9.6 LAN Protocols

The local area network (LAN) connects computers that are relatively close to each other. The different topologies discussed earlier are all possible ways of connecting computers to create a LAN, but the bus and ring arrangements are the most popular methods. Again, to facilitate the flow of information in a LAN network, LAN protocols are needed. In this section two of the popular protocols are discussed: the Ethernet protocol and the token ring protocol.

Ethernet

The Ethernet protocol operates on a bus topology (see figure 9.4). The Ethernet protocol is based on a technique known as CSMA/CD (Carrier Sense Multiple Access with Collision Detection). Though the earlier versions of the Ethernet protocol provided data transfer rates or bandwidth in the range of 10 Mbps to 100 Mbps, the newer versions referred to as gigabit Ethernet can achieve bandwidths in the range of 1 Gbps to 10 Gbps. The gigabit versions have somewhat progressed from the initial CSMA/CD technique. Carrier sense describes the process by which each of the devices attached to the Ethernet network ‘listen’ to the network or bus to see if any other device is transmitting on the bus. This is necessary as only one device can transmit at a time on a shared bus, or else there would be a collision amongst the multiple signals. Multiple access recognizes the fact that all devices or PCs attached to the Ethernet network have equal access to the network or the shared bus. There is no hierarchy, or higher priority given to any one device on the Ethernet network. What collision detect involves is that when one device (e.g. PC1) is transmitting, and a second device (e.g. PC2) also begins to transmit at the same time, then Ethernet rules forces the device that recognizes this impending collision first to stop transmitting immediately and to send out what is known as a jamming signal on the bus. This alerts the second device of the collision that has taken place on the bus, forcing that device to stop transmitting immediately. Both devices then hold-off from transmitting for an individually and randomly selected duration after which they try again to transmit on the shared bus. Given that each device holds off from transmitting for a random period of time that is different from the other, the chances are that the next time they try transmitting, they will not encounter a collision. Of course as you increase the amount of devices attached to the shared bus, the amount of traffic increases, resulting in an exponential increase in collisions. This could ultimately reduce the efficiency of the network down to less than fifty percent.

Token Ring

The token ring protocol operates on a ring topology (see figure 9.5). In this approach a bit pattern called the token is passed around the ring continuously. When a computer is in possession of the token it is permitted to transmit data by attaching the data to the token and then passing it on to the next computer on the ring. If a computer receiving the token has any data attached to it that is addressed to it, then it will remove it. Once the data is removed by the computer, it can attach its own transmitting data and then pass the token and the data to the next computer and subsequently to the other computers on the ring. Through this process, data communication is established between the computers on the ring.

Token Bus

The token passing concept of the token ring, where a token is passed systematically between computers, could also be implemented on a bus network. In such an arrangement only the computer with the token is permitted to transmit data on the network.  If a computer needs to transmit data, it must wait to receive the token before it can begin transmission.   In this approach a systematic data transfer method is used instead of the random process used by the Ethernet protocol.