As companies extend into wireless networking, they need to carefully define the purpose and goals of their new technology. For almost any organisation, WLAN’s must provide large coverage areas; handle extensive traffic, and support a continuously clear, reliable signal. Increasing numbers of companies want their new wireless network to manage converged voice and data communications, or accommodate high data rate transmissions. To assure needs are met, companies should perform a radio frequency (RF) site survey prior to the wireless installation. A site survey provides a detailed specification addressing coverage, equipment placement, power considerations, and wiring requirements. It also serves as a guide for the network design and for installing and verifying the wireless communication infrastructure.

The importance of a site survey

The site survey’s primary objective is to ensure mobile workers – the wireless LAN’s ‘clients’– will not find that the radio frequency signal drops out as they move around the facility, thus becoming disconnected from the host device or other mobile computing devices and their work – and potentially causing data loss. The survey also helps customers to clearly understand the impact of the addition of wireless local area networking on their overall networking and system requirements. Important questions it will answer include: Does the existing backbone have adequate bandwidth to handle the overlaid wire-less network? In a cellular architecture, are there sufficient resources to include wireless capabilities? Performing a site survey therefore provides a realistic understanding of the wireless installation. It determines whether a site has unusually high interference issues to resolve, or capacity is greater than anticipated. And an accurate site survey enables accurate quotes to guide financial decision making.

Site survey considerations

There are four major areas to be considered when developing a wireless installation design and should all be considered in terms of future expansion as well as current needs.

• Range and Coverage: The physical area and characteristics of the space where wireless coverage is needed must first be defined.
• Data Rate and Capacity: Determining the required data rate and capacity are essential to assessing the number of access points needed for the site.
• Interference Immunity: It is important to assess potential sources of interference, both now and in the future.
• Connectivity and Power Requirements: Networking constraints within the environment are among the most commonly overlooked issues covered by the site survey.

Site survey tools

The measurement step of a site survey consists of setting up temporary antennas and access points. The consultant measures every part of the site with a software survey tool – installed on a laptop – that monitors the signal strength and identifies failure zones. Such site survey tools measure performance between access points, identify sources of interference, and help determine access point placement. It helps customers to both determine the location and, even more essential for high data rate communications, to calculate the number of access points needed for good coverage and adequate bandwidth.

Choosing the right antenna

The role of a wireless antenna is to direct radio frequency (RF) power from a radio into the coverage area. Different antennas produce different coverage patterns, however, and need to be selected and placed according to site coverage requirements. There are two types of antennas:

• Omni-directional antennas: These have a 360° coverage pattern on a horizontal plane. The coverage pattern is torus-shaped (doughnut-shaped), making them ideal for square or fairly square areas.
• Directional antennas: These concentrate the coverage pattern in one direction. This produces an almost conical-shaped coverage pattern, similar to that of a flashlight. The antenna directionality is specified by the angle of the beam width. Typical beam width angles are from 90° (somewhat directional) to as little as 20° (very directional). The directed beam allows for a longer but narrower coverage pattern, which is ideal for elongated areas, corners and outdoor point-to-point applications.

Calculating antenna site coverage

An increase in coverage within the beam width is called “antenna gain” and is measured in decibels (dB). Gain improves the range of the signal for better communication. Each 1dB increase in antenna gain results in a range increase of approximately 2.5%. For an unobstructed outdoor site, each 1dB increase in gain results in a range increase of approximately 5%. Actual results vary depending on the amount and type of obstructions at the site.

Optimising performance with antenna diversity

For sites with many obstructions, performance and signal strength may be improved by placing two antennas at the site.

Positioning antennas for best results

The proper positioning (orientation) of antennas at each site ensures maximum coverage area. Antennas should generally be mounted as high and as clear of obstructions as is practically possible. Best performance is attained when the transmitting and receiving antennas are both located at the same height and within direct line of sight of each other. Access point antennas should be positioned on or close to the ceiling. Omni-directional antennas should be placed in the centre of the coverage area whenever possible. If mounted below the coverage area (on a table, desk, etc.), point the top of the antenna up. If mounted above the coverage area (from a ceiling), the top of the antenna must be pointing down. Directional antennas should be pointed in the direction of the coverage area.

Do you need a site survey?

Site surveys differ in their complexity and level of effort, based on the technology and space being surveyed. Small facilities probably do not require one at all; customers can rely on wireless vendor specifications and a good look at the office blueprint. However, for larger installations, the site survey offers a level of security in an arena in which – unlike wired networks – there are many variables to consider. A thoughtful, accurate, and effective assessment of coverage and system requirements will ease the wireless LAN installation process, while laying the groundwork for ongoing expansion.