Service Request Procedure | Core Network: Architecture and Signaling Procedures

6.3. Service Request Procedure

6.3.1. UE-Initiated Service Request Procedure

6.3.1.1. Technical Overview

The UE-initiated Service Request procedure lets a UE in ECM-IDLE mode establish a NAS signaling connection with the network, and the UE starts the procedure when it has pending uplink user data or pending uplink signaling.

The procedure establishes the radio bearers and the S1 user-plane bearers for the UE’s active EPS bearers, moving the UE from ECM-IDLE mode to ECM-CONNECTED mode.

The procedure also synchronizes the EPS bearer contexts between the UE and the MME.

6.3.1.2. Call Flow

The call flow has three phases: Service Request, Radio Bearer Setup, and Path Switch and Data Delivery.

Figure 16. UE-initiated service request procedure call flow

6.3.1.3. Step-by-Step Description

The UE sends a Service Request message to the eNodeB, carrying this NAS message inside the RRC connection establishment procedure. The message carries the Key Set Identifier (KSI), the NAS sequence number, and the NAS-MAC.
The eNodeB forwards the NAS message to the MME inside an S1AP Initial UE Message, which establishes the UE-associated logical S1 connection. The message carries the NAS-PDU, the S-TMSI, the TAI and the ECGI of the serving cell, and the RRC establishment cause, for example "mo-Data" or "mo-Signalling".
If the UE’s security context is missing or no longer valid, the MME and the UE perform the Authentication and Security Procedures, carrying the RAND, the AUTN, and the selected NAS security algorithms, and activating NAS integrity protection and ciphering.
The MME sends an S1AP Initial Context Setup Request message to the eNodeB, providing it with the security context and the Serving GW address for the uplink. The message carries the MME UE S1AP ID, the eNB UE S1AP ID, the E-RAB To Be Setup List, with the S1-U S-GW Fully Qualified Tunnel Endpoint Identifier (F-TEID) and the QoS parameters, the UE Aggregate Maximum Bit Rate, and the Security Context.
The eNodeB sets up the radio bearer by sending an RRC Connection Reconfiguration message to the UE, establishing the data radio bearers for the requested E-RABs.
The UE completes the radio reconfiguration and returns an RRC Connection Reconfiguration Complete message to the eNodeB, after which the UE is ready to send user data.
The eNodeB sends an S1AP Initial Context Setup Response message to the MME, providing it with the eNodeB’s downlink tunnel information. The message carries the eNB UE S1AP ID, the MME UE S1AP ID, and the E-RAB Setup List, with the S1-U eNodeB F-TEID for the downlink.
Once the radio bearer setup completes, the eNodeB can forward uplink user-plane packets to the S-GW, using the S-GW F-TEID from step 4.
To update the user-plane path with the eNodeB’s downlink F-TEID, the MME sends a GTPv2 Modify Bearer Request message to the S-GW for each PDN connection, carrying the Bearer Contexts to be modified, that is the EPS Bearer Identity (EBI) and the S1-U eNodeB F-TEID, the User Location Information, and the RAT type.
If the network deploys dynamic PCC, the S-GW forwards the GTPv2 Modify Bearer Request message to the P-GW. The P-GW then starts a Diameter Credit-Control-Request/ Credit-Control-Answer (Update) exchange with the PCRF over the Gx interface, carrying the CC-Request-Type set to UPDATE_REQUEST, the 3GPP-User-Location-Info, and the RAT-Type, before returning a GTPv2 Modify Bearer Response message to the S-GW.
Whether or not the network deployed dynamic PCC, the S-GW returns a GTPv2 Modify Bearer Response message to the MME, carrying a Cause set to "Request Accepted" and the S-GW F-TEID for the control plane.
Using the downlink F-TEID from step 7, the S-GW delivers buffered or newly arrived downlink user data to the eNodeB, which delivers the data to the UE over the radio interface.

