There's a longstanding need in wired networks to verify the quality of connectivity of wire, cable and fibre with appropriate test equipment. But with wireless nets, this requirement extends beyond initial site surveys and occasional troubleshooting.
Managers of wireless networks have an ongoing need for spectral analysis, viewing not just protocol-based traffic but also energy in a given band. And this monitoring needs to be constant, because interference can appear at any time.
We refer to the analysis of energy at the physical layer to improve performance as spectral assurance. With the announcement of Cisco's CleanAir technology, which includes access points with spectral sensing as well as new management software features, spectral assurance is for the first time available as an integral part of the wireless LAN infrastructure.
We call this style of spectral analysis the centralised infrastructure model (CIM), differentiating it from the earlier but still-useful walking around model (WAM) typically implemented on notebook computers.
But instead of having to send out a trained engineer to detect, classify, localise and (perhaps) remedy a given interference challenge, all this can now be done continually and from a central location. So, in addition to getting round-the-clock coverage, the CIM strategy can help lower WLAN operating expences.
We tested CleanAir's functionality in four key areas:
- Detection – identifying interferers and examining the duty cycle and intensity of their energy.
- Classification – identifying a specific interferer by its electromagnetic signature. This enabled us to look for specific types of devices as we proceeded further through these steps.
- Localisation – via the power of a CIM-based installation to determine the location of the interferer, a new application of the location and tracking capabilities already present in many WLAN implementations.
- Remediation – Eliminating the interferer, usually via automatically reconfiguring channel assignments. Localisation can also be used to find and physically disable an interferer.
Our test configuration consisted of a Cisco 5500-series WLAN controller, three Aironet 3500-series access points with CleanAir Technology, an identical 3500 AP operating in "monitor mode'' and a 3300-series Mobility Services Engine, which is an applications server used to perform location calculations.
We also used version 7.0.164 of Cisco's Wireless Control System (WCS) management software via a notebook PC. The 3500 AP is equipped with a Cisco-designed radio chipset that incorporates the sensor capabilities originally implemented in Cisco's notebook-based (WAM) Spectrum Expert product, and the version of WCS we used incorporates much of the software capability in Spectrum Expert as well.
Integrating spectral sensing and analysis directly into the infrastructure allows automated remediation of interference situations, which is not possible with Spectrum Expert alone.
We used the following commercial products as interference sources:
- Panasonic KX-TG3031S 2.4 GHz cordless phone
- GE 28021EF3-A 5.8 GHz cordless phone
- QSWLMCR 2.4 GHz analogue video camera
- RC5833A 5.8 GHz analogue wireless camera
- Nuts About Nets' AirHORN signal generator, which puts out Wi-Fi-based interference on a pulsed, swept or continuous basis.
We put one access point into what Cisco calls Monitor Mode, which allows it to scan all channels continuously rather than also serving as an access point. A typical access point supporting CleanAir only monitors its own channel, and then only while receiving, so the added assurance of monitoring all of the local airspace can be quite valuable especially in detecting unauthorised devices.