5G/NR - NAS

5GSM(5G Session Management)

Very simply put, 5GSM is a layer in core network that handles all the procedures for PDN. According to 24.501 - 6 Elementary procedures for 5GS session management, The main function of the 5GSM sublayer is to support the PDU session handling in the UE and in the SMF (transferred via the AMF).

Overall Functions of 5GSM

Overall description of 5GSM is well described in 24.501 - 6.1.1 as follows :

The 5GSM comprises procedures for:

  • the authentication and authorization, establishment, modification and release of PDU sessions; and
  • request for performing handover of an existing PDU session between 3GPP access and non-3GPP access, or to transfer an existing PDN connection in the EPS to the 5GS.

Each PDU session represents a PDU session established between the UE and an SMF. PDU sessions can remain established even if the radio and network resources constituting the corresponding PDU session between the UE and the SMF are temporarily released.

5GSM procedures can be performed only if a 5GMM context has been established between the UE and the AMF, and the secure exchange of NAS messages has been initiated by the AMF by use of the 5GMM procedures described in 5GMM Section. Once the UE is successfully registered to a PLMN, a PDU session can be established. If no 5GMM context has been established, the 5GMM sublayer has to initiate the establishment of a 5GMM context by use of the 5GMM procedures as described in 5GMM Section.

The UE can request the network to modify or release PDU sessions. The network can fulfil such a request from the UE

by modifying a PDU session or releasing a PDU session using network-requested procedures

5GSM Statemachine

5GSM is very complicated process described in about 100 pages for overall functionaligy in 24.501-6 and around 40 pages for NAS signaling message handled by 5GSM in 24.501-8.3. You would easily get lost if you directly jumping into reading the details. My recommendation is to try to get familiar with the following statemachine to the point where you can draw the statemachine by hands and then read the specification as-you-need basis.

PDU Session Handling in UE

Following state machine diagram illustrates the lifecycle of a PDU session in the 5G System happending in UE. It captures the various phases a PDU session undergoes, from inactivity to potential modifications and active data transmission. The transitions represent events or commands that cause the session to change its state, showing the dynamic nature of session management in 5G networks.

< 24.501-Figure 6.1.3.2.1.1: The 5GSM sublayer states for PDU session handling in the UE (overview) >

PDU SESSION INACTIVE : This is the state where the PDU session is not active, meaning there's no ongoing data transmission for this session.

Followings are transition to other states:

  •   To PDU SESSION ACTIVE PENDING: Triggered by "PDU SESSION ESTABLISHMENT REQUEST".

PDU SESSION ACTIVE PENDING : This state indicates that the PDU session is in the process of becoming active.

Followings are transition to other states:

  •   To PDU SESSION ACTIVE: Triggered by "PDU SESSION ESTABLISHMENT ACCEPT"
  •   To PDU SESSION INACTIVE: Triggered by "PDU SESSION ESTABLISHMENT REJECT"

PDU SESSION ACTIVE : The PDU session is active and data transmission is possible in this state.

Followings are transition to other states:

  •  To PDU SESSION INACTIVE: Triggered by "PDU SESSION RELEASE COMMAND".
  • To PDU SESSION INACTIVE PENDING: Triggered by "PDU SESSION RELEASE REQUEST".
  • To PDU SESSION MODIFICATION PENDING: Triggered by "PDU SESSION MODIFICATION REQUEST"
  • Self-looping transition: Triggered by PDU SESSION ESTABLISHMENT REQUEST or PDU SESSION MODIFICATION COMPLETE or "PDU SESSION MODIFICATION REJECT including cause value = 43".

PDU SESSION INACTIVE PENDING : This state indicates that the PDU session is in the process of transitioning to an inactive status.

Followings are transition to other states:

  •   To PDU SESSION INACTIVE: Triggered by PDU SESSION RELEASE COMMAND or "PDU SESSION RELEASE REJECT including cause value = 43".

PDU SESSION MODIFICATION PENDING : This state signifies that modifications to the PDU session parameters or settings are being processed.

Followings are transition to other states:

  •   To PDU SESSION ACTIVE: Triggered by PDU SESSION MODIFICATION COMMAND or PDU SESSION MODIFICATION REJECT including cause value = 43.
  •   To PDU SESSION INACTIVE: Triggered by "PDU SESSION MODIFICATION REJECT including cause value = 43".

PDU Session Handling in Network

The Following state machine diagram shows the lifecycle of a PDU session in the 5G System happening in the network side. It encapsulates the various states and transitions a PDU session undergoes, including initiation, modification, and termination. Through this structure, the network manages and adapts to the dynamic requirements and conditions of data sessions, ensuring efficient communication and resource allocation in a 5G environment.

< 24.501-Figure 6.1.3.3.1.1: The 5GSM sublayer states for PDU session handling in the network (overview) >

PDU SESSION INACTIVE: This is a state where the PDU session is not currently active, indicating there's no ongoing data transmission for this session.

Followings are transition to other states:

  • To PDU SESSION ACTIVE: Triggered by "PDU SESSION ESTABLISHMENT ACCEPT", indicating that the session setup request was successful.
  • Self-looping transition: Triggered by "PDU SESSION ESTABLISHMENT REJECT"

PDU SESSION ACTIVE: In this state, the PDU session is active, and data transmission is feasible.

Followings are transition to other states:

  • To PDU SESSION INACTIVE: Triggered by "PDU SESSION MODIFICATION REJECT including cause value = 43"
  • To PDU SESSION INACTIVE PENDING : Triggered by PDU SESSION RELEASE COMMAND
  • To PDU SESSION MODIFICATION PENDING: Triggered by "PDU SESSION MODIFICATION COMMAND", indicating an intent to modify certain session parameters.

PDU SESSION MODIFICATION PENDING: This state signifies that there's an ongoing process to alter the PDU session's parameters or characteristics.

Followings are transition to other states:

  • To PDU SESSION INACTIVE: Triggered by "PDU SESSION MODIFICATION COMMAND REJECT including cause value = 43", indicating that the session modification was rejected.
  • To PDU SESSION ACTIVE: Triggered by "PDU SESSION MODIFICATION COMPLETE" or "PDU SESSION MODIFICATION COMMAND REJECT including cause value = 43"

PDU SESSION INACTIVE PENDING: This state represents that the PDU session is in transition towards an inactive status.

Followings are transition to other states:

  • To PDU SESSION INACTIVE: Triggered by PDU SESSION ESTABLISHMENT REJECT. This can be inferred as a natural progression once all processes leading to inactivity are complete.

NAS Signaling Procedures Handled by 5GSM

Followings are the list of NAS signaling (OTA procedure) handled by 5GSM. I think these message (process) list would give more concrete idea to most of the readers since analyzing OTA message would be pretty common practice. I will create separate notes for each of these process and put the link as I complete the note... but not sure how long it will take to complete all of them, but I will try to complete at least 5G specific process as early as possible. For most of other procedures, I think you can apply your LTE knowledge. Each of NAS messages involved in each of these procedures are described in 24.501-8.3. As you see, 5GSM is just all about PDU session.

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