Core Network: Architecture and Signaling Procedures v2.2.0 PDF

6.2. Detach Procedure

6.2.1. UE-Initiated Detach Procedure

6.2.1.1. Technical Overview

The UE-initiated Detach procedure lets a UE inform the network that it no longer needs access to the EPS, and it deactivates all active EPS bearer contexts, including the default bearer.

The procedure removes the UE’s mobility management and security contexts from the MME and from the UE, moving the UE to the EMM-DEREGISTERED state.

Across the S1, S11, and S5/S8 interfaces, the EPC releases all associated user-plane and control-plane resources.

6.2.1.2. Call Flow

The call flow has three phases: Detach Request, Resource Deactivation, and Detach Acceptance and S1 Release.

UE-initiated detach procedure call flow
Figure 14. UE-initiated detach procedure call flow
6.2.1.3. Step-by-Step Description

  1. The UE sends a Detach Request message to the eNodeB, carrying this NAS message inside an RRC message; if the UE was in ECM-IDLE mode, this message triggers the establishment of a NAS signaling connection. The message carries the GUTI, the Detach type, which shows the kind of detach, for example EPS detach or combined EPS/IMSI detach, the Switch off indicator, which shows whether the UE is powering down, and the NAS key set identifier, that is the eKSI.

  2. The eNodeB forwards the NAS message to the MME inside an S1AP Uplink NAS Transport message, including the User Location Information of the serving cell, that is the ECGI and the TAI, and the NAS-PDU.

  3. If ISR is active for the UE, the MME sends a GTPv2 Detach Notification message to the associated SGSN to detach the UE from GSM EDGE Radio Access Network (GERAN) and Universal Terrestrial Radio Access Network (UTRAN) access as well. The message carries a Cause value set to "Complete Detach" and the Detach Type.

  4. On receiving the Detach Notification, the SGSN deactivates the UE’s active Packet Data Protocol (PDP) contexts and sends a GTPv2 Delete Session Request message to the S-GW. The message carries the LBI, the User Location Information, that is the Cell Global Identification (CGI) and the Service Area Identifier (SAI), and the UE Time Zone.

  5. The S-GW acknowledges the deletion of the SGSN-related bearer contexts by returning a GTPv2 Delete Session Response message, carrying a Cause, to the SGSN.

  6. The SGSN confirms the completion of its local detach procedure by sending a GTPv2 Detach Acknowledge message, carrying a Cause, to the MME.

  7. To deactivate all active EPS bearers, the MME sends a GTPv2 Delete Session Request message to the S-GW for each PDN connection, carrying the LBI, the User Location Information, that is the ECGI, the Secondary RAT usage data, and the UE Time Zone. If ISR was active, the S-GW does not release its control-plane TEID until it receives this message.

  8. For each PDN connection that the procedure releases, the S-GW sends a GTPv2 Delete Session Request message to the P-GW, telling the P-GW to delete all bearers for that PDN connection. The message carries the LBI, the User Location Information, and the Secondary RAT usage data.

  9. If the network deploys dynamic PCC, the P-GW starts an IP-CAN Session Termination procedure with the PCRF, using Diameter, letting the PCRF release the policy and charging rules for the session. The Diameter message carries a CC-Request-Type set to TERMINATION_REQUEST and the Used-Service-Unit, for usage monitoring reporting.

  10. The P-GW acknowledges the session deletion by returning a GTPv2 Delete Session Response message to the S-GW, carrying a Cause and the APN Rate Control Status.

  11. The S-GW releases the UE’s context information and forwards the acknowledgement to the MME with a GTPv2 Delete Session Response message, carrying a Cause and the APN Rate Control Status.

  12. If the UE indicated that Switch off is false in the Detach Request, the MME sends a NAS Detach Accept message to the eNodeB inside an S1AP Downlink NAS Transport message, carrying the NAS-PDU and the MME UE S1AP ID.

  13. The eNodeB delivers the Detach Accept message to the UE over the radio interface, and the UE stops timer T3421 on receiving it.

  14. The MME starts the release of the S1-MME signaling connection by sending an S1AP UE Context Release Command message to the eNodeB, carrying the UE S1AP IDs and a Cause set to "Detach".

  15. The eNodeB sends an RRC Connection Release message to the UE and deletes the UE’s RAN context after the UE acknowledges it.

  16. The eNodeB confirms the release of the S1 connection and the user-plane resources by sending an S1AP UE Context Release Complete message to the MME, carrying the MME UE S1AP ID, the eNB UE S1AP ID, and the User Location Information.

