The present document specifies the standards for Signalling Transport to be used across the W1 interface. The W1 interface provides means for interconnecting a ng-eNB-CU and a ng-eNB-DU of a ng-eNB within a NG-RAN. The present document describes how the W1AP signalling messages are transported over W1.

The following documents contain provisions which, through reference in this text, constitute provisions of the present document.

• References are either specific (identified by date of publication, edition number, version number, etc.) or non-specific.
• For a specific reference, subsequent revisions do not apply.
• For a non-specific reference, the latest version applies. In the case of a reference to a 3GPP document (including a GSM document), a non-specific reference implicitly refers to the latest version of that document in the same Release as the present document.

The support of any suitable Data Link Layer protocol, e.g. PPP, Ethernet, etc., shall not be prevented.

The ng-eNB-CU and ng-eNB-DU shall support IPv6 (RFC 8200) and/or IPv4 (RFC 791). The IP layer of W1-C only supports point-to-point transmission for delivering W1AP message. The ng-eNB-CU and ng-eNB-DU shall support the Diffserv Code Point marking as described in RFC 2474.

SCTP (RFC 4960) shall be supported as the transport layer of W1-C signalling bearer. The Payload Protocol Identifier assigned by IANA to be used by SCTP for the application layer protocol W1AP is 73. SCTP refers to the Stream Control Transmission Protocol developed by the Sigtran working group of the IETF for the purpose of transporting various signalling protocols over IP network. The ng-eNB-DU and ng-eNB-CU shall support a configuration with a single SCTP association per ng-eNB-DU/ng-eNB-CU pair. The ng-eNB-DU shall establish the SCTP association. The SCTP Destination Port number value assigned by IANA to be used for W1AP is 37472. Within the set of SCTP associations established between one ng-eNB-CU and ng-eNB-DU pair, a single SCTP association shall be employed for W1AP elementary procedures that utilize non-UE-associated signalling with the possibility of fail-over to a new association to enable robustness. Between one ng-eNB-CU and ng-eNB-DU pair:

• A single pair of stream identifiers shall be reserved over an SCTP association for the sole use of W1AP elementary procedures that utilize non UE-associated signalling.
• At least one pair of stream identifiers over one or several SCTP associations shall be reserved for the sole use of W1AP elementary procedures that utilize UE-associated signalling. However, a few pairs (i.e. more than one) should be reserved.
• For a single UE-associated signalling, the ng-eNB-DU shall use one SCTP association and one SCTP stream, and the association/stream should not be changed during the communication of the UE-associated signalling unless TNL binding update is performed.

Transport network redundancy may be achieved by SCTP multi-homing between two end-points, of which one or both is assigned with multiple IP addresses. SCTP end-points shall support a multi-homed remote SCTP end-point. For SCTP endpoint redundancy an INIT may be sent from a ng-eNB-CU or ng-eNB-DU, at any time for an already established SCTP association, which shall be handled as defined in Section 5.2 of RFC 4960. The SCTP congestion control may, using an implementation specific mechanism, initiate higher layer protocols to reduce the signalling traffic at the source and prioritise certain messages.