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NAS

Author: Frédéric Firmin, 3GPP MCC

This article gives an overview of the Non-Access Stratum (NAS) and its protocols EMM and ESM.

The Non-Access Stratum is a set of protocols in the Evolved Packet System. The NAS is used to convey non-radio signalling between the User Equipment (UE) and the Mobility Management Entity (MME) for an LTE/E-UTRAN access.

NAS

From a protocol stack perspective, the NAS is the highest stratum of the control plane, as shown in figure 1.

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Figure 1: EPS control plane for E-UTRAN access

The NAS procedures are grouped in two categories:

  • the EPS Mobility Management (EMM), and
  • the EPS Session Management (ESM).

EPS Mobility Management

The EPS Mobility Management protocol refers to procedures related to mobility over an E-UTRAN access, authentication and security. It is the equivalent in EPS of MM in GSM and GMM in GPRS.

3GPP specifications make distinction between EMM common, specific and connection management procedures:

EMM common procedures

EMM common procedures refer to various network-initiated mechanisms.

  • GUTI (Global Unique Temporary ID) reallocation
  • Authentication
  • Security mode control
  • Identification
  • EMM information

EMM specific procedures

EMM specific procedures are UE-initiated.

These procedures define attach/detach (to/from the EPC) mechanisms. They also introduce the Tracking Area Update (TAU) mechanism, which update the location of the UE within the network. Tracking Area is the equivalent in EPS of Location Area in GSM and Routing Area in GPRS.

In EPS, a UE initiates a Tracking Area Update when it detects that it enters into a new Tracking Area. EMM specific procedures also define periodic Tracking Area Update.

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Figure 2: tracking areas

EPS also introduces the concept of Tracking Area List (TAL), which allows the provision of lists of Tracking Areas in UEs. A UE does not need to initiate a TAU if it enters a Tracking Area which is included in its TAL.

Provisioning different lists to the UEs within the network avoid signalling peaks for instance when all UEs on a train cross a TA border.

EMM connection management procedures:

(also known as ECM: “EPS Connection Management“)

EMM connection management procedures provide several functions to support the connection of the UE to the network (see ESM procedures below):

  • Service request: initiated by the UE to start the establishment of NAS signalling connection
  • Paging: initiated by the network in case of downlink NAS signalling to indicate to the UE to start a service request
  • Transport of NAS messages: used for SMS (CS fallback)
  • Generic transport of NAS messages: Various other applications (e.g. LCS)

EPS Session Management

The EPS Session Management protocol offers support to the establishment and handling of user data in the NAS.

In EPS, two concepts are introduced to define the IP connectivity between a UE and a packet data network (PDN):

  • PDN connection, and
  • EPS bearer

As shown in figure 3, a PDN connection is composed of a default EPS bearer and possibly additional ones called "dedicated bearers".

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Figure 3: PDN connection and EPS bearer

A default bearer is created upon establishment of a PDN connection. If a service (e.g. video streaming) requires specific handling in terms of quality of service (QoS), dedicated bearers can be established.

Within a PDN connection, all EPS bearers share a same UE IP address and an APN.

EPS supports multiple simultaneous PDN connections. For instance, a UE can have a PDN connection to the Internet (with just a default EPS bearer) and one to the operator’s IMS (with additional dedicated bearers, if required by the service).

ESM procedures are grouped into two categories:

  • EPS bearer procedures, which are network-initiated and provide mechanisms for activation, deactivation or modification of EPS bearers, and
  • Transaction-related procedures, which are UE-initiated and provide mechanisms for:
  • requests for PDN connection establishment and disconnection,
  • requests for bearer resources allocation and modification, and
  • release requests

Further reading:

General:

The 3GPP website contains all 3GPP specifications. They can be downloaded for free at www.3gpp.org/specifications

Descriptions of all 3GPP releases can be found at ftp://ftp.3gpp.org/Information/WORK_PLAN/Description_Releases/

An overview of the EPC can be found at: http://www.3gpp.org/The-Evolved-Packet-Core

NAS:

3GPP TS 23.401 defines the architecture of the EPC for an LTE access
3GPP TS 24.301 defines the protocol details of the NAS
3GPP TS 24.008 defines inter-system mobility between LTE and 2G/3G
3GPP TS 24.303 defines inter-system mobility between LTE and non-3GPP accesses.

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