People working everyday with networks are familiar with the term ‘collision’.
About Ethernet Collisions
– Any event that takes place over an Ethernet network when two data stations happen to speak simultaneously on the cable is termed as a collision.
– Experiencing collisions is a very common thing for an Ethernet network.
– Also, under a number of circumstances it is not considered to be a problem.
– There are two major types of collisions occurring over the network as mentioned below:
1. The early collision and
2. The late collision
About Early Collisions
– Consider an imaginary Ethernet network.
– This network consists of two particular stations namely station A and station B and let there exist other n number of stations.
– The current status of the network is idle i.e., the wire is current not under use or nobody is talking.
– Suppose a time period of 9.6 microseconds has passed away since the last talking on the wire.
– Now a collision that would occur before putting 512 bits of the frame on to the wire is called an ‘early collision’.
Let us see how it actually happens.
– The idle state of the wire for 9.6 microseconds is detected by the station A.
– Therefore, it begins transmitting data frame with a starting preamble of 64 bits.
– During the transmission phase, the station also comes to detect an abnormal voltage.
– This is signal of occurrence of a collision.
– Between this time period and before the data frame is propagated down to B from A, station B also recognizes the idle state of the wire.
– Therefore, it also begins transmitting its data again with a 64 bit size to A.
– Station B also comes to know about the collision on the wire.
– There will be a point where the electrical signals from A and B would overlap, thus giving rise to an abnormal voltage.
– But since the signals continue with their propagation, this abnormal voltage also propagates towards both the stations.
– The collision will be detected by the station closest to the point of overlap.
– Let us say A detects collision, therefore it will transmit a jam signal on the wire to stop the transmission from both ends.
About Late Collisions
– They are not usual and usually occur because of out of spec, adapter malfunction or cabling.
– This collision takes place after 512 bits of the frame have been put on to the wire.
– For this consider the same network but the wire has been violated.
– Both the stations detect the idle state of the wire and start transmitting.
– B starts transmitting just before the electrical signal from A is about to reach it.
– Later, after a little amount of time collision takes place.
– Since B is closer to the point of collision, it detects it and transmits a jam signal.
– The collision then moves towards station A followed by the jam signal.
– But by then the station A is finished with its transmission and will not detect collision since the wire was out of spec.
– The station A has no awareness regarding the occurrence of collision.
Why these collisions are considered to be a problem?
– This is because even if once the fact concerning the occurrence of collision is missed by the NIC, the task of recovering and re-transmitting is left to the upper layers which cause a tremendous increase in the recovery time.
– For example, if an NIC takes at most 3 ms for recovering and re-transmitting, it would take 10 – 100 times more.
– Malfunctioning of NIC can also cause late collisions.