X2 signalling bearer provides the following functions:

• Provision of reliable transfer of X2-AP message over X2 interface.
• Provision of networking and routeing function
• Provision of redundancy in the signalling network
• Support for flow control and congestion control

L2 - Data link layer

Support of any suitable data link layer protocol, e.g. PPP, Ethernet

IP layer

• The eNB supports IPv6 and/or IPv4
• The IP layer of eNB-eNB only supports point-to-point transmission for delivering X2-AP message.
• The eNB supports the Diffserv Code Point marking

Transport layer

SCTP is supported as the transport layer of eNB-eNB signalling bearer.

• 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.
• There is only one SCTP association established between eNB pairs.
• The eNB establishes the SCTP association. The SCTP Destination Port number value assigned by IANA to be used for X2AP is 36422.

Protocol specification

3GPP TS 36.420 - Evolved Universal Terrestrial Radio Access Network (E-UTRAN); X2 general aspects and principles

3GPP TS 36.421 - Evolved Universal Terrestrial Radio Access Network (E-UTRAN); X2 layer 1

3GPP TS 36.422 - Evolved Universal Terrestrial Radio Access Network (E-UTRAN); X2 signalling transport

IETF RFC 2460: "Internet Protocol, Version 6 (IPv6) Specification"

IETF RFC 791 (September,1981): "Internet Protocol"

IETF RFC 2474 (December 1998): "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers"