5.3.5 S1 release procedure

23.4013GPPGeneral Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) accessRelease 18TS

This procedure is used to release the logical S1-AP signalling connection (over S1-MME) and all S1 bearers (in S1-U) for a UE. This Procedure releases the S11-U bearer in Control Plane CIoT EPS Optimisation (except in the case of buffering in MME), instead of the S1-U bearer. The procedure will move the UE from ECM-CONNECTED to ECM-IDLE in both the UE and MME, and all UE related context information is deleted in the eNodeB. When the S1-AP signalling connection is lost, e.g. due to loss of the signalling transport or because of an eNodeB or MME failure, the S1 release procedure is performed locally by the eNodeB and by the MME. When the S1 release procedure is performed locally by the eNodeB or by the MME each node performs locally its actions as described in the procedure flow below without using or relying on any of the signalling shown directly between eNodeB and MME.

If Service Gap Control shall be applied for the UE (see clause 4.3.17.9) and the Service Gap timer is not already running, the Service Gap timer shall be started in MME and UE when entering ECM-IDLE, unless the connection was initiated after a paging of an MT event, or after a TAU procedure without any active flag set or signalling active flag set.

The initiation of S1 Release procedure is either:

– eNodeB-initiated with cause e.g. O&M Intervention, Unspecified Failure, User Inactivity, Repeated RRC signalling Integrity Check Failure, Release due to UE generated signalling connection release, CS Fallback triggered, Inter-RAT Redirection, UE using satellite access moved out of PLMN serving area, etc. as defined in TS 36.413 [36]; or

– MME-initiated with cause e.g. authentication failure, detach, not allowed CSG cell (e.g. the CSG ID of the currently used CSG cell expires or is removed from the CSG subscription data), etc.

Both eNodeB-initiated and MME-initiated S1 release procedures are shown in Figure 5.3.5-1.

Figure 5.3.5-1: S1 Release Procedure

1a. In certain cases the eNodeB may release the UE’s signalling connection before or in parallel to requesting the MME to release the S1 context, e.g. the eNodeB initiates an RRC Connection Release for CS Fallback by redirection. If the reason for the release is that the eNodeB received Release Assistance Indicator in Access Stratum as defined in TS 36.321 [87], the eNodeB should not immediately release the RRC connection, instead send S1 UE context Release Request with appropriate cause value e.g. user inactivity.

1b. If the eNodeB detects a need to release the UE’s signalling connection and all radio bearers for the UE, the eNodeB sends an S1 UE Context Release Request (Cause) message to the MME. Cause indicates the reason for the release (e.g. O&M intervention, unspecified failure, user inactivity, repeated integrity checking failure, or release due to UE generated signalling connection release).

If the PLMN has configured secondary RAT reporting and the eNodeB has Secondary RAT usage data to report, the Secondary RAT usage data is included in S1 UE Context Release Request.

NOTE 1: Step 1 is only performed when the eNodeB-initiated S1 release procedure is considered. Step 1 is not performed and the procedure starts with Step 2 when the MME-initiated S1 release procedure is considered.

The MME upon receiving S1 UE Context Release Request with Cause=user inactivity continues with the S1 release procedure unless the MME is aware of pending MT traffic or signalling and/or NAS Release Assistance Information that may have been received in NAS PDU when Control Plane CIoT EPS Optimisation is used, which indicates that downlink data is expected.

The MME upon receiving S1 UE Context Release Request with Cause indicating the release is requested due to a UE using satellite access moved out of PLMN serving area, may detach the UE as described in clause 5.3.8.3 before continuing with the S1 Release procedure.

For Control Plane CIoT EPS Optimisation with data buffering in the MME, step 2 and step 3 are skipped.

2. The MME sends a Release Access Bearers Request (Abnormal Release of Radio Link Indication, Secondary RAT usage data) message to the S‑GW that requests the release of all S1-U bearers for the UE, or the S11-U in Control Plane CIoT EPS Optimisation if buffering is in the S-GW. This message is triggered either by an S1 Release Request message from the eNodeB, or by another MME event. The Abnormal Release of Radio Link Indication is included if the S1 release procedure is due to an abnormal release of the radio link. If secondary RAT usage data was received in Step 1b, the MME includes Secondary RAT usage data in this message.

