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."