NAS layer : Non-Access Stratum: This is used to communicate control plane information between UE and MME.
NAS layer protocols functions are to support 1. Mobility of the UE 2. Session management procedures to establish and maintain IP connectivity between UE and PDN GW.
There are 2 types of procedures available in NAS layer:
1. EPS Mobility Management (EMM)
2. EPS Session Management (ESM).
1. EPS Mobility Management(EMM)
EMM is used to provide support for UE to network related to eNB access, authentication and security.
EMM has 2 states:
1. EMM registered
2. EMM De-registered
If UE is registered to MME then it is EMM registered else it will be in de-registered state.
EMM common procedures
The below EMM common procedures are network-initiated:
* GUTI (Global Unique Temporary ID) reallocation
* Security mode control
* EMM information
EMM specific procedures
The below EMM specific procedures are UE-initiated:
* attach procedure
* detach procedure
* Tracking Area Update
EMM connection management procedures also known as ECM: “EPS Connection Management“:
These services provide support the connection of the UE to network.
* Service request
* Transport of NAS messages: used for SMS (CS fallback)
* Generic transport of NAS messages: Various other applications (e.g. LCS)
2. EPS Session Management (ESM)
The EPS Session Management protocol is used to establishment and handling of user data in the NAS.
ESM has 2 states:
1. ESM Idle
2. ESM Connected
If there is an end to end session that is established for data transmission then UE NAS will be in ESM connected else idle.
Below are the concept that define IP connectivity between a UE and PDN.
Procedures related to EPS bearer contexts:
These procedures are initiated by the network and are used for the manipulation of EPS bearer contexts:
* default EPS bearer context activation;
* dedicated EPS bearer context activation;
* EPS bearer context modification;
* EPS bearer context deactivation.
Transaction related procedures
* PDN connectivity procedure;
* PDN disconnect procedure;
* bearer resource allocation procedure;
* bearer resource modification procedure.
To learn more about NAS layer it is defined in TS 24.301