The purpose of LAPDm is to convey information between layer 3 entities across the GSM PLMN radio interface using the Dm channel. Specifically LAPDm will support:

• multiple layer 3 entities;
• multiple physical layer entities;
• broadcast control channel (BCCH) signalling;
• paging channel (PCH) signalling;
• access grant channel (AGCH) signalling;
• dedicated control channel (DCCH) signalling.

The frame structure of data link layer messages is defined in TS 44.006. The Dm channel between a network and a specific MS may be distorted on several control channels, e.g. PCH, SDCCH and FACCH during a connection (See also TS 44.003). Selection and activation of these channels is performed by layer 3. LAPDm includes functions for:

1. the provision of one or more data link connections on a Dm channel. Discrimination between the data link connections is by means of a Data Link Connection Identifier (DLCI);
2. allowing recognition of frame types;
3. allowing layer 3 message units to be passed transparently between layer 3 entities;
4. sequence control, to maintain the sequential order of frames across a data link connections;
5. detection of format and operational errors on a data link;
6. notification to the layer 3 entity of unrecoverable errors;
7. flow control; and
8. contention resolution when establishing a data link after an access request has been made on the RACH.

Two types of operation of the data link layer are defined for layer 3 information transfer: unacknowledged operation and acknowledged (multiple frame) operation. They may co exist on a Dm channel. The BCCH and the (PCH + AGCH) will only support unacknowledged operation. The SDCCH, SACCH and FACCH will support both types of operation.

In unacknowledged operation, layer 3 information is transmitted in Unnumbered Information (UI) frames. At the data link layer, the UI frames are not acknowledged. Flow control mechanisms and error recovery mechanisms are not defined. Applicability of unacknowledged operation to different types of control channels is specified in subclause 6.1.

In acknowledged operation, layer 3 information is transmitted in frames that are acknowledged by the receiving data link layer. Error recovery procedures based on retransmission of unacknowledged frames are specified. In case of errors which cannot be corrected by the data link layer, a report is issued to the layer 3 entity. Flow control procedures are also defined. Applicability of acknowledged operation to different types of control channels is specified in subclause 6.1. Only one form of acknowledged information transfer is defined, i.e. multiple frame operation. For multiple frame operation, layer 3 information is sent in numbered Information (I) frames. In principle, a number of I frames may be outstanding at the same time. However, for many applications (e.g. signalling) a window size of 1 is required. Multiple frame operation is initiated by a multiple frame establishment procedure using a Set Asynchronous Balanced Mode (SABM) command. If an access request has been made on the RACH, the establishment procedure also contains functions for resolving any ambiguity that may arise as a result of this access method.

Multiple frame operation may be released in the following ways:

• normal release by exchange of commands/responses. This type of release is initiated by layer 3;
• local end release, i.e. without exchange of commands/responses, initiated and controlled by layer 3;
• abnormal local end release, i.e. without exchange of commands/responses, commanded by layer 3.

The release mode is indicated by layer 3. No release mechanism using exchange of commands/responses is defined for unacknowledged operation.

The data link layer provides services to layer 3 and utilizes the services provided by the physical layer. In the present document and TS 44.006 the following general syntax is used for describing primitives. where XX designates the layer providing the services. In the present document XX is DL and MDL for the data link layer and PH for the physical layer.

The services provided by the physical layer are described in detail in TS 44.004. They are summarized in the following:

1. physical layer connection for transparent transmission of frames. The bits of a frame are to be delivered to the peer data link entity in the same order in which they were submitted to the physical layer by the sender;
2. indication of the physical status of the Dm channel;
3. transmission of data link layer message units in the same order as they were issued by the data link layer;
4. provision of frame synchronization;
5. provision of error protection to ensure a low residual bit error rate at the data link layer;
6. transmission (in the MS) and reception (in the network) of random access bursts.

The primitives between the data link layer and the physical layer are:

1. Data transfer:
   PH DATA REQUEST/INDICATION
   These primitives are used to request that a message unit be sent and to indicate the arrival of message unit. Parameters associated with these primitives are the data link layer message unit, the priority and the type of channel being used.
2. Random access:
   PH RANDOM ACCESS REQUEST/INDICATION/CONFIRM
   The REQUEST primitive is used to request (in the MS) that a random access frame be sent and the INDICATION primitive is used to indicate (in the network) the arrival of a random access frame. A parameter associated with these primitives is the random access message unit. The CONFIRM primitive is used (in the MS) to confirm in which time slot the random access burst was sent.
3. Connection establishment:
   PH CONNECT INDICATION
   This primitive is used to indicate that a specific physical resource has been established on the physical layer. The parameter associated with this primitive is the type of channel.
4. Transmission synchronization:
   PH READY TO SEND INDICATION
   This primitive is used to enable the data link layer to synchronize to the next instant of physical transmission. The parameter associated with this primitive is the type of channel.
   PH EMPTY FRAME REQUEST
   This primitive is used by the data link layer instead of the PH DATA REQUEST primitive when no frame has to be sent after receiving the PH READY TO SEND indication. The parameter associated with this primitive is the type of channel.

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 = 0 and the data link connection for a SACCH when associated with a TCH, or a SDCCH otherwise, 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.

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 TS 44.018 and TS 24.010: SAPI = 0;
• short message services as defined in TS 24.011: SAPI = 3.

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

There is at most one 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 TS 44.006) 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.

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).

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.

The various types of channels shall support SAPIs and modes of operation as follows.