3. If the S-GW has received a Release Access Bearers Request, the S‑GW releases all eNodeB related information (address and TEIDs), or the MME TEIDs related information in Control Plane CIoT EPS Optimisation (address and TEIDs), for the UE and responds with a Release Access Bearers Response message to the MME. Other elements of the UE’s S‑GW context are not affected. The S‑GW retains the S1-U configuration that the S‑GW allocated for the UE’s bearers. The S‑GW starts buffering downlink packets received for the UE and initiating the "Network Triggered Service Request" procedure, described in clause 5.3.4.3, if downlink packets arrive for the UE. In Control Plane CIoT EPS Optimisation Downlink data triggers Mobile Terminated Data transport in NAS signalling defined in clause 5.3.4B.3.

NOTE 2: Based on operator policy any received Indication of "Abnormal Release of Radio Link" may be used by Serving GW in subsequent decisions to trigger PDN charging pause if the feature has been enabled on that PDN.

4. The MME releases S1 by sending the S1 UE Context Release Command (Cause) message to the eNodeB.

5. If the RRC connection is not already released, the eNodeB sends a RRC Connection Release message to the UE in Acknowledged Mode. Once the message is acknowledged by the UE, the eNodeB deletes the UE’s context.

6. The eNodeB confirms the S1 Release by returning an S1 UE Context Release Complete (ECGI, TAI, Secondary RAT usage data) message to the MME. With this, the signalling connection between the MME and the eNodeB for that UE is released. This step shall be performed promptly after step 4, e.g. it shall not be delayed in situations where the UE does not acknowledge the RRC Connection Release.

If Dual Connectivity was activated by that eNodeB at the time of the release or earlier by that eNodeB, the eNodeB shall include the last known PSCell ID and the time elapsed since the Dual Connectivity was released.

If the eNodeB supports WUS, the eNodeB should include the Information On Recommended Cells And eNodeBs For Paging in the S1 UE Context Release Complete message; otherwise, the eNodeB may include the Information On Recommended Cells And eNodeBs For Paging in the S1 UE Context Release Complete message. If available, the MME shall store this information to be used when paging the UE.

The eNodeB includes Information for Enhanced Coverage, if available, in the S1 UE Context Release Complete message.

If the PLMN has configured secondary RAT usage data reporting, the eNodeB has not included Secondary RAT usage data at step 1b, and the eNodeB has Secondary RAT usage data to report, the Secondary RAT usage data is included in this message. If Secondary RAT usage data was included at step 1b then MME ignores Secondary RAT usage data at step 6.

The MME deletes any eNodeB related information ("eNodeB Address in Use for S1-MME", "MME UE S1 AP ID" and "eNodeB UE S1AP ID") from the UE’s MME context, but, retains the rest of the UE’s MME context including the S‑GW’s S1-U configuration information (address and TEIDs). All non-GBR EPS bearers established for the UE are preserved in the MME and in the Serving GW.

If the cause of S1 release is because of User I inactivity, Inter-RAT Redirection, the MME shall preserve the GBR bearers. If the cause of S1 release is because of CS Fallback triggered, further details about bearer handling are described in TS 23.272 [58]. Otherwise, e.g. Radio Connection With UE Lost, S1 signalling connection lost, eNodeB failure the MME shall trigger the MME Initiated Dedicated Bearer Deactivation procedure (clause 5.4.4.2) for the GBR bearer(s) of the UE after the S1 Release procedure is completed.

NOTE 3: EPC does not support the GPRS preservation feature with setting the MBR for GBR bearers to zero.

NOTE 4: The MME can defer the deactivation of GBR bearers for a short period (in the order of seconds) upon receipt of an S1AP UE Context Release Request due to radio reasons, so as to allow the UE to re-establish the corresponding radio and S1-U bearers and thus avoid deactivation of the GBR bearers.

If LIPA is active for a PDN connection, the HeNB informs the collocated L-GW by internal signalling to releases the direct user plane path to the HeNB. After the direct user plane path is released, if downlink packets arrive for the UE, the L-GW forwards the first packet on the S5 tunnel to the S‑GW to initiate the "Network Triggered Service Request" procedure, as described in clause 5.3.4.3.

7. If the eNodeB provided and MME accepted Secondary RAT usage data in step 6 (i.e. MME initiated S1 release), the MME initiates the Secondary RAT usage data reporting procedure in clause 5.7A.3 as illustrated in figure 5.7A.3-2 (steps 7a – 7d). If PGW secondary RAT usage reporting is active, steps 7b and 7c are performed, otherwise only steps 7a and 7d are performed.

If the eNodeB provided Secondary RAT usage data in step 1b (i.e. eNodeB initiated S1 release) and PGW secondary RAT usage data reporting is active, the MME initiates the Secondary RAT usage data reporting procedure in clause 5.7A.3 as illustrated in figure 5.7A.3-2.