always .transmission delay >= 2 * propagation delay
because sender has capacity of collision detection until sender do not transmit all data.
Assume
At time t=1:00 pm, a frame is sent on the idle medium by A.
At time t=1:59 pm, NIC B also transmits.(In this case, the medium, as observed by the B happens to be idle too).
At t=2:00 pm, collision detection.
At t=3:00 pm, A detect collision
After one complete round trip propagation time (twice the one way propagation delay), both station are aware of the collision. B will shortly cease transmission of the Jam Sequence, however A will continue to transmit a complete Jam Sequence. Finally the cable becomes idle.
