| A collision doesn't propagate; what a repeater does is to enforce the
collision throughout its' collision domain by sending out a
96-microsecond 'jam' signal. Digital Equipment repeaters simply use
alternating 1's and 0's for 96 bit times. This jam signal prevents any
other stations on those segments from transmitting (unless they're
broken) until everyone else in the collision domain has stopped.
Repeaters can't "create" collisions on segments; a collision by
definition is the existence of more than one station transmitting,
detected by DC levels on the media (for coax) or simultaneous transmit
and receive activity (for twisted pair or inter-repeater links). When a
repeater senses a collision on one segment (the 'over 50% collisions
rate' you mentioned), it sends out a jam signal for 96 microseconds
(minimum) to ensure that no one on any other port tries to transmit and
thus perpetuate the collision. So the DECrepeater 900FP is working the
same way all IEEE802.3-compliant repeaters work: your LAN analyzer
doesn't report collisions on the other ports, but you will see activity
on those ports for the duration of the jam signal.
Hope this helps.
Harry Sutton
Multivendor Systems Engineering
|