This above diagram shows the Ethernet architecture, LLC sub-layer and MAC sub-layer, which are
responded to the Data Link layer, and transceivers, which are responded to the Physical layer in OSI
model. In this section, we are mainly describing the MAC sub-layer.
3-2. Logical Link Control (LLC)
Data link layer is composed of both the sub-layers of MAC and MAC-client. Here MAC client may
be logical link control or bridge relay entity.
Logical link control supports the interface between the Ethernet MAC and upper layers in the
protocol stack, usually Network layer, which is nothing to do with the nature of the LAN. So it can operate
over other different LAN technology such as Token Ring, FDDI and so on. Likewise, for the interface to
the MAC layer, LLC defines the services with the interface independent of the medium access technology
and with some of the nature of the medium itself.
The table 3-1 is the
format of LLC PDU. It comprises four fields,
DSAP, SSAP, Control and Information. The DSAP address field identifies the one or more service access
points, in which the I/G bit indicates it is individual or group address. If all bit of DSAP is 1s, it’s a global
address. The SSAP address field identifies the specific services indicated by C/R bit (command or
Coaxial/STP/UTP
IEEE 802.2 LLC
IEEE802.3 CSMA/CD MAC
IEEE 802.3 PLS
ANSI X3T9.5 PMD
CS
IEEE 802.3
MAU
Physical
Layer
Data
Link
Layer
MII
Fiber
Table 3-1 LLC Format
