Contention

Contention

In pure contention-based access control, any computer can transmit at any time. This system breaks down when two computers attempt to transmit at the same time, in which case a collision occurs (see Figure 4.1). Eventually, when a network gets busy enough, most attempts to transmit result in collisions and little effective communication can take place.

Mechanisms, therefore, usually are put into place to minimize the effects of collisions. One mechanism is carrier sensing, whereby each computer listens to the network before attempting to transmit. If the network is busy, the computer refrains from transmitting until the network quiets down. This simple “listen before talking” strategy can significantly reduce collisions.

Another mechanism is carrier detection. With this strategy, com-puters continue to listen to the network as they transmit. If a computer detects another signal that interferes with the signal it’s sending, it stops transmitting. Both computers then wait a random amount of time and attempt to retransmit. Unless the network is extremely busy, carrier detection along with carrier sensing can manage a large volume of transmissions.

Carrier detection and carrier sensing used together form the protocol used in all types of Ethernet: Carrier Sense Multiple Access with Collision Detection (CSMA/CD). CSMA/CD limits the size of the network to 2,500 meters. At longer distances, the broadcast sensing mechanisms don’t work—a node at one end can’t sense when a node at the other end starts to broadcast.

Apple’s LocalTalk network uses the protocol Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA). Collision avoidance uses additional techniques to further reduce the likelihood of collisions. In CSMA/CA, each computer signals a warning that says it is about to transmit data, and then the other computers wait for the broadcast. CSMA/CA adds an extra layer of order, thereby reducing collisions, but the warning broadcasts increase network traffic, and the task of constantly listening for warnings increases system load.

Although it sounds as if contention methods are unworkable due to the damage caused by collisions, contention (in particular CSMA/CD in the form of Ethernet) is the most popular media access control method on LANs. (In fact, no currently employed LAN standards utilize pure contention access control without adding some mechanism to reduce the incidence of collisions.)

Contention is a simple protocol that can operate with simple network software and hardware. Unless traffic levels exceed about 30 percent of bandwidth, contention works quite well. Contention-based networks offer good performance at low cost.

Because collisions occur at unpredictable intervals, no computer is guaranteed the capability to transmit at any given time. Contention-based networks are called probabilistic because a computer’s chance of being permitted to transmit cannot be predicted. Collisions increase in frequency as more computers use the network. When too many computers use the network, collisions dominate network traffic, and few frames are transmitted without error.

All computers on a contention-based network are equal. Consequently, it’s impossible to assign certain computers higher priorities and, therefore, greater access to the network.

Contention access control is well-suited for networks that experience bursts in traffic—for instance, from large intermittent file transfers—and have relatively few computers.