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Chapter 4: Basic Concepts and Management
Figure 4-2. MAC sub layer in physical layer in OSI model.
This diagram shows the Ethernet architecture, LLC sub-layer and MAC sub-layer, which operate at the Data Link layer, and
transceivers, which work at the Physical layer in the OSI model. In this section, we describe the MAC sub-layer.
4.2 Logical Link Control (LLC)
The data link layer consists of both the sub-layers of MAC and MAC-client. The MAC client may be logical link control or the
bridge relay entity.
Logical link control supports the interface between the Ethernet MAC and upper layers in the protocol stack, such as Network
layer. It can operate over different LAN technologies, such as Token Ring, FDDI, and so on. For the interface to the MAC layer, LLC
defines the services with the interface, independent of the medium access technology.
Table 4-1. LLC PDU format.
Parameter Value Description
DSAP address 8 bits Destination service access point address field
SSAP address 8 bits Source service access point address field
Control 8 or 16 bits
Control field (16 bits for formats that include
seqeuence numbering, and 8 bits for formats that
do not)
Information M*8 bits Information
* = Multiplication
M = An integer value equal to or greater than 0. (Upper bound of M is a function of the medium access control
methodology used.)
Table 4-1 shows the format of LLC PDU. It consists of four fields: DSAP, SSAP, Control, and Information. The DSAP address field
identifies one or more service access points that have an I/G bit for an individual or group address. If all bits of DSAP are 1s, it’s a
global address. The SSAP address field identifies the specific services indicated by the C/R bit (command or response). The DSAP
and SSAP pair with some reserved values indicate some well-known services listed in Table 4-2.