Independent tests prove lack of frequency hopping with XPS

[cadconversions]

I would like to see the test ran on the other systems just to see if they lock-out or continue to work. If all three systems completely lock-out then none is better than the others. If the futaba continues to work through everything throwed at it - then it's the best... If spektrum continues to work as well, then that makes XPS the worse....etc... However, I wonder if the spektrum (after getting two clear channels) would fail if both channels were swamped? Probably (right?) then in a high traffic area wouldn't that be fairly likely? If so, why have we not seen a lot of crashes reported from both channels being swamped or have we?

[Daemon]

	Quote: Originally Posted by cadconversions
	However, I wonder if the spektrum (after getting two clear channels) would fail if both channels were swamped? Probably (right?) then in a high traffic area wouldn't that be fairly likely? If so, why have we not seen a lot of crashes reported from both channels being swamped or have we?

It would, but it depends entirely on what's doing the swamping. Spektrum will
generally pick a couple of channels that aren't close to each other. If something
like a high powered 2.4Ghz video transmitter comes along, it may spread its signal
over 3-4 channels (actually probably closer to 8-10 of Spektrum's narrower channels),
but as long as it doesn't step on both of the channels Spektrum has selected, it'll be
fine. And even if it does, one of them is liable to be on the edge of the noise, and still
function. The type of noise that wipes out the whole 2.4Ghz spectrum and steps
on both of Spektrum's chosen frequencies at once, is going to hurt everyone else
as well. If there is any clear area, FASST will find it, but because it hops all
the time, it'll hop back into the noise as well. So if 90% of the spectrum is swamped
then FASST will lose 90% of its packets.

And keep in mind *none* of the 2.4Ghz *SS systems use 100% of the time on
channel. Typically it's closer to 10% utilization, and 90% of the time that frequency
is clear. That's why at even an extremely busy field, XPS, Spektrum, FASST,
Assan etc, all work happily. Even if they're on the same channel they're only
using it 10% of the time. If they lose a packet, they have different ways of dealing
with it. Some just ignore the lost packet and continue on (like FASST), and some
like XPS, lose the pack or the ACK from the Rx and send it again up to 3 times
before the next PPM frame. For Spektrum or XPS to be hurt by each other at all,
there'd need to be probably 5-6+ systems transmitting all on the same frequency
(XPS says up to 10 per channel, and from networking experience, we usually see
the collision rate starts to increase real fast at about 50% utilization of the wire).

ian

[wrightme]

	Quote: Originally Posted by Daemon
	And BTW, the reason people keep asking you to run FASST is not so much to test it, but to test XPS with it. There's only been one reported case of XPS hopping, and that was with someone touching the FASST Tx antenna to the XPS Rx antenna. XPS hopping in response to FASST is of course pointless, since FASST goes everywhere, but if it happens it should show up on your spectrum analyzer. Looking forward to the results of further tests. ian

Do not forget that the "FASST/XPS" test could almost be considered to be a "destructive" test, since the performance of the module was questionable after the antenna touch.

Frankly, Kiwi is the one testing, so the tests should be what he wants them to be.

[wrightme]

NOTE: From the start, Kiwi has been intending to test MORE than simply XPS hop.

	Quote: Originally Posted by Kiwi
	Some really simple facts. All receivers including 72 Mhz have low voltage cutoff. From memory its 3.5 volts, but I might be wrong there but thats close. Some 2.4 Ghz Rx´s take a lifetime to re connect. ( note I did not say reboot). Two channels is better than one ( Murphies law alone proves that without any independent testing.) A hopping system where the Tx knows where to go and look if comunication is lost is equal to or as robust as two channels. A single channel hopping system that does does not have a pre scanned and acknowledged next position is not as robust if swamped in a noisy environment. A single channel non hopping system is the weakest of them all. Likely OK for park flyers and small helis etc but again if they are flown in very noisy environments they will likely crash. FACT: 2.4 is not immune to interference. Thats pure BS if anyone thinks that. However the strenght of 2.4 is the abilty of the system to defeat the interference by moving to a clean environment. Fact : 2.4 Ghz is the most uncontrolled non regulated frequency in use today. (I qualify that by saying while 2.4 is supposedly controlled by FCC regulations etc etc etc the junk being imported that does not and never will comply with FCC would horrify you all.) Anyone can put a sticker on a device saying its FCC, EU etc etc approved) Microwave ovens??? Just one example. Now here´s my question and I need some help. Can anyone in this forum prove to me beyond any doubt that XPS skips. This is not bash against anyone or any system. Please can any of you show me XPS skipping. Please dont confuse this question with XJET testing to see if it does. Thats not my question. We have a wealth of intelligence on this web site so please, who can show us the system skipping?? If not ,does anyone have a Futaba XPS system they would lend me or what ever so I can put it over the scanner and see if we can force it to hop and what conditions it takes to make it do just that. I know XJET has a system taken from a crashed plane. I dont want that as I really want him to post what he finds independently with what he has. So guys if you have a setup collecting dust and sitting in the bottom of your field box, if you trust me to test it and send it back, then you have my word I will do so. I have in my collection of 2.4 systems so far. FASST, SPEKTRUM, ASSAN so this will be a fair and level playing field for what ever we can test. I have the facitlies to do a decent job, I have very qualified technicians who dont know diddly squat about RC planes etc. You cannot get a more level playing field than that. Someone for sure is going to ask why we are doing this and what is the purpose. It´s simple. We have no bone to pick, we just want to fairly and honestly deliver what we find. I hope all the systems pass with flying colors. There´s no pre formed opinion here guys. Lets take a totally open minded look at what and how these setups work. In fact the idea is not even to say what is best or what is worst. Its just to show (hopefuly) that each system performs as close as we can acertain to what the manufacturer advertises. No hidden agenda here. no opinions, just facts. Forums open guys, I think it is actually getting vbe very sensible now and Sweetpea your contributions are much appreciated. The only thing I ask from anyone who wants to debate at lenght here is lets restrain the opinions and state the facts. Then we all benifit from the experience.

