How LAN Protocols Relate to OSI

LAN protocols utilize only OSI layers 1 and 2, the physical and data-link layers, respectively. The data-link layer is split into two sublayers: logical link control (LLC) and

OS I

OSI simplified

7 Application

Network layers

Network layers not defined

6 Presentation

5 Session

4 Transport

Data link

Logical link control

3 Network

Media access control

2 Data link

1 Physical

Physical

Physical

OS I

OSI simplified

IEEE 802

IEEE 802

802.2

Figure 11.2 LAN 802 architecture related to OSI.

medium access control (MAC). These relationships are shown in Figure 11.2. The principal functions of OSI layer 3, namely switching, relaying, and network end-to-end control, are not necessary in this simple, closed network. The remaining layer 3 functions that are necessary are incorporated in layer 2. The two layer 2 sublayers (LLC and MAC) carry out four functions:

1. They provide one or more service access points (SAPs). A SAP is a logical interface between two adjacent layers.

2. Before transmission, they assemble data into a frame with address and error-detection fields.

3. On reception, they disassemble the frame and perform address recognition and error detection.

4. They manage communications over the link.

The first function and those related to it are performed by the LLC sublayer. The last three functions are handled by the MAC sublayer.

In the following subsections we will describe four common IEEE and ANSI standardized protocols. Logical link control (LLC) is common to all four. They differ in the medium access control (MAC) protocol.

A station on a LAN may have multiple users; oftentimes these are just processes, such as processes on a host computer. These processes may wish to pass traffic to another LAN station that may have more than one "user" in residence. We will find that LLC produces a protocol data unit (PDU) with its own source and destination address. The source address, in this case, is the address of the originating user. The destination address is the address of a user in residence at a LAN station. Such a user is connected through a service access point (SAP) at the upper boundary of the LLC layer. The resulting LLC PDU is then embedded in the information field of a MAC frame. This is shown in Figure 11.3. The MAC frame also has source and destination addresses. These direct traffic to a particular LAN station or stations.

User data

1

1

LLC header

LLC info field

MAC header

MAC info field

To physical layer

To physical layer

Figure 11.3 A user passes traffic to an LLC where encapsulation takes place forming an LLC PDU. The LLC PDU is embedded in a MAC frame info(rmation) field. The resulting MAC frame is passed to the physical layer, which transmits the traffic on the LAN.

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