Surveying your site for peak wireless LAN performance
Gary Singh / Symbol Technologies Inc.
Conducting an RF sight survey prior to installing a wireless LAN can result in a more effective system.
From the initial result of a site survey with a single access point, the space was small enough that the user thought one would suffice. This site has an unusually high number of signal obstacles, however, and clearly large coverage gaps occur even fairly close to the access point.
Imagine that it's the "go-live" moment of your new wireless local-area-network (LAN) installation. Your company is creating its first mobile workforce, enabling associates to work wirelessly by providing instant access and interaction with information. The results will be invaluable: increased productivity, better communications, and faster service. But wait a minute. The radio-frequency (RF) signal is dropping out as staff members move about. Your newest technology investment isn't performing as promised. Users are being disconnected and data lost.
Can such a scenario really occur? It can and it has. But it can also be avoided. To help ensure wireless LAN success, take the time to perform an RF site survey before installing your new wireless solution.
An RF site survey provides a detailed specification addressing network design, equipment placement, power considerations, and wiring requirements. It serves as a guide for installing and verifying the wireless communications infrastructure and enables the work to be accurately quoted.
A site survey provides assurance for your wireless infrastructure investment and lays the groundwork for future expansion. Without an effective understanding of your site's requirements, your wireless LAN installation can become more expensive and time-consuming than necessary.
What is a site survey?
A good site survey addresses two issues: First, it examines the RF physical-coverage requirements for your specific site. In other words, how far does the signal go for each coverage area or cell?
Second, the site survey enables you to understand your overall networking and system requirements. Does the existing backbone have adequate bandwidth to handle the overlaid wireless network? In a cellular architecture, are there sufficient resources to include wireless capabilities?
You can perform a site survey yourself using tools and equipment provided by wireless vendors. Or you can hire a wireless vendor or value-added reseller to make the site assessment for you. If you are introducing wireless LANs to your organization for the first time, it might be preferable to contract a specialist. Later, you may feel ready to expand your system using in-house resources.
There are five major areas to consider when developing your site's design. As you consider the following issues, keep in mind that you may want to scale up the system and its applications as your business grows or changes.
Range and coverage. First, you need to define the area in which you need wireless coverage, and also its characteristics. Is it inside or outside? Do you need coverage to be isolated to certain spaces? A wireless access point's typical range is spherical, and each area will generally need four access points, with cells overlapping by 30% to achieve optimum coverage. Access point and antenna ranges differ by vendor specification.
The final configuration of three access points ensures excellent coverage throughout the facility.
Data rate and capacity. For how many users are you planning? What sorts of applications will they run? What types of equipment do you plan to use? These factors affect the number of access points you need as well as the data rate you select. Site capacity can be scaled simply by adding more access points, so set up your initial installation to allow for future growth.
Today's wireless LANs generally operate at 1 to 2 Mbits/sec, which enables both robust data transmission and the new wireless voice/data technologies. Some users need higher data rates—hospitals transmitting X-rays, for example, or organizations using wireless multimedia applications—and should consider the value of the 11-Mbit/sec IEEE 802.11 specification. These higher data rates have a much shorter range, however, and such installations generally require more access points.
Interference immunity. What are your potential sources of interference now and in the future? These can include sensitive hospital equipment, previously installed RF systems and the upcoming Bluetooth initiatives for very-short-range mobile communications.
In outdoor installations, moving vehicles such as trucks, planes, or other equipment may also be large enough to temporarily block RF signals. For example, in a busy transportation yard you may decide to place access points at very high locations for good vertical signal penetration. Or, access points may be placed at opposite sides of the yard, so that if one is shadowed, the signal may still be transmitted through the other.
You also need to examine potential antenna performance patterns to decide on an omnidirectional or directional antenna. Interference avoidance design requires a combination of sufficient signal strength, the appropriate technology and intelligent access point and antenna placement.
Connectivity and power requirements. What are your environment's networking constraints? All your networks must be based on the same standards—usually Ethernet or Token Ring. One cannot assume the current environment will be able to handle the increased workload, and you don't want 40 new access points suddenly overstraining your system. You should also calculate your installation based on the network device with the least range, so that every device is sure to transmit clearly.
In terms of IP segmentation, it is advantageous to keep all networks, wired and wireless, as flat as possible for the best performance. Switched environments add a layer, and wireless performance will depend partly on the quality of the switches and how quickly they update the moves, adds, and changes tables.
Connecting to the wired infrastructure may also require different cabling alternatives. Wireless access points are typically installed in ceilings for better coverage, which means that AC power and data cabling will need to be run in some unconventional places.
Cost and ease of installation. Performing a site survey provides a realistic understanding of your wireless-installation investment before it happens. Perhaps your site has unusually high interference issues to resolve, or capacity is greater than you anticipated. Based on a good site survey, you can get more-accurate quotes to guide financial decision-making.
Many companies try to implement their new LANs in stages and find themselves in the midst of a communications disaster as sessions drop out and cell locations are lost. Because wireless systems simply fit in on top of existing environments, however, you can anticipate limited work interruptions during the installation process.
The final step of a site survey consists of setting up temporary antennas and access points (available from any wireless vendor). This process includes assessing every part of the site with a software survey tool—installed on a handheld computer or laptop—that monitors the signal and identifies failure zones. Such site-survey tools measure performance between access points, identify sources of interference, and help determine access-point placement.
Site surveys differ in their complexity and level of effort, based on technology and space. Small facilities probably do not require one at all, and you can rely on wireless vendor specifications and a good look at your office blueprint.
However, larger installations are another story. The site survey offers a level of security in an arena in which, unlike wired networks, there are many variables to consider and few fixed rules. A thoughtful, accurate, and effective assessment of coverage and system requirements will make your wireless LAN installation more straightforward and less expensive, while laying the groundwork for ongoing expansion.
Gary Singh is the director of technical marketing for Symbol Technologies Inc.'s wireless LAN systems division (Holtsville, NY).
This article is reprinted from the Nov/Dec 1999 issue of Wireless Integration, a sister publication.