ENTERPRISE NETWORKS I: LOCAL AREA NETWORKS
LAN 802 architecture related to OSI.
medium access control (MAC). These relationships are shown in Figure 11.2. The princi-
pal 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-
3. On reception, they disassemble the frame and perform address recognition and
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 stan-
dardized 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.