7.3a CIoT signalling reduction optimisations

36.3003GPPEvolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN)Overall descriptionRelease 17Stage 2TS

7.3a.1 General

Which solution of CIoT signalling reduction optimisations to be used is configured over NAS signalling between the UE and the MME or the AMF.

For NB-IoT, PDCP is not used while AS security is not activated.

7.3a.2 Control Plane CIoT EPS/5GS optimisation

The RRC connection established for Control Plane CIoT EPS optimisation, as defined in TS 24.301 [20], and Control Plane CIoT 5GS Optimisation, as defined in TS 24.501 [91], are characterized as below:

– A UL NAS signalling message or UL NAS message carrying data can be transmitted in a UL RRC container message (see Figure 7.3a.2-1). A DL NAS signaling or DL NAS data can be transmitted in a DL RRC container message;

– for NB-IoT:

– RRC connection reconfiguration is not supported;

– Data radio bearer (DRB) is not used;

– AS security is not used;

– A non-anchor carrier can be configured for all unicast transmissions during RRC connection establishment or re-establishment.

– There is no differentiation between the different data types (i.e. IP, non-IP or SMS) in the AS.

Figure 7.3a.2-1: The RRC connection established for Control Plane CIoT EPS/5GS Optimisations

7.3a.3 User Plane CIoT EPS/5GS optimisations

The RRC connection established for User Plane CIoT EPS Optimisation, as defined in TS 24.301 [20], and User Plane CIoT 5GS Optimisation, as defined in TS 24.501 [91], are characterized as below:

– A RRC connection suspend procedure is used at RRC connection release, the (ng-)eNB may request the UE to retain the UE AS context including UE capability in RRC_IDLE;

– A RRC connection resume procedure is used at transition from RRC_IDLE to RRC_CONNECTED where previously stored information in the UE as well as in the (ng-)eNB is utilised to resume the RRC connection. In the message to resume, the UE provides a Resume ID (for EPS) or I-RNTI (for 5GS) to be used by the (ng-)eNB to access the stored information required to resume the RRC connection;

– At suspend-resume, security is continued. Re-keying is not supported in RRC connection resume procedure. The short MAC-I is reused as the authentication token at RRC connection reestablishment procedure and RRC connection resume procedure by the UE. For EPS, the eNB provides the NCC in the RRCConnectionResume message as well. And also the UE resets the COUNT;

– Multiplexing of CCCH and DTCH in the transition from RRC_IDLE to RRC CONNECTED is not supported;

– For NB-IoT, a non-anchor carrier can be configured for all unicast transmissions when an RRC connection is re-established, resumed or reconfigured additionally when an RRC connection is established.

The RRC connection suspend and resume procedures are illustrated in Figures 7.3a.3-1/7.3a.3-1a and 7.3a.3-2/7.3a.3-2a, respectively. Note that the description here is only intended as an overview and all parameters are therefore not listed in the message flows.

Figure 7.3a.3-1: RRC Connection Suspend procedure in EPS

Figure 7.3a.3-1a: RRC Connection Suspend procedure in 5GS

1. Due to some triggers, e.g. the expiry of a UE inactivity timer, the (ng-)eNB decides to suspend the RRC connection.

2. In EPS, the eNB initiates the S1-AP UE Context Suspend procedure to inform the MME that the RRC connection is being suspended. In 5GS, the ng-eNB initiates the NG-AP UE Context Suspend procedure to inform the AMF that the RRC connection is being suspended.

3. In EPS, the MME requests the S-GW to release all S1-U bearers for the UE. In 5GS, the AMF requests the SMF to suspend the PDU session and the SMF requests the UPF to release the tunnel information for the UE.

4. MME/AMF Acks step 2.

5. The (ng-)eNB suspends the RRC connection by sending an RRCConnectionRelease message with the releaseCause set to rrc-Suspend. For EPS, the message includes the Resume ID which is stored by the UE and optionally, for EDT and transmission using PUR, the message also includes the NextHopChainingCount which is stored by the UE. For 5GS, the message includes the I-RNTI and NextHopChainingCount which are stored by the UE.

