I`m doing experimental for Micaz in one large environment that crowded with Access Points... I`m sending data from 2 different places with different distances. what surprise me, the average round trip time (RTT) for longer distance (40 ft) is more lower than the closer one (24 ft).
40ft = 30 sec RTT average.
24ft = 65 sec RTT average.
Suppose 24ft will have lower average RTT than 40ft distance. The antenna power i used in this both experiment is 0 dbm. I already doing another experiment using different channel (non overlap with wifi channel) but still get nearly same result.. So I wonder if, if i change the antenna power to lower one will get better result or worse?
Any ideas, I`ll really appreciate it. TQ.
I'm not clear on the geometry here. Normally, you would expect the shorter distance to create the lower round trip times. The only times this result won't apply would be if the shorter distances, in certain cases, required more hops to go from A to B?
I might start by reducing the number of access points, just to get an idea of what's going on. Reduce the number, until you see effects that you would expect.
In general, I would expect that increasing transmit power would result in more direct paths, for A to reach B, if the lower power meant that multiple hops were necessary. But there will be other effects at play, if you have a very crowded access point environment, right? You will have co-channel interference problems, where access points will have to share their channel with other access points on the same channel. So you would expect queuing delays to play a part. Very messy.
If transmit power level is higher, it will definitely enable data from A to reach B with less hops. The assumption is that there is no degradation in receiver performance due to co-channel interference.
Low power means more hops leading to delays in each hop. In actual network condition, the number of access points and power level are to be optimized.
Albert, i`m doing point to point communication which involve one sensor node and one base station.About the environment that i`m currently doing experiment is crowded with access point from other rooms. Mostly of the APs assigned with channel 1 and 6. The Micaz, i set it to operate under channel 20. Sometimes i still got uneven result. Later, I`ll try change my sensor nodes position and the APs is not under my control so i cannot turn it off (used by other researcher in other labs). Thanks for the information Albert.
Thanks Prabir ,if low power leading to higher delays cause by data hops... I better stick to high power configuration...
RTT is affected by packet length. I had some very surprising results while testing early WiMAX implementation. You should try ping with different packet lengths. There is an interplay between packets and packet scheduling on the air.
Lauris, currently I`m doing with one sensor node and one base station. I run the experiment with 3 different distance between the sensor node and the base station.
For my packet configuration, the maximum allowable size for the payload is set to 65 bytes. Most the packets sent from the sensor node in 37 bytes per packet (sensor), 29 byte for health packet and 17 bytes for acknowledgement packet.
Dear Azizul Lau
Some time ago we developt a test with a group of student using movile phone with wifi in a soccer field, from the distance of 10 to about 90 mts we got a normal decrease of the signal power, as could be espected , but near the 100 mts to 120 mts we deteted a increase in the signal power, it was bizarre for us, until we saw that the last measures was near the stadium stands, where people sit to see the play. We had a increase of power reception because we got the direct signal plus the reflexion of adition signals that came from the concrete stans.
Maybe the geometry of the place that you are using is being helping to focus the longer distance node, and is not helping the nearest one. Verify is you can collect others metric addiotional to the RTT, such as the power of the received signal, with micaz is possible to use the RSSI metric.
You can also repeat the experiment in a place without interference of Wifi or using a Channel not used by nearest Access Point. There are some apps that you can download on an android phone that can be used to detect access point and Wifi signal around you.
Lauris, the RTT is measure in millisecond provide by the data logger software. But some of the RTT measurements reach more than 10,000 milliseconds. That`s why I`m also surprise with the results. Changing the node position to the other place seems make very small improvement of the RTT (not a lot).
Ricardo, mostly of my RSSI reading for short distance is around -55 ~ -60 and also can reach -80 for long distance. Sometimes even with better RSSI reading, i still get high RTT values with link quality 100 for for both sensor node and base station.
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