TechNote

Number: 08301-04
Date: May 1994

LAN standards

Within the communications industry, one of the biggest concerns is to design devices so that they can communicate with each other regardless of who manufactured them. The International Standards Organisation (ISO) developed a model which provides a framework for the design of networking protocols and services. This model is known as the International Standards Organisation's Open Systems Interconnection (ISO OSI) reference model. Theoretically, any computer systems, from any vendors, will be able to communicate if they comply with the ISO OSI model. This model incorporates seven layers, with each layer providing a certain set of services for overall communication. Each layer is independent of the others, which allows it to be changed without affecting the other layers. Figure 1 illustrates the seven layers of this model and explains the function of each layer. 

OSI Layer
Application     Provides services such as file transfer and resource 
                sharing to the application program running

Presentation    Translates data formats so that stations with different 
                data formats can communicate. For example, it provides 
                code conversion where one station uses ASCII code
                for character storage and the other uses EBCDIC.

Session         Responsible for establishing and maintaining a session. 
                The period of time two or more stations remain logically 
                connected together is known as a session.

Transport       Hides the network-dependent characteristics from the 
                layers above it. It provides connection management, that 
                is, establishes and maintains a logical connection between 
                it and corresponding transport layers in another station.

Network         Provides routing capabilities across a network; including 
                standardizing addressing.

Data link       Defines how the medium is accessed and provides error 
                control.

Physical        Defines the transmission medium, transmission method, and 
                transmission rates available for the network.
Figure 1: The OSI 7-layer model and the function each layer performs

IEEE 802 standards

IEEE set up a committee, called 802, to produce standards for LANs. The model that they designed contains three layers. The standards that make up these three layers provide the services necessary for stations on the LAN to communicate with each other. Each layer also provides one or more Service Access Points (SAPs) which enables a layer to communicate with the layer above and below it. Figure 2 shows how the IEEE model corresponds to the OSI 7-layer model. 

OSI Layer      IEEE 802 Layer
Application

Presentation

Session

Transport

Network

Data link      Logical link control
               Medium access control

Physical       Physical
Figure 2: The OSI and IEEE layer models

Physical layer

The lowest IEEE layer, the Physical layer, is responsible for:

The physical medium can use one of two signalling techniques: broadband or baseband:

Broadband:
The medium can carry more than one signal simultaneously by using different frequency channels for different data.

Baseband:
The signal is digitally transmitted with the whole medium carrying one signal.

MAC layer

The layer above the physical layer is called the Medium Access Control (MAC layer). It is responsible for access to the transmission medium. IEEE specify three mutually exclusive standards for this layer:

802.3   Defines an access method called CSMA/CD, 
        commonly known as Ethernet

802.4   Defines a token-passing access method that 
        runs on a bus topology

802.5   Defines a token-passing access method that 
        runs on a ring topology, commonly
        known as Token ring
Table 2: MAC Standards 

IEEE standard   802.3 CSMA/CD       802.4 Token-bus   802.5 Token-ring

Bandwidth       10 Mbps             1, 5 or 10 Mbps   4 or 16 Mbps

Cable           10 Base 5 -         75 ohm coaxial    STP (type 1)
                thick wire coaxial
                                                      UTP (type 3)
                10 Base 2 -
                thin wire coaxial

                10 Base T - UTP
Table 3: IEEE standards

LLC layer

The Logical Link Control Layer (LLC), corresponds to the upper half of the data-link layer of the OSI model. The LLC layer is responsible for establishing, maintaining, and terminating the logical link, and also for error checking.