6. The UE stores the AS context, suspends all SRBs and DRBs, and enters RRC_IDLE.

Figure 7.3a.3-2: RRC Connection Resume procedure in EPS

Figure 7.3a.3-2a: RRC Connection Resume procedure in 5GS

1. At some later point in time (e.g. when the UE is being paged or when new data arrives in the uplink buffer) the UE resumes the connection by sending an RRCConnectionResumeRequest to the (ng-)eNB. The UE includes its Resume ID (for EPS) or I-RNTI (for 5GS), the establishment cause, and authentication token. The authentication token is calculated in the same way as the short MAC-I used in RRC connection re-establishment and allows the (ng-)eNB to verify the UE identity. For 5GS, the UE resumes SRB1, derives new security keys using the NextHopChainingCount provided in the RRCConnectionRelease message of the previous RRC connection and re-establishes the AS security.

2. Provided that the Resume ID (for EPS) or I-RNTI (for 5GS) exists and the authentication token is successfully validated, the (ng-)eNB responds with an RRCConnectionResume. For EPS, the message includes the Next Hop Chaining Count (NCC) value which is required in order to re-establish the AS security.

3. For EPS, the UE resumes all SRBs and DRBs and re-establishes the AS security. For 5GS, the UE resumes all other SRBs and all DRBs. The UE is now in RRC_CONNECTED.

4. The UE responds with an RRCConnectionResumeComplete confirming that the RRC connection was resumed successfully, along with an uplink Buffer Status Report, and/or UL data, whenever possible, to the (ng-)eNB.

5. For EPS, the eNB initiates the S1-AP Context Resume procedure to notify the MME about the UE state change. For 5GS, the ng-eNB initiates the NG-AP Context Resume procedure to notify the AMF about the UE state change.

6. For EPS, the MME requests the S-GW to activate the S1-U bearers for the UE. For 5GS, the AMF requests the SMF to resume the PDU session and the SMF requests the UPF to establish the tunnel information for the UE.

7. MME/AMF Acks step 5.

An RRC connection can also be resumed in an (ng-)eNB (the new (ng-)eNB) different from the one where the connection was suspended (the old (ng-)eNB). Inter (ng-)eNB connection resumption is handled using context fetching, whereby the new (ng-)eNB retrieves the UE context from the old (ng-)eNB over the X2/Xn interface. The new (ng-)eNB provides the Resume ID (for EPS) or I-RNTI (for 5GS) which is used by the old (ng-)eNB to identify the UE context. This is illustrated in Figure 7.3a.3-3/7.3a.3-3a.

Figure 7.3a.3-3: RRC Connection Resume procedure in different eNB in EPS

Figure 7.3a.3-3a: RRC Connection Resume procedure in different ng-eNB in 5GS

1. Same as step 1 in the intra (ng-)eNB connection resumption.

2. The new (ng-)eNB locates the old (ng-)eNB using the Resume ID (for EPS) or I-RNTI (for 5GS) and retrieves the UE context by means of the X2-AP (for EPS) or Xn-AP (for 5GS) Retrieve UE Context procedure.

3. The old (ng-)eNB responds with the UE context associated with the Resume ID (for EPS) or I-RNTI (for 5GS).

4. Same as step 2 in the intra (ng-)eNB connection resumption.

5. Same as step 3 in the intra (ng-)eNB connection resumption.

6. Same as step 4 in the intra (ng-)eNB connection resumption.

7. For EPS, the new eNB initiates the S1-AP Path Switch procedure to establish a S1 UE associated signalling connection to the serving MME and to request the MME to resume the UE context. For 5GS, the new ng-eNB initiates the NG-AP Path Switch procedure to establish a NG UE associated signalling connection to the serving AMF and to request the AMF to resume the UE context.

8. For EPS, the MME requests the S-GW to activate the S1-U bearers for the UE and updates the downlink path. For 5GS, the AMF requests the SMF to resume the PDU session and the SMF requests the UPF to create the tunnel information for the UE and update the downlink path.

9. MME/AMF Acks step 7.

10. For EPS, after the S1-AP Path Switch procedure the new eNB triggers release of the UE context at the old eNB by means of the X2-AP UE Context Release procedure. For 5GS, after the NG-AP Path Switch procedure the new ng-eNB triggers release of the UE context at the old ng-eNB by means of the Xn-AP UE Context Release procedure.

For a NB-IoT UE that supports Control Plane CIoT EPS optimisation and S1-U data transfer or User Plane CIoT EPS optimisation, as defined in TS 24.301 [20], and for a NB-IoT UE that supports Control Plane CIoT 5GS Optimisation and NG-U data transfer or User Plane CIoT 5GS Optimisation, as defined in TS 24.501 [91], PDCP is not used until AS security is activated.