Designing WLANs with Simulation

By Jim Geier

August 13, 2003

Factors such as irregular propagation, RF interference, and varying utilization significantly impact the operation of a wireless LAN. Learn why simulation can help you optimize your network as you design it.

As wireless LAN applications require higher performance, the design of a wireless LAN needs greater attention. In some cases, however, the decision on what settings to include in the access points becomes somewhat of a guessing game, mainly because of the difficulty in determining the impact of varying levels of user activity, RF interference, and moving propagation boundaries. In order to implement a wireless LAN that meets expectations, consider the use of simulation.

Simulation Concepts

Simulation electronically imitates a wireless LAN under different situations. With simulation, you can artificially represent any part of the network, such as access points, radio cards, software, the amount of traffic, etc. A simulation calculates resulting throughput, range, collisions, or almost anything else that you want to know. This enables designers to determine the results of configuration settings for various access point configuration elements, such as RTS/CTS, fragmentation, RF channels, and power settings.

Traditionally, you design the wireless LAN to meet specific requirements. An issue is that the actual outcome of a wireless LAN design is difficult to predict when users start pounding on wireless applications. This is not the best time to discover design issues. Simulation greatly reduces headaches, such as an inadequate number of access points and less than optimum fragmentation threshold, which you may encounter when deploying a wireless LAN.

Simulation Tools in Action

Most simulation tools follow a fairly straight-forward setup process. For instance, Opnet's Modeler (the only 802.11 simulation tool I've found on the market) uses a "process level" to model the behavior of objects and a "node level" that connects the objects to form devices. Opnet also has a "network level" that connects the devices to form networks.

This enables the testing of multiple network designs to determine which ones provide optimum performance. You can program in almost every aspect of the network, including physical interferences, environmental factors, network availability, packet delay, and throughput.

Once the testing is complete, viewing the results is very easy. The results include graphs, histograms, probability equations, and confidence intervals, among others. Opnet Modeler also includes financial functions, helping you determine which project provides the most "bang for the buck."

When to Use Simulation

Simulation saves a great deal of time during the testing phase of a project by condensing days of actual testing down to minutes of computer simulation. In fact, it's often not practical to fully test an actual WLAN installation, especially when verifying performance at near maximum capacity. If you're deploying a relatively large WLAN, and you can't stand to be tweaking performance for weeks after users are active on the network, then strongly consider simulation.

For example, you might design a WLAN on paper to support 300 users having access to a particular corporate application. In order to test a live network, you'd need to find 300 users to hammer on the network for a period of time while measuring actual performance. This is not an easy task.

You're often left waiting until the wireless LAN is operational before finding out whether it will actually satisfy user needs. With a simulation tool, though, you can create a scenario that includes any number of virtual users with typical and peak utilization levels. At a push of a button, the simulation results will tell you how the network will behave.

Simulation is also very useful for shaping performance specifications during the definition of requirements. This will enable network administrators to streamline a network so that all of the users have adequate performance. If you're having trouble defining what throughput levels to specify in the requirements, then consider running simulations with various numbers of users and utilization to determine the right numbers.

Negatives to Consider

Something to keep in mind is that simulation tools are rather costly, with prices in the tens of thousands of dollars. This puts simulation out of reach of many companies, but realize that it's generally the bigger companies with more money that deploy larger networks (which can benefit from simulation). If a smaller company doesn't have the resources to purchase simulation tools, they can hire an integrator that already owns a simulation tool to do the analysis.

Another downside of simulation tools is that their results are only as good as the model they are given. Many times it is difficult to accurately estimate the number of users and especially their activity. Because the tool is based on assumptions, there is a chance that the network will not provide optimum performance for the actual operational users. As a result, you need to define ranges of user activity in order to capture all possible scenarios.

Also, most simulation software is rather complex. The newer simulation tools include graphical representation of the network in addition to the use of a programming language, but you'll find a need to spend weeks learning how to develop effective simulation models.

If you're deploying large or complex wireless LANs, though, you'll likely benefit from simulation, and these negatives will not be significant.

Jim Geier provides independent consulting services to companies developing and deploying wireless network solutions. He is the author of the book, Wireless LANs and offers computer-based training focusing on wireless LANs.

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