Core Network: Architecture and Signaling Procedures v2.2.0 PDF

6.5. Bearer Modification Procedure

6.5.1. P-GW-Initiated Bearer Modification Procedure

6.5.1.1. Technical Overview

The P-GW-initiated Bearer Modification procedure updates the parameters of an active default or dedicated EPS bearer, and the P-GW starts the procedure.

The procedure changes the QoS of the bearer, that is the QCI, the GBR, the MBR, and the ARP, and it can also update the TFT, the APN-AMBR, and the PCO.

The procedure keeps the radio and core network resources aligned with the PCC rules that the PCRF authorizes, reconfiguring the DRB at the UE and updating the EPS bearer contexts at the P-GW, the S-GW, and the MME.

6.5.1.2. Call Flow

The call flow has four phases: Policy Decision, Bearer Modification Request, Radio Bearer Modification, and Bearer Modification Acknowledgement.

P-GW-initiated bearer modification procedure call flow
Figure 19. P-GW-initiated bearer modification procedure call flow
6.5.1.3. Step-by-Step Description

  1. If the network deploys dynamic PCC, the PCRF starts the procedure by sending a Diameter Re-Auth-Request (RAR) message to the P-GW over the Gx interface, to provision new PCC rules, and the P-GW returns a Diameter Re-Auth-Answer (RAA) message to acknowledge the new policy decision.

  2. The P-GW derives the new EPS Bearer QoS from the PCC rules and sends a GTPv2 Update Bearer Request message to the S-GW, carrying the PTI, the EBI, the EPS Bearer QoS, the APN-AMBR, and the TFT.

  3. The S-GW forwards the GTPv2 Update Bearer Request message to the MME, without changes.

  4. If the UE is in ECM-IDLE mode, the MME pages the UE, unless the procedure only changes the ARP, by sending an S1AP Paging message to the eNodeB, which pages the UE over the radio interface with an RRC Paging message. The UE responds with an RRC Service Request message to the eNodeB, and the eNodeB forwards the request to the MME inside an S1AP Initial UE Message.

  5. The MME builds a NAS Modify EPS Bearer Context Request message and sends it to the eNodeB inside an S1AP E-RAB Modify Request message, carrying the E-RAB ID, the E-RAB Level QoS Parameters, the NAS-PDU, and, if the APN-AMBR changed, the UE-AMBR.

  6. The eNodeB maps the new EPS Bearer QoS to the Radio Bearer QoS and sends an RRC Connection Reconfiguration message to the UE, carrying the Radio Bearer QoS and the Session Management Request, that is the NAS Modify EPS Bearer Context Request message.

  7. The UE reconfigures its radio resources and returns an RRC Connection Reconfiguration Complete message to the eNodeB.

  8. The eNodeB confirms the successful radio bearer modification by returning an S1AP E-RAB Modify Response message to the MME, carrying the E-RAB ID and, if the network uses Dual Connectivity, the PSCell ID.

  9. The UE accepts the new EPS bearer context by sending a NAS Modify EPS Bearer Context Accept message, inside an RRC Direct Transfer message, to the eNodeB, which forwards it to the MME inside an S1AP Uplink NAS Transport message.

  10. When the MME receives both the S1AP and the NAS acknowledgements, it sends a GTPv2 Update Bearer Response message to the S-GW, carrying the EBI, a Cause set to "Request Accepted", the ULI, that is the ECGI, and the PSCell ID.

  11. The S-GW forwards the GTPv2 Update Bearer Response message to the P-GW over the S5/S8 interface, carrying the EBI and the ULI.

  12. If the network deploys dynamic PCC, the P-GW confirms the enforcement of the PCC rules to the PCRF by sending a Diameter Credit-Control-Request (CCR) message to provide the provisioning acknowledgement, and the PCRF returns a Diameter Credit-Control-Answer (CCA) message, concluding the IP-CAN Session Modification procedure.

6.5.1.4. Possible Failures and Required Actions
Radio resource allocation rejection by eNodeB

If the eNodeB cannot meet the requested QoS, for example because of congestion or a lack of GBR capacity, it returns an S1AP E-RAB Modify Response message, with the bearer in the E-RAB Failed to Modify List and S1AP Cause #20, "Radio resources not available". The MME reports the failure to the S-GW and the P-GW with a GTPv2 Update Bearer Response message, with GTP Cause #89, "Service denied", and the P-GW informs the PCRF that the network cannot enforce the PCC decision.

