You might want to check out a latency predictor tool like NetPredictor from
www.netpredict.com.
Priscilla
At 09:22 PM 11/7/00, Laurent Lange wrote:
>Irwin, or anyone else
>
>I appreciate your answer. In the same order of idea, is there any formula
>for the latency on a serial interface depending on the load. Serialization
>delay 8*packet_size/speed_of_link is only applicable for a low average
>usage of the bandwidth, is it?
>
>thanks
>Laurent
>
>>From: Irwin Lazar <[EMAIL PROTECTED]>
>>Reply-To: Irwin Lazar <[EMAIL PROTECTED]>
>>To: "'[EMAIL PROTECTED]'" <[EMAIL PROTECTED]>
>>Subject: RE: How to determine bandwidth requirements for an application ?
>>Date: Tue, 7 Nov 2000 09:12:34 -0700
>>
>> > Hello,
>> > can anyone give a hint or advice `?
>> >
>> > We have to determine how much line bandwidth is needed to run some
>> > application over WAN.
>> > That is, we were asked: "Here is a client application X. We
>> > plan to install
>> > 75 machines running X at remote office. Is 128K line enough
>> > for this or do
>> > we need 1M line ?" - something like this.
>> >
>> > The question is: which approaches exist for this problem ?
>>
>>This is kind of long, but here is a column I wrote on the topic a few months
>>ago:
>>I'd also recommend getting a copy of "Wide-Area Data Network Performance
>>Engineering"
>>by Robert G. Cole, Ravi Ramaswamy, which directly addresses this topic.
>>
>>Irwin
>>
>>----------
>>The bandwidth size that is required for any given
>>connection is a function of the following three factors,
>>number of users, requirements of specific
>>applications, and how the application is used. For
>>example, a site with five users that all access a highly
>>interactive application for twelve hours per day may
>>require more bandwidth than a site in which a dozen
>>users sporadically access a client-server application
>>in which most of the processing is performed by the
>>remote server.
>>
>>In addition, another concern in the bandwidth selection
>>process is delay. Certain applications such as voice
>>and video may require a low level of delay (latency) as
>>well as a low variability in delay (jitter). These
>>requirements may add significant complexity to the
>>design process.
>>
>>The first step in sizing bandwidth is to determine the
>>requirements for the specific applications that will be
>>deployed. During this step, a sniffer is useful in tracing
>>application sessions to determine the average packet
>>size and the average number of packets for a given
>>transaction. Once you have these values, the next
>>step is to factor in the number of users, the required
>>latency, and the amount of time that typically exists
>>between transactions.
>>
>>Once you have obtained these values, you can use
>>the following formula (created by Ravi Ramaswamy of
>>AT&T Solutions) to determine bandwidth requirements:
>>
>> 8 x N x K x M / (K x P + T)
>>
>>Where:
>> N = number of active users at a location (the number of users that
>>will simultaneously use an application)
>> T = User think time (how much time typically exists between
>>inquiries)
>> K = number of packets per transaction in any given direction
>> M = number of bytes per packet in any one direction
>> P = one-way network latency
>>
>>Note that this calculation must be performed for both
>>directions of the connection. The required bandwidth
>>is then the maximum bandwidth estimated by this
>>formula (unless you are deploying a technology such
>>as Frame Relay which allows for different bandwidth
>>allocations for each direction of the connection).
>>
>>Note also that this formula only applies to
>>client-server type applications in which there is a
>>substantial amount of two-way traffic. For additional
>>information on bandwidth sizing, please see
>>"Optimizing Client-Server Application Performance on
>>the WAN" in the November 1999 issue of "Network
>>Magazine."
>>
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Priscilla Oppenheimer
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