### 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:

- Encoding and decoding signals
- Transmitting and receiving bits
- Defining the transmission medium

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.