Core Network: Architecture and Signaling Procedures v2.2.0 PDF

6.4. Dedicated Bearer Activation Procedure

6.4.1. Technical Overview

The network starts the Dedicated Bearer Activation procedure to establish an additional EPS bearer for an IP-CAN session. A dedicated bearer never exists on its own: the network always links it, through the LBI, to the default bearer that the UE already holds for that IP-CAN session. The new bearer has specific QoS characteristics, for example a GBR or a non-GBR profile, together with a TFT.

PCC usually triggers the procedure when the PCRF detects a service data flow (SDF) that needs prioritized treatment, for example IMS Voice over LTE (VoLTE) voice media with the QCI set to 1.

The procedure establishes end-to-end user-plane resources that map the traffic matching the packet filters of the TFT to dedicated radio tunnels and dedicated network tunnels. If the UE is in ECM-CONNECTED mode, the network establishes these resources immediately over the existing S1 signaling connection. If the UE is in ECM-IDLE mode, the MME first pages the UE and waits for a NAS Service Request message, because the eNodeB needs an active S1 and RRC connection before it can set up the new Data Radio Bearer (DRB). The procedure has these high-level effects:

On the UE

The UE establishes a new DRB, associates the DRB with the assigned EBI, and stores the uplink packet filters in its TFT to enforce the correct uplink bearer binding.

On the EPC

The nodes establish logical GTP-U tunnels: one tunnel connects the eNodeB and the S-GW over the S1-U interface, while the other connects the S-GW and the P-GW over the S5/S8 interface. The user-plane nodes, that is the PGW-U and the SGW-U, allocate new Fully Qualified Tunnel Endpoint Identifiers (F-TEIDs) to route the downlink data and the uplink data. All nodes obey the authorized QoS parameters, that is the QCI, the ARP, the GBR, and the Maximum Bit Rate (MBR).

6.4.2. Call Flow

The call flow has six phases: Gx Policy Provisioning, GTPv2 Bearer Resource Allocation, Paging and Service Request, E-UTRAN Resource Establishment and NAS Signaling, GTPv2 Tunnel Confirmation, and Gx Enforcement Reporting.

Dedicated bearer activation procedure call flow
Figure 18. Dedicated bearer activation procedure call flow

6.4.3. Step-by-Step Description

  1. If the network deploys dynamic PCC, the PCRF starts the IP-CAN session modification by sending a Diameter Re-Auth-Request (RAR) message over the Gx interface to the PCEF, which is collocated in the P-GW. The message installs new PCC rules and provisions the authorized QoS profile and the packet filters for the SDF. The message carries these Attribute-Value Pairs (AVPs): the Session-Id, the Auth-Application-Id, set to the vendor-specific Gx Application ID 16777238, the Re-Auth-Request-Type, set to AUTHORIZE_ONLY, and the Event-Trigger, set to request user location reporting, if the PCRF needs this report. The message also carries the Charging-Rule-Install AVP, which encapsulates the Charging-Rule-Definition AVP containing the Charging-Rule-Name, the Flow-Information with the IP 5-tuple filters, and the QoS-Information that defines the target QCI, the ARP, the GBR, and the MBR.

  2. The P-GW acknowledges the policy provisioning request by returning a Diameter Re-Auth-Answer (RAA) message to the PCRF. This message confirms that the PCEF received the PCC rule installation command and will continue with the bearer mapping and with the access network signaling. The message carries the Session-Id, the Origin-Host, the Origin-Realm, and the Result-Code, set to DIAMETER_SUCCESS. This result code shows that the transaction routing and the command parsing were successful.

  3. The P-GW maps the received PCC rules to a dedicated bearer context and sends a GTPv2-C Create Bearer Request message to the S-GW over the S5/S8 control-plane interface to start the tunnel allocation. The message establishes the downlink S5/S8-U user-plane path at the S-GW and carries these IEs: the IMSI, the Procedure Transaction Identity (PTI), set to "no procedure transaction identity assigned" unless the P-GW correlates a UE-requested modification, the LBI, which identifies the associated default bearer, and the Protocol Configuration Options (PCO), which carry application-level parameters. The message also carries the Bearer Contexts grouped IE, which contains the EPS Bearer ID, set to 0 for a new allocation, the Bearer TFT with the packet filters, the S5/8-U P-GW F-TEID, which contains the uplink IP address and the uplink TEID for user data that the PGW-U allocates, the Bearer Level QoS, which defines the QCI, the ARP, the GBR, and the MBR, and the Charging Id.

