7 Communication Primitives for the Iu UP protocol layer

25.4153GPPRelease 17TSUTRAN Iu interface user plane protocols

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

7.1 Modelling Principle

The principle illustrated by figure 30 is used for modelling the primitives towards the protocol layer.

Figure 30: Modelling principle

7.2 Primitives towards the upper layers at the RNL SAP

7.2.1 General

The Iu UP protocol layer interacts with upper layers as illustrated in the figure above. The interactions with the upper layers are shown in terms of primitives where the primitives represent the logical exchange of information and control between the upper layer and the Iu UP protocol layer. They do not specify or constrain implementations.

The following primitives are defined:

– Iu-UP-DATA;

– Iu-UP-STATUS;

– Iu-UP-UNIT-DATA.

Table 1: Iu UP protocol layer service primitives towards the upper layer at the RNL SAP

Primitive	Type	Parameters	Comments
Iu-UP-DATA	Request	Iu-UP-payload	Subflow 1 SDU, …, Subflow n SDU
		Iu-UP-control	RFCI
			FQC
			Frame Number
	Indication	Iu-UP-payload	Subflow 1 SDU, …, Subflow n SDU
		Iu-UP-control	RFCI
			FQC
			Frame Number
Iu-UP-Status	Indication	Iu-UP-Procedure-Control	Error Cause, Error Distance
			Complete protocol data for Initialisation, Rate Control, Time Alignment and Error Event


	Request	Iu-UP-Procedure-Control	Error Cause
			Complete protocol data for Initialisation, Rate Control, Time Alignment and Error Event

Iu-UP-UNIT-DATA	Request	Iu-UP-payload
	Indication	Iu-UP-payload

Primitive usage is a function of the mode of operation of the Iu UP protocol. Table 2 provides the association between Iu UP primitives towards the upper layers and the Iu UP modes of operation.

Table 2: Iu UP protocol layer service primitives related to the Iu UP mode
of operation and function within the mode of operation

Primitive	Type	Mode of Operation
Iu-UP-DATA	Request	SMpSDU
Iu-UP-DATA	Indication	SMpSDU
Iu-UP-Status	Request	SMpSDU
	Indication	SMpSDU
Iu-UP-UNIT-DATA	Request	TrM
	Indication	TrM

7.2.2 Iu-UP-DATA-REQUEST

This primitive is used as a request from the upper layer Iu NAS Data Stream entity to send the RAB subflow SDU(s) on the established transport connection. This primitive includes the RFCI, the Frame Number and FQC information of the payload.

The Iu UP protocol layer forms the Iu UP data frame, the Iu Data Stream DU being the payload of the Iu UP frame, and transfers the frame by means of the lower layer services.

7.2.3 Iu-UP-DATA-INDICATION

This primitive is used as an indication to the upper layer entity to pass the Iu NAS Data Stream User Plane information of a received Iu UP frame.

This primitive includes the RFCI, the Frame Number and FQC information of the payload.

This primitive may also include information aiming at informing the upper layers of a faulty situation that relates to the payload included in the primitive.

7.2.4 Iu-UP-STATUS-REQUEST

This primitive is used to pass control procedure information from the upper layer.

This primitive shall contain the complete protocol data for the respective procedure.

7.2.5 Iu-UP-STATUS-INDICATION

This primitive is used to pass control procedure information to the upper layer.

This primitive shall contain the complete protocol data for the respective procedure.

7.2.6 Iu-UP-UNIT-DATA-REQUEST

This primitive is used as a request from the upper layer to send an Iu UP payload on the established transport connection.

The Iu UP protocol layer transfers the Iu Data Stream DU by means of the lower layer services without adding any protocol header overhead.

7.2.7 Iu-UP-UNIT-DATA-INDICATION

This primitive is used as an indication to the upper layer entity to pass the Iu UP payload.

7.3 Primitives towards the transport layers at TNL SAP

7.3.1 General

Access to the Transport network Layer is performed through a generic SAP: TNL-SAP.

When the Transport Network upper layer consists of AAL2, the TNL SAP maps onto the AAL-SAP through which communication is performed using specific AAL primitives.

When the Transport Network upper layer consists of GTP-U, the TNL SAP maps onto the GTP-U SAP through which communication is performed using generic primitives.

The choice of communication, specific or generic, through the TNL SAP is fixed by the Radio Network Layer control plane logic. This choice is based on the requirements placed by e.g. the RAB characteristics, the CN domain requesting the RAB establishment or other operator’s choice.

7.3.2 ATM/AAL2 based Transport layer

7.3.2.1 General

When the Iu UP protocol layer uses the services of an ATM/AAL2 transport (ITU-T Recommendation I.363.2 [7]), it uses an established AAL2 connection for transferring frames between the peer TNL-SAPs at both ends of the Iu User plane access points. The Transport Network Control Plane over Iu handles the signalling to establish and release the AAL2 call connections.

7.3.2.2 AAL2 Service Primitives used by the Iu UP protocol

AAL2 services and primitives used at the Service Access Point from the AAL2 layer are shown in table 3.

Table 3: AAL2 primitives and parameters

Primitive	Type	Parameters	Comments
SSSAR-UNITDATA	Request	SSSAR-INFO
SSSAR-UNITDATA	Request	SSSAR-UUI	Not used (note)
SSSAR-UNITDATA	Indication	SSSAR-INFO
SSSAR-UNITDATA	Indication	SSSAR-UUI	Not used (note)
NOTE: The setting of this field is set to not used i.e. decimal value 26 according to ITU-T Rec. I.366.1 [8].

The primitives of table 3 are the standard primitives of ITU-T Rec. I.366.1 [8]. These primitives are intended to be used in the Iu UP.

7.3.3 GTP-U based Transport Layer

7.3.3.1 General

When the Iu UP protocol layer uses the services of a GTP-U transport, it uses an established GTP-U tunnel for transferring frames between the GTP-U tunnel endpoints at both ends of the Iu User plane access points. The RANAP Control Plane signalling over Iu handles the signalling to establish and release the GTP-U tunnels.

7.3.3.2 Generic Service Primitives used by the Iu UP protocol

Generic primitives are used at the GTP-U SAP. They are shown in table 4.

Table 4: Generic primitives and parameters to and from GTP-U layer

Primitive	Type	Parameters	Comments
Iu-UP-UNITDATA	Request	Iu-UP-payload
Iu-UP-UNITDATA	Indication	Iu-UP-payload

7.3.4 RTP based Transport Layer

7.3.4.1 General

When the Iu UP protocol layer uses the services of a RTP/UDP/IP transport, it uses an established RTP session for transferring frames between the two RTP endpoints at both ends of the Iu User plane access points as defined in IETF RFC 1889 [14]. A single Iu-UP PDU shall be transported as RTP payload. A dynamic Payload Type (IETF RFC 1890 [15]) shall be used (see TS 25.414 [4]).

7.3.4.2 Generic Service Primitives used by the Iu UP protocol

Generic primitives are used at the RTP SAP. They are shown in table 5.

Table 5: Generic primitives and parameters to and from RTP layer

Primitive	Type	Parameters	Comments
Iu-UP-UNITDATA	Request	Iu-UP-payload
Iu-UP-UNITDATA	Indication	Iu-UP-payload