书名

3GPP TS 36.423 X2 application protocol (X2AP)

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3GPP协议文档，对X2协议的描述

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S1AP相关信息。 Services, Functions, Procedures, Elements etc.

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Definitions, symbols and abbreviations

Elementary Procedure: X2AP protocol consists of Elementary Procedures (EPs). An X2AP Elementary Procedure is a unit of interaction between two eNBs. Two kinds of EPs are used:

Class 1: Elementary Procedures with response (success or failure),
Class 2: Elementary Procedures without response.

General

Procedure the first letters in each word in upper case characters followed by the word “procedure”, e.g. Handover Preparation procedure.
Message the MESSAGE NAME is written with all letters in upper case characters followed by the word “message”, e.g. HANDOVER REQUEST message.
IE the Information Element Name is written with the first letters in each word in upper case characters and all letters in Italic font followed by the abbreviation “IE”, e.g. E-RAB ID IE.
Value of IE “Value” is written as it is specified in sub clause 9.2 enclosed by quotation marks, e.g. “Value”.

X2AP services

The present clause describes the services an eNB offers to its neighbours.

X2AP procedure modules

The X2 interface X2AP procedures are divided into two modules as follows:

X2AP Basic Mobility Procedures;
X2AP Global Procedures;

The X2AP Basic Mobility Procedures module contains procedures used to handle the UE mobility within E-UTRAN.
The Global Procedures module contains procedures that are not related to a specific UE. The procedures in this module are in contrast to the above module involving two peer eNBs.

Parallel transactions

Unless explicitly indicated in the procedure specification, at any instance in time one protocol peer shall have a maximum of one ongoing X2AP procedure related to a certain UE.

Services expected from signalling transport

The signalling connection shall provide in sequence delivery of X2AP messages. X2AP shall be notified if the signalling connection breaks.

Functions of X2AP

Function	Elementary Procedure(s)
Mobility Management	a) Handover Preparation b) SN Status Transfer c) UE Context Release d) Handover Cancel
Load Management	a) Load Indication b) Resource Status Reporting Initiation c) Resource Status Reporting
Reporting of General Error Situations	Error Indication
Resetting the X2	Reset
Setting up the X2	X2 Setup
eNB Configuration Update	a) eNB Configuration Update b) Cell Activation
Mobility Parameters Management	Mobility Settings Change
Mobility Robustness Optimisation	a) Radio Link Failure Indication b) Handover Report
Energy Saving	a) eNB Configuration Update b) Cell Activation

X2AP procedures

Handover Preparation

This procedure is used to establish necessary resources in an eNB for an incoming handover.
UE-associated signalling.

SN Status Transfer

The purpose of the SN Status Transfer procedure is to transfer the uplink PDCP SN and HFN receiver status and the downlink PDCP SN and HFN transmitter status from the source to the target eNB during an X2 handover for each respective E-RAB for which PDCP SN and HFN status preservation applies.
The procedure uses UE-associated signalling.

UE Context Release

The UE Context Release procedure is initiated by the target eNB to signal to indicate the source eNB that radio and control plane resources for the handed over UE context are allowed to be released.
The procedure uses UE-associated signalling.

Handover Cancel

The Handover Cancel procedure is used to enable a source eNB to cancel an ongoing handover preparation or an already prepared handover.
The procedure uses UE-associated signalling.

Global Procedures

Load Indication

The purpose of the Load Indication procedure is to transfer load and interference co-ordination information between eNBs controlling intra-frequency neighboring cells.
The procedure uses non UE-associated signalling.

Error Indication

The Error Indication procedure is initiated by an eNB to report detected errors in one incoming message, provided they cannot be reported by an appropriate failure message.
If the error situation arises due to reception of a message utilising UE associated signalling, then the Error Indication procedure uses UE-associated signalling. Otherwise the procedure uses non UE-associated signalling.

X2 Setup

The purpose of the X2 Setup procedure is to exchange application level configuration data needed for two eNBs to interoperate correctly over the X2 interface.
This procedure erases any existing application level configuration data in the two nodes and replaces it by the one received.
This procedure also resets the X2 interface like a Reset procedure would do.
The procedure uses non UE-associated signalling.

Reset

The purpose of the Reset procedure is to align the resources in eNB1 and eNB2 in the event of an abnormal failure. The procedure resets the X2 interface. This procedure doesn’t affect the application level configuration data exchanged during the X2 Setup procedure.
The procedure uses non UE-associated signalling.

eNB Configuration Update

The purpose of the eNB Configuration Update procedure is to update application level configuration data needed for two eNBs to interoperate correctly over the X2 interface.
The procedure uses non UE-associated signalling.

Resource Status Reporting Initiation

This procedure is used by an eNB to request the reporting of load measurements to another eNB.
The procedure uses non UE-associated signalling.

Resource Status Reporting

This procedure is initiated by eNB 2 to report the result of measurements requested by eNB 1 using the Resource Status Reporting Initiation.
The procedure uses non UE-associated signalling.

Mobility Settings Change

This procedure enables an eNB to negotiate the handover trigger settings with a peer eNB controlling neighbouring cells.
The procedure uses non UE-associated signalling.

Radio Link Failure Indication

The purpose of the Radio Link Failure Indication procedure is to transfer information regarding RRC re-establishment attempts between eNBs controlling neighbouring cells. The signalling takes place from the eNB at which a re-establishment attempt is made to an eNB to which the UE concerned may have previously been attached prior to radio link failure. This may aid the detection of handover failure cases (TS 36.300 [15]).
The procedure uses non UE-associated signalling.

Handover Report

The purpose of the Handover Report procedure is to transfer mobility related information between eNBs controlling neighbouring cells.
The procedure uses non UE-associated signalling.

Cell Activation

The purpose of the Cell Activation procedure is to request to a neighbouring eNB to switch on one or more cells, previously reported as inactive due to energy saving reasons.
The procedure uses non UE-associated signalling.

Elements for X2AP Communication

abbr.

Information Element definitions

The GTP Tunnel Endpoint IE identifies an X2 transport bearer or the S-GW endpoint of the S1 transport bearer associated to an E-RAB. It contains a Transport Layer Address and a GTP Tunnel Endpoint Identifier. The Transport Layer Address is an IP address to be used for the X2 user plane transport or for the S1 user plane transport.
The GTP Tunnel Endpoint Identifier is to be used for the user plane transport between eNB and the S-GW or between eNBs.

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