OSI Data Link Layer Concepts

OSI Data Link Layer Concepts

As you learned in the preceding section, the OSI Physical layer is concerned with moving messages at the machine level. Network communication, however, is considerably more involved than moving bits from one device to another. In fact, dozens of steps must be performed to transport a message from one device to another.

Real messages consist not of single bits but of meaningful groups of bits. The Data Link layer receives messages, called frames, from upper layers. A primary function of the Data Link layer is to disassemble these frames into bits for transmission and then to reconstruct the frames from the bits received.

The Data Link layer has other functions as well, such as addressing, error control, and flow control for a single link between network devices. (The adjacent Network layer, described later in this chapter, handles the more complex tasks associated with addressing and delivering packets through routers and across an internetwork.)

The IEEE 802 standard divides the Data Link layer into two sublayers:

Media Access Control (MAC). The MAC sublayer controls the means by which multiple devices share the same media channel. This includes contention methods (see Chapter 4) and other media access details. The MAC layer also provides addressing information for communication between network devices.
Logical Link Control (LLC). The LLC sublayer establishes and maintains links between communicating devices.

Further Information