Content for TS 37.473 Word version: 18.0.0

3.1 Terms 3.2 Symbols 3.3 Abbreviations 4.1 Procedure specification principles 4.2 Forwards and backwards compatibility 4.3 Specification notations
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1 Scope p. 10

The present document specifies the 5G radio network layer signalling protocol for the W1 interface. The W1 interface provides means for interconnecting a ng-eNB-CU and a ng-eNB-DU of a ng-eNB within an NG-RAN. The W1 Application Protocol (W1AP) supports the functions of W1 interface by signalling procedures defined in the present document. W1AP is developed in accordance to the general principles stated in TS 38.401 and TS 37.470.

2 References p. 10

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.

[1]

TR 21.905: "Vocabulary for 3GPP Specifications".

[2]

TS 36.331: "Evolved Universal Terrestrial Radio Access (E-UTRAN); Radio Resource Control (RRC) Protocol Specification".

[3]

TS 36.401: "E-UTRAN Architecture Description".

[4]

TS 38.401: "NG-RAN; Architecture Description".

[5]

TS 37.470: "E-UTRAN and NG-RAN; W1 general aspects and principles".

[6]

TS 38.300: "NR; Overall description; Stage-2".

[7]

TS 38.423: "NG-RAN; Xn application protocol (XnAP)".

[8]

TS 23.501: "System Architecture for the 5G System (5GS)".

[9]

TS 38.321: "NR; Medium Access Control (MAC) protocol specification".

[10]

TS 23.401: "General Packet Radio Service (GPRS) enhancements for Evolved Universal Terrestrial Radio Access Network (E-UTRAN) access".

[11]

TS 23.203: "Policy and charging control architecture".

[12]

TS 36.423: "Evolved Universal Terrestrial Radio Access Network (E-UTRAN); X2 Application Protocol (X2AP)".

[13]

TS 36.211: "Evolved Universal Terrestrial Radio Access (E-UTRA); Physical channels and modulation".

[14]

TS 36.104: "Base Station (BS) radio transmission and reception".

[15]

TS 36.321: " Evolved Universal Terrestrial Radio Access (E-UTRAN); Medium Access Control (MAC) protocol specification".

[16]

TS 38.331: "NR; Radio Resource Control (RRC); Protocol specification".

[17]

TS 29.281: "General Packet Radio System (GPRS); Tunnelling Protocol User Plane (GTPv1-U) ".

[18]

TS 38.414: "NG-RAN; NG data transport".

[19]

TS 38.413: "NG-RAN; NG Application Protocol (NGAP)".

[20]

TS 36.304: "Evolved Universal Terrestrial Radio Access (E-UTRA), User Equipment (UE) procedures in idle mode".

[21]

ITU-T Recommendation X.691 (2002-07): "Information technology - ASN.1 encoding rules - Specification of Packed Encoding Rules (PER)".

[22]

ITU-T Recommendation X.680 (07/2002): "Information technology - Abstract Syntax Notation One (ASN.1): Specification of basic notation".

[23]

ITU-T Recommendation X.681 (07/2002): "Information technology - Abstract Syntax Notation One (ASN.1): Information object specification".

3 Definitions of terms, symbols and abbreviations p. 11

3.1 Terms p. 11

For the purposes of the present document, the terms 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.

elementary procedure:

W1AP consists of Elementary Procedures (EPs). An Elementary Procedure is a unit of interaction between ng-eNB-CU and ng-eNB-DU. These Elementary Procedures are defined separately and are intended to be used to build up complete sequences in a flexible manner. If the independence between some EPs is restricted, it is described under the relevant EP description. Unless otherwise stated by the restrictions, the EPs may be invoked independently of each other as standalone procedures, which can be active in parallel. The usage of several W1AP EPs together is specified in stage 2 specifications (e.g., TS 37.470).

An EP consists of an initiating message and possibly a response message. Two kinds of EPs are used:

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

For Class 1 EPs, the types of responses can be as follows:

Successful:

A signalling message explicitly indicates that the elementary procedure successfully completed with the receipt of the response.

Unsuccessful:

A signalling message explicitly indicates that the EP failed.
On time supervision expiry (i.e., absence of expected response).

Successful and Unsuccessful:

One signalling message reports both successful and unsuccessful outcome for the different included requests. The response message used is the one defined for successful outcome.

Class 2 EPs are considered always successful.

NGEN-DC operation:

Used in the present document when the W1AP is applied for ng-eNB-CU and ng-eNB-DU in NG-RAN.

ng-eNB:

As defined in TS 38.300.

ng-eNB-CU:

As defined in TS 37.470.

ng-eNB-DU:

As defined in TS 37.470.

ng-eNB-CU UE W1AP ID:

As defined in TS 38.401.

ng-eNB-DU UE W1AP ID:

As defined in TS 38.401.

