What are the LAN access methods? Describe their working.

Solution

LAN Access Methods

LAN access methods are involved in determining who gets to use the network and when they get to use it. There are two main approaches to LAN access: token passing and Carrier Sense Multiple Access/Collision Detection (CSMA/CD).

Token Passing

Token passing is used with the token-ring and FDDI architectures. Here is how token ring works:

1. The active master, chosen through a process called beaconing, inserts a token (i.e., a specially formatted packet) into the ring.

2. The token circulates around the ring and is regenerated by each workstation it passes. (In token-ring networks, a workstation obtains data from only its upstream neighbor, regenerates it, and sends it to its downstream neighbor.)

3. When a workstation has data to send, it waits for the token to pass by and grabs it off the wire, holding it until it has finished its transmission. The station then injects its data packets onto the wire.

4. The packets circulate around the ring and are examined and regenerated by each workstation. When the receiving workstation receives the packets, it marks them as received when it regenerates and reinjects them. This marking notifies the sender that the data was in fact received when it finally returns to the sender. The sender then generates a new token and injects it into the ring. To prevent any particular workstation from hogging the ring, a transmitting station can hold the token (and thus transmit data packets) for a specific interval, called the token hold time. If the time expires before the station has transmitted all its information, it must stop transmitting and put a new token back on the ring so that other stations have a chance to communicate. When the token gets back to the waiting workstation, it can resume transmitting.

A benefit of token passing is that it is a deterministic technique: You can always calculate the maximum delay that you'll encounter in moving information between any two points on that network, and this is especially important for applications with defined response times, such as process control. For example, an oil pipeline may have sensors in the pipelines to detect minute leakage. The oil company wants to know exactly how long it will take for the alarm from the sensor to reach the control station to shut off the valve, in order to avoid leaking oil into the community. In a LAN where there's a need to determine the delay, token passing works very well. The disadvantage of token passing is that it occurs in a unidirectional ring, so it takes time to pass the tokens. A device has to wait until it receives a token before it can send it, and if the ring is broken because a device goes down, the ring is then unable to send tokens until the ring is recovered (i.e., the failed device is either taken out of commission or reinitiated). Today, almost all token-ring interfaces include an electrical shunt to prevent this from happening.

CSMA/CD

CSMA/CD is used with the Ethernet standard. CSMA/CD is a nondeterministic access method, meaning that any device can send whenever it determines that the network is clear. However, each device must listen to the network at all times because there is the potential for a collision. If a collision occurs, both sending devices back off the network and wait a random number of nanoseconds or milliseconds before attempting to retransmit.

CSMA/CD works as follows:

1. The terminals listen to the network.

2. Both of the terminals assume that the network is clear. They both start sending, but they continue listening because they know there's a chance that their messages will collide.

3. The messages do collide, so both terminals produce a jam signal, which carries a bit pattern sent by a data station to inform the other stations that they must not transmit.

4. One terminal waits 20 milliseconds, and the other waits 50 milliseconds, and then they again attempt to transmit assuming that their messages won't bump into each other. Because the terminals waited different amounts of time, most likely the messages don't collide and therefore get transmitted. However, if the messages collide again, the terminals repeat the back-off and random-wait procedure up to 16 times before the frames are dropped.

In this type of environment, the more devices added and the greater the traffic volumes, the more likely collisions will occur. When collisions start to exceed the available throughput, you need to start segmenting the LAN into smaller clusters, called collision domains in Ethernet parlance, or consider making the move to switched Ethernet LANs.