Independent tests prove lack of frequency hopping with XPS

[Kiwi]

OK gentlemen before this one goes off the rails how about we stick to what Xjet started here and that is the evident fact that XPS does not channel hop as advertised and the fact that guys are and have and likely will loose aircraft until its fixed or disappears.

Its not about anyone having to prove why 2.4 is better than 72, 50, 40, 35 or 36 Mhz as all those frequencies are used in some part of the world every day on RC airplanes.

Kris, let this thread run as the guy desired please, dont turn it into a personal thing.

[jonkoppisch]

I switched so that I wouldn't get shot down... and I got shot down

[3ddd]

well i for one am still waiting on xjet to post just that.post count on pure bs=76,post count of proof xps is dangerous=0,we will keep waiting.beeb flying xps in three planes all year and not one misshap,not even a slow response time

[XJet]

	Quote: Originally Posted by Kiwi
	So Xjet what Mr Drew wrote here and these are his own words is totally BS.?? (Quote Jim Drew on RCU) I am not sure what answers you are looking for. The bottom line is that we don't have a problem with the system, and I don't expect to ever see a problem. We are the ONLY 2.4GHz system that has the ability to monitor the available frequencies in the 2.4GHz range and change as necessary, in real time.

JD forgot to add "under ideal circumstances" to that.

Remember that JD also said:

	Quote:
	If the noise becomes so high that it saturates the channel, the hopping counter is reset and the system will not switch until there is a decrase in the noise level

and

	Quote:
	The frquency hop requires a rise in the noise floor over a number of frames. A sudden saturation will reset the noise coutner and not switch at all

In checking back over the earlier claims made on the XPS website I see he's been *very* careful with his use of words, claiming only that the system would frequency hop when it detected potential interference. In fact, the exact words were:

	Quote:
	Only changes frequencies when potential interference is encountered

It doesn't claim that *actual* interference will produce a frequency hop.

The reason for this (and what JD is now saying) is that if there's a gradual rise in noise (ie: an interfering signal of slowly increasing strength) on the frequency XPS is using, it will do a quick scan and jump to another (quieter) frequency.

Now that sounds fine -- and indeed it may well happen (something I'm going to be testing shortly) -- but the reality is that (despite what JD claims) interfering signals don't always (or even often) introduce themselves by slowly ramping up from nothing -- this allowing XPS the time to change to another frequency.

As I've stated before, the big problem with single-frequency non-agile systems is that 2.4GHz is very much a "line of sight" system. When you're down at ground-level in the pits, there's a good chance that some very strong interfering 2.4GHz signals are completely blocked by what's called ground-clutter. Trees, buildings, vehicles and even people all act like a very effective RF screen that means when XPS does its initial frequency scan at start-up time, it may be totally unaware that a powerful 2.4GHz transmitter is operating just a mile away. The signal from that 2.4GHz interference source is simply soaked up by the ground clutter between it and your model.

To see what I mean -- get down to "plane level" (ie: lay down on the runway" and look around. You can't see much else that might be at ground level beyond the confines of the flying field can you? That's the view your RC gear gets when it sets about looking for a "clean" channel to use.

After locating what it thinks is a clear channel (one out of the 12 available), XPS will then start providing a link between transmitter and receiver.

But, what happens when you take off?

Suddenly it's like climbing to the top of a tall building. You can see things in the distance or just past the runway fence, that you couldn't see before while laying on the runway.

At the same time you're flying away from the transmitter (ie: its signal is getting weaker) strong interfering signals are suddenly becoming "visible" to the receiver.

What if one of the things you can now see is a very strong source of 2.4GHz signal that just happens to be right smack dab on the channel your XPS system is using?

If you're lucky, the strength of the signal from that source will rise slowly enough to trigger XPS's alarm bells, allow it to scan the other 11 channels it can use, advise the transmitter it wishes to change, then perform the frequency change -- a process that does take time and if interrupted, will result in a failed hop attempt.

If you're not lucky, the receiver experiences a very sudden increase in the noise level, something which constantly resets the noise counter which JD admits is a problem. It then then unable to change frequencies so goes into failsafe -- and your plane becomes a very expensive shovel.

Another scenario is that your XPS system is flying quite happily, receiving a steady stream of data from the transmitter and sending back very short "ACK" signals from its 60mW onboard transmitter. A short (or not so short) distance away, someone turns on another 2.4GHz device with 600mW or more of power. It may be a video transmitter, it may be some other data link.

Suddenly the signal from XPS's own 10dB weaker 60mW transmitter on the ground is lost, overwhelmed by the much stronger signal from a transmitter that may have the benefit of greater transmitter power, a directional antenna, lower path-loss and extra visibility arising from a matching polarization.

In an instant, the XPS receiver sees this as a massive increase in the noise level - sufficient to throw it into this counter-reset mode from which it can't hop frequencies. Lockout time.

A satellite receiver and/or diversity antennas would help mitigate this somewhat by avoiding the situation where the transmitter and receiver are cross-polarized or the receiver's null-point is oriented directly towards the transmitter. The single-antenna setup makes XPS doubly vulnerable.

	Quote:
	We don't conflict with any other 2.4GHz systems, and they don't conflict with us.

I think that ought to read: we dont' conflict with any other 2.4GHz RC systems"

There are plenty of other systems out there that will conflict with XPS (video senders for a start).

