EPS session management procedures are used at the radio interface "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; and
- the request for resources (IP connectivity to a PDN or dedicated bearer resources) by the UE.
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. The first default EPS bearer context, however, is activated during the EPS attach procedure. Once the UE is successfully attached, 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.
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 additional EPS bearer resources. The network decides whether to fulfil a request for additional resources by activating a new dedicated EPS bearer context or modifying an existing dedicated or default EPS bearer context.
Types of ESM procedures
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.
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.
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.