You use a 1 Mbps link to send a series of 1250 byte messages across your city using the ARQ protocol. The propagation delay (D) of this link is 5 ms. What is the maximum RATE at which data can be delivered over the link?
62.5 kbytes/sec
1000 kbps
7.8 kbytes/sec
41.6 kbytes/sec

Question

You use a 1 Mbps link to send a series of 1250 byte messages across your city using the ARQ protocol. The propagation delay (D) of this link is 5 ms. What is the maximum RATE at which data can be delivered over the link?
62.5 kbytes/sec
1000 kbps
7.8 kbytes/sec
41.6 kbytes/sec

doc.brown · Answer

The RTT (round-trip time) for a long link as 2D (twice the propagation delay)

doc.brown · Answer

So I figure I need to find out how long 1250B takes at 1Mb/s
1250B * (8b / B) * (s / Mb) * (Mb / 10^6b) * (10^3ms / s)
=10ms
Now the ACK takes another 10ms to come back, plus the 5ms propagation.
10ms + 10ms + 5ms = 25ms

So every 25ms I can send one 1250B message.
1250B / 25ms
= 400kb/s
which isn't a choice.
What am I doing wrong?

doc.brown · Answer

Do I add a propagation delay to each end?
1250B / 30ms
41.6kB/s?

oh