Independent tests prove lack of frequency hopping with XPS

[zoomer260]

	Quote: Originally Posted by dick hanson
	Somehow-- all of the problems being researched seem to have faded out by the time they reached SLC Ut - Tho I use Spektrum exclusively -some friends use the XPS and their stuff works fine If it didn't ----I would hear about it at the weekly lunch n fact finding and general BS meeting --but then the only packets discussed are packets of Sweet n Low. and I still don't understand how anyone can say that dreck tastes like sugar.

Sweet'n low ? YUCK !! I'd rather have my coffee straight !

[Daemon]

	Quote: Originally Posted by XJet
	While it's true that the use of a reliable-transport layer has the potential to improve performance in such situations, it's not necessarily going to work that way in practice. The XBeePro modules use a CSMA approach to transmitting data-packets so if the interference is strong enough, it will cause the transmitter *not* to transmit, even if it hasn't received an ACK packet before the timeout occurs. With a high (enough) general noise floor the XBee modules will just sit and wait for the noise to drop before they even try to transmit. This means that if there's a strong source of wideband noise visible to the transmitter but not the receiver, the XPS is at a decided disadvantage over something like FASST which will keep transmitting regardless.

That does not match the results of my testing. I have two pretty good indicators
that tell me when XPS Tx is transmitting, and when it's retransmitting.
The first is the voltage display on my Tx. It drops a tenth or two when it
starts retransmitting packets.
The second is a side effect of what I'm using to generate noise, which is my
500mW 2.4Ghz video system. With the video receiver, I can actually *see* 2.4Ghz
RC systems transmitting on overlapping frequencies (Spektrum is the worst).
Watching it, while XPS is on, I see a little bit of visible noise on the video image
from XPS Tx/Rx when they're transmitting normally, and *a lot* of noise when
the XPS Tx starts retransmitting. A spectrum analyzer may not be able to see this,
because the noise generators most people are using often just swamp the XPS
signal on the spectral display. My video signal doesn't have that problem, because
it is both the noise for XPS, and shows noise *from* XPS.

Anyway, what I've found is that at no time, does the XPS Tx stop transmitting,
and re-transmitting when the link goes bad, and is ultimately totally swamped
by my video system. We're talking putting the half watt transmitter inches away
from either XPS Tx or Rx, sometimes both close together, and sometimes
separated by a great distance (so it only swamps the Rx, or only swamps the
Tx) and I can see it re-transmitting furiously any time the link is broken for any
reason. The Tx never gives up. Unfortunately if you mix failsafe and a power supply
problem together, the Rx does sometimes give up, but that's a different problem.

	Quote:
	The other issue is that an XPS system can use a lot more than the usual 10% of its timeline whenever it fails to get an ACK from the receiver. XPS claim that up to 10 users can share the same channel (a theoretical maximum, as anyone working with CSMA networks will tell you, things can start to degrade well before the "theoretical maximum" is reached) which means that if you have (say) four XPS systems working on the same channel and the noise floor rises, those four transmitters alone can completely monopolize the time-line because of all the retries that are forced. In fact, with four transmitters each issuing three retries, the timeline would be well and truly saturated -- meaning that even if packets did get through, the latency would rise significantly. The apparent inability to hop means that none of those four transmitters would be able to switch to a cleaner channel either.

That's true although it'd be rare to find 4 XPS on the same channel, as the Rx
does try to find an open channel when it boots. You'd have to have pretty much
saturated the rest of the 2.4Ghz band to force that many XPS systems together
onto the same frequency, in which case there may not be anywhere safe for it
to hop to, even if it tried.

	Quote:
	This might well be true -- although it must be remembered that with a 10% duty-cycle, the XPS data packets (in fact the data packets of all 2.4Ghz systems) are a pretty small target so the effect of short-burst impulse noise tends to be minimal (or at least significantly less than on 100%-duty-cycle systems like PPM/PCM) anyway.

Again, I was thinking something along the lines of fully swamped 50% of
the time. 10ms worth of noise on and 10ms off would cut the other
systems down to 50% of their packets getting through, while XPS would
retransmit through the holes.

Ultimately we don't know what the noise looks like though. I think XPS will
work better in some environments and worse in others.

ian

[Rolling Thunder]

	Quote: Originally Posted by zoomer260
	Sweet'n low ? YUCK !! I'd rather have my coffee straight !

I agree. Also Rats now gain weight when given a diet of sweet N low instead of sugar.