6.3.1.4. Possible Failures and Required Actions

Authentication failure: If the UE fails the authentication check that the MME starts during the Service Request procedure, the MME sends a NAS Authentication Reject message, with EMM Cause #3, "Illegal UE", or EMM Cause #6, "Illegal ME". The UE then stops the procedure, considers its Universal Subscriber Identity Module (USIM) or Subscriber Identity Module (SIM) invalid for EPS services, and enters the EMM-DEREGISTERED state.
MME congestion: If the MME is overloaded and cannot process the Service Request, it sends a NAS Service Reject message, with EMM Cause #22, "Congestion", and a value for timer T3346. The UE starts timer T3346 and does not start a new Service Request of the same category until the timer expires.
E-UTRAN resource allocation failure: If the eNodeB cannot admit one or more E-RABs because radio resources are not available, it returns an S1AP Initial Context Setup Failure message, with S1AP Cause "Radio resources not available". The MME maps this internally to GTP Cause #73, "No resources available", and, if it rejects the UE, can map it further to NAS EMM Cause #26, "Insufficient resources". The MME locally deactivates the EPS bearer contexts that failed to establish, while the UE stays in its current state and can start timer T3346.

6.3.1.5. References

Specification	Clauses and coverage
TS 23.401	Clause 5.3.4.1 — Service Request procedure Clause 5.3.4.2 — Abnormal cases Clause 4.3.7.4.2 — Congestion control
TS 24.301	Clause 5.6.1 — Service Request procedure Clause 8.2.24 — Service Reject message content Clause 8.2.25 — Service Request message content Clause 9.9.3.9 — EMM cause
TS 36.413	Clause 8.2.1 — Initial Context Setup Clause 8.6.2.1 — Initial UE Message Clause 9.1.4.1 — Initial Context Setup Request
TS 29.274	Clause 7.2.7 — Modify Bearer Request/Response Clause 8.4 — Cause values
TS 29.213	Clause 4.3.1.1 (PCC rule update procedures).

Specification

Clauses and coverage

TS 23.401

Clause 5.3.4.1 — Service Request procedure
Clause 5.3.4.2 — Abnormal cases
Clause 4.3.7.4.2 — Congestion control

TS 24.301

Clause 5.6.1 — Service Request procedure
Clause 8.2.24 — Service Reject message content
Clause 8.2.25 — Service Request message content
Clause 9.9.3.9 — EMM cause

TS 36.413

Clause 8.2.1 — Initial Context Setup
Clause 8.6.2.1 — Initial UE Message
Clause 9.1.4.1 — Initial Context Setup Request

TS 29.274

Clause 7.2.7 — Modify Bearer Request/Response
Clause 8.4 — Cause values

TS 29.213

Clause 4.3.1.1 (PCC rule update procedures).

6.3.2. Network-Triggered Service Request Procedure

6.3.2.1. Technical Overview

The Network-Triggered Service Request procedure moves a UE from ECM-IDLE mode to ECM-CONNECTED mode when the EPC has downlink user data or downlink signaling to deliver. The S-GW starts the procedure when it receives a downlink packet for a UE that has no active S1 user-plane connection.

The procedure establishes the radio bearers and the S1 bearers for the UE, synchronizes the EPS bearer contexts between the UE and the MME, and delivers the buffered downlink data to the UE.

6.3.2.2. Call Flow

The call flow has four phases: Downlink Data Notification, Paging and Service Request, Radio Bearer Setup, and Path Switch and Data Delivery.

