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Tip of the Month

Network Analysis Tip of the Month – February 2006

Jump on the Bandwidth Wagon

By Jeff Trawick, WildPackets Professional Services

So you’ve made up your mind. You’ve decided to extend existing LAN services to one or more remote locations over WAN links. The question is: Will those WAN circuits handle the traffic that has traditionally been isolated to high-speed LAN segments??? Can all those bits, bytes, and packets that currently traverse your bandwidth-rich LAN live in peace and harmony as they travel through a more narrow and more crowded data pipe?

WildPackets analyzers provide a variety of “what-if” capabilities you can use to simulate transactions and traffic streams across links with specific bandwidths. We’ll save the more advanced of these features for a future tip (teaser!!!). But there is one really quick and easy way to model how traffic from one network will look on another network of differing bandwidth.

For example, assume that you want to examine traffic that now lives on a 100 Mbps LAN segment to determine how it will look and behave on a T-1 WAN link with much lower bandwidth. Here’s how to get that view without having to invest in a complex network simulation solution.

First, attach the WildPackets analyzer to the 100 Mbps LAN segment (using mirrored ports, taps, etc.). Then, go to the File menu click on New, or click on the New Capture button on the toolbar. This will open the Capture Options dialog. In the left-hand pane of the Capture Options, click on Adapter. Next right-click on the adapter you will use for this capture session. You should now see a popup menu that looks like the screen shot shown here.

Click on thumbnail for larger view

Now, click on the Network Speed option. A smaller dialog box will appear. Normally, the analyzer Auto Senses the network speed for the capture adapter. But you can set this to anything you like. In this case, we need to set the bandwidth to match our T-1 WAN link, which is 1.544 Mbps. OK, let digress into math for a second. 1 Mbps = 1,048,576 bps. Multiplied by 1.544, we get about 1,619,001 bps for a T-1. Divide that by 1,000 to covert to kilobits per second, which equals 1,619 kbps. You can do similar calculation for any circuit speed.

Click on thumbnail for larger view

Once you click OK to confirm your entry and complete any other capture option selections, you are ready to capture! And here’s the cool thing: the analyzer now believes it is capturing from a circuit with only 1.5 Mbps of bandwidth. All of the utilization statistics, graphs, and other data will now be computed based on this bandwidth, so you can get a good estimate of what the LAN data stream will look like over a smaller (or larger) bandwidth circuit.

This is just one of many ways you can use WildPackets analyzers for “What-If” analysis. In future tips, we’ll look at more complex and accurate modeling based on other data communications variables, such as latency and packet size. But for now, you can jump aboard the “bandwidth wagon,” and set the speed as needed for your environment.