[XJet]

	Quote: Originally Posted by Daemon
	That does not match the results of my testing. Anyway, what I've found is that at no time, does the XPS Tx stop transmitting, and re-transmitting when the link goes bad, and is ultimately totally swamped by my video system. We're talking putting the half watt transmitter inches away from either XPS Tx or Rx, sometimes both close together, and sometimes separated by a great distance (so it only swamps the Rx, or only swamps the Tx) and I can see it re-transmitting furiously any time the link is broken for any reason. The Tx never gives up. Unfortunately if you mix failsafe and a power supply problem together, the Rx does sometimes give up, but that's a different problem.

Then XPS is not setting the CCA (Clear Channel Assesment) threshold to a sensible value, which is a bad thing.

Why is it a bad thing?

Because it means that XPS isn't listening to see that the channel is clear before it transmits its data packets.

If this is the case then XPS will never even come close to reaching its theoretical maximum number of users because packet collisions will be significantly higher than when a proper CSMA protocol is used.

I will test for this and see if the CCA value has been set to something other than 0 (disable).

	Quote:
	That's true although it'd be rare to find 4 XPS on the same channel, as the Rx does try to find an open channel when it boots. You'd have to have pretty much saturated the rest of the 2.4Ghz band to force that many XPS systems together onto the same frequency, in which case there may not be anywhere safe for it to hop to, even if it tried.

Not as unlikely as you might think.

If you're in an area where there's a reasonable amount of other 2.4GHz traffic (public-access WiFi, video transmitters, etc) then there may only be a few of the XBeePro's 13 channels that have a reasonably low noise floor. In that case it would make sense to put more than one XPS user on each channel if, in doing so, the total noise level on that channel remained less than the other channels.

It's not likely but not inconceivable that you might end up with four (or more) XPS users on the same XBeePro channel.

However, like so many of the issues surrounding the XPS, it's not going to affect operation under normal/favorable conditions and (as we've seen) XPS can work fine under normal circumstances. What we're looking at though is how well XPS copes with the exceptional circumstances that can and do occur from time to time. We need to see which of the current 2.4GHz systems is best able to deal with such "exceptional circumstances" and thus which is most likely to let you return home with a model in one piece or maybe just a fright -- rather than a trash bag full of bits.

	Quote:
	Again, I was thinking something along the lines of fully swamped 50% of the time. 10ms worth of noise on and 10ms off would cut the other systems down to 50% of their packets getting through, while XPS would retransmit through the holes.

The problem is that if XPS have disabled the CCA and aren't checking for a clear channel before sending their data packets then it won't know where the holes are -- so the chances of its packets getting corrupted (and thus lost) by being sent over the top of other legitimate 2.4GHz traffic are increased.

It's kind of like trying to get across the freeway by just running out into the traffic and hoping you don't get hit. Not a good idea -- much better to look before you leap (which is what the CCA setting is supposed to allow.

	Quote:
	Ultimately we don't know what the noise looks like though. I think XPS will work better in some environments and worse in others.

You are right -- each of the systems will have its strengths and weaknesses, it's just a matter of weighting them to get a reasonably accurate indication of their relative performances under normal and exceptional (but not impossible) conditions.

[KrisW]

	Quote: Originally Posted by Rolling Thunder
	I agree. Also Rats now gain weight when given a diet of sweet N low instead of sugar.

Actually ALL "artificial sweeteners". . aspartame, sweetnlow, that stuff from the 60's that was supposed to cause cancer. . .pink blue, or yellow packets .. they ALL make your body want to gain weight. Which is why "Diet Soda" makes you hungry and why all those fat people who drink diet drinks STAY fat.

On an unrelated topic, but something really cool. . . .close your fingers together and look at the length of the fingertips. If your index finger is longer than your ring finger, you had a low testosterone level during development in the womb, and tend to have a lower level as an adult. . if the index finger is shorter than the ring finger, the opposite is true. (actually proven through scientific research, not a gag)

Hmmmmmm . my ring finger is only 3/16" shorter than my middle finger. . . my index finger is over 1/2" shorter than that. Funny. . I don't FEEL macho. . . though I am a bit furry. .arh arh arh arh.

Back to our regularly scheduled "beat up on XPS" thread

[Kiwi]

OK,

Lets leave the sugar versus sweet n low and the fat rats to another thread.

Kris, old mate, this is not a beat up on XPS , in fact looking at the other forums I think this one is still the most focused and the most open.

The good news is the FASST system arrived in Chile today and is on the way to my office as I write this. Spektrum will be in my hands today or Saturday so we have them all in one place.

That has not been as easy as it sounds.

Now the fun begins.

[sweetpea]

I like the idea of another thread with just the results for each system posted without all the other B.S.

But keep going Kiwi.

