to-back NAS messages to be delivered, does the the UE open a connection to MME and receive all the messages at once. Or does it open close the connection to get each of the messages separately.

Answer 1

MME doesn't have a direct connection with the UE. MME is connected to enodeB via S1 interface and enodeB to UE on Uu

1. NAS message transferred from MME to enodeB on S1 interface (downlink NAS transport) and enodeB forword this message to UE in a encapsulated way in RRC message.

2. all NAS messages forwarded one by one from MME to enodeB to UE and vice-versa.

3. If UE is in Idle mode then RRC connection is established first then message is forwarded.

Answer 2

If MME is sending "NAS massage" to an UE by using S1-AP "Downlink NAS Transport" , it means UE is connected with MME/Network. When an UE is not connected to network and network wants to send any message, it initiates paging for that UE. There is an User Inactivity timer which runs at eNodeB, it significance is that if there is no data trans/receive to/from UE, release the rrc connection and also informs to MME.