Figure 17. Network-triggered service request procedure call flow

6.3.2.3. Step-by-Step Description

The P-GW forwards a downlink PDU to the S-GW over the S5/S8 interface. Because the UE is in ECM-IDLE mode, the S-GW has no S1-U tunnel, that is no eNodeB F-TEID, for this UE, and it buffers the incoming data.
The S-GW identifies the MME that serves the UE and sends it a GTPv2 Downlink Data Notification (DDN) message to start the paging process. The message carries the ARP, derived from the bearer on which the data arrived, and the EBI.
The MME acknowledges the DDN message by returning a GTPv2 Downlink Data Notification Acknowledge message to the S-GW, carrying a Cause set to "Request Accepted" to confirm that the MME attempts to reach the UE.
The MME sends an S1AP Paging message to every eNodeB in the UE’s registered Tracking Area List. The message carries 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.
During the UE’s paging occasion, the eNodeB broadcasts the paging indication over the radio interface by sending an RRC Paging message to the UE.
The UE matches its S-TMSI in the Paging message and starts the RRC connection establishment procedure, sending a 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.
The eNodeB forwards the NAS message to the MME inside an S1AP Initial UE Message.
If the UE’s security context is missing or no longer valid, the MME and the UE perform the Authentication and Security Procedures, carrying the RAND, the AUTN, and the selected NAS security algorithms.
The MME sends an S1AP Initial Context Setup Request message to the eNodeB to establish the S1-U connection and the radio bearers, carrying the MME UE S1AP ID, the E-RAB To Be Setup List, with the S-GW F-TEIDs for the uplink, the UE Aggregate Maximum Bit Rate, and the Security Context.
The eNodeB sends an RRC Connection Reconfiguration message to the UE to activate the data radio bearers for the requested EPS bearers, allocating the radio resources for the user-plane traffic.
The eNodeB confirms the successful setup of the radio and S1 bearers by returning an S1AP Initial Context Setup Response message to the MME, carrying the eNB UE S1AP ID, the MME UE S1AP ID, and the E-RAB Setup List, with the eNodeB F-TEIDs for the downlink.
To provide the eNodeB’s downlink F-TEID and show that the UE is reachable, the MME sends a GTPv2 Modify Bearer Request message to the S-GW for each PDN connection. The message carries the Bearer Contexts to be modified, that is the EBI and the S1-U eNodeB F-TEID for the downlink, the User Location Information, and the RAT type.
If the network deploys dynamic PCC, for example for location reporting or RAT change reporting, the S-GW forwards the GTPv2 Modify Bearer Request message to the P-GW. The P-GW then starts a Diameter Credit-Control-Request/ Credit-Control-Answer (Update) exchange with the PCRF over the Gx interface, carrying the CC-Request-Type set to UPDATE_REQUEST, the 3GPP-User-Location-Info, and the RAT-Type, before returning a GTPv2 Modify Bearer Response message to the S-GW.
Whether or not the network deployed dynamic PCC, the S-GW updates its bearer contexts and returns a GTPv2 Modify Bearer Response message to the MME, carrying a Cause set to "Request Accepted" and the S-GW F-TEID for the control plane.
Using the downlink F-TEID from step 11, the S-GW transmits the buffered downlink packets to the eNodeB, which delivers the data to the UE over the radio interface.

6.3.2.4. Possible Failures and Required Actions

Paging failure, UE not responding: If the MME completes its paging repetition strategy and receives no Service Request from the UE, it sends a GTPv2 Downlink Data Notification Reject message to the S-GW, with Cause #87, "UE not responding", and the S-GW deletes all buffered packets for the UE. Based on operator policy, the MME can also start a Dedicated Bearer Deactivation procedure for any preserved Guaranteed Bit Rate (GBR) bearers.
Resource allocation rejection by eNodeB: If the eNodeB cannot allocate radio resources for some or all bearers during the Initial Context Setup because of congestion, it returns an S1AP Initial Context Setup Failure message, with S1AP Cause "Radio resources not available". The MME maps this internally to GTP Cause #73, "No resources available", and, if it rejects the UE, can map it further to NAS EMM Cause #26, "Insufficient resources". The MME can locally deactivate the EPS bearer contexts that failed to establish, while the UE stays in its current state and the NAS layer can start back-off timer T3346.
MME-triggered congestion control: If the MME receives a Service Request in response to paging but is too overloaded to establish the user-plane radio bearers, it sends a NAS Service Reject message, with EMM Cause #22, "Congestion", and a value for timer T3346. The UE aborts the procedure, enters the EMM-REGISTERED state, and starts timer T3346, while the MME notifies the S-GW of the failure so that the S-GW discards the buffered data.

6.3.2.5. References

Specification	Clauses and coverage
TS 23.401	Clause 5.3.4.1 — Service Request procedure Clause 5.3.4.3 — Network-triggered Service Request Clause 5.3.4.2 — Abnormal cases
TS 24.301	Clause 5.6.1 — Service Request procedure Clause 5.6.2 — Paging procedure Clause 8.2.24 — Service Reject message content Clause 9.9.3.9 — EMM cause
TS 29.274	Clause 7.2.7 — Modify Bearer Request/Response Clause 7.2.11 — Downlink Data Notification Table 8.4-1 — Cause values Annex C — Cause mapping
TS 29.272	Annex A (Mapping between S6a and NAS causes).
TS 36.413	Clause 8.2.1 — Initial Context Setup Clause 9.1.6 — Paging Clause 9.2.1.3 — S1AP Cause values