	Quote:
	Our systems are special, one of a kind radio configurations and no two radio modules have the same ID. It is physically impossible to have a conflict unless two identical ID's were available. We can't even get MaxStream to do that for us for testing the theory, so we know that it will not happen

This is irrelevant to the frequency hopping issue -- it simply refers to the fact that each module is bound to the other such that the data packets carry a unique identifier. That'll stop one XPS system from responding to the transmission from another (because if the numbers don't match, the data is ignored) but it won't stop a strong interfering signal from causing a lockout. The best analogy is that a JR PCM receiver won't respond to a Futaba PCM transmitter -- but they can still shoot each other down if they're on the same frequency because lockout will occur.

	Quote:
	Anything is prone to a problem. 2.4GHz will not transmit through large sections of earth (hills, etc.). So, as long as you are flying LOS (line of sight) you will never encounter and issue with the radio link that is related to our system or any other 2.4GHz system available.

This is the ground-clutter I was referring to earlier. The fact that ground clutter obscures what may be powerful nearby sources of interference is exactly what makes a single-frequency non-agile system so vulnerable.

	Quote:
	Even 2.4GHz cameras are not a problem as we can move around their frequency, even with huge drift that occurs in the cheap overseas versions. Our system is extremely well thought out for safety reasons.

There are plenty of posts on the Net which dispute this. In my testing, XPS can operate if turned on in the presence of an already operating video transmitter but if you turn the transmitter on after the XPS system, control is sometimes immediately lost and failsafe results.

	Quote:
	People have witnessed and flown the system first hand. We did a range check with the 40% gas plane, and it was so high that we lost it in the camera and could no longer hear the engine running (and it was!) That is some distance. The highest recorded flight to date was 4947 feet, but I believe we have gone higher than that, just not recorded the flights with the altimeter.

This is a total red herring, range has never been a problem with 2.4GHz systems and has nothing to do with the frequency-hopping issue that's being debated.

	Quote:
	If you care to argue, I have no reason to reply.

That kind of sums things up right now. JD won't argue the facts or provide substantiating proof for his claims.

Some will believe as an act of faith, others will be more objective and recall the many other claims from XPS that have since been debunked.

[XJet]

	Quote: Originally Posted by DesertHucker
	PS XJet did you get the XPS i sent?

Yep, turned up just before Christmas.

I'll be back to working on this first-thing in the New Year.

[sweetpea]

As posted on RCgroups

"Originally Posted by JimDrew
As stated from the beginning, the frequency hopping is predictive and based on the noise level increase over a period of time. If there is a sudden burst of noise, it does not makes sense to change the frequency as we have found in nearly every case that a sudden burst does not occur on a single frequency, but on the entire band (as well as above and below 2.405GHz and 2.485GHz). "

Xjet quote:

"JD won't argue the facts or provide substantiating proof for his claims."


Well to date neither have you! But I'm waiting, along with others for you to produce it.

I'm very interested in Xjet's test of the gradual increase of noise......But I still want to see the data for both.

[sweetpea]

I'm not trying to say who is right and who is wrong.......I just want truth, backed with data

But so far the only facts we have.

-Some people have crashed or saved due to failsafe (why it was in failsafe has yet to be determined)
-Some people have had sluggish controls (which this appears to be from high noise levels but cannot be confirmed)
-Lots of people are flying trouble free
-Xjet has said testing was done on freq hopping and posted results with no data or crediantials of those doing the testing
-XPS has done testing and got FCC approval
-XPS has stated why the freq would not hop and why it would
-XPS has stated that they did not expect to see noise levels as high as like the LV area (I agree with everyone that this is disheartening and should have been looked at in intial testing)
-We have no testing on Spektrum or FASST to determine if the same "noise" level from Xjet's testing would cause problems with them.
-Their are plenty of issues running 1.2-2.4ghz wireless cameras on any 2.4 system......(use 900mhz and solve the problem)


And though I know there is the common talk of ......"it works till it doesn't"

But come on.....72mhz does exactly the same. Unless you confirm someone turned on your freq you only guess as to what your lockout is caused by with several good flights.

I should know more than anyone.....after 2 yrs of flying my plane on 72mhz.....one day it stopped working. took 3 months and 3 different freqs to get it to work, but I had no confidence in the system anymore.

[Kiwi]

Something I find a bit unnerving here is I remember clearly in the early days of the Web spamming on the benifits of XPS was the fantastic amount of channels available to this setup. I seem to recall it being stated that you could technically fly hundreds of models at the same time and get no interference between them.

I am neither for nor against any manufacturers attempts to provide a reliable and robust RC system but I do find 12 channels a seriously small number of available frequencies. I know someone is going to say yeah but!!! You never have 12 planes flying at the same time but by god you can easily have more than 12 transmitters turned on in the pits if everyone thinks they are never ever going to shoot someone down or cause a problem.

What happens when transmitter No 13 gets turned on.?? Does it just fail to link up or does it squeeze someone out with a weaker transmitter signal.??

Again I am not taking sides, nor do I want to open pandoras box but seeing as we have someone with a seriously indepth knowledge of 2.4 Ghz radio systems its an opportune time to ask I think??

[jonkoppisch]

Good question about the 13th transmitter. Also, Does the transmitter broadcast any kind of signal before it hears from the receiver? Ie, you turn on the transmitter (not broadcasting), turn on the receiver (broadcasts signal to trans), establishes link then the trans starts to broadcast 2.4 signal? I assume that the receiver is broadcasting a 2.4 signal so.. Another question. If 12 people are flying (the limit and fills the frequencies) and