Difference Between Good and Bad Coverage

 I thought it would be helpful to be able to show the difference between what is considered good and bad coverage as viewed through the lens of the I/O graph. It is a very handy tool to understand how the test device is performing in the environment and gives a visual representative of the trace and makes it easier to figure out where to begin looking for issues. Below is an I/O graph in a facility where I discovered low coverage on their third floor. The Ekahau heat map made that abundantly clear, but this data shows how the device actually performed in that environment at that time and foreshadows how the devices will perform without the necessary remediation.

Figure 1. Example of Low Coverage on 3^rd Floor

The BLACK lines represent the upstream audio frames from the badge to the wireless access point, the BLUE dots represent the downstream audio frames from the access point to the badge, the RED lines represent packet retries, and the GREEN lines represent the probe requests captured from every device in the area. The graph represents about 45 seconds in time where the badge is roaming quickly through an area. The device starts out with a very good signal strength on channel 64. At 14 seconds the device roams to channel 149 but it has a very poor signal. The device suffers through this association for 10 seconds, losing audio for the last three, before roaming to channel 40, which also has a poor signal. At 29 seconds the device roams to an equally poor signal on channel 36 and remains there for the rest of the trace.

Figure 2. Amount of frames below -65dBm

This graph shows the downstream frames in signal strength measured in dBm; most of the frames are well below the -65dBm threshold. This is the downstream frames from the last graph with the field set to dBm instead of percentage.

This is the primary coverage where that trace was captured, not exactly a surprise:

Figure 3. Example of Low Coverage Area 3rd Floor

So now that we have seen what bad looks like, here is an example of another facility where the wireless was set up correctly and working perfectly :

Figure 4. Example of Good Coverage and Proper Roaming

In this graph, you can see how the downstream frames in BLUE start at the top of the graph, indicating good signal, then move down the graph until such a time as the device picks up another good signal and roams to another access point. Notice how the blue dots shoot up to the top of the graph on each roam; also notice that there are very few retries shown in RED. This is the rarest of the type of wireless you will see out on the road and as such, it is very exciting to see a graph that looks this good. It was hard not to walk around and show it to everyone, mostly to people who already knew their wireless was in good shape. Needless to say, this was a much easier assessment then most others you will run into in your travels.