UE-associated signalling:

When W1AP messages associated to one UE uses the UE-associated logical W1-connection for association of the message to the UE in ng-eNB-DU and ng-eNB-CU.

UE-associated logical W1-connection:

The UE-associated logical W1-connection uses the identities ng-eNB-CU UE W1AP ID and ng-eNB-DU UE W1AP ID according to the definition in TS 38.401. For a received UE associated W1AP message the ng-eNB-CU identifies the associated UE based on the ng-eNB-CU UE W1AP ID IE and the ng-eNB-DU identifies the associated UE based on the ng-eNB-DU UE W1AP ID IE. The UE-associated logical W1-connection may exist before the W1 UE context is setup in ng-eNB-DU.

3.2 Symbols p. 12

Void.

3.3 Abbreviations p. 12

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.

5GC

5G Core Network

5QI

5G QoS Identifier

AMF

Access and Mobility Management Function

Core Network

Cell Group

CGI

Cell Global Identifier

Downlink

EN-DC

E-UTRA-NR Dual Connectivity

NE-DC

NR-E-UTRA Dual Connectivity

NGEN-DC

NG-RAN E-UTRA-NR Dual Connectivity

NSSAI

Network Slice Selection Assistance Information

RANAC

RAN Area Code

RRC

Radio Resource Control

S-NSSAI

Single Network Slice Selection Assistance Information

TAC

Tracking Area Code

TAI

Tracking Area Identity

4 General p. 12

4.1 Procedure specification principles p. 12

The principle for specifying the procedure logic is to specify the functional behaviour of the terminating node exactly and completely. Any rule that specifies the behaviour of the originating node shall be possible to be verified with information that is visible within the system.

The following specification principles have been applied for the procedure text in clause 8:

The procedure text discriminates between:
1. Functionality which "shall" be executed.
  The procedure text indicates that the receiving node "shall" perform a certain function Y under a certain condition. If the receiving node supports procedure X but cannot perform functionality Y requested in the REQUEST message of a Class 1 EP, the receiving node shall respond with the message used to report unsuccessful outcome for this procedure, containing an appropriate cause value.
2. Functionality which "shall, if supported" be executed.
  The procedure text indicates that the receiving node "shall, if supported," perform a certain function Y under a certain condition. If the receiving node supports procedure X, but does not support functionality Y, the receiving node shall proceed with the execution of the EP, possibly informing the requesting node about the not supported functionality.
Any required inclusion of an optional IE in a response message is explicitly indicated in the procedure text. If the procedure text does not explicitly indicate that an optional IE shall be included in a response message, the optional IE shall not be included. For requirements on including Criticality Diagnostics IE, see clause 10.

4.2 Forwards and backwards compatibility p. 13

The forwards and backwards compatibility of the protocol is assured by mechanism where all current and future messages, and IEs or groups of related IEs, include ID and criticality fields that are coded in a standard format that will not be changed in the future. These parts can always be decoded regardless of the standard version.

4.3 Specification notations p. 13

For the purposes of the present document, the following notations apply:

Procedure

When referring to an elementary procedure in the specification the Procedure Name is written with the first letters in each word in upper case characters followed by the word "procedure", e.g. Handover Preparation procedure.

Message

When referring to a message in the specification the MESSAGE NAME is written with all letters in upper case characters followed by the word "message", e.g. HANDOVER REQUEST message.

When referring to an information element (IE) in the specification 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 an IE

When referring to the value of an information element (IE) in the specification the "Value" is written as it is specified in the specification enclosed by quotation marks, e.g. "Value".

5 W1AP services p. 13

W1AP provides the signalling service between ng-eNB-DU and the ng-eNB-CU that is required to fulfil the W1AP functions described in clause 7. W1AP services are divided into two groups:

Non UE-associated services:

They are related to the whole W1 interface instance between the ng-eNB-DU and ng-eNB-CU utilising a non UE-associated signalling connection.

UE-associated services:

They are related to one UE. W1AP functions that provide these services are associated with a UE-associated signalling connection that is maintained for the UE in question.

Unless explicitly indicated in the procedure specification, at any instance in time one protocol endpoint shall have a maximum of one ongoing W1AP procedure related to a certain UE.

6 Services expected from signalling transport p. 13

The signalling connection shall provide in sequence delivery of W1AP messages. W1AP shall be notified if the signalling connection breaks.

7 Functions of W1AP p. 13

The functions of W1AP are described in TS 37.470.