04.053GPPData Link (DL) Layer General AspectsRelease 1999TS
The general layering principles used in the present document and other specifications in the 04 series are given in 3GPP TS 04.01.
The data link layer is the next to lowest layer of the OSI reference model. The data link layer receives services from the physical layer and provides services to layer 3.
The services provided by the data link layer are the combination of the services and functions provided by both the data link layer and the physical layer.
A data link layer Service Access Point (SAP) is the point at which the data link layer provides services to layer 3. The Service Access Point is identified by a Service Access Point Identifier (SAPI). One or more data link connection endpoints can be associated with each data link layer SAP. See figure 1. A data link connection endpoint is identified by a data link connection endpoint identifier (as seen from layer 3) and by a Data Link Connection Identifier DLCI (as seen from the data link layer).
SAPIs and DLCIs used by LAPDm are defined in section 5.2.
Figure 1: Entities, service access points and endpoints
Co‑operation between data link layer entities is governed by a peer‑to‑peer protocol specific to the layer. For information exchange between two or more layer 3 entities, an association must be established between the layer 3 entities in the data link layer using a data link layer protocol. This association is called a data link connection. Data link connections are provided by the data link layer between two or more SAPs (see figure 2).
Figure 2: Peer‑to‑peer relationship
Data link layer message units are conveyed between data link layer entities by means of physical connection.
Layer 3 requests services from the data link layer via service primitives. The same applies for the interaction between the data link layer and the physical layer. The primitives represent, in an abstract way, the logical exchange of information and control between the data link layer and its adjacent layers. They do not specify or constrain implementations.
The primitives that are exchanged between the data link layer and adjacent layers are of the following four types (see also figure 3).
NOTE: The same principle applies for data link layer‑ physical layer interactions.
Figure 3: Primitive action sequence
The REQUEST primitive type is used when a higher layer is requesting a service from the next lower layer.
The INDICATION primitive type is used by a layer providing a service to notify the next higher layer of activities related to the primitive type REQUEST.
The RESPONSE primitive type is used by a layer to acknowledge receipt, from a lower layer, of the primitive type INDICATION.
The CONFIRM primitive type is used by the layer providing the requested service to confirm that the activity has been completed.
The precise specification of Layer‑to‑layer interactions is given in 3GPP TS 04.06.
Information between peer entities and between entities in adjacent layers attached to the same SAP is transferred in two different types of message units:
‑ message units of a peer‑to‑peer protocol; and
‑ message units that contain layer‑to‑layer information concerning status and specialized service requests.
The message units of the layer 3 peer‑to‑peer protocol are carried by the data link connection. The message units containing layer‑to‑layer information concerning status and specialized service requests are never conveyed over a data link connection or a physical connection.
The present document introduces (see also figure 4):
a) the peer‑to‑peer protocol for the transfer of information and control between any pair of data link layer service access points;
b) the interactions between the data link layer and layer 3, and between the data link layer and the physical layer.
Figure 4: Data link layer reference model