  4. After processing the request of the P-GW, the S-GW reserves local downlink user-plane resources on the S5/S8-U interface and forwards the GTPv2 Create Bearer Request message to the MME over the S11 interface. The message carries the IMSI, the LBI, the PTI, the PCO, and the Bearer Contexts grouped IE, which contains the EPS Bearer ID, set to 0, the Bearer TFT, the S1-U S-GW F-TEID, which contains the SGW-U IP address and the downlink TEID that the S-GW allocates for downlink user-plane data, the S5/8-U P-GW F-TEID, and the Bearer Level QoS.

  5. If the UE is in ECM-IDLE mode when the MME receives the Create Bearer Request, the MME buffers the request and pages the UE before it continues the dedicated bearer signaling. The MME sends an S1AP Paging message to every eNodeB in the UE’s registered Tracking Area List, carrying the UE Identity Index Value, for calculating the paging frame, the UE Paging ID, that is the S-TMSI, the Paging DRX cycle, and the List of TAIs.

  6. The eNodeB broadcasts the paging indication over the radio interface, during the UE’s paging occasion. The eNodeB sends an RRC Paging message to the UE.

  7. The UE matches its S-TMSI in the Paging message and starts the RRC connection establishment procedure, sending a NAS Service Request message to the eNodeB inside this procedure. The message carries the Protocol Discriminator, the NAS KSI, the NAS sequence number, and the NAS-MAC.

  8. The eNodeB forwards the NAS message to the MME inside an S1AP Initial UE Message, which completes the UE’s transition to ECM-CONNECTED mode.

  9. The UE’s security context can be missing or no longer valid. In this case, the MME and the UE perform the Authentication and Security Mode Control procedures, carrying the RAND, the AUTN, and the selected NAS security algorithms, before the MME resumes the buffered dedicated bearer signaling.

  10. The MME selects an unassigned EBI from its pool, that is a value from 5 to 15 because the network reserves values 0 to 4, and builds an S1AP E-RAB Setup Request message, which the MME sends to the eNodeB to trigger the resource allocation at the eNodeB and to encapsulate the NAS message for the UE. The MME starts the session management guard timer T3485 when it sends this message. The message requests the eNodeB to assign Uu radio resources and S1-U uplink transport resources, and carries the Message Type, the MME UE S1AP ID, the eNB UE S1AP ID, the UE Aggregate Maximum Bit Rate (UE-AMBR), and the E-RAB to Be Setup List grouped IE. Each item of this list contains the allocated E-RAB ID, that is the EBI, the E-RAB Level QoS Parameters, that is the QCI, the ARP, and the GBR and MBR limits, the Transport Layer Address, that is the S1-U S-GW IP address, the GTP-TEID, that is the S1-U S-GW downlink TEID for the user plane, and a NAS-PDU that encapsulates the NAS Activate Dedicated EPS Bearer Context Request message.

  11. After receiving the S1AP E-RAB Setup Request message, the eNodeB maps the QoS parameters to the Radio Bearer QoS and sends an RRC Connection Reconfiguration message to the UE over the Uu air interface. The message establishes a new DRB and configures the physical layer parameters, the Medium Access Control (MAC) layer parameters, and the Radio Link Control (RLC) layer parameters. The message carries the rrc-TransactionIdentifier and the radioResourceConfigDedicated container, which includes the drb-ToAddModList with the assigned DRB identity, the EBI, the rlc-Config, and the logicalChannelConfig. The message also carries the dedicatedInfoNASList, which contains the NAS Activate Dedicated EPS Bearer Context Request message. This NAS sub-layer message carries the EPS bearer identity, the LBI, the PTI, the EPS QoS, which excludes the ARP parameter, the TFT, and the PCO.