NAS syntactical error in the TFT

If the UE’s NAS layer finds a syntax error in the packet filters of the modification request, the UE returns a NAS Modify EPS Bearer Context Reject message to the MME, with ESM Cause #45, "syntactical error in the TFT operation". The MME stops timer T3486 and sends a GTPv2 Update Bearer Response message to the S-GW, with GTP Cause #77, "Syntactic errors in packet filter(s)", and, if it detects a mismatch, can start a procedure to re-align the bearer context.

UE temporarily not reachable

When the MME tries to modify a bearer for a UE in ECM-IDLE mode with Extended Discontinuous Reception (eDRX) active, the UE can fail to respond to paging before the paging timer expires. In this case, the MME sends a GTPv2 Update Bearer Response message to the S-GW, with GTP Cause #123, "UE is temporarily not reachable due to power saving", and sets the "Pending Network Initiated PDN Connection Signalling" flag for the UE. The P-GW retries the procedure after it receives an indication that the UE is reachable, for example in a later Modify Bearer Request message.

6.5.1.5. References
Specification Clauses and coverage

TS 23.401

  • Clause 5.4.2.1 — PGW-initiated bearer modification without bearer QoS update

  • Clause 5.4.3 — Bearer modification with bearer QoS update

  • Figures 5.4.2.1-1 and 5.4.3-1

TS 24.301

  • Clause 6.4.3 — Modification of EPS bearer context

  • Clause 8.3.18 — Modify EPS Bearer Context Request/Accept/Reject message content

  • Table 9.9.4.4 — ESM cause values

  • Annex B

TS 29.274

  • Clauses 7.2.15 and 7.2.16 — Update Bearer Request/Response

  • Clause 8.4 — Cause values

  • Tables 7.2.15-1 and 8.4-1

  • Annex C — Cause mapping

TS 29.272

Annex A (Mapping between S6a, Gx, and NAS/S1AP cause values).

TS 36.413

  • Clause 8.2.2 — E-RAB Modify procedure

  • Clause 9.1.3.3 — E-RAB Modify Response

  • Clause 9.2.1.3 — S1AP Cause values

TS 29.213

Clause 4.3.2 (IP-CAN Session Modification).

6.5.2. HSS-Initiated Subscribed QoS Modification Procedure

6.5.2.1. Technical Overview

The HSS-initiated Subscribed QoS Modification procedure updates the QoS of an active EPS bearer after a change in the subscriber’s subscription data at the HSS, and the HSS starts the procedure.

The procedure changes the Subscribed UE Aggregate Maximum Bit Rate (Subscribed-UE-AMBR), the Subscribed APN-AMBR, or the QCI and the ARP of the default bearer.

The procedure synchronizes the UE’s radio and session management contexts with the new subscription parameters across the MME, the S-GW, and the P-GW, keeping the bearer compliant with the PCC rules.

6.5.2.2. Call Flow

The call flow has three phases: Subscription Update, Bearer Modification Command, and, if the QCI, the ARP, or the APN-AMBR changed, Bearer Modification Request and Bearer Modification Acknowledgement.

HSS-initiated subscribed QoS modification procedure call flow
Figure 20. HSS-initiated subscribed QoS modification procedure call flow
6.5.2.3. Step-by-Step Description

  1. After an administrative change to the subscription, the HSS starts the procedure by sending a Diameter Insert-Subscriber-Data-Request (IDR) message to the MME, carrying the IMSI and the Subscription-Data, that is the new Subscribed-UE-AMBR, APN-AMBR, QCI, or ARP.

  2. The MME updates its local UE context with the new subscription data and returns a Diameter Insert-Subscriber-Data-Answer (IDA) message to the HSS, carrying a Result-Code set to DIAMETER_SUCCESS.

  3. If the new subscription data affects an active PDN connection, for example a changed QCI or APN-AMBR, the MME sends a GTPv2 Modify Bearer Command message to the S-GW, carrying the EBI of the default bearer, the Bearer Level QoS, and the APN-AMBR.

  4. The S-GW forwards the GTPv2 Modify Bearer Command message to the P-GW, carrying the EBI, the Bearer Level QoS, and the APN-AMBR.

  5. If the network deploys dynamic PCC, the P-GW reports the updated subscribed QoS to the PCRF by sending a Diameter Credit-Control-Request (CCR) message over the Gx interface, with the CC-Request-Type set to UPDATE_REQUEST, carrying the Subscription-Id, the QoS-Information, and the APN-Aggregate-Max-Bitrate.

  6. The PCRF authorizes the modification, adjusting the parameters based on local policy if needed, and returns a Diameter Credit-Control-Answer (CCA) message to the P-GW, carrying a Result-Code, the QoS-Information, and the APN-Aggregate-Max-Bitrate.

  7. To update the bearer context, the P-GW sends a GTPv2 Update Bearer Request message to the S-GW, carrying the EBI, the Bearer Level QoS, the TFT, and the APN-AMBR.

