# How to understand CSMA/CD algorithm from this graph?

I am currently studying Computer Network, and I have some problems when encountered CSMA/CD, I can understand how it works, but I am struggling in understanding this graph, anyone can give me some help?

This is a space-time diagram so, going from top to bottom, the time elapsed is increasing.
Here, A and B are two nodes that are broadcasting messages to the other nodes present with it on the same channel.
At tA, A senses that the channel is idle and then starts transmitting its message as shown in the diagram below. As A's message travels away from it, the time increases which is why the message section in the channel space is shown slanting downwards on both sides of A. At tB, B senses that the channel is still empty as A's message hasn't reached it yet and so, assuming the channel is idle, it also starts transmitting. At time tC, First bit of A's message and B's message reach the same point and this is where "collision" happens.

• In the diagram posted on the question, B detects the collision first then after back off limit it senses the channel and finds its busy as A's msg is present, now I don't understand why B senses the channel after a 2×512 bit time? Jan 17 at 2:22