Creating a historical record of network utilization is an important tool for optimizing your applications, planning hardware and software upgrades, and setting a baseline for many other types of future projects. Network monitoring and performance logging should be part of my administrator's toolbox, but often, busy network admins don't put these tools in place until they are absolute required to.
There are many packages that will automate the process of developing a historical record, with very fine granularity. A first decision you have to make is determining the data point intervals. Evaluate carefully the sampling interval; that interval should be long enough so that it won't add network overhead. In most instances seconds and even minutes may be suitable. Depending upon your needs, you should record enough data points to measure the ebb and flow of your network over several periods of repetitions -- for most applications that will probably be weeks, months, or years. You may only require measurement of data during a single week every month to create the appropriate historical record that you need.
The two most important measurements are average utilization and peak utilization. You can use both protocol analyzers and network management packages to measure these factors. Most operating systems like Windows and Solaris ship with built-in tools for network analysis based on counters that are either automatically installed or that you can activate later. Many applications such as SQL Server also provide counters that you can turn on for specific analysis, so check with an application vendor if you are trying to measure a particular aspect of that application's performance that isn't obviously covered by standard counters. The vendor may have one in his product -- often vendors develop counters as part of their optimization routines during product development.
In taking measurements you should consider measuring the traffic at several key points: your application server's network interface, throughput through a hub or switch, and at a sampling of your clients.
An historical utilization study, even a rudimentary one, is a persuasive tool when arguing for new equipment or for the size of your next annual budget.
Barrie Sosinsky is president of consulting company Sosinsky and Associates (Medfield MA). He has written extensively on a variety of computer topics. His company specializes in custom software (database and Web related), training and technical documentation.