ation may transmit. Because of the propagation delay on the wire, two or more stations may sense an idle channel and attempt to transmit simultaneously. This results in a collision. In order that all stations (including the one transmitting the packet) can "hear" the collision it is required that all packets be greater than a certain minimum size. That size is determined by a parameter called the "slot time". The slot time is slightly greater than the round trip propagation delay. Any station involved in a collision must stop sending the packet and reschedule the transmission. The algorithm used to determine when the next attempt should be made is called 88 TELECOMMUNICATIONS the backoff algorithm. Basically, every time a station is involved in a collision it backs off (i.e., waits) a random amount of time that increases every time it experiences a collision. The backoff time is reset after a successful transmission. One of the main reasons for Ethernet's popularity is because it uses a passive broadcast medium. This results in very reliable op- eration. Ethernet inferfaces can be built using VLSI technology and thus made fairly inexpensive. Multivendor environments can be im- plemented by adhering to interface specifications at any of several levels. For instance, one may choose to provide compatibility at the wire tap, transceiver cable or port. Ethernet is a shared bus network topology. The shared physical coaxial cable is passive and therefore highly reliable. The key active component which interfaces nodes is the transceiver. Ethernet is all- digital; there are no analog-to-digital conversions nor modems needed for transmission of data on the coaxial cable. Maintenance of the network and repair of faulty components can be accomplished without interrupting network operation. For example, any node can be powered off, repaired, and then powered back on again, without causing a disruption of service. Coaxial cable, in conjunction with distributed CSMA/CD net- work control, enables communications at ten million b