The set of rules defining how a computer puts data onto the network cable and retrieves data from the cable is called an access method. Access methods prevent simultaneous access to the cable. By ensuring that only one computer can put the data on the network cable, access methods keep the transmission of network data an orderly process.
In the Carrier-Sense Multiple Access with Collision Detection (CSMA/CD) method, each computer on the network senses any signal passing on the cable. The computer can only transmit if the cable is free. If two or more computers send data at exactly the same time, it will result in data collision. When a collision occurs, the two computers involved stop transmitting and then attempt to re-transmit after a certain period of time. The collision detection method is only effective up to 2,500 meters.
With more traffic, collisions tend to increase, which slows down the network. So, CSMA/CD can be a slow access method.
In the Carrier-Sense Multiple Accesses with Collision Avoidance (CSMA/CA) method, each computer signals its intent to transmit before it actually transmits data. This way, the computer can avoid collisions. CSMA/CA is a slower access method and is less popular than CSMA/CD.
In Token Passing, a special packet called a token circulates around the ring network in one direction. When any computer on the network wants to send data, it takes control of a free token. The computer can then transmit data. Until the token has been released by the computer sending data, no other computer can transmit. As only one computer can use the token at a time, the possibility of a collision is ruled out.
Demand priority is an access method designed for the 100 Mbps Ethernet standard called 100 VG-AnyLAN. The 100VG-Anyl_AN network comprises repeaters and end nodes. An end node could be a computer, bridge, router or switch. The function of the repeater is to search for requests from all nodes on the network. In a demand priority network, computers can receive and transmit at the same time. This is because four pairs of wires are used in the cabling scheme defined for this method. In demand priority, communication is only between the sending computer, the hub or the repeater, and the destination computer. This method is more efficient than CSMA/CD, which sends transmission to the entire network.
Access method is a set of rules defining how computer puts data onto the network cable and retrieves data from the cable.