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Re: Network Sizing Guidelines?

  • From: Marshall Eubanks
  • Date: Mon Mar 05 23:09:56 2001

These guidelines will be badly misleading if you have serious
streaming use, where K, T and P are basically undefined or irrelevant.

BTW, I have asked numbers of people about the underprovisioning
(or oversubscribing) they use, and have received responses
ranging from a factor of 5 to a factor of 20 or more.

                                   Marshall Eubanks

Irwin Lazar wrote:
> I'd recommend taking a look at: Wide-Area Data Network Performance
> Engineering By Cole, Robert G. / Ramaswamy, Ravi
> ISBN: 0890065691
> Here is a guideline based on Ravi's methodology that I wrote for our
> customer newsletter some time ago:
> 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 issue of "Network
> Magazine."
> -----
> Irwin Lazar - <>
> Senior Consultant, The Burton Group
> Office: 703-742-9659
> Cell: 703-402-4119
> "The Ultimate Resource For Network Architects"
> -----Original Message-----
> From: Daniel R Glover []
> Sent: Friday, March 02, 2001 5:00 PM
> To:
> Subject: Network Sizing Guidelines?
> Can you point me to some current network sizing advice for Internet service
> (WAN and LAN)?  I am looking for rules-of-thumb, guidelines, equations,
> books, or even anecdotal evidence that would help me in evaluating future
> network design concepts.  I have somewhat unconventional network
> constraints, but I would like to start from current conventional
> assumptions on, say, numbers of users (home or office) that can be
> supported by various bandwidths.
> I've tried the related links off the NANOG pages, but some are old or
> broken.  I have found some advice like "a T1 will support 200 to 300 28K
> modem users," but I hope there may be more current advice somewhere
> especially with regard to QoS, future trends, and to larger networks.  Any
> relevant pointers or advice would be welcome.
> R/
> Dan Glover


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