Bus structures differ from the others in that a link connects several sites, rather than just two. This means that a site cannot transmit over the link whenever it wants to, but must instead wait until the link becomes free.
Such systems must deal with the possibility of collision: if two sites are both waiting for the link to become free, they might both begin transmitting at the same time when the previous user finishes. To deal with this, a site must listen for a collision when it is transmitting, and if it detects one must stop transmitting, wait for a while, then start again. To prevent repeated collisions, each site waits for a random time before restarting.
An alternate approach to collisions, if the bus is logically (but not necessarily physically) structured as a ring, is token passing.