Section: 6.1 Overview 6.1.1 General
Content: 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.