The Data Link layer deals with issues on a single segment of the network, the Network layer deals with issues spanning networks.

Regenerates the signal at the packet level per the 802 standards, including the topics covered in 802.2, 802.3, and 802.5, see next page for more on 802 standards.

802.2 - LLC (Logical Link Control manages link control and defines SAPs); Adds header fields to identify upper-layer protocols. It provides reliable, intelligent information to otherwise dumb frames. Also, acts as a switch board to make sure MAC frames find their way to the right Network layer process. 802.3 - (MAC communicates with adapter card)

Responsibilities of the Data Link layer:

The lower sub-layer of the Data Link layer, the Media Access Control (MAC), performs Data Link layer functions related to the Physical layer, such as controlling access and encoding data into a valid signaling format.

The upper sub-layer of the Data Link layer, the Logical Link Control (LLC), performs Data Link layer functions related to the Network layer, such as providing and maintaining the link to the network.

MAC

The Media Access Control (MAC) sub-layer contains methods that logical topologies can use to regulate the timing of data signals and eliminate collisions.

The MAC address concerns a device's actual physical address, which is usually designated by the hardware manufacturer. Every device on the network must have a unique MAC address to ensure proper transmission and reception of data.

Bridges and LAN switches, which deal with the Data Link layer, specifically filter or forward traffic based on the MAC addresses.

LLC - Logical Link Control

The MAC and LLC sub-layers work in tandem to create a complete frame. The portion of the frame for which LLC is responsible is called a Protocol Data Unit (LLC PDU or PDU).

LLC supports the following three types of connections for transmitting data:
- Unacknowledged connectionless service:does not perform reliability checks or maintain a connection, very fast, most commonly used
- Connection-oriented service. Once the connection is established, blocks of data can be transferred between
nodes until one of the nodes terminates the connection.
- Acknowledged connectionless service provides a mechanism through which individual frames can be acknowledged

For more information on Networking Basics, the OSI Model and Server Administration:
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