Forum

I've got my CWNA exam coming up in a couple of weeks and there's something I just can't get my head round - this might be too in-depth for the exam but I would like to understand it anyway...

The CWNA third edition book says that when a frame is fragmented, each fragment requires its own ACK (pg. 388 2nd sentence). Also, when a frame is fragmented, the time between each fragment being transmitted is one SIFS (pg. 369 last line). If this is the case when does the ACK get to the sending station.

Let's say that Station1 is sending a fragmented frame. It sends fragment 0, waits for one SIFS, sends fragment 1, waits one SIFS, sends fragment 2 etc. The AP would have received fragment 0, waited for one SIFS and then sent the ACK. That means that both the ACK and fragment 1 would have been transmitted at exactly the same time and would collide. Or does the AP not need to wait for the SIFS before sending the ACK?

Can someone please clear this up for me?

Any enlightenment greatly appreciated.

Andy

The proper sequence for example with three fragments is:

DIFS,Fragment#1,SIFS,ACK,SIFS,Fragment#2,SIFS,ACK,SIFS,Last Fragment,SIFS,ACK(SIFS-PIFS-DIFS as required)


Every fragment is followed by SIFS and and ACK prior to the next fragment being transmitted.

CWAP page 103

Bryan,

Thanks for the explanation. Another question that arises from that is isn't it possible that another station might start sending data before the SIFS, ACK, SIFS has completed? I don't know the actal lengths of a SIFS or DIFS interval. Is it the case that DIFS is more than a SIFS, ACK and SIFS?

Andy

A DIFS is longer than a SIFS, so that helps protect the network. However, the real protection is with the duration values in the fragments and ACK's. I'm not sure if duration values are covered in the CWNA, so it may be beyond what you need right now. If it is something you want to know about, I can help or point you in the right direction. I don't want to overfill your brain before test time. :)

Oh, something you should think about is getting a CWAP study guide. It is a great book and can be had from this site for a nice price.

I agree with you Gene. The duration field may be beyond the scope of CWNA and everyone that does not already have a CWAP book should buy at least one before they go out of print.

Gene,

Thanks for the info - maybe I should be concentrating about my exam rather than these topics that aren't going to be in the test!!

I think I'll take a look at getting a CWAP book as hopefully it will fill in any holes in my mind like this one.

Thanks also to Bryan for your input.

Andy