  12. The UE processes the reconfiguration parameters, activates the dedicated DRB, binds the DRB to the new dedicated ESM bearer context, and returns an RRC Connection Reconfiguration Complete message to the eNodeB over the air interface. This message confirms that the UE reconfigured its radio parameters and established the new radio bearer, and carries the rrc-TransactionIdentifier and the rrcConnectionReconfigurationComplete-r8 field.

  13. The eNodeB sends an S1AP E-RAB Setup Response message to the MME over the S1-MME interface to confirm the successful access-stratum bearer establishment, delivering the uplink S1-U transport path information that the eNodeB allocates. The message carries the Message Type, the MME UE S1AP ID, the eNB UE S1AP ID, and the E-RAB Setup List grouped IE, which contains the E-RAB ID, the Transport Layer Address, that is the eNodeB downlink user-plane IP address, and the downlink GTP-TEID, that is the eNodeB downlink TEID for the user plane. If the network uses Dual Connectivity, the message also carries the ID of the Primary SCG Cell (PSCell), that is the primary cell of the Secondary Cell Group (SCG).

  14. At the same time as the radio acknowledgement, or immediately after it, the NAS layer of the UE builds the ESM response and encapsulates this response in an RRC Direct Transfer message, which the UE sends to the eNodeB over the control-plane signaling connection. The message delivers the NAS ESM session acceptance and carries the rrc-TransactionIdentifier and the dedicatedInfoNASList, which contains the NAS Activate Dedicated EPS Bearer Context Accept message. This NAS message includes the Protocol discriminator, the EPS bearer identity, the PTI, and optional PCO elements.

  15. The eNodeB extracts the encapsulated NAS-PDU from the RRC message and forwards the NAS-PDU without changes to the MME, inside an S1AP Uplink NAS Transport message that completes the control-plane ESM signaling link between the UE and the MME. The message carries the Message Type, the MME UE S1AP ID, the eNB UE S1AP ID, the NAS-PDU with the Activate Dedicated EPS Bearer Context Accept message, the ECGI, and the TAI of the serving cell.

  16. When the MME receives both the S1AP E-RAB Setup Response message of step 13 and the Uplink NAS Transport message with the NAS Accept of step 15, the MME stops timer T3485, moves the bearer context to the BEARER CONTEXT ACTIVE state, and acknowledges the bearer activation by sending a GTPv2 Create Bearer Response message to the S-GW over the S11 interface. The message transmits the downlink transport parameters of the eNodeB to the S-GW and carries the Bearer Contexts grouped IE, which contains the EPS Bearer ID, the Cause, set to "Request accepted" to show success, the F-TEID, that is the eNodeB downlink IP address and the downlink user-plane S1-U TEID, and the User Location Information (ULI) with the ECGI and the TAI.

  17. The S-GW matches the downlink transport parameters from the MME with its S5/S8 user-plane parameters and sends a GTPv2 Create Bearer Response message to the P-GW over the S5/S8 control-plane interface, which completes the end-to-end user-plane tunnel establishment. The message carries the Bearer Contexts grouped IE, which contains the EPS Bearer ID, the Cause, the S5/8-U S-GW F-TEID, that is the downlink IP address and the downlink user-plane S5/S8-U TEID that the SGW-U allocates, the ULI with the ECGI and the TAI, and, optionally, the Secondary RAT Usage Data Report.

  18. If a policy provisioning step triggered the procedure, the P-GW, in its PCEF role, reports the enforcement outcome of the installed PCC rules to the PCRF by sending a Diameter Credit-Control-Request (CCR) message with the CC-Request-Type set to UPDATE_REQUEST. The message confirms that the access network activated the requested QoS policy, and carries the Session-Id, the CC-Request-Type, set to UPDATE_REQUEST, the CC-Request-Number, the Event-Trigger, set to SUCCESSFUL_RESOURCE_ALLOCATION, and the Charging-Rule-Report, which contains the Charging-Rule-Name and the PCC-Rule-Status, set to ACTIVE. The message also carries the Access-Network-Charging-Identifier-Gx, which carries the charging ID of the dedicated bearer to bind the bearer to the charging records, and the 3GPP-User-Location-Info.

  19. The PCRF acknowledges the rule activation report and closes the dynamic policy modification transaction by sending a Diameter Credit-Control-Answer (CCA) message to the P-GW. This message completes the PCRF-initiated IP-CAN Session Modification procedure and permits the start of the user-plane traffic forwarding. The message carries the Session-Id, the Origin-Host, the Origin-Realm, and the Result-Code, set to DIAMETER_SUCCESS.

