This article is from the Ethernet FAQ, by James Messer James@NetworkUptime.com with numerous contributions by others.
When a collision is recognized by a transmitting station, a bit
sequence called jam is transmitted. This jam is 32 bits long, which
is long enough to traverse the entire collision domain so that all
transmitting stations can detect the collision.
Interestingly enough, the actual format of jam is unspecified in
the 802.3 specifications. Most manufacturers have used alternating
1s and 0s as jam, which is displayed as 0x5 (0101) or 0xA (1010)
depending on when the jam is captured in the data stream.
In many Fast Ethernet implementations, the jam has been seen as
other arbitrary values, such as 1101000 (0xD0) or 10000110 (0x43).
The reasoning for this particular jam pattern isn't very obvious. If
anyone has more information on this jam sequence, please email