Network Access Methods

This section describes the two most commonly used network access methods: carrier sense multiple access with collision detection ( CSMA / CD ) and token ring . Also briefly mentioned here is a middle-ground method called token bus .

CSMA/CD

Carrier sense multiple access with collision detection is the most commonly used access method in ethernet networks and is described in the IEEE 802.3 Ethernet standard. Since ethernet relies on broadcasts, any of the hosts connected to the network can transmit at any time, but the CSMA part listens to the network to see whether anyone else is transmitting. If the network is empty, the host starts sending its data; if it's busy, it waits for a random time before trying again. Having started sending, the CD part of the protocol detects whether any other host starts sending. If it detects activity, a collision is said to have occurred and both hosts stop sending, wait a random time, and then try again. The random time is based on time slots, which are usually set to 512-bit times, which is 51.2 microseconds (the time it would take to send 512 bits). The number of time slots increases exponentially if further collisions occur, thereby reducing the chances of another collision. If a host fails to send a data packet 10 times, the number of time slots to wait is fixed at 1,023. If it fails 16 times, it gives up and reports a failure. The algorithm used to determine how long a host waits before trying to retransmit is called binary exponential backoff .

CSMA/CD-based networks perform very well on lightly loaded networks, but they suffer higher collision rates when the network is heavily used. This leads to more retransmissions of data and, hence, worse performance.

Token-Ring

The token-ring access method, described in the IEEE 802.5 standard, is used mainly in IBM environments in which a token travels around the network in a predetermined order. If a host has data to send, it inserts its data onto the network when it has the token. Token-ring networks perform very well when the network is heavily used because there are no collisions and only one host sends data at a time. The disadvantage of using token-ring networks is that performance is not very good when the network is lightly loaded because each host has to wait for the token to reach it.

Token Bus

The token bus access method, described in the IEEE 802.4 standard, uses a bus topology, but also implements a token passing system that is similar to the token-ring access method. One main difference, however, is that because a broadcast medium is being used, all packets are received by all hosts. The token bus method also supports four classes of priority, enabling a host to send its highest-priority data first. As with token-ring networks, each host can send only a finite amount of data before relinquishing the token to the next host.