I believe Netstumbler has some those features.
You could use a packet capture program such as kismet (or kismac if you have a mac) or airsnort to work out bytes transferred.
First I add that some of these replies need clarification.
There are two kinds of overlap, one is channel overlap where the frequencies overlap, and the second being signal overlap.
You MUST have signal overlap for all devices to have coverage in all areas, or even most devices in most areas.
Secondly, there are various schools of thought for frequency overlap and some manufacturers even suggest putting all APs on a common channel. In the case of roaming IP phones this case becomes even stronger as a phone may hop across APs while in a call. This of course depends much on the hardwae of the phones and antenna placement and design.
Let us assume that we had a large open area that we wanted wifi coverage in. Now lets take a pole and place it in the middle of the area. Now we place 4 directional 90 degree antennas on the pole, each 90 degrees from the other . In this situation one may make a strong case for having all APs on the same channel to facilitate roaming. In theory there is little signal overlap but all frequencies overlap.
Now we have an open area with walls on four sides. and place an AP on each of the four walls. The signals WILL overlap from each of the 90 degree antennas , so we may want to consider using separate non overlapping channels on each AP , however there are only 3 non overlapping channels. 1, 6 , and 11. So instead we do the best we can in North America this might be 1, 4, 7, and 11 , each AP having SOME necessary frequency overlap. Of course in a perfect world this might be better accomplished with three APS in a triangular configuration.
In my home I have toyed with APs on Same channel and separate channels and in the end I see little coverage difference., I do see however that some devices such as wireless IP phones can more easily hop to another AP while in a phone call. I see that in most areas I do not have more than 2 overlapping signals and each on channel 4 at present. As I sit here I can launch wifi seeker on my android and see either of the 2 available APs and even connect to either. This of course is easier to test with separate SSIDs but more practical to use common SSIDs fopr everyday use.
Best Answer
Move it closer to the Base station. Everything you send in typical wifi links goes to/from the base station. Ad-hoc connections are different, but not many use those.
Really, though, I expect your problem has to do with interference. That is much more likely to be the problem than distance. Here's the kicker: that interference may be your own signal.
With wifi, you may have a base station can do a hypothetical 65 Mbit connection. Unfortunately, that is not 65 Mbit for each node: that is 65 Mbit total, shared among not only nodes A and B, but also any other clients on that same channel in the same area. Worse, let's say one of your nodes is only able to get an 18 Mbit signal, and is actively using 3Mbit of that signal. That use scales proportionally to the max theoretical number for the base station. The client is using air time, not bandwidth, and so 3 Mbit of a total available 18 Mbit (one sixth) means it's using one sixth of the total theoretical 65 Mbit supported by the base station, or about 11 Mbit worth of air time. This leaves at most 54 Mbit for all other clients combined on the same channel in the same area. Worse than that, you can even get interference from devices on different channels, because the channel frequency ranges overlap (this is why 2.4Ghz radios should only ever use channels 1, 6, or 11 in the US).
In your situation, when A streams to B, you must upload the data to the base station, which must then resend it to B. That means you cut your available wireless bandwidth in half, because you're having to share. If A is also downloading its data for the stream from the internet you take a share away again, and you're down to one third of the original total. We also need to account for command and control information from the protocols used that must be transmitted. Worse than that, the bandwidth is not shared perfectly. Different nodes can try to send at the same time, resulting in collisions. When that happens, all colliding nodes must re-send the packet. As the traffic increases, the number of collisions increase. As the number of collisions increase, the amount of data needed to be re-transmitted increases, and the odds of additional collisions go even higher. This doesn't even begin to account for other interference sources like cordless phones, video game controllers, microwave ovens, wireless keyboards/mice, running water, etc. In the end, you may only have a small fraction of the original and reported 65 Mbit actually usable. Newer 5Ghz radios can help with this, but it's not a panacea; if you're sharing a base station, you're still sharing a single channel and still sharing your theoretical max among all clients of that base station.
If you really want good performance here, go wired or go home. Wired connections can fix the issues described above in three ways: they can provide a connection that is switched, full-duplex, and that is almost completely insusceptible to outside interference. Switched means that if each node has a 100 Mbit connection to the base, that is 100 Mbit devoted exclusively to that node. If two nodes try to send at the same time, the base is able to hold packets from one and forward them when the line is clear, reducing collisions and therefore reducing the need to re-transmit the same data. Full-duplex means that nodes are able to both send and receive at the same time... again, reducing collisions. Here, node A could be downloading stream data from the internet at the same time that it is sending it back towards B, with no interference or collisions.
In this case, because of all the re-transmission of the same data, you might see dramatic performance improvement if even one of nodes A or B has a wired connection.
A recent example where I'm at is that we deployed iPads to all the faculty this term at the college where I work. To support these devices, during the trial we deployed a few AppleTV devices to classrooms and connected them to the projector to support AirPlay mirroring from an iPad to the front of the classroom. We learned from this that leaving both the AppleTV and the iPad wireless did not work well, especially as we may have two instructors in neighboring rooms both wanting to do mirroring. The solution for us was to install software on the PCs in each room to support AirPlay mirroring to the PC, which is wired. We had to make some network changes so the classroom PCs were on the same subnet as iPads, but the result is much more reliable and with much better video quality.