Wireless Network Site Survey Overview

By Shaun Hummel

Overview

The site survey is an assessment of signal coverage per each access point within a specific building. It confirms with the client 100 percent coverage at a specified signal guaranteeing employees will have no issues with sending wireless packets across the network. Each building structure will have a design that determines signal characteristics.

The most often issues that cause problems with decreasing wireless signals include water, metal, building contours and devices that emit electro-magnetic interference. Some access points can send a pattern of coverage extending 100 - 170 feet while some will emit a signal 50 feet with same characteristics. The building structural design has a lot to do with that.

The following is an overview of the site survey process:

1. Discuss Signal / Noise and Data Rate with Client
2. Obtain Floor Plans
3. Survey Buildings Noting Signal and Coverage
4. Document Access Point Placement

What then is an acceptable signal? Signal to noise separation of around 20 - 25 dBm at 54 Mbps is a minimum with most designs. The site survey software measuring a continuous 75 dBm signal and 95 dBm noise (75/95) would be enough to have employees connected and sending packets with same service level as wired clients. As signal weakens it becomes more difficult to distinguish between a signal of 80 dBm and noise of 95 dBm (80/95). There is less separation between signal and noise making the signal difficult to detect. It is counter intuitive that a larger number is a weaker signal however the key with signal and noise measurements is the signal is a negative number and hence a lower value is a bigger number.

Discuss with the client what specific client adapter, 802.11a⁄b⁄g wireless standard and data rate they'll implement and survey with that. Deploying the 802.11a standard doesn't require a separate survey if you implement dual band and surveyed with 802.11g. Some companies with higher bandwidth demands will improve cell coverage with a separate 802.11a survey. It isn't as cost effective with additional access points deployed per cell coverage. Configure the access point with the selected 802.11a⁄b⁄g protocol, transmit power and data rate. Set the data rate and transmit power of the client adapter to maximum during the survey.

The site survey of a building involves walking around with a laptop on a powered cart configured with a Cisco wireless client adapter and the Cisco Aironet Desktop Utility (ADU). The software is a utility available with Cisco client adapters. The Cisco access point is placed at a specific spot on the ceiling or above the ceiling tile, wall, desktop or cubicle and measurements of signal coverage is noted. The access point is moved until optimal coverage occurs for that cell. Select ADU active mode to examine data rate, link status and errors while surveying.

The cell coverage can be transferred to a Visio drawing floor plan with specific access placement and signal boundaries. The access point is moved around until the entire building is mapped. The minimum signal strength should be recorded per coverage cell. Be as specific as possible as you describe where access points should be implemented. Some clients will have floor plans which makes the site survey much easier to document coverage. Scan each floor plan as a Visio drawing noting specific placement of access points and assignment of channels.

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