Specifications

Ethernet Media Access Method 5-3
Packet Involved in a Collision
When a collision is detected, both stations will transmit a jam signal that is
long enough to ensure that the collision is detected by all stations on the
network. Then, each station involved in the collision will wait for a
random period of time and then attempt transmission again. The station
will attempt again transmission up to 16 consecutive times before an error
is sent to the upper layer protocols notifying the station of a serious
communication problem.
Collision Detection on Point-to-Point Media
On Point-to-Point media, it is not possible to detect a collision by listening
to the transmission as with multi-point media. Point-to-point media
transceivers use a method called Transmit Mode Collision Detection. With
this method, the transceivers will monitor their receive ports while
transmitting. If they receive a signal while transmitting, then a collision
has occurred.
Out-Of-Window Collision
As mentioned in Chapter 3, from any station on the network, a
transmitted frame has 25.6 µs to get to the end of the collision domain. If a
collision were to happen at the farthest point from the transmitting station
(25.6 µs away), the collision signal will take an additional 25.6 µs to
propagate back to the transmitting station for a total of 51.2 µs round trip
time (or the time it takes to transmit a minimum size frame). If a station is
able to transmit for 51.2 µs without detecting a collision, the station should
have acquired the communications channel and its signal should be the
only one using the network. If a collision is detected after the station has
transmitted the required minimum frame size, an Out-Of-Window (OOW)
collision has occurred. In other words, the station has transmitted for
51.2 µs without a collision but senses a collision after 51.2µs has passed.
Out- Of-Window collisions indicate abnormal network operation. They
are usually caused by the network being too long where the round trip
propagation delay is greater than 51.2 µs, a station somewhere on the
network is transmitting at will, or a cable somewhere on the network
failed during the transmission of the frame.
In the following chapters we will look at the importance of propagation
behavior. Propagation delay is discussed in more detail in Chapter 8,
Propagation Delay.