4. 2. Effect on neighboring Wi-Fi networks
High cumulative channel occupancy transmissions.
Po WiFi leverages the inherent fairness of the Wi-Fi Medium
Access Control to ensure that it is fair to other Wi-Fi networks. As a worst-case evaluation, we consider a situation
where PoWiFi always tries to achieve high cumulative
channel occupancies at all times. To do this, we place
our PoWiFi router in the vicinity of a neighboring Wi-Fi
router–client pair operating on channel 1. We configure
the Po WiFi router to transmit power packets at the highest achievable channel occupancies using our algorithm
on all three nonoverlapping channels. We run iperf with
UDP traffic on the neighboring router–client pair at the
highest data rate and measure the achievable throughput as before. We repeat the experiments for different
Wi-Fi bit rates at the neighboring Wi-Fi router–client
pair. We compare three schemes: BlindUDP where our
router transmits UDP packets at 1Mbps, EqualShare
where we set our router to transmit the UDP packets
at the same Wi-Fi bit rate as the neighboring router–
client pair, and finally Po WiFi. EqualShare provides a baseline when every router in the network gets an equal share of
the wireless medium.
Figure 7a shows the throughput for the three schemes,
averaged across five runs. As expected, BlindUDP
significantly degrades the neighboring router–client performance. Further, this deterioration is more pronounced at
the higher bit rates. With Po WiFi, however, the throughput
achieved at the neighboring router–client pair is higher
than EqualShare. This is because Po WiFi transmits power
packets at 54 Mbps; transmissions at such high rates occupy
the channel for a smaller duration than, say, a neighboring
router transmitting at 16Mbps. This property means that
Po WiFi provides better than equal-share fairness to other
Wi-Fi networks. We note that while our experiments are
with 802.11g, Po WiFi’s power packets use the highest bit
rate available for Wi-Fi. Thus, the above fairness property
would hold true even with 802.11n/ac.
Rectifier-aware power transmissions. Next, we evaluate
the potential of our rectifier-aware technique, to signifi-
cantly reduce the average channel occupancy of the power
transmissions, while efficiently delivering power to the sen-
sors. To do this, we place our battery-free temperature sen-
each run, we run five sequential copies of iperf, 3 s apart, and
compute the achievable throughput over 500ms intervals,
with all the schemes described above.
Figure 5b plots CDFs of the measured throughput values
across all the experiments. The plot shows that BlindUDP significantly degrades TCP throughput. As before, since NoQueue
does not prioritize the client traffic over the power packets, it
roughly halves the achievable throughput. Po WiFi sometimes
achieves higher throughput than the baseline. This is due to
changes in channel conditions that occur during the 3-h experiment period. The general trend however points to the conclusion that PoWiFi does not have a noticeable effect on TCP
throughput at the client.
Figure 6b plots the CDFs of the channel occupancies for
Po WiFi during the above experiments. The figure shows that
PoWiFi has a mean cumulative occupancy of 100.9% and
hence can efficiently deliver power.
Effect on PLT. We develop a test harness that uses the
PhantomJS headless browser11 to download the front pages
of the 10 most popular websites in the US1 100× each. We
clear the cache and pause for 1 s in between page loads. The
traffic is recorded with tcpdump and analyzed offline to determine PLT and channel occupancy. The router uses the default rate adaptation to modify its Wi-Fi bit rate. The experiments were performed during a busy weekday at UW CSE
over a 2-h duration.
Figure 5c shows that BlindUDP significantly increases
the PLT. This is expected because the 1 Mbps power traffic
occupies a much larger fraction of the medium and hence
increases packet delays to clients. NoQueue improves PLT
over BlindUDP, with an average delay of 294ms over the
baseline. PoWiFi further minimizes the delay to 101ms,
averaged across websites. This residual delay is due to the
computational overhead of PoWiFi from the per-packet
checks performed by the kernel. This slows down all the
processes in the OS and hence results in additional delays.
However, increasing processing power and moving these
checks to hardware can further reduce these delays. In our
home deployments (Section 6), users did not perceive any
noticeable effects on their web performance.
For completeness, we plot the CDFs of channel occupancies for Po WiFi in Figure 6c. The plot shows the same trend
as before, with a mean cumulative occupancy of 87.6%.
0 10 20 30 40 50 Ach
Wi-Fi bit rate (Mbps)
0 5 10 15 20 25
TCP throughput (Mbps)
0 5 10 15 20 25 30 35 40 45 50
Per channel occupancy (%)
Po WiFi transmitters
(a) (b) (c) (d)
Figure 7. Effect of Po WiFi, rectifier aware and concurrent power transmissions on neighboring Wi-Fi networks. The plots show that Po WiFi
power transmissions provide better than EqualShare throughput performance. Rectifier aware power transmissions further improve the
throughput by reducing the per channel occupancy by a factor of 10. Additionally, increasing the number of concurrently transmitting Po WiFi
devices does not degrade the performance of neighboring Wi-Fi devices. (a) Po WiFi bit-rates, (b) Rectifier aware throughput, (c) Rectifier
aware occupancies, and (d) Concurrent transmissions.