Ethernet and IEEE 8023

Ethernet was invented by Xerox Corporation in 1972. It was further redefined by Digital, intel and Xerox in 1980 and renamed as Ethernet Version I or DIX (Digital, Intel and Xerox). Ethernet is the most commonly used LAN technology today, accounting for more than 83 percent of the installations world wide. Ethernet still enjoys continued popularity and growth. Some of the reasons are :

1. Least expensive

2. It is fast enough for the vast majority of applications in use

3. It continue to keep pace with other LAN technologies

4. Its various standards supporting a wide variety of media

5. Sufficiently defined standards

6. Ethernet is considered to be the most user friendly.

Ethernet Reference Model. The IEEE developed the standards for Ethernet in 1984. It is called IEEE 802.3 and uses the bus topology. The function of Ethernet is as follows.

The Ethernet system consists of three basic elements. They are the physical media (the ether) used to carry signals, a set of rules, embedded in each Ethernet interface and the Ethernet frame that consists of bits used to carry data, control and address information. Each station in the network consists of Network Interface Card (NIC). This card is connected to the Ethernet cable via a transceiver cable. The NIC is also called as Ethernet controller. As there is no central controller, Ethernet LAN operates independently of all other stations.

When a station transmits a frame on the bus, all stations connected to the network will copy the frame. Ethernet signals are transmitted serially, one bit at a time over the bus. Each station checks the address of the frame. If it matches the station's NIC address, it accepts the frame, otherwise the station discards the frame.

Fig. 11.20 shows the Ethernet reference model. It shows, how Ethernet fits into the OSI


Network layer

Logical link control (LLC) IEEE 802-2

Media access control (MAC) IEEE 802-3

Physical layer

/ Data link layer

Fig. 11.20. Ethernet reference model. The data link layer is divided into two sublayers. They are :

1. Logical link control (LLC) and

2. Media Access control (MAC).

LLC and MAC work together to formulate an ISO data-link layer protocol. The LLC and MAC field together with data and additional fields form the final message format.

LLC layer. The LLC layer is designed to establish a logical connection between source and destination. The IEEE standard for LLC is IEEE 802.2. The responsibilities of LLC sublayer are :

(a) establishing and terminating a communication link

(b) Supplying a frame format for the pay load

(c) detecting and correcting errors and

(d) Maintaining control over the traffic flow.

The establishment of communication link at LLC may be connection oriented (LLC 1) or connectionless (LLC 2). In LLC 1, data link connection is established before data is sent. LLC 2 establishes the connection path when the first frame being transmitted. The basic format of LLC is shown in Fig. 11.21.



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