19.2.0 General

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

The S1 control plane interface (S1-MME) is defined between the eNB and the MME. The control plane protocol stack of the S1 interface is shown on Figure 19.2-1. The transport network layer is built on IP transport, similarly to the user plane but for the reliable transport of signalling messages SCTP is added on top of IP. The application layer signalling protocol is referred to as S1-AP (S1 Application Protocol).

Figure 19.2-1: S1 Interface Control Plane (eNB-MME)

The SCTP layer provides the guaranteed delivery of application layer messages.

In the transport IP layer point-to-point transmission is used to deliver the signalling PDUs.

A single SCTP association per S1-MME interface instance shall be used with one pair of stream identifiers for S1-MME common procedures. Only a few pairs of stream identifiers should be used for S1-MME dedicated procedures. MME communication context identifiers that are assigned by the MME for S1-MME dedicated procedures and eNB communication context identifiers that are assigned by the eNB for S1-MME dedicated procedures shall be used to distinguish UE specific S1-MME signalling transport bearers. The communication context identifiers are conveyed in the respective S1-AP messages.

If the S1 signalling transport layer notifies the S1AP layer that the signalling connection broke:

– the MME

– either locally changes the state of the UEs which used this signalling connection to the ECM-IDLE state as described in TS 23.401 [17] and removes suspended UE Context data for UEs in ECM-IDLE which have used the S1 signalling connection before it was broken; or

– keep those UEs in ECM_CONNECTED and keep the suspended UE Context data for UEs in ECM-IDLE which have used the S1 signalling connection before it was broken;

– the eNB

– either releases the RRC connection with those UEs and removes suspended UE Context data for UEs in RRC_IDLE which have used the S1 signalling connection before it was broken; or

– keep those UEs in RRC_CONNECTED and keep the suspended UE Context data for UEs in ECM-IDLE which have used the S1 signalling connection before it was broken.

If the S1 signalling transport layer notifies the S1AP layer that the signalling connection is operational again and the eNB and the MME have decided to keep UEs in ECM-CONNECTED and RRC_CONNECTED respectively and keep the suspended UE Context data for UEs in ECM-IDLE while the signalling connection was broken, handling of the UE-related contexts and related signalling connections during the S1 Setup procedure that attempts to re-establish the broken signalling connection is described in clause 19.2.2.8.

RNs terminate S1-AP. In this case, there is one S1 interface relation between the RN and the DeNB, and one S1 interface relation between the DeNB and each of the MMEs in the MME pool. The S1 interface relation between the RN and the DeNB carries non-UE-associated S1-AP signalling between RN and DeNB and UE-associated S1-AP signalling for UEs connected to the RN. The S1 interface relation between the DeNB and an MME carries non-UE-associated S1-AP signalling between DeNB and MME and UE-associated S1-AP signalling for UEs connected to the RN and for UEs connected to the DeNB.