  8. The S-GW forwards the GTPv2 Update Bearer Request message to the MME, without changes.

  9. The MME builds a NAS Modify EPS Bearer Context Request message and sends it to the eNodeB inside an S1AP E-RAB Modify Request message, carrying the E-RAB ID, the E-RAB Level QoS Parameters, the NAS-PDU, and the UE-AMBR.

  10. The eNodeB reconfigures the radio bearer by sending an RRC Connection Reconfiguration message to the UE, carrying the Radio Bearer QoS and the Session Management Request.

  11. The UE confirms the successful reconfiguration of its radio resources by returning an RRC Connection Reconfiguration Complete message to the eNodeB.

  12. The eNodeB informs the MME of the outcome of the resource modification by returning an S1AP E-RAB Modify Response message to the MME, carrying the E-RAB ID and a Cause set to "Request Accepted".

  13. The UE accepts the new EPS bearer context by sending a NAS Modify EPS Bearer Context Accept message, carrying the EBI, inside an RRC Direct Transfer message, to the eNodeB, which forwards it to the MME inside an S1AP Uplink NAS Transport message.

  14. When the MME receives both the eNodeB and the UE acknowledgements, it sends a GTPv2 Update Bearer Response message to the S-GW, carrying the EBI, a Cause set to "Request Accepted", and the ULI.

  15. The S-GW forwards the GTPv2 Update Bearer Response message to the P-GW, carrying the EBI, the Cause, and the ULI.

  16. If the network deploys dynamic PCC, the P-GW confirms the enforcement of the authorized QoS to the PCRF by sending a Diameter Credit-Control-Request (CCR) message, with the Event-Trigger set to SUCCESSFUL_RESOURCE_ALLOCATION, and the PCRF returns a Diameter Credit-Control-Answer (CCA) message to acknowledge the final status.

6.5.2.4. Possible Failures and Required Actions
HSS registration rejection

If the MME sends an Update Location for a subscriber unknown to the HSS, the HSS returns a Diameter Result-Code DIAMETER_ERROR_USER_UNKNOWN (5001). When the HSS returns this error during an Update Location, the MME rejects the UE’s Attach or Tracking Area Update request, with EMM Cause #2, "IMSI unknown in HSS", whereas when it returns this error during an Insert Subscriber Data procedure for an active UE, the MME locally detaches the UE and removes its context.

Radio resource allocation rejection by eNodeB

If the eNodeB cannot provide the requested GBR or QCI because of congestion, it returns an S1AP Cause #20, "Radio resources not available", or a cause of "Not supported QCI Value". The MME maps this to GTPv2 Cause #89, "Service denied", or GTPv2 Cause #90, "No resources available", in the Update Bearer Response message to the P-GW, which informs the PCRF that the modification failed and, if the modification is mandatory, can delete the affected bearer.

NAS syntactical error in the TFT

If the UE’s NAS layer finds a syntax error in the packet filters of the modification request, the UE returns a NAS Modify EPS Bearer Context Reject message, with ESM Cause #45, "syntactical error in the TFT operation", which maps to GTPv2 Cause #77, "Syntactic errors in packet filter(s)". The MME stops timer T3486, aborts the procedure, and can start a procedure to re-align the bearer context between the EPC and the UE.

6.5.2.5. References
Specification Clauses and coverage

TS 23.401

  • Clause 5.3.9.2 — Subscribed QoS modification

  • Clause 5.4.2.1 — PGW-initiated bearer modification without bearer QoS update

  • Clause 5.4.2.2 — Bearer modification with bearer QoS update procedures using GTP

  • Figure 5.4.2.2-1

TS 24.301

  • Clauses 6.4.3.2, 6.4.3.3, and 6.4.3.4 — Network-requested EPS bearer context modification

  • Table 9.9.4.4 — ESM cause values

TS 29.272

  • Clause 5.2.2.1 — Insert Subscriber Data procedure

  • Clause 7.2.9 — IDR/IDA commands

  • Tables 5.2.2.1.1/1 and A.1

TS 29.274

  • Clause 7.2.14 — Modify Bearer Command

  • Clauses 7.2.15 and 7.2.16 — Update Bearer Request/Response

  • Tables 7.2.14.1-1 and 8.4-1

TS 36.413

  • Clause 8.2.2 — E-RAB Modify procedure

  • Clause 9.1.3.3 — E-RAB Modify Response

  • Clause 9.2.1.3 — S1AP Cause values

TS 29.213

  • Clause 4.3.2 — IP-CAN Session Modification

  • Clause 7.4.1.2.2.2 — QoS negotiation at Subscribed QoS modification

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