Scott’s Weblog

Characterising errors in wireless links, continued.

April 25th, 2007

After spending most of last week manually validating the packet matcher I’ve decided that it’s at a pretty good state. I’ve settled (for now at least) on calculating the hamming-distance over 128 bits from various parts of TCP, IPv4 and 802.11MAC headers. Any frames that are transmitted and received within a certain time period and have a hamming-distance of 10 or less are marked as matches. This appears to work “quite well”, but I’ll need to come up with some hard numbers if I’m going to make any arguments based on it.

After convincing myself that the packet matcher was in a good state, I started to look at packet errors. On Monday, I took several traces. Each trace consisted of a 10 minute bi-directional TCP iperf between two nodes which were placed about 4 meters apart in the lab. One acted as an AP and the other acted as a station. I took a separate trace for each of the 802.11b rates, 1, 2, 5.5 and 11 Mbits. I did this so I could look at how different encodings affect error patterns within the packet. I’ll take more traces later with 802.11g and a rates.

Interestingly the error rates were very asymmetric. From the station to the AP, each trace showed a Packet Error Rate (PER) of > 10%. However, from the AP to the station the PER was ~1%. This suggests a hidden terminal which is surprising given that the nodes are only approximately 4 meters apart. Also, the PER decreased as the rate increased, which was surprising.

Over the different rates, the errors patterns within packets were very distinct. In the 1 and 2 Mbit traces (DBPSK and DQPSK), errors within packets were highly clustered, however in the 5.5 and 11 Mbit traces (CCK) the errors were more “spread out”.

Another interesting point is that although the PER decreased as the bitrate increased, the rate of errors within each packet increased. So, at higher rates we had less packets with errors, but the packets were more corrupted.

My next objective is to come up with some statistical models of these bit errors so that they can be applied in simulation. I’d also like to get some “proper” traces from real-world links to work on. Hopefully I’ll get some graphs up to illustrate this data within the next few days.

Entry Filed under: Ph.D