5 Overview of data link layer structure

04.053GPPData Link (DL) Layer General AspectsRelease 1999TS

5.1 Functional composition

Figure 5 is an example of a functional block diagram of the data link layer in the MS. In the example the data link connection for all physical channels terminates at the SAP identified by SAPI = O but only the data link connection for a SACCH terminates at the SAP identified by SAPI = 3. Other arrangements are possible depending on the capabilities of the MS.

The network will contain a similar arrangement with one (PCH + AGCH), SDCCH, SACCH, as required, for each active MS.

Figure 5 illustrates three procedural types: the data link procedure, the data link distribution procedure and the random access procedure.

5.2 Identification of data link end points

The data link endpoints are identified by a Data Link Connection Identifier (DLCI).

The DLCI consists of two elements:

‑ the Service Access Point Identifier (SAPI) which is carried in the address field of each frame;

‑ the type of control channel on which the data link connection is or is to be established. This information is not carried in frames between data link layer peer entities but is managed locally in each end system and is carried in primitives between the layers.

When a layer 3 message unit is to be sent, layer 3 will select the appropriate SAP and data link connection end point. Layer 3 will indicate to the data link layer which data link connection end point has been chosen.

When receiving a frame containing a layer 3 message unit, the data link layer will receive from the physical layer an indication concerning the type of channel on which the frame was received. This information together with the SAPI contained in the frame enables the data link layer to deliver the layer 3 message unit to the required data link connection end point of the indicated SAP.

The SAP takes a specific value for each of the following functions carried on the Dm channel:

‑ call control signalling, mobility management signalling, supplementary services signalling and radio resource management signalling information as defined in 3GPP TS 04.08 and 04.10: SAPI = O;

‑ short message services as defined in 3GPP TS 04.11: SAPI = 3.

Other functions requiring specific SAPI values may be defined in the future.

5.3 Data link procedure

There is at mostone instance of the data link layer procedure for each SAPI on each type of channel supported on that SAPI.

For some combinations of SAPI and type of channel only a subset (e.g. unacknowledged operation) of the overall data link layer procedure is required.

The procedure analyses the control field and the length indicator field of the received frame (see 3GPP TS 04.06) and provides appropriate peer‑to‑peer responses and layer‑to‑layer indications. In addition, it analyses the data link layer service primitives and transmits the appropriate peer‑to‑peer commands and responses.

The procedure also performs segmentation and concatenation of layer 3 message units.

5.4 Data link distribution procedure

This procedure is only required if there are more than one SAPI supported on a channel associated with a specific MS.

The procedure analyses the address field of a received frame and the type of physical channel contained in the primitive received from the physical layer. It then distributes the frames to the appropriate data link procedure block.

On frame transmission, the procedure delivers the frames to the required channel after layer 3 has established the respective association between the channel and its physical parameters. The procedure also provides for resolution of conflicts between the various data link procedure blocks on the same physical channel. The conflict resolution is based on the SAPI and the priority requested by layer 3 (see also note to subclause 4.2.2).

5.5 Random access procedures

This procedure is used for data links on the random access channel (RACH). The procedure in the MS formats the random access frames and initiates transmission of them. The procedure in the network receives the random access frames and provides the appropriate indication to layer 3.

Figure 5: Example of the data link layer configuration in the MS