6.4.4. Possible Failures and Required Actions

Radio resource allocation rejection by the eNodeB

The eNodeB can lack the cell capacity or the GBR hardware channels to admit the QoS that the E-RAB Setup Request demands, for example the guaranteed bit rate of a QCI 1 VoLTE bearer during a busy hour. In this case, the eNodeB returns an S1AP E-RAB Setup Response message that lists the bearer in the E-RAB Failed to Setup List, with S1AP Cause #20, "Radio resources not available". The MME stops timer T3485, leaves the bearer context in the BEARER CONTEXT INACTIVE state, and reports the failure to the S-GW with a GTPv2 Create Bearer Response message that carries GTP Cause #89, "Service denied". The S-GW relays this response to the P-GW, which, if the network deploys dynamic PCC, reports the enforcement failure to the PCRF over Gx with a Charging-Rule-Report that carries a Rule-Failure-Code.

TFT validation failure at the UE

The UE’s NAS layer can find a semantic or a syntactical error in the packet filters of the received TFT, for example a filter that conflicts with an existing precedence value. In this case, the UE returns a NAS Activate Dedicated EPS Bearer Context Reject message, inside an S1AP Uplink NAS Transport message, with ESM Cause #45, "syntactical errors in packet filter(s)", or ESM Cause #41, "semantic error in the TFT operation". The MME stops timer T3485, releases the radio resources that the eNodeB already reserved with an S1AP E-RAB Release Command message, and reports the rejection to the S-GW with a GTPv2 Create Bearer Response message that carries the matching GTP Cause, #77, "Syntactic errors in packet filter(s)", or #74, "Semantic error in the TFT operation". The bearer context stays in the BEARER CONTEXT INACTIVE state at the UE and at the network.

Paging timeout

The UE can be unreachable, for example because it is out of coverage or has a depleted battery, and does not respond to the S1AP Paging message before the MME’s paging repetition strategy completes. In this case, the MME cancels the buffered dedicated bearer signaling and sends a GTPv2 Create Bearer Response message to the S-GW with GTP Cause #87, "UE not responding", or, when extended idle-mode DRX applies, 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 dedicated bearer activation after it learns that the UE is reachable again, for example from a later Modify Bearer Request message.

6.4.5. References

Specification Clauses and coverage

TS 23.401 (v18.5.0)

  • Clause 5.4.1 — Dedicated bearer activation, and Figure 5.4.1-1

  • Clauses 4.7.2 and 4.7.3 — The EPS bearer with GTP-based S5/S8, and bearer level QoS parameters

  • Clause 5.3.4.3 — Network-triggered Service Request, for the paging case

TS 24.301 (v18.6.0)

  • Clause 6.4.2 — Dedicated EPS bearer context activation procedure, and Figure 6.4.2.2.1

  • Clauses 8.3.1 and 8.3.3 — Activate dedicated EPS bearer context accept and Activate dedicated EPS bearer context request message definitions

  • Clause 5.6.2 — Paging procedure

  • Table 9.9.4.4 — ESM cause values

TS 29.274 (v18.6.0)

  • Clause 7.2.3 — Create Bearer Request, and Tables 7.2.3-1 and 7.2.3-2

  • Clause 7.2.4 — Create Bearer Response

  • Table 8.4-1 — Cause values

TS 29.212 (v18.0.0)

  • Clause 4.5.2 — PCC Rule Provisioning

  • Clauses 5.6.1 and 5.6.2 — Gx Application messages, including the RAR/RAA and CCR/CCA message syntax

TS 36.413 (v18.1.0)

  • Clause 8.2 — E-RAB Management Procedures, that is E-RAB Setup Request and E-RAB Setup Response

  • Clauses 9.1.3.1 and 9.1.3.2 — E-RAB SETUP REQUEST and E-RAB SETUP RESPONSE message content tables

  • Clause 9.1.6 — Paging

  • Clause 9.2.1.3 — S1AP Cause values

TS 24.007

Clause 11.2.3.1.5 (EPS bearer identity value range).

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