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New Member

How do you dimension WAN links?


I´m currently trying to figure out how to dimension WAN links to fulfil the very vague requirements of my client. They want the network to operate brilliantly on their busiest days during the year.

I´ve been trying to find information in the Internet about how people ussually go about this.

The data I have is the following:

- Known number and size of company sites (Let´s call it Y)

- Known Number of file transfers/busy day between the Head and the Branch Offices.(Let´s call it Z)

- Size of the data involved in each transaction (Let´s call this X)

- Max number simultaneous transactions per hour (limited to the number of PCs per office because it´s a human involved process)



How do you relate throughput with volume of data? (For instance, I want to FTP a file of 40 KB, how much BW is enough?)

Does the question depend on user perception? (something like 3 seconds for the transfer is Ok but 5 is too much)

What is the typical overhead introduced by all the layers below the file transfer application? (Transport, Network, etc) This is to relate link speed to actual payload. I guess this depends on the WAN tech emploied.

If you know of any book, web page, etc. with info on the subject I would appreciate the data.

Thank you very much in advance for your help.


New Member

Re: How do you dimension WAN links?

Ok...let's see if we can put something together.

Bandwidth is just the amount of data the can be physical sent in a time period. Usually we represent bandwith in seconds, but it really doesn't's just a function of time. So, in your case

BW * TimePeriod = Z * X

Thus BW needed :

BW = (Z * X) / TimePeriod

Now an example. Let's just assume you have an 8 hour work day, each data file averages 500K and you do 10000 transfers per day.

So BW = (10000 * (500000 * 8)) / (8 * 60 * 60)

BW = 1,388,888 bps

But now we have a performance issue...this will get all our traffic through in the 8 hour work day, but how long for each set of data (remember, we don't want our users upset about slow response).

So, at 1,388,888 bps it will take about 3 seconds :

time for 1 file = (500000 * 8) / 1388888

= 2.88 seconds

Not too bad....

This may seem long and boring, but may be getting you to what your looking for.

Also, be sure to remind you client that traffic is inherently bursty....1.4 Mbps may be good for all day, but not in the morning when everyone is logging in and downloading their e-mail. You may to consider some Qos to handle the peaks.

As for overhead, that really depends on packets size. The larger your packets the better performance. That's why you want to send at 1500 bytes on Ethernet, no 64k packets. This is really an application issue. If you can tune your application to send at maximum segment size you will get the best performance and pay the lowest overhead "tax".

Good luck.

Michael J. Morris

CCIE #11733, MCSE

New Member

Re: How do you dimension WAN links?

Thanks for the help Michael, :-) It´s more or less what I was thinking about.

Special thanks for the performance pointer.