Здравствуйте, коллеги.

Прохожу стэнфордский курс по сетям.
Не могу решить одну задачу, нереально туплю.

Suppose that a wireless link layer using a CSMA-like protocol backs off 1ms on average.
A packet’s link and physical layer headers are always set at the same bitrate and take a
total of 125us to transmit. If a packet is sent with a link layer payload of 1000 bytes at
a bitrate of 1Mbps, what overhead do the physical and link layers introduce? Calculate
overhead as the fraction of the complete packet time taken up by backoff and link/physical layer headers.

На форуме уже все разжевали:

Time to transmit headers = 125 micro seconds
Back off delay = 1ms = 1000 micro seconds
Total overhead = 1125 micro seconds
Time to transmit payload = 8ms = 8000 micro seconds
Total time to transmit the entire packet (header + payload + back off) = (125 + 8000 + 1000) = 9125 micro seconds.
Now you can calculate the fraction!

Но реально не могу понять, какое отношение им нужно и причем тут битрейт?
Буду рад помощи. Заранее благодарю.