Bus Access Methods

Ethernet uses the CSMA/CD access method (Carrier Sense Multiple Access / Collision Detection). With this method, the station that wants to transmit data, first “listens” to the carrier whether data are currently being transmitted by another station (carrier sense). If the carrier is busy, the station later tries to access the carrier again. If the carrier is idle, the station starts the transmission.

With this method, particularly in greater networks, it can happen that several stations try to transmit at the same time (multiple access). As a result, they “listen” to the carrier, detect that the carrier is free and correspondingly start the transmission. This can cause collisions between the different data packages. This is why each station verifies whether a collision occurred during transmission (collision detection). If this is the case, the station aborts the transmission and then tries to send its data again after a wait time which is determined by a random generator.

Collisions within an Ethernet network do not cause loss of data, but they reduce the available bandwidth of the network. In practice, for a network with 30 stations on the bus, a net bandwidth of approx. 40 % is assumed. This means that a bandwidth of only approx. 4 Mbit/s is available in a network with a theoretical bandwidth of 10 Mbit/s, for instance. This has to be considered when planning an Ethernet network. The number of collisions can be reduced to a minimum if the network is carefully planned and if only suitable network components are used (refer to 1.4 Cabling and 1.5.4 Media converters).