Layer 2 The Data Link Layer

The data link layer focuses on reliably getting data across any particular kind of link. Flow control and error notifications are also functions of the data link layer. The data link layer applies to all access methods, whether they are LAN or WAN methods. Information processed at this layer is commonly known as frames.

The IEEE further complicated matters by subdividing the data link layer into two sublayers: the Logical Link Control (LLC) sublayer and the MAC sublayer.

Figure 1-1 displays the IEEE definition compared to the ISO definition.

Figure 1-1 IEEE Sublayers Versus ISO Definitions

IEEE 802 Definition ISO Standard 3

Logical Link Control, LLC

Data Link Layer

MAC Sublayer

Physical Medium (Layer 1)

Physical Medium (Layer 1)

The LLC sublayer manages and ensures communication between end devices, and the MAC sublayer manages protocol access to the physical layer.

Examples of data link layer frame types include the following:

■ Integrated Services Digital Network (ISDN)

■ Synchronous Data Link Control (SDLC)

■ High-Level Data Link Control (HDLC)

■ Ethernet version 2

■ Bridge protocol data units (BPDUs) in the Spanning Tree Protocol (STP)

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