Independent tests prove lack of frequency hopping with XPS

[zoomer260]

KIWI, 16 gigs of RAM ? All I can say is WOW ! My first computer only had 250 Megs of HARD DRIVE !! I thought I had enough with one gig ( RAM ) now ! You must be some serious power user of some kind ? Did you do work on something like the Matrix movie ?

Seriously though, I am one that really, really appreciates your work. I'm in the process of getting back into the Hobby and can't afford to make too many mistakes when I spend my hobby dollars.

I am pleased that you could not get Spektrum or Fasst to fail although I would like to know just how bad it needs to be for them to go into failsafe.

Again THANKS !

[XJet]

	Quote: Originally Posted by Kiwi
	OK I find the receiver quiet a complex little device and have to take my hat off to the guys that designed it. I mean if it really did hop away from bad interference this thing has some serious ability built into it.

The XBeePro modules are indeed a very nice (and powerful) device, which is one of the reasons I use them for small UAV applications with great success.

However, when it comes to designing systems, it's usually the case that the more complex/powerful the building bock the more compromises and limitations are associated with it.

To use a programming analogy for those who understand computers...

You can write some really cool programs using a high-level language like Visual Basic. Accounting software, simple games, etc are a breeze.

However, you won't find any of today's hi-powered 3D shootem-up games written in Visual Basic. Nor will you find any operating systems written in Visual Basic.

Why?

Because in return for ease of use and rapid programming capabilities, you have to give up the kind of nitty-gritty low level stuff and performance that is needed for more intensive applications.

And so it is with the XBeePro modules.

They are like a high-level languages -- *most* of the funcationality you need to do most things is built in and a standard-feature that can be accessed incredibly easily.

However, if you want to get real hard-core about producing a system that needs very finely-grained control then you find that the XBeePro has a lot of limitations -- useful dynamic frequency agility being one of the big ones.

Sure, you can tell the modules to change frequencies -- but there's no robust way to find a new frequency and negotiate a simulataneous switch at both ends -- that kind of thing requires access to far more primitive functions than are accessible through the XBeePro's programmer interface.

What XPS have done (to their credit) is take a powerful off-the-shelf module (XBeePro) and tack a simple microcontroller onto it then call it a 2.4GHz RC system.

Plenty of amateurs have also done this and flown their own creations -- it's not a big job for any talented amateur or professional with microcontroller experience.

JD is well aware of the limitations of the XBeePro modules in an RC context (no way to provide diversity, no robust frequency agility, etc) but rather than acknowledge them, has instead opted to come up with a whole raft of BS to try and fool people into thinking that these things don't matter. That's where the mythical spherical antennas, predictive hopping, etc has come from.

Well they *do* matter -- or the best-performing systems (JR/Spektrum and Futaba) wouldn't have built them into their own systems would they? If a single-frequency DSSS was adequate for all models then Spektrum wouldn't have gone to all the expense and cost of using two and Futaba wouldn't have opted for FHSS.

I just wish that JD would fess-up and say "this is a great 2.4GHz system for small to medium sized models operated in relatively benign RF environments". If he was honest enough to say this then he'd probably find that all those "naysayers" would get off his back and everyone would be happy.

[Four Stroker]

XJet, doesn't a Spektrum system work just like a FHSS system with a lot of busted channels ?

[XJet]

	Quote: Originally Posted by Four Stroker
	XJet, doesn't a Spektrum system work just like a FHSS system with a lot of busted channels ?

I guess you could say that.

But it's more of a stereo DSSS system than a FHSS one :-)

Depending on whether the two channels are used simultaneously or alternately (I haven't checked) then it may or may not offer better latency than an FHSS when exposed to high noise levels on a part of the 2.4GHz band.

Unfortunately I only have very intermittent access to JR/Spektrum and FASST systems and that often means that when I've got the time to do some testing, I don't have the RC sets to test - or v/v.

[Four Stroker]

XJet; Spektrum's claim to link robustness seems to be the statistical improbability of two widely spaced channels being jammed at the same time. Does this agree with your experience ?

[XJet]

	Quote: Originally Posted by Four Stroker
	XJet; Spektrum's claim to link robustness seems to be the statistical improbability of two widely spaced channels being jammed at the same time. Does this agree with your experience ?

Yep, that's about it.

This means that FASST ought to be *more* resilient than Spektrum but Spektrum is a lot more resilient than XPS/Assan/iMax.

What Spektrum has on its side however, are the satellite receivers you can add. Futaba has far more limited diversity -- relying on two closely-spaced antennas that may not provide total visibility in all installations.

I'd be happy flying either FASST or JR/Spektrum in most models however.

[Panzlflyer]

Surely if you find a device that jams ALL frequencies, then it doesnt matter if it hops or uses 2 channels, neither will be able to work although the Faast would be able to find a hole in the noise sooner, but you would be down in the dirt by then with either??? Or am I being to optimistic

[adjonym]

Hmmm, wouldn't it be possible to make XPS decide a "random" sequense for channel-hopping at start-up?
I mean, "rx/tx starting up; tx decides a random sequence of channels that are "next in line", tx tells rx what the sequence are"
Next step for this to work needs the (if I have understood things correctly) is for the rx to send a signal to the tx(again, if I have understood XPS system design, this is something it can do).
This signal is to be sent in X millisecond intervalls. The TX is sending signals all the time, so that's not a problem.

Now, if the tx doesn't get a signal, we can assume that the signal is jammed, and vice versa


If the RX is jammed, it reacts to the fact that no packet is comming through, and switch to next channel in the list. Same with the TX.

This of course would rely on that the next channel is "clean", but then again, that's what spectrum does. The advantage of the XPS with such a configuration is that it could switch again, if the new channel becomes jammed as well, to the next channel in the list.

