Download Lessons Learned Constructing the NG-Mesh Wireless

4.6. Discussion
4.6
McMaster - Elec. Eng. - M.A.Sc. - S.W.K. Ng
Discussion
Consumer-grade Wi-Fi interfaces such as the popular Ralink chip-sets are capable
of capturing RSSI metrics across the ISM band. From the driver source code, it is
possible to periodically retune the Ralink radio to each of the eleven North American
Wi-Fi channels and read their RSSI levels. Logic tells us that if a file transfer was occurring in isolation on Wi-Fi channel 1, then the highest RSSI reading will be detected
there. And moving away to incrementally higher channels will lead to the detection
of ever lower RSSI levels as the distance from the frequency of transmission activity
increases. Additionally, this rate of decrease in RSSI levels should be proportional to
the IEEE 802.11b spectral mask requirement. Based on this logic, it should be possible to construct a software-based spectrum analyzer from a rt2570 Wi-Fi transceiver.
It was found through the experiments of this thesis that the software-based spectrum
analyzer developed provided unsatisfactory results due to the following reasons.
• The RSSI readings were much higher than the actual levels in the 2.4 GHz
spectrum as verified by the commercial spectrum analyzer.
• There is a noticeable latency in the updates to the RSSI registers whenever the
radio is retuned to a new frequency.
For these reasons, the software-based spectrum analyzer was only able to identify
the communication channel adequately.
Capturing accurate RSSI data in real-time was possible using the retail spectrum
analyzer and allowed us to analyze co-channel interference patterns from file transfers occurring on each Wi-Fi channel. It also permitted us to develop the AutoMin
algorithm to implement automatic congestion avoidance for our test-bed.
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