Планирование и проектирование беспроводных сетей: обследование радиочастотного покрытия.

Planning and designing a wireless network
A wireless site survey, sometimes called an RF (Radio Frequency) site survey or wireless survey, is the process of planning and designing a wireless network, to provide a wireless solution that will deliver the required wireless coverage, data rates, network capacity, roaming capability and quality of service (QoS). The survey usually involves a site visit to test for RF interference, and to identify optimum installation locations for access points. This requires analysis of building floor plans, inspection of the facility, and use of site survey tools. Interviews with IT management and the end users of the wireless network are also important to determine the design parameters for the wireless network. As part of the wireless site survey, the effective range boundary is set, which defines the area over which signal levels needed support the intended application. This involves determining the minimum signal to noise ratio (SNR) needed to support performance requirements. Wireless site survey can also mean the walk testing, auditing, analysis or diagnosis of an existing wireless network, particularly one which is not providing the level of service required.

There are three main types of wireless site surveys: passive, active, and predictive. During a passive survey, a site survey application passively listens to WLAN traffic to detect active access points, measure signal strength and noise level. However, the wireless adapter being used for a survey is not associated to any WLANs. For system design purposes, one or more temporary access points are deployed to identify and qualify access point locations. This used to be the most common method of pre deployment Wi Fi survey. During an active survey, the wireless adapter is associated with one or several access points to measure round trip time, throughput rates, packet loss, and retransmissions. Active surveys are used to troubleshoot Wi Fi networks or to verify performance post deployment. During a predictive survey, a model of the RF environment is created using simulation tools. It is essential that the correct information on the environment is entered into the RF modeling tool, including location and RF characteristics of barriers like walls or large objects. Therefore, temporary access points or signal sources can be used to gather information on propagation in the environment. Virtual access points are then placed on the floor plan to estimate expected coverage and adjust their number and location. The value of a predictive survey as a design tool versus a passive survey done with only a few access point is that modeled interference can be taken into account in the design. Additionally, some survey application allow the user to collect RF spectrum data using portable hardware spectrum analyzers, which is beneficial in case of high RF interference from non 802.11 sources, such as microwave ovens or cordless phones.

Depending on the survey type, a number of software and software/hardware options are available to WLAN surveyors.

Passive and active surveys are performed using software and typically require only a compatible off the shelf Wi Fi adapter; no additional specialized hardware is required. Predictive surveys require no hardware at all, as no wireless data collection is needed. Currently, professional level site survey applications exist primarily for Microsoft Windows. Some site survey applications for other platforms, including iOS and Android, also exist, however they are limited in functionality due to the limitations of the underlying platform API. For example, signal level measurements cannot be obtained on iOS without jailbreaking. The feasibility of creating professional level applications for non Windows tablets is debated.

Unlike passive and active surveys, RF spectrum surveys require specialized RF equipment. There are various types of spectrum analyzers ranging from large and expensive bench top units to portable ("field units") and USB based analyzers. Because portability is a decisive factor in conducting wireless site surveys, spectrum analyzers in USB form factor are widely used today.