If two or more stations on a shared
media segment do transmit at the same time, a collision results, and the frames are
destroyed. When the sending stations involved with the collision recognize the collision
event, they will transmit a special ???jam??? signal, for a predetermined time, so that all devices
on the shared segment will know that the frame has been corrupted, that a collision has
occurred, and that all devices on the segment must stop communicating. The sending
stations involved with the collision will then begin a random countdown timer that must be
completed before attempting to retransmit the data.
As networks become larger, and devices each try to use more bandwidth, it becomes more
likely that end devices will each attempt to transmit data simultaneously, and that will
ultimately cause more collisions to occur. The more collisions that occur, the worse the
congestion becomes, and the effective network throughput of actual data can become slow.
Eventually, with suf?¬?cient collisions, the total throughput of actual ???data??? frames becomes
almost nonexistent.
Adding a hub to an Ethernet LAN can overcome the segment length limits and the distances
that a frame can travel over a single segment before the signal degrades, but Ethernet hubs
cannot improve collision issues.
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