It will be nice to put any questions to rest on how the other systems react to each scenario as well.

We all think we know the answers......but it will be great to see what actually happens

[sweetpea]

Sorry Double Post

[Kiwi]

OK Sweetpea,

I will start a new thread when I get my butt back home on the weekend.

The only thing I will ask is just for once lets leave all the brand loyalty at the front door. This has nothing to do with team red, orange, blue or black. Its all about seeing how this stuff works and if it can live harmoniously with each other.

Its intended to show that they all function as advertised and live up to those expectations.

I am not personally looking for a winner here, I do want to see what works well and if I trust my $XX,XXXX dollar scale aircraft to them.

Apart from that I just enjoy learning new stuff and 2.4 seems to be something a whole lot more technical than 72Mhz PPM or PCM.

[Woketman]

Please post a link to the results thread, when appropriate. Thanks.

[wrightme]

	Quote: Originally Posted by Kiwi
	OK Sweetpea, I will start a new thread when I get my butt back home on the weekend. The only thing I will ask is just for once lets leave all the brand loyalty at the front door. This has nothing to do with team red, orange, blue or black. Its all about seeing how this stuff works and if it can live harmoniously with each other.

Kiwi, Is it possible for you to start a thread, post results, then "close" or "lock" it so that you can block the arguing? (then re-open it to post new results?)

[KrisW]

	Quote: Originally Posted by Kiwi
	OK, Lets leave the sugar versus sweet n low and the fat rats to another thread. Kris, old mate, this is not a beat up on XPS , in fact looking at the other forums I think this one is still the most focused and the most open.

Sorry, Kiwi. . obviously my Testosterone level is too high for me to be able to make the distinction.. .

[Wojcigitty]

XJet, thanks for starting this thread. Your posts have been tremendously enlightening, and I truly appreciate your efforts.

Kiwi, I hope the rest of the testing goes well. The more we know, the closer we are to finding or implementing the safest possible control solution for large RC stuff.

Earlier in the thread the inherent issues with 2.4 were brought up. That made me think. Suppose I am using a 2.4 Ghz device, and then, quite legally, the channel(s) on which I am operating are flooded, by some other device. Now, if I was using the 2.4 band to browse the internet, I lose my connection, swear, and have to refresh the FG homepage. Inconvenient, but no big deal. Or I might lose my cordless phone connection, or the bluetooth headset I'm using doesn't work, or whatever. My point is, none of these things are mission-critical. They temporarily don't work, and nothing really happens.

But if this happens to me when I'm flying my RC model on 2.4, it could cause a crash, safety hazard, etc (I won't go there).

My question is this: is there anything that operates in a manner similar to our RC stuff on 2.4 (ie the connection absolutely can not fail)? If no, then maybe 2.4 really isn't a good choice; maybe it's not meant for something like RC aviation.

[XJet]

Thanks for the kind words...

The ideal solution for RC would be to have some dedicated spectrum dedicated solely for RC use at a high-enough frequency that we could use still SS techniques.

Right now, 2.4GHz is a battlefield where it's only thanks to the technical merits of SS technology that we're able to use it for anything at all.

There are some advantages to 2.4GHz though:

It's a short enough wavelength (high enough frequency) that distance is a great barrier to interference. If you're out in the middle of nowhere, miles from any other 2.4GHz devices or services then you're pretty damned safe. The path loss for 2.4GHz is quite high and it's also fairly much line of sight which means stuff that's below the horizon or behind a mountain isn't going to hurt you. Even stuff you can see will be of no matter -- if it's far enough away that its signal is simply absorbed or dissipated before it gets to you.

Of course since the RC hobby is a relatively non-essential use of the radio spectrum, it's unlikely that there'll be any band of frequencies allocated specifically for it and, even if there was, the cost of RC gear on those frequencies would likely be a *lot* more than the 2.4GHz stuff we're now using.

The only reason that 2.4GHz has taken off for RC use is that there are plenty of "solutions" (chipsets and modules) designed for other things on 2.4GHz (such as WiFi etc) which can be pushed into service for RC links. Of course this also means that there are limitations to some of the current generation of 2.4GHz radio gear because few of those "solutions" were actually designed for robust, resilient, highly mobile, real-time telecontrol applications.

With the ever-growing flood of cheap "gadgets" operating on 2.4GHz coming out of China I can't see the band getting any friendlier for RC users. This means there will always be fields that are simply too risky for 2.4GHz RC use, while others are interference and trouble-free on this band.

[airborneSGT]

So we could also stick with the "old" system with more frequency control? I don't see why the old system would not work with some type of frequency hopping in which it would only recognize the model you fly.