OSI Models Data Link Layer

The data link layer reliably transits data across a physical network link. Different data link layer specifications define different network and protocol characteristics, including physical addressing, network topology, error notification, frame sequencing, and flow control. Physical addressing (as opposed to network addressing) defines how devices are addressed at the data link layer. A network topology consists of the data link layer specifications that often define how devices are to be connected physically, such as in a bus or 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. Finally, flow control moderates data transmission so that the receiving device is not overwhelmed with more traffic than it can handle at one time.

The Institute of Electrical and Electronics Engineers (IEEE) has subdivided the LAN data link layer into two sublayers: Logical Link Control (LLC) and Media Access Control (MAC), as illustrated in Figure C-7.

Figure C-7 LAN Data Link Layer Contains Two Sublayers

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LLC

Sublayer

Data Link

Layer

MAC

Sublayer

->-

The data link layer's LLC sublayer manages communications between devices over a single network link. LLC, which is defined in the IEEE 802.2 specification, 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. The data link layer's MAC sublayer manages protocol access to the physical network medium. The IEEE MAC specification defines MAC addresses, thereby enabling multiple devices to uniquely identify each other at the data link layer.

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