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ESM (EPS Session Management)

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Introduction:

EPS (Evolved Packet System) is a wireless communication technology used in 4G and 5G cellular networks. EPS Session Management (ESM) is a component of EPS that is responsible for establishing, maintaining, and terminating user sessions in the EPS network. ESM manages the signaling and control protocols that are used to set up, modify, and release user sessions.

ESM Architecture:

ESM architecture consists of two entities: the EPS bearer level and the session level. The EPS bearer level manages the Quality of Service (QoS) parameters and the traffic flow between the user equipment (UE) and the packet data network (PDN) gateway. The session level manages the UE's user plane and control plane connections to the PDN gateway.

The ESM signaling is divided into two categories: bearer management signaling and session management signaling. The bearer management signaling is used to establish, modify, and release EPS bearers. The session management signaling is used to establish, modify, and release sessions. The signaling messages are exchanged between the UE and the network through the radio access network (RAN) and the core network.

ESM States:

ESM defines several states that a session can be in during its lifetime. These states are:

  1. Inactive state: In this state, the UE is not connected to any PDN. This state is entered when the UE is switched on or after the release of an EPS bearer.
  2. Active state: In this state, the UE is connected to a PDN and has an EPS bearer allocated. This state is entered when the UE requests an EPS bearer.
  3. Modified state: In this state, the QoS parameters of an existing EPS bearer are modified. This state is entered when the UE requests a modification of an existing EPS bearer.
  4. Deactivated state: In this state, the UE has released an EPS bearer but is still connected to the PDN. This state is entered when the UE requests the release of an EPS bearer.

ESM Procedures:

ESM defines several procedures that are used to establish, modify, and release EPS bearers and sessions. These procedures are:

  1. Activate Default EPS Bearer: This procedure is used to establish a default EPS bearer for the UE. The default bearer is used for all non-QoS-guaranteed traffic.
  2. Activate Dedicated EPS Bearer: This procedure is used to establish a dedicated EPS bearer for the UE. The dedicated bearer is used for QoS-guaranteed traffic.
  3. Modify EPS Bearer: This procedure is used to modify the QoS parameters of an existing EPS bearer.
  4. Deactivate EPS Bearer: This procedure is used to release an EPS bearer.
  5. PDN Connectivity: This procedure is used to establish a connection between the UE and a PDN.
  6. PDN Disconnect: This procedure is used to release the connection between the UE and a PDN.

ESM Information Elements:

ESM uses several information elements (IEs) to convey the necessary information between the UE and the network during signaling. These IEs are:

  1. EPS Bearer Identity (EBI): This IE identifies the EPS bearer.
  2. Traffic Flow Template (TFT): This IE specifies the QoS parameters for the EPS bearer.
  3. Linked EPS Bearer Identity (LBI): This IE links a dedicated bearer to a default bearer.
  4. Protocol Configuration Options (PCO): This IE carries information about the UE's protocol configuration.
  5. Access Point Name (APN): This IE specifies the name of the PDN to which the UE wants to connect.
  6. Quality of Service (QoS): This IE specifies the QoS parameters for the session.

Conclusion:

In conclusion, EPS Session Management (ESM) is an important component of EPS that is responsible for managing user sessions in the network. ESM is responsible for establishing, modifying, and releasing EPS bearers and sessions. ESM architecture is divided into two entities, the EPS bearer level and the session level. The signaling messages are exchanged between the UE and the network through the RAN and the core network.

ESM defines several states that a session can be in during its lifetime, including the inactive state, active state, modified state, and deactivated state. ESM procedures include activating a default EPS bearer, activating a dedicated EPS bearer, modifying an EPS bearer, deactivating an EPS bearer, establishing PDN connectivity, and releasing PDN connectivity.