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Calculating bandwidth needs

By carola ·
Basic question -- and apologies for this. But we all have to start somewhere, don't we?

I need to estimate bandwidth upgrade needs on a number of Frame Relay links each of which are at present 128 access speed (consisting of 2x64k channels), and 2 x 64K PVC's each. The links are Point to Point connections to a Corporate Office. Corporate office Frame connection to Comms Provider is 1280k.

The Corp. Office provides access to the Internet for each Centre. Our need is to provide access to each Centre to an additional service: Browser based access to an internet training site. Vendor states that 28.8 K fine but ideally 50kbps is needed.

Surely, I am miscalculating: I have calculated that 25 simultaneous users at any one Centre needing 50Kbps, totals 1250kbps additional bandwidth needs to be added to each 128k Frame Link (and of course the Corp Office link upgrade is equal to a total of all these sep. upgrades).

Clearly I missing something -- and I have a feeling (can you tell yet I'm not a comms person ?) -- that the key is that 128K Frame allows more than 128kbps. It's got to be a simple misunderstanding of Frame capacity. IF not, we are upgrading each link by more than 9 times its present size, for 25 users to simultaneously access slightly more than a standard single 28.8 connection.

What have I missed?

Thanks,

clarrie

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Calculating bandwidth needs

by ckilday In reply to Calculating bandwidth nee ...

Clarrie,
I have tried several times to post a response via TR. I have been rejected each time. I will send you a response via what I presume is your e-mail... clavis@tig.com.au
- ck

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Calculating bandwidth needs

by ckilday In reply to Calculating bandwidth nee ...

Clarrie,
I'm trying again. Here is my reply.
Just about all data circuits base their ability to support users on the fact that people read/think/type/enter information at different rates of speed. Transmissions across a line occur at random intervals, so a 128kbps link can support 20-30 users at a remote site without too much trouble. Here's a suggestion on how to plan for your upgrade.
1. Contact your telco and obtain utilization statistics for each of the frame relay links. Theyshould be able to provide you with some sort of % vs. time graph for a month. If you have an internal network organization, they should be able to provide the same thing for you by querying the router interfaces for utilization statistics, or by putting a "packet sniffer" on the line.
2. Perform a test with a particular site by having, say, 10 users begin using the new application. Monitor the increase in traffic and extrapolate for the total number of users at a site.
3. Add this new figure to the busiest, or "peak" utilization period. You want to make sure you can provide adequate response during your times of heaviest use.
4. If you are seeing utilization rates in excess of 70% of your CIR, upgrade by at least the next logical jump. For 128kbps, that would be a move to 192kbps CIR. If you are hitting 100% of your "burst" rate limit, I'd recommend jumping up two levels to say 256kbps.
...more to follow...

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Calculating bandwidth needs

by ckilday In reply to Calculating bandwidth nee ...

5. If you don't have the luxury of setting up a prototype site, talk with the application vendor and see if he can help you ballpark the amount of data passed between the application and the user (in both directions) over a period of say an hour. Divide the total number of Mbytes by 3600 (the number of seconds in an hour) and this will give you roughly how many kbytes/second need to be transmitted. Multiply this by the number of users accessing the application and you have a ballpark bandwidth requirement. You will still need to know the utilization stats for your existing network to plan adequately.
Here's some additional information on frame relay
Frame relay is typically sold with a CIR or committed information rate which isguaranteed bandwidth, and a Burst rate, or the maximum bandwidth you are allowed to transmit at provided there's extra bandwidth available at the time. The important thing to remember with frame relay is that the vendors also count on the randomness of data transmission, so they typically oversubscribe their lines. The burst rate on any particular T-1 for all users may equal many times what is available. But the CIR can only equal the actual T-1 bandwidth (1.544Mbps in the US and 2.032Mbps in Europe, called an E-1). What happens with frame relay is that in slow periods, users can transmit up to their burst rate without any problem. When traffic gets heavy, they can only transmit up to their CIR. Any traffic beyond that is dropped from the line. Telcos oftentimes sell frame relay lines with a 0 (zero) CIR. The user gets a great price, but then is totally screwed during the busiest times of the day because his data is transmitted on an "as available" basis.
I hope this helps. If you have more questions, drop me a line.
Best wishes,
- ck

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Calculating bandwidth needs

by csmith In reply to Calculating bandwidth nee ...

The assumption is that the twenty five assumed users do not click a link at the same time.
This assumption is also used in the electric and water utilities.
It sometimes fails, but not often enough to worry about.
Regards, Chris

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Calculating bandwidth needs

by carola In reply to Calculating bandwidth nee ...

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