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.
: "Vocabulary for 3GPP Specifications".
(2017-07): "Internet Protocol, Version 6 (IPv6) Specification".
(1981-09): "Internet Protocol".
(1998-12): "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers".
(2007-09): "Stream Control Transmission Protocol".
: "NR; Overall description; Stage-2".
For the purposes of the present document, the terms and definitions given in TR 21.905
and the following apply. A term defined in the present document takes precedence over the definition of the same term, if any, in TR 21.905
interface between a ng-eNB-CU and a ng-eNB-DU, providing an interconnection point between the ng-eNB-CU and the ng-eNB-DU.
Reference point for the control plane protocol between ng-eNB-CU and ng-eNB-DU.
For the purposes of the present document, the abbreviations given in TR 21.905
and the following apply. An abbreviation defined in the present document takes precedence over the definition of the same abbreviation, if any, in TR 21.905
Internet Assigned Number Authority
Point to Point Protocol
Stream Control Transmission Protocol
The W1-C signalling bearer provides the following functions:
Provision of reliable transfer of W1AP messages over the W1-C interface.
Provision of networking and routeing function.
Provision of redundancy in the signalling network.
Support for flow control and congestion control.
The protocol stack for W1-C Signalling Bearer is shown in Figure 4.1-1 and details on each protocol are described in the following clauses.
Figure 4.1-1: W1-C signalling bearer protocol stack
The Transport Network Layer is based on IP transport, comprising SCTP on top of IP.
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 (IETF RFC 8200
) and/or IPv4 (IETF 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 IETF RFC 2474
SCTP (IETF 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 IETF RFC 4960
in subclause 5.2.
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.