27.4 Access Methods

An access method is used to access physical transmission medium and send data over it. Computers connected to a network don't start sending data over network transmission media of their own volition. Before a host on the network starts sending data, it must obey some rules to access the transmission media. This is to ensure that the transmission media are used by all hosts in an orderly fashion.

Different types of access methods are used. CSMA/CD and token ring are discussed here.

CSMA/CD

The Carrier Sense Multiple Access with Collision Detection (CSMA/CD) method is used to check the medium before starting the transmission. When a host wants to send data on the network, it first senses if another host is already sending data at that time. In case another host is sending data, it waits. If no host is sending data, it starts sending data on the network transmission medium.

Sometimes it may happen that two or more hosts listen to the transmission medium and find that no one is transmitting data. They then start sending their own data simultaneously . A collision occurs in such a case. To resolve such a condition, each host that sends data also listens simultaneously to detect if any collision occurred. In case of a collision, all hosts wait for a random time and retry data transfer.

The CSMA/CD method is a broadcast method and is useful for connecting large numbers of hosts. It is the standard access method for Ethernet and IEEE 802.3 networks.

Token Passing

The token passing method uses a data frame as the token. This token is passed from one host to another in a network in an orderly fashion. When a host needs to transmit data, it waits for the token. When a host gets the token, it can transmit its data. If a host has no data to transmit, it passes the token to the next host.

The token passing method guarantees network access to every host. It can be used to transmit data at a higher rate. Priorities can also be used in token passing networks. IEEE 802.5 networks use the token passing access method. It can also be used in a bus topology, known as a token bus network.

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