# Bandwidth Calculation Question

May 12th, 2009

I have around 4000 users and i have 2 CORE 6500 switches supporting all the VLANs and from there to reach the internet and remote resources we have a DS3 (45 Meg) connection .

And would i like to know is how to calculate the amount of bandwidth that will be available for each user once it goes out the serial connection (DS3).

Thank you

Vic

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## Replies

lamav Tue, 05/12/2009 - 09:59
• Blue, 1500 points or more

Hi, Victor:

If you look at it linearly, you have 45,000,000 bps divided by 4000 users. That is the maximum amount of bandwidth available to each user if every user decides to access the Internet and remote resources all at once. That's about 11 kbps per user.

Is this what you're after?

Victor

Victor Fabian Tue, 05/12/2009 - 10:10

That's what i though at the beginning but that kind of looks to easy for some reason.

Thanks

Vic

vmiller Tue, 05/12/2009 - 12:23
• Gold, 750 points or more

Its a good theoretical maximum. Your milage will vary based on workstation configs, application behavior....

Giuseppe Larosa Tue, 05/12/2009 - 12:43
• Super Silver, 17500 points or more
• Hall of Fame,

Founding Member

Hello Victor1 and Victor2,

>> you have 45,000,000 bps divided by 4000 users. That is the maximum amount of bandwidth available to each user

From a maths point of view this is the average rate per user if all of them are accessing the internet at the same time.

without any QoS configuration the whole 45 Mbps is available to each user.

However, from a design point of view this average rate per user is the parameter that allows to decide if the internet link is fast enough or not.

Hope to help

Giuseppe

Joseph W. Doherty Tue, 05/12/2009 - 15:56
• Super Bronze, 10000 points or more

As the others have noted, if you had some kind of fair bandwidth allocation active management, you would just take your total bandwidth and divide by number of active flows. Each flow should have that as a minimum.

However, often fair bandwidth allocation active management isn't enabled, and you can (at worse) have one flow attempt to monopolize the bandwidth at the expense of all other flows. Effectively, one flow could use your 45 Mbps and leave every other flow no bandwidth.

Fortunately, the worst case is very rare. Normally, without fair bandwidth active management, active flows will obtain variable proportions of the bandwidth (which usually results in variable network performance - of which users so often notice).

PS:

If your question really is, whether 45 Mbps is sufficient for your 4,000 users, that depends on how they utilize network bandwidth (and how you manage it).