5 Elements for layer-to-layer communication

3GPP48.016Base Station System (BSS) - Serving GPRS Support Node (SGSN) interfaceGeneral Packet Radio Service (GPRS)Network serviceRelease 17TS

This sub-clause presents the Network Service in a generic way, no assumptions are made regarding the real protocols providing the network services.

5.1 Service primitive model

The service primitive model shown in figure 5.1.1 is applicable to both BSS and SGSN.

Figure 5.1.1: Network Service primitive model

The network services are provided at the Network Service-Service Access Point (NS-SAP).

5.2 Service primitives and parameters

The Network Service primitives are summarized in table 5.2.1. The general syntax of the Network Service primitives is:

NS ‑ Generic name ‑ Type (Parameters).

Table 5.2.1: Network Service primitives

Generic name		Type			Parameters
	Request	Indication	Response	Confirm
UNITDATA	X	X			– BVCI and NSEI – NS SDU – NS Change IP endpoint – Link Selector Parameter
CONGESTION		X			– BVCI and NSEI – congestion cause
STATUS		X			– BVCI and NSEI – NS affecting cause – transfer capability

5.2.1 Primitives

5.2.1.1 NS-UNITDATA-Request

This primitive is used by the NS user entity to send a NS SDU to its peer entity via a BVC. The NS entity sends the NS SDU in unacknowledged mode. The Link Selector Parameter is used to identify the NS SDUs which must be sent in order relatively to each other. The NSEI is used by the NS entity to select the group of NS-VCs corresponding to the addressed remote entity. The NS Change IP endpoint is used to request the NS entity to indicate whether a "request change flow" or "confirm change flow" indication needs to be sent to its peer entity.

5.2.1.2 NS-UNITDATA-Indication

This primitive is used by the NS entity to provide the NS user entity with a NS SDU received on a virtual connection. The NS SDU are received in unacknowledged mode. BVCI together with NSEI indicate which BVC the NS SDU was received on. The NS Change IP endpoint is used to indicate to the user of the NS entity whether a "request change flow" or "confirm change flow" indication was received from the peer entity. In case a "request change flow" is received from the peer entity the Link Selector Parameter on which the NS SDU was received is sent to the higher layer.

5.2.1.3 NS-CONGESTION-Indication

The NS entity shall be able to detect when a congestion situation starts and ends on an NS-VC.

This primitive is used by the NS entity to report to the NS user entity that congestion is detected or that the congestion situation has disappeared. The BVCI and NSEI of the affected BVC and the congestion cause are reported to the NS user entity. Typically, the cause indicates the direction of transmission affected by the congestion.

5.2.1.4 NS-STATUS-Indication

There may be situations where an NS-VC becomes unavailable for NS user traffic. When this occurs, the NS user is informed of the degradation of the transfer capacity by means of this primitive including the "transfer capability" parameter.

When an NS-VC previously unavailable for NS user traffic becomes available again, the NS user entity is also informed by means of this service primitive, indicating the current transfer capability.

This primitive may be used in response to an NS-UNITDATA-Request primitive which the NS is unable to serve because of e.g. NS-VC failure.

This primitive may also be used upon recovery after a failure affecting the NS.

5.2.2 Parameters

5.2.2.1 NS SDU

The NS SDUs are specified in 3GPP TS 48.018. They shall never be inspected by the Network Service entity.

5.2.2.2 Link Selector Parameter

The Link Selector Parameter is included in the NS-UNITDATA-Request primitive for load sharing purposes as described in sub-clause "Requirements on load sharing function".

The Link Selector Parameter is included in the NS-UNITDATA-Indication primitive and in the NS-UNITDATA-Request primitive for resource distribution purposes as described in sub-clause 4.4a.1. The Link Selector Parameter shall include a reference to the local IP endpoint and the remote IP endpoint for resource distribution purposes if the NS Change IP endpoint parameter has the cause value of "request change flow".

5.2.2.3 BVCI I and NSEI

BVCI and NSEI parameters are included in the service primitives to identify the BVC for which the service is provided. These parameters are used by the NS entity to multiplex the NS SDUs over the NS-VCs.

5.2.2.4 Congestion cause

The congestion cause shall indicate the affected direction of transmission and may be set to the following values:

a) congestion detected, backward;

b) end of congestion, backward;

c) congestion detected, forward;

d) end of congestion, forward.

5.2.2.5 Transfer capability

This parameter is used to report to the NS user entity the current transfer capacity available for a BVC, in terms of bandwidth. This parameter may be set to any value from "0" (zero) to the maximum bandwidth provided by the complete set of NS-VCs associated to the BVC.

5.2.2.6 NS affecting cause

This parameter is used to indicate to the NS user entity the reason which caused the NS-STATUS-Indication primitive to be used. The cause values are:

NS-VC failure: a failure is affecting one or more NS-VCs, the NS is still available.

NS-VC recovery: one or more NS-VCs which were in failure are available again.

NS failure: the NS can not provide data transfer services to the NS user.

NS recovery: the NS can provide data transfer services again.

5.2.2.7 NS change IP endpoint

The NS change IP endpoint parameter is included in the NS-UNITDATA-Request and NS-UNITDATA-Indication primitive for resource distribution purposes. This parameter is used in an IP sub-network to indicate to the NS user entity a request for change in IP endpoint or a response to a change in IP endpoint. The cause values are:

Request change flow: Request to change the IP endpoint at which to receive NS SDUs associated with a subscriber or MBMS session.

Confirm change flow: Confirmation to a request for change of IP endpoint of NS SDUs associated with a subscriber or MBMS session.