Bit times and propagation delay

Question

Nodes A and B are accessing a shared medium using CSMA/CD, with a propagation delay of 245-bit times between them (i.e., propagation delay equals the amount of time to transmit 245 bits). The minimum frame size is 64 bytes. Suppose node 𝐵 begins transmitting a frame at 𝑡 = 0 bit time. Before A finishes, node B begins transmitting a frame. Assume no other nodes are active.

What is the latest time by which B can begin its transmission?

Attempt: Considering that t=512 bit times is the time at which A finishes transmission, shouldn't the latest time by which B begins its transmission be at t=245 bit time? (At t=1, 1st bit is pushed on the link, and at t=245 bit time, this is the last "timing" that B can start transmitting before it senses that A is transmitting). However, the answer given is 244-bit time and I do not understand why.

Answer 1

I would say that at t=0 the first bit would begin getting pushed on the link, which would imply that the 245th bit would begin getting pushed at t=244. Thus you'd want to start pushing at t=244.

Disclaimer: I know nothing of networking and little of comp sci but hey, I thought I'd give the logic a whirl. I could be completely wrong.

Answer 2

B does carrier sensing, and, if the medium is not free, it does not transmit.

Since A sent its bit at t=0, at t=245 this bit should reach B, and B would sense that the medium is not free anymore. So I would say 244 is the last time B would sence the medium as free.