[Julez]

I guess it would be insightful to ramp up the noise until the XPS RX is *just* not glitching, and then bring all kinds of wires near its antenna, to see how their influence will be.

[xed]

	Quote: Originally Posted by Daemon
	BTW, the test you didn't do that, Jim claimed would induce hopping, is a gradual rise of the noise floor to the point where it does harm to the link, followed by a small decrease. I think the logic being, that it needs to know it's in harm's way, but still be able to tell the Tx that it needs to move. ian

Which is basically the only condition which XPS could possibly "hop" for since any other case means that it cannot communicate with the receiver/transmitter (either side can "need" to hop - noise may be in the air but not on the ground with the pilot or vice versa).

I suppose it could establish a pre-determined next "channel" for both sides to decide to "hop" to in the event of loss of communication but it doesn't mean that channel is free/has low noise floor when it comes time to hop (i.e. broken link) and pray that everything is OK after 400ms. It would also need to store this next channel in persistent memory to survive a potential reboot on the receiver (low battery condition causing a broken link).

Of all the testing performed so far on RC Groups and the snake oil replies made by JD, I'm not convinced that XPS has ever been tested in a situation that JD claims it will "hop", which is so specific that it is likely never to happen anyways before the sky comes falling down.....

I agree with you Ian, it would be really nice to see of all the present designs (XPS, FASST, Spektrum), which ones perform the best in a noisy environment or an environment where the noise floor changes over time (from great to bad and bad to great). In theory, I believe that FASST with it constantly hoping about the spectrum will behave the best, with Spektrum copping to some degree if the noise isn't swamping both of its "selected" channels and XPS being a complete failure in anything but a clean environment with other devices playing nicely (i.e. not video transmitters, ground transmitters, weather stations, etc)... Now we just need to have the theory proven...

[Daemon]

I can still think of a couple scenarios where XPS might beat the other two
with regard to noise, even if it never hops. One would be if the noise
floor rises across the whole band to the point that they're all
losing some packets (say 50%, for the sake of argument). Spektrum and FASST will
just simply lose them and work with what what gets through showing decreased
responsiveness. XPS Tx will retransmit up to 3 times between every PPM frame when it
doesn't see ACKs from the Rx, increasing the odds that at least one valid packet
will get through for each PPM frame. If 50% of the packets were lost, it should
still show close to 100% responsiveness due to the retries.
Second would be a source of noise that produces loud short transient bursts
across the whole spectrum (spark gap generator comes to mind).
Again the other systems will just lose those packets while XPS may retransmit
around the problem.

A "reliable data link" with acks and retries can make up for a lot of evils in the
underlying communications layer. TCP/IP is one demonstration of this.
The real shame is that JD doesn't really promote and demonstrate
this, instead of coming up with additional marketing claims that don't
really help anything.


ian

[XJet]

	Quote: Originally Posted by Daemon
	I can still think of a couple scenarios where XPS might beat the other two with regard to noise, even if it never hops. One would be if the noise floor rises across the whole band to the point that they're all losing some packets (say 50%, for the sake of argument). Spektrum and FASST will just simply lose them and work with what what gets through showing decreased responsiveness. XPS Tx will retransmit up to 3 times between every PPM frame when it doesn't see ACKs from the Rx, increasing the odds that at least one valid packet will get through for each PPM frame. If 50% of the packets were lost, it should still show close to 100% responsiveness due to the retries.

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.

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.

	Quote:
	Second would be a source of noise that produces loud short transient bursts across the whole spectrum (spark gap generator comes to mind). Again the other systems will just lose those packets while XPS may retransmit around the problem.

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.

Most other systems cope by transmitting the data more frequently anyway -- FASST is a good example of this.

	Quote:
	A "reliable data link" with acks and retries can make up for a lot of evils in the underlying communications layer. TCP/IP is one demonstration of this. The real shame is that JD doesn't really promote and demonstrate this, instead of coming up with additional marketing claims that don't really help anything.

As I've said, XPS is not without its stronger points but other systems manage perfectly well without the need for a reliable transport layer and in fact it's this layer that contributes to the the difficulties that XPS has with implementing any kind of robust frequency-hopping capabilities.

The main reason that XPS is viable at all (with its lack of receiver diversity) is the reliable transport layer. Without the ability to re-send data lost to multi-pathing nulls caused by the lack of diversity, the XPS would be a whole lot worse than it is. However, it's my personal opinion that a reliable transport layer is no real substitute for diversity and frequency agility in an RC environment where you're talking about realitime systems and a really harsh environment (propagation-wise).

[zoomer260]

Man! This is nothing short of amazing ! How far modeling has come and the advancements continue. And to have a forum like FG to find all the people who can evaluate something like this is incredible. What can I say ? I'm old. RC has grown without me.

[DadsToysBG]

Gee Zoomer, If your lost at 49, just think where I am at 60. Dennis

[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.

[zoomer260]

	Quote: Originally Posted by DadsToysBG
	Gee Zoomer, If your lost at 49, just think where I am at 60. Dennis

Ha! I might have to remove my age in the future LOL! I just FEEL so out of touch even though I can grasp the meat of it because these guys talk pretty much in laymans terms (thanks guys) I have been doing my modeling from a keyboard and flat screen for too long.
And the advancements never cease to keep my attention. Heck, I just bought my first NiMH battery for crying out loud ! They were passe' by the time I got back into it. And I remember all the "problems" and backlash they got when they came out. I'm gonna skip the 7.2 Lipo's altogether and go A123's on my next one.