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The Data Link layer provides reliable transit of data across a physical network link.  Different Data Link layer specifications define different network and protocol characteristics, including physical addressing, network topology, error notification, sequencing of frames, and flow control.  The Data Link layer translates messages from the Network layer into bits for the Physical layer to transmit.  It formats messages into data frames and adds a customized header containing the source and destination hardware addresses.  Data Link layer is responsible for uniquely identifying each device on a local network.

• Physical addressing (as opposed to network addressing) defines how devices are addressed at the data link layer.
• Network topology consists of the data link layer specifications that often define how devices are to be physically connected, such as in a bus or a ring topology.
• Error notification alerts upper-layer protocols that a transmission error has occurred, and the sequencing of data frames reorders frames that are transmitted out of sequence.
• Flow control moderates the transmission of data so that the receiving device is not overwhelmed with more traffic than it can handle at one time.

-When a packet is sent between routers, it is framed with control information at the Data Link layer.  The information is removed at the destination router and only the original packet remains.  If the packet is to go to another router, the framing process is repeated until it gets to the receiving host.  The packet is never altered, only encapsulated with control information to be passed on to the different media type.

The IEEE has subdivided the data link layer into two sublayers: Logical Link Control (LLC) and Media Access Control (MAC).

MAC (Media Access Control)

The Media Access Control (MAC) sublayer of the data link layer manages protocol access to the physical network medium. The IEEE MAC specification defines MAC addresses, which enable multiple devices to uniquely identify one another at the data link layer.

--The MAC describes how a station schedules, transmits and receives data on a shared media environment.

--Ensures reliable transfer of information across the link, synchronizes data transmission, recognizes errors (doesn't correct them), and controls the flow of data.

--Defines how packets are placed on the media.

--Physical addressing is defined here as well as local topologies.

--MAC example is Ethernet/802.3 and Token Ring/802.5

--Line discipline, error notification, ordered delivery of frames, and optional flow control can be used at this layer.

--In General, MACs are only important in shared medium environments where multiple nodes can connect to the same transmission medium.

LLC (Logical Link Control)

The Logical Link Control (LLC) sublayer of the data link layer manages communications between devices over a single link of a network.  LLC is defined in the IEEE 802.2 specification and supports both connectionless and connection-oriented services used by higher-layer protocols.  IEEE 802.2 defines a number of fields in data link layer frames that enable multiple higher-layer protocols to share a single physical data link.

--Responsible for identifying Network layer protocols and encapsulating them.

--A LLC header tells the Data Link layer what to do with a packet once it is received.

Switches and Bridges

• Work at Data Link layer and filter network using MAC addresses.  Layer 2 switching is hardware based switching because it uses an ASIC (Application Specific Integrated Circuit).
• Switches and Bridges read each frame as it passes through, it then puts the source address in a filter table and keeps track of which port it was received on.  This tells the switch where that device is located.
• After a filter table is built, the device will only forward frames to the segment where the destination address is located.  If the destination device is on the same segment as the frame, the layer 2 device will block it from being forwarded.  If the destination is on another segment, the frame will only be forwarded to that segment (transparent bridging).
• When a layer 2 device receives a frame and the destination is unknown to the device's filter table, it will forward the frame to all connected segments.  If the unknown device replies, the filter table is updated with that device's location.

-Layer 2 devices (Switches / Bridges) propagate broadcast storms and the only way to prevent them is with a router.

-Each port on a switch is in its own collision domain.

-Switches allow all segments to transmit simultaneously.

-Switches can't translate different media types.