How NAS Procedures Works In LTE

Question

Lot of discussions are happening on NAS but nothing clear and crisp.
Can someone describe in detail on how NAS procedure works in LTE in details.

Answer 1

The NAS ( Non Access Stratum) protocol, is being used to communicate directly between the UE and the MME. It is called Non Access , in order to distinguish it from the AS ( Access Stratum) protocol that carries information over the air between the UE and the eNodeB..

The NAS protocol supports :

• Mobility Management Functionality , also called EPS Mobility Management ( EMM)
• The EPS Session Management (ESM) including:
  - User Plane Bearer Activation
  - User Plane Modification
  - User Plane Deactivation

The interface used for the NAS is the S1-MME interface , and the transport protocol that carries the NAS protocol to the MME is S1-AP.

As an example we can take the procedure of Identification initiation by the network

• The network initiates the identification procedure by sending an IDENTITY REQUEST message to the UE and starting the timer T3470 The IDENTITY REQUEST message specifies the requested identification parameters in the Identity type information element.

• The UE answers with an IDENTITY RESPONSE

• When the MME receives the rsponse it stops timer T3470

More on the NAS protocol can be found in ETSI TS 124 301
LTE; Non-Access-Stratum (NAS)protocol for Evolved Packet System (EPS)

Answer 2

NAS is like a big ocean. Its not fair to describe it in just few words.

For a high level info, you can follow the following link below -
http://lteworld.org/specification/non-access-stratum-nas-protocol

And as suggested by Avigdor, the best place to understand NAS will be (Bible) 24.301 :-)