As I discussed in my last column, requirements are a foundation for defining a wireless LAN's function. The next step is to specify the technical aspects and components of a wireless LAN -- e.g. access points, radio cards, and associated software -- that satisfy those requirements.
To ensure a successful design, execute the following steps:
Select a standard. Determine which wireless-LAN standard best satisfies your requirements. Choose between IEEE 802.11 at 1 or 2 Mbps and IEEE 802.11b at 11 Mbps. You'll soon be able to add IEEE 802.11a products, with data rates up to 54 Mbps, to the list.
Your choice of a standard depends mostly on your data rate requirements, as I said in my last column. With 802.11 alternatives, you must take into account frequency hopping vs. direct sequence tradeoffs. Most designers now choose 802.11b because it offers Ethernet-equivalent speeds and is priced only slightly higher than 802.11 equipment.
Define optional features. Many IEEE-802.11-compliant wireless LANs implement optional features, giving system designers more choices. For example, designers can choose to implement 802.11's Point Coordination Function (PCF), which delivers time-bounded data via synchronous communications using station-polling mechanisms; in other words, it better supports real-time transmissions of video and voice.
Another optional feature is Wired Equivalent Privacy (WEP), which provides frame transmission privacy similar to that of wired networks by generating secret shared-encryption keys for source and destination stations. Consider using this feature if you need a higher security level.
Properly size the network. Be sure to determine the number of access points necessary to provide adequate throughput, and locate them to give your users complete coverage. Perform an RF site survey. Also, if you need higher throughput, consider collocating access points that operate on different channels -- this increases the bandwidth available in specific areas.
Define the wired backbone. If your wireless LAN's users need to communicate with users at other access points, you need to specify a wired backbone, such as Ethernet or Token Ring, to provide physical interconnections between access points. If you have a wired LAN, you can connect the access points to open ports on your switch or hub.
Define application connectivity software. These are the tools for interfacing devices that operate on the wireless LAN with host systems. For instance, if you need to reach applications that run on an AS/400 or Unix box, you'll probably need to specify terminal emulation software for your handheld devices. Consider using wireless middleware if high reliability -- or the ability to interface with data located on multiple, dissimilar host systems -- is required.
Choose products. Consider the products' technical specifications, such as standards compliance, optional features beyond the scope of the 802.11 standard, price, warranties, and technical support capability.
Verify the design. This includes verifying that the technologies, configurations, and product selections truly satisfy your requirements. To do this, you can run simulations and set up a physical prototype.A simulation uses software models, like Opnet, that artificially represent an 802.11 network's hardware, software, traffic flows, and utilization. You can run simulations and check results quickly; days of network activity go by in minutes of simulation runtime. Simulation tools, however, are generally costly, with prices in the tens of thousands of dollars. You might be better off hiring a company that already owns a simulation tool.
Run a physical prototype by constructing and testing part of the system you wish to verify. You get real results because you're using the actual hardware and software. Be prepared to reconfigure your prototype as you go. Don't skip this step if you need to ensure compatibility among 802.11-compliant products from different vendors.
Document the design. As with requirements and other phases of a system implementation project, you should document the details of the design before you implement it. Be certain to document schematics, building-layout drawings, and bills of materials or parts lists.
Once you finish these steps, you'll be ready to procure the components.
Next time, we'll take a close look at configuring an 802.11b- compliant access point.