6.1 Overview

24.3013GPPNon-Access-Stratum (NAS) protocol for Evolved Packet System (EPS)Release 18Stage 3TS

6.1.1 General

This clause describes the procedures used for EPS session management (ESM) at the radio interface (reference point "LTE-Uu").

The main function of the ESM sublayer is to support the EPS bearer context handling in the UE and in the MME.

The ESM comprises procedures for:

– the activation, deactivation and modification of EPS bearer contexts;

– the request for resources (IP connectivity to a PDN or dedicated bearer resources) by the UE; and

– the transport of user data via the control plane between the UE and the MME.

Each EPS bearer context represents an EPS bearer between the UE and a PDN. EPS bearer contexts can remain activated even if the radio and S1 bearers constituting the corresponding EPS bearers between UE and MME are temporarily released.

An EPS bearer context can be either a default bearer context or a dedicated bearer context.

A default EPS bearer context is activated when the UE requests a connection to a PDN.

Generally, ESM procedures can be performed only if an EMM context has been established between the UE and the MME, and the secure exchange of NAS messages has been initiated by the MME by use of the EMM procedures described in clause 5. The first default EPS bearer context, however, can be activated during the EPS attach procedure (see clause 4.2). Once the UE is successfully attached, and the first default EPS bearer context has been activated during or after the attach procedure, the UE can request the MME to set up connections to additional PDNs. For each additional connection, the MME will activate a separate default EPS bearer context. A default EPS bearer context remains activated throughout the lifetime of the connection to the PDN.

A dedicated EPS bearer context is always linked to a default EPS bearer context and represents additional EPS bearer resources between the UE and the PDN. The network can initiate the activation of dedicated EPS bearer contexts together with the activation of the default EPS bearer context or at any time later, as long as the default EPS bearer context remains activated. However, the network shall not initiate a dedicated bearer context activation procedure for established PDN connection(s) of "non IP" PDN type.

Default and dedicated EPS bearer contexts can be modified. Dedicated EPS bearer contexts can be released without affecting the default EPS bearer context. When the default EPS bearer context is released, then all dedicated EPS bearer contexts linked to it are released too.

The UE can request the network to allocate, modify or release EPS bearer resources. The network can fulfil such a request from the UE by activating a new dedicated EPS bearer context, modifying an EPS bearer context or deactivating an EPS bearer context.

6.1.2 Types of ESM procedures

Two types of ESM procedures can be distinguished:

1) 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.

This procedure is initiated by the network or by the UE and is used for the transport of user data via the control plane:

– transport of user data via the control plane procedure.

2) Transaction related procedures:

These procedures are initiated by the UE to request for resources, i.e. a new PDN connection or dedicated bearer resources, or to release these resources:

– PDN connectivity procedure;

– PDN disconnect procedure;

– bearer resource allocation procedure;

– bearer resource modification procedure.

This procedure is initiated by the ProSe UE-to-network relay and is used for the manipulation of EPS bearer contexts:

– remote UE report.

When combined with the attach procedure, the PDN connectivity procedure can trigger the network to execute the following transaction related procedure:

– ESM information request procedure.

When combined with the attach procedure, if EMM-REGISTERED without PDN connection is supported by the UE and the network and no PDN connectivity procedure is initiated during the attach procedure, the UE or the network executes the following transaction related procedure:

– ESM dummy message procedure.

A successful transaction related procedure initiated by the UE triggers the network to execute one of the procedures related to EPS bearer contexts. The UE treats the start of the procedure related to the EPS bearer context as completion of the transaction related procedure.

During procedures related to EPS bearer contexts, the MME and the UE shall not initiate the transport of user data via the control plane procedure until the ongoing procedure is completed.

NOTE 1: The UE determination of the completion of the transport of user data via the control plane procedure is left to the implementation.

NOTE 2: NAS signalling prioritization with respect to the transport of user data via the control plane cannot be guaranteed in the downlink direction in this version of the specification.

Except for the remote UE report procedure and ESM information request procedure, during transaction related procedures, the MME and the UE shall not initiate the transport of user data via the control plane procedure until the ongoing procedure is completed.

The following ESM procedures can be related to an EPS bearer context or to a procedure transaction:

– ESM status procedure;

– notification procedure.

6.1.3 ESM sublayer states

6.1.3.1 General

In this clause the possible states of EPS bearer contexts in the UE and on the network side are described. Each EPS bearer context is associated with an individual state.

6.1.3.2 ESM sublayer states in the UE

6.1.3.2.1 BEARER CONTEXT INACTIVE

No EPS bearer context exists.

6.1.3.2.2 BEARER CONTEXT ACTIVE

The EPS bearer context is active in the UE.

Figure 6.1.3.2.2.1: The ESM sublayer states for EPS bearer context handling in the UE (overview)

6.1.3.2.3 PROCEDURE TRANSACTION INACTIVE

No procedure transaction exists.

