I'm a regular over at Stack Overflow (Software developer) that is trying to get through a networking course. I got a homework problem I'd like to have a sanity check on. Here is what I got.

Q:

A 3000-km-long T1 trunk is used to transmit 64-byte frames using Go-Back-N protocol. If the propagation speed is 6 microseconds/km, how many bits should the sequence numbers be?

My Answer:

For this questions what we need to do is lay the base knowledge. What we are trying to find is the size of the largest sequence number we should us using Go-Back-N. To figure this out we need to figure out how many packets can fit into our link at a time and then subtract one from that number. This will ensure that we never have two packets with the same sequence number at the same time in the link.

Length of link: 3,000km Speed: 6 microseconds / km Frame size: 64 bytes T1 transmission speed: 1544kb/s (http://ckp.made-it.com/t1234.html)

Propagation time = 6 microseconds / km * 3000 km = 18,000 microseconds (18ms). Convert 1544kb to bytes = 1544 * 1024 = 1581056 bytes Transmission time = 64 bytes / 1581056bytes / second = 0.000040479 seconds (0.4ms)

So then if we take the 18ms propagation time and divide it by the 0.4ms transmission time we will see that we are going to be able to stuff ( 18 / 0.4) 45 packets into the link at a time. That means that our sequence number should be 2 ^ 45 bits long!

Am I going in the right direction with this?

Thanks, Mike