6.2.1.4. Possible Failures and Required Actions
Authentication failure during S1 establishment

When the UE is in ECM-IDLE mode, it first needs to set up the S1 connection to send the Detach Request, and the MME can start authentication at that point. If authentication fails, the UE receives a NAS Authentication Reject message, stops the Detach procedure, stops all related EMM timers, for example T3421, and enters the EMM-DEREGISTERED state. The MME can deactivate all non-emergency bearers and mark the UE as detached.

Detach through an unauthorized CSG cell

The UE can send a Detach Request, without the Switch off indicator, while connected to a CSG cell for which its subscription has expired. In this case, the MME responds with EMM Cause #25, "Not authorized for this CSG", and starts a network-initiated detach procedure. The UE removes the CSG ID from its Allowed CSG list and, if needed, performs a new Public Land Mobile Network (PLMN) selection.

T3421 timer expiry

The UE can send a Detach Request, without the Switch off indicator, and not receive a Detach Accept message before timer T3421 expires. In this case, the UE retransmits the Detach Request message, up to four times, and on the fifth expiry stops the signaling procedure, performs a local detach, and enters the EMM-DEREGISTERED state.

6.2.1.5. References
Specification Clauses and coverage

TS 23.401

  • Clause 5.3.8.1 — Detach procedure

  • Clause 5.3.8.2.1 — UE-initiated detach

  • Clause 5.3.5 — ISR

TS 24.301

  • Clause 5.5.2.1 — Detach procedure

  • Clause 5.5.2.2.1 — UE-initiated detach, general

  • Clause 5.5.2.2.2 — UE-initiated detach procedure

  • Clause 8.2.11 — Detach Request message content

TS 29.274

  • Clause 7.2.9.1 — Delete Session Request

  • Clause 7.2.10.1 — Delete Session Response

  • Clause 7.3.12 — Detach Notification/Acknowledge

TS 29.213

Clause 4.2.2.1 (IP-CAN Session Termination).

TS 36.413

  • Clause 8.3.3 — UE Context Release

  • Clause 9.1.4.6 — UE Context Release Command

  • Clause 9.1.4.7 — UE Context Release Complete

6.2.2. HSS-Initiated Detach Procedure

6.2.2.1. Technical Overview

The HSS-initiated Detach procedure lets the HSS remove a subscriber’s MM context and EPS bearer contexts from the network; the HSS, not the UE, triggers this procedure.

The HSS starts the procedure for operator-determined reasons, for example a subscription withdrawal, a profile change that no longer allows PS services, or a Core Network access restriction.

The procedure removes the contexts from the MME and, if the network deploys an SGSN for the subscriber, from the SGSN as well.

The procedure moves the UE to the EMM-DEREGISTERED state and releases all active PDN connections and their EPS bearers, and the MME, the S-GW, and the P-GW remove the subscriber’s session information.

6.2.2.2. Call Flow

The call flow has four phases: Cancellation at the MME, UE Notification, Resource Release, and Detach Acknowledgement and S1 Release.

HSS-initiated detach procedure call flow
Figure 15. HSS-initiated detach procedure call flow
6.2.2.3. Step-by-Step Description

  1. The HSS starts the procedure by sending a Diameter Cancel-Location-Request (CLR) message to the MME, asking the MME to delete the subscriber’s MM context and EPS bearer contexts. The message carries the IMSI, the Cancellation-Type, for example "Subscription Withdrawal", and, optionally, the CLR-Flags, including a Reattach-Required indicator.

  2. The MME verifies the IMSI and confirms that it accepted the cancellation by returning a Diameter Cancel-Location-Answer (CLA) message to the HSS, carrying a Result-Code and the Supported-Features.

  3. If the UE is in ECM-IDLE mode, the MME starts the Paging procedure by sending an S1AP Paging message to the eNodeB, carrying the UE Identity Index Value, the Paging Discontinuous Reception (DRX) cycle, and the S-TMSI, and the eNodeB pages the UE over the radio interface with an RRC Paging message.

  4. The UE responds to the page with an RRC Service Request message to the eNodeB, which forwards the request to the MME inside an S1AP Initial UE Message, carrying the Service Request; this exchange establishes a NAS signaling connection.

  5. The MME sends a NAS Detach Request message to the eNodeB inside an S1AP Downlink NAS Transport message, carrying the Detach type, which shows whether the UE needs to reattach, and an EMM cause, for example cause #2, "IMSI unknown in HSS". The eNodeB delivers the Detach Request to the UE inside an RRC Downlink Direct Transfer message, telling the UE that the network detached it.