6.1.3.2.4 PROCEDURE TRANSACTION PENDING

The UE has initiated a procedure transaction towards the network.

Figure 6.1.3.2.4.1: The procedure transaction states in the UE (overview)

6.1.3.3 ESM sublayer states in the MME

6.1.3.3.1 BEARER CONTEXT INACTIVE

No EPS bearer context exists.

6.1.3.3.2 BEARER CONTEXT ACTIVE PENDING

The network has initiated an EPS bearer context activation towards the UE.

6.1.3.3.3 BEARER CONTEXT ACTIVE

The EPS bearer context is active in the network.

6.1.3.3.4 BEARER CONTEXT INACTIVE PENDING

The network has initiated an EPS bearer context deactivation towards the UE.

6.1.3.3.5 BEARER CONTEXT MODIFY PENDING

The network has initiated an EPS bearer context modification towards the UE.

Figure 6.1.3.3.5.1: The ESM sublayer states for EPS bearer context handling in the network (overview)

6.1.3.3.6 PROCEDURE TRANSACTION INACTIVE

No procedure transaction exists.

6.1.3.3.7 PROCEDURE TRANSACTION PENDING

The network has initiated a procedure transaction towards the UE.

Figure 6.1.3.3.7.1: The procedure transaction states in the network (overview)

6.1.4 Coordination between ESM and SM

For inter-system change from S1 mode to A/Gb mode or Iu mode, SM uses the following parameters from each active EPS bearer context:

– EPS bearer identity to map to NSAPI;

NOTE 1: If the UE and the MME support signalling for a maximum number of 15 EPS bearer contexts, any active EPS bearer contexts using EPS bearer identity 1 to 4 are not mapped but deactivated locally before inter-system change to A/Gb mode or Iu mode.

– linked EPS bearer identity (if available) to map to linked TI;

– PDN address and APN of the default EPS bearer context to map to PDP address and APN of the default PDP context;

– TFT of the default EPS bearer context, if any, to map to the TFT of the default PDP context;

– TFTs of the dedicated EPS bearer contexts to map to TFTs of the secondary PDP contexts; and

– GERAN/UTRAN parameters as provided by the MME while on E-UTRAN access, i.e. R99 QoS, LLC SAPI, radio priority, packet flow identifier, transaction identifier and BCM (if available). If the MME also provided an Extended EPS QoS IE, then the bit rates signalled with this IE are also applicable to the R99 QoS.

NOTE 2: Some networks not supporting mobility from S1 mode to A/Gb mode or Iu mode or both do not provide the UE with the GERAN/UTRAN parameters. However, for this case there is no need for the UE to perform mapping to GERAN/UTRAN parameters (i.e. the PDP contexts cannot be transferred to A/Gb mode or Iu mode).

The MME performs the mapping from EPS to R99 QoS parameters according to 3GPP TS 23.401 [10], annex E.

At inter-system change from S1 mode to A/Gb mode, SM shall not activate the PDP context(s) if SM does not have the following parameters from the active EPS bearer context(s):

– LLC SAPI;

– radio priority;

– transaction identifier; and

– R99 QoS.

At inter-system change from S1 mode to Iu mode, SM shall not activate the PDP context(s) if SM does not have the following parameter from the active EPS bearer context(s):

– transaction identifier; and

– R99 QoS.

For inter-system change from A/Gb mode or Iu mode to S1 mode, ESM uses the following parameters from each active PDP context:

– NSAPI to map to EPS bearer identity;

– NSAPI of the default PDP context to map to linked EPS bearer identity;

– PDP address and APN of the default PDP context to map to PDN address and APN of the default EPS bearer context;

– TFT of the default PDP context, if any, to map to the TFT of the default EPS bearer context; and

– TFTs of the secondary PDP contexts to map to the TFTs of the dedicated EPS bearer contexts.

The MME and the UE perform the mapping from R99 to EPS QoS parameters according to 3GPP TS 23.401 [10], annex E. In particular the MME derives the APN-AMBR for the corresponding PDN connection from the MBR of the R99 subscribed QoS profile and the UE maps the MBR of its default PDP context to the APN-AMBR of the corresponding PDN connection.

6.1.4A Coordination between ESM and 5GSM

See clause 6.1.4 in 3GPP TS 24.501 [54].

6.1.5 Coordination between ESM and EMM for supporting ISR

The UE with its TIN set as "RAT‑related TMSI" for which ISR is activated shall change its TIN to "GUTI" to locally deactivate ISR and stop the periodic routing area update timer T3312 or T3323, if running:

– upon modification of any EPS bearer context which was activated before the ISR is activated in the UE;

– at the time when the UE changes from S1 mode to A/Gb mode or Iu mode not due to PS handover procedure (see 3GPP TS 24.008 [13]), if any EPS bearer context activated after the ISR was activated in the UE exists; or

– upon deactivation of the last non-emergency EPS bearer context in the UE, if the UE has only a PDN connection for emergency bearer services remaining.

ISR remains activated on the network side in the above cases.