Black Box LGB2124A manual Logical Link Control LLC, MAC sub layer in physical layer in OSI model

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 field (16 bits for formats that include
Control	8 or 16 bits	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.

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