Phhewww, i hope you understand what I mean; reading it now i can surely see that it might sound confusing;-)

From what I can understand of the function of the module it's using(and the possibility of me understanding it wrong is big;-)), this should be possible, without to much hassle, or?

The one problem I can see with it though, is, if you get a low voltage on the rx side, the channel map could be erased from the memory, and that would not be a good thing.

[Kiwi]

Panzlflyer,

You touched on something I cannot do so far and that is jam the entire 2.4Ghz spectrum. Short of hauling the door of a microwave oven and bypassing the lockout I canr find a way to do it. Its actually very hard, much harder than jamming 72Mhz for example,

Correct me if I['m wrong here XJET as I can be with this stuff.

2.4 Ghz is like a square box. That is the channels maybe 13 wide but they can also be 10 deep. 72 Mhz on the other hand is 50 wide for example but one deep. So if you put out a blast of RF on 72 that covers all 50 channels then 72Mhz is down for the count.

Now 2.4Ghz to be successfully jammed requires enough power to cover the width of the band and the depth. In other words it takes a lot of power to do it. The jammers you see on the internet are very weak and really only work in a 30 ft radius. They are little like cell phone jammers and they also only work with a 20 to 30 ft radius. They are also 800 or 900 Mhz so dont worry about those things with your 2.4 Ghz equipment.

I'm sure that its possible to saturate the 2.4Ghz band in some very specific locations. Built up area with large microwave communications towers for example but honestly I dont ever see the band being fully saturated under normal conditions.

It is however very easy to saturate one channel and thats where I see the weakness in XPS. The Vegas field seems to be a very good example of this. Apparently the noise levels in Vegas are high and when you get a single channel DSSS system up in the open where you have a clear view of all the towers and WiFi access points it falls over. It would be interesting to see if anyone is flying SPEKTRUM or FASST at that same field with problems.

But as for totally saturating the 2.4Ghz bandwidth I think its a hell of a challenge unless your the military or you drive round with Microwave ovens turned on with the door interlock removed. The second option would look a little obvious at the field as well I think!!!!

[Kiwi]

	Quote: Originally Posted by adjonym
	Hmmm, wouldn't it be possible to make XPS decide a "random" sequense for channel-hopping at start-up? I mean, "rx/tx starting up; tx decides a random sequence of channels that are "next in line", tx tells rx what the sequence are" Next step for this to work needs the (if I have understood things correctly) is for the rx to send a signal to the tx(again, if I have understood XPS system design, this is something it can do). This signal is to be sent in X millisecond intervalls. The TX is sending signals all the time, so that's not a problem. Now, if the tx doesn't get a signal, we can assume that the signal is jammed, and vice versa If the RX is jammed, it reacts to the fact that no packet is comming through, and switch to next channel in the list. Same with the TX. This of course would rely on that the next channel is "clean", but then again, that's what spectrum does. The advantage of the XPS with such a configuration is that it could switch again, if the new channel becomes jammed as well, to the next channel in the list. Phhewww, i hope you understand what I mean; reading it now i can surely see that it might sound confusing;-) From what I can understand of the function of the module it's using(and the possibility of me understanding it wrong is big;-)), this should be possible, without to much hassle, or? The one problem I can see with it though, is, if you get a low voltage on the rx side, the channel map could be erased from the memory, and that would not be a good thing.

Thats what XPS already claim it can do. Facts are however it does not and so far no one has ever been able to prove otherwise.

[adjonym]

I know that XPS claims that, and I know that it doesn't
I have no idea on how JD implemented it though, since he refuses to say(and how to make it jump). I just thought about it, and honestly don't see how it can be so hard to implement(make it work as advertised).

The part that changed channel should be able to send "i changed channel" on the "old" channel in a sequence where it changes between the "old" and "new" channel until it recives an "ack" packet saying that the channel switch is complete.

I have no experience with the modules at hand though, so again, i might have missunderstood or even not understood the problems that occur with such a setup.

[Judge]

	Quote: Originally Posted by Kiwi
	Panzlflyer, You touched on something I cannot do so far and that is jam the entire 2.4Ghz spectrum.

Which was the main reason SS was developed in the first place - to prevent jamming.

[RappyMan]

So the ultimate RC system would be a FAAST module "y-harnessed" to a functioning 72 mhz transmitter. It would be a dual-RX setup but also have 2.4 and 72 working simultaneously! Then someone would have to jam both 2.4 and 72 on your channel to take the plane down. Doesn't Powerbox have a Royal version that takes the functioning RX if one goes down & spreads it to the whole plane.

Just sort of for fun here, but it could work right?

[Edge 540]

.

[sweetpea]

	Quote: Originally Posted by Kiwi
	It is however very easy to saturate one channel and thats where I see the weakness in XPS. The Vegas field seems to be a very good example of this. Apparently the noise levels in Vegas are high and when you get a single channel DSSS system up in the open where you have a clear view of all the towers and WiFi access points it falls over. It would be interesting to see if anyone is flying SPEKTRUM or FASST at that same field with problems.

There are 2 Vegas fields. Bennett field (old TOC) has had issues with 72 and Spektrum and XPS. I would say XPS and 72 have had the most problems though. Its hit and miss on both of these. Spektrum has been the best at that field so far.

At the North Field....we have had a couple instances of Spektrum issues and a few XPS as well (I'd venture to say exactly the same amount). Though there is more Spektrum flying at this field than XPS for whats that is worth. We have both XPS and Spektrum flying in everything from small electrics to 50% size planes. 72 hasn't had any real problems other than the turn on your freq issues.

As for FASST I don't have much data on it.........it took so long to get out that 90% of the fliers moved to something else. If I was staying here longer I'd be able to tell you more.