  6. To deactivate the associated EPS bearers, the MME sends a GTPv2 Delete Session Request message to the S-GW for each active PDN connection, carrying the LBI, the User Location Information, that is the ECGI, and, if policy permits, the NAS Release Cause.

  7. For each PDN connection that the procedure releases, the S-GW forwards the GTPv2 Delete Session Request message to the P-GW, carrying the LBI, the User Location Information, and the Indication Flags, with the Operation Indication flag set.

  8. If the network deploys dynamic PCC, the P-GW starts an IP-CAN Session Termination procedure with the PCRF by sending a Diameter Credit-Control-Request (CCR) message to report the final usage, carrying a CC-Request-Type set to TERMINATION_REQUEST, the Subscription-Id, and the Used-Service-Unit.

  9. The PCRF acknowledges the termination by returning a Diameter Credit-Control-Answer (CCA) message, carrying a Result-Code, to the P-GW, letting the P-GW release the session’s policy and charging resources.

  10. The P-GW acknowledges the deletion by returning a GTPv2 Delete Session Response message to the S-GW, carrying a Cause set to "Request Accepted" and the APN Rate Control Status.

  11. The S-GW releases its local context for the UE and returns a GTPv2 Delete Session Response message to the MME, carrying a Cause and the Recovery counter.

  12. The UE acknowledges the Detach Request with a NAS Detach Accept message to the eNodeB inside an RRC Direct Transfer message, and the eNodeB forwards it to the MME inside an S1AP Uplink NAS Transport message, carrying the NAS-PDU and the User Location Information.

  13. When the MME receives both the Detach Accept and the Delete Session Response, it starts the release of the S1-MME signaling connection by sending an S1AP UE Context Release Command message to the eNodeB, carrying the UE S1AP IDs and a Cause set to "Detach".

  14. The eNodeB sends an RRC Connection Release message to the UE and deletes the UE’s RAN context after the UE acknowledges it.

  15. The eNodeB confirms the release of the S1 connection by sending an S1AP UE Context Release Complete message to the MME, carrying the MME UE S1AP ID and the eNB UE S1AP ID.

6.2.2.4. Possible Failures and Required Actions
T3422 timer expiry

The MME sends the Detach Request and starts timer T3422, with a default value of 6 seconds. If the MME does not receive a Detach Accept message before the timer expires, it retransmits the Detach Request, up to four times, and on the fifth expiry stops the signaling procedure and enters the EMM-DEREGISTERED state for the UE.

Detach in a forbidden satellite E-UTRAN area

In a network that uses satellite E-UTRAN access, the MME can determine that the UE is in a location where the PLMN cannot operate. In this case, the MME sends the Detach Request with EMM Cause #78, "PLMN not allowed to operate at the present UE location", and the UE enters the EMM-DEREGISTERED.LIMITED-SERVICE state and searches for a new suitable cell.

Detach procedure collision

The UE can start a UE-initiated Detach procedure, for example due to power off, while an HSS-initiated Detach procedure is in progress at the MME, sending a Detach Request with the Switch off indicator before the MME completes the HSS-initiated procedure. In this case, the MME considers both Detach procedures complete at the same time.

6.2.2.5. References
Specification Clauses and coverage

TS 23.401

  • Clause 5.3.8.4 — HSS-initiated detach

  • Clause 5.3.5 — S1 release

  • Clause 5.10.3 — satellite E-UTRAN access restrictions

TS 24.301

  • Clause 5.5.2.3 — Network-initiated detach procedure

  • Clause 8.2.11 — Detach Request message content

  • Clause 9.9.3.7 — Detach type

TS 29.272

  • Clause 5.2.1.2 — Cancel Location procedure

  • Clause 7.2.7 — CLR command

  • Clause 7.3.24 — Cancellation-Type values

TS 29.274

  • Clause 7.2.9.1 — Delete Session Request

  • Clause 7.2.10.1 — Delete Session Response

  • Clause 8.4 — Cause values

TS 29.213

Clause 4.2.2.1 (IP-CAN Session Termination).

TS 36.413

  • Clause 8.3.3 — UE Context Release

  • Clause 9.1.4.6 — UE Context Release Command

  • Clause 9.1.4.7 — UE Context Release Complete

← Previous: Attach Procedure | ↑ Up: Functional Description and Information Flows | ⌂ Home: Core Network: Architecture and Signaling Procedures | Next: Service Request Procedure →