Finite Propagation Delays

• Collisions can still occur even with carrier sensing because of finite propgation delays.
• The time it takes for a signal to travel from one end of the cable to the other is not zero but on the order of a few bit periods.
• Hence, a station at one end of the ethernet cable, can not detect that a station at the other end has started to transmit until a few bit periods later.
• In the mean-time, it may have found the channel to be idle and started to transmit.
• Notice, also that the station at the other end will not be able to find out that a collision has occured until the colliding signal has propagated the entire length of the channel.
• Therefore, it takes at most twices the time for a signal to travel the length of the cable until all stations are aware of the collision.
• To ensure reliable operation, the propagation delay must be kept small.
• For that reason, an ethernet cable is not allowed to be longer than 500 m.
• The ethernet protocol is standardized in IEEE 802.3.