Data Link Layer
Data Link layer
provides reliable delivery of data across the network medium & translates messages from the network layer in bits for the physical layer. Its specifications define different network and protocol characteristics, including physical addressing, error notification, network topology and sequencing of frames.
Data link protocols provide the delivery across individual links and are concerned with the different media types such as 802.2 and 802.3. The data link layer is responsible for putting 1s and 0s into a logical group. These 1s and 0s are then put on the physical wire. Some examples of data link layer implementations are IEEE 802.2/802.3, IEEE 802.5/802.2, packet trailer (for Ethernet, this is the FCS or CRC), HDLC and Frame Relay.
Two sub layers of data link layer defined by IEEE are ….
- Logical Link Control (LLC) sub layer
- Media Access Control (MAC) sub layer
Logical Link Control
The Logical Link Control (802.2) sub layer is responsible for identifying different network layer protocols and then encapsulating them to be transferred across the network. There are two types of LLC frames: Service Access Point (SAP) and Subnetwork Access Protocol (SNAP) An LLC header conveys to the data link layer what to do with a packet once its received.
Media Access Control
The MAC sublayer specifies how data is placed and transported over the physical wire. The LLC layer communicates with the network layer, but the MAC layer communicates downward directly to the physical layer addresses), network topologies, error notifications and delivery of frames are defined at this sublayer