Math issue on Qos lesson

having some issues on math on lesson.

when talking about committed burst rate (Bc)

the have the formula which I have confirmed is right from my book “end to end Qos network design” second edition.

Bc= Tc * CIR

in the example they give the following:

In the example above we have a Tc of 125 ms and we are shaping to 64 kbps (that’s the CIR) so the formula will be:
125 ms * 64 kbps = 8.000 bits.

ho do they get bits? by using dimensional analysis which I am very rusty on. I can see them getting


125 milliseconds * (64 kilobits / second) = 8 kilobits


but to get the answer given in bits you would have to multiple each side by kbps as well as ms.

So you would need to do the following dimensional analysis

(1/1000ms) (125ms)(64kbps)(1000bps/1kbps) = 8000. bps they they have 8.0 bits am I doing something wrong or is this answer just recorded incorrectly?

here is the link:

If he just was getting rid of the ms then you would have:

(1s/1000ms) (125ms) (64kbps) = 8.000 kbps now that makes sense to me mathematically and looks like he just mislabeled it as bits instead of kbps.

that has to be what happened. I only looked at it so much endepth because I am studying to see about fixing one of my links.

I have a 4500 cisco switch gigabyte interface and it goes into a MPLS circuit with a CIR of 20Mb so I was going to shape the interface to best practices in our enterprise network so that it fits to the best practices which the book and Renee said is a Tc of 10ms and the book said:

“A 10-ms Tc interval value can be achieved (Tc = Bc/CIR) by setting the Bc equal to CIR/100.”

Since this is a live enterprise I was trying to make sure I was understanding correctly o really going over Renee post at a granular level as I was trying to use it with the book to make sure I was understanding. I dont even know how to change the “Bc” yet I will still have to figure that out after I make sure I am understanding this.

forgot to add yes 10ms because we have VOIP going over this. I didnt know anything about Qos a few days ago ordered the book which is suppose to be the bible of Qos and then trying to use Renee website to help me understand parts of the book which is:

End-to-End Qos Network Design
Quality of service for Rich-Media & Cloud Networks second edition

Tim Szigeti, CCIE No. 9794
Robert Barton, CCIE No. 6660
Christina Hattingh
Kenneth Briley Jr., CCIE No. 9754

You can blame the confusion on the difference between how Europeans and Americans separate decimals and thousands.

For “eight thousand”, some Europeans would use “8.000” while Americans would use “8,000” The same is true for decimals. Pi for some Europeans is 3,14159 while Americans it is 3.14159

So with that cleared up, let’s look at the formula again:

125 ms * 64 kbps = 8,000 bits

125 ms = 1/8 second, so 64 kbps times 1/8 seconds = 8 kb = 8,000 bits.