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RE: Network Sizing Guidelines?

  • From: Irwin Lazar
  • Date: Fri Mar 02 19:13:13 2001

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

Once you have obtained these values, you can use the following formula
(created by Ravi Ramaswamy of AT&T Solutions) to determine bandwidth

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

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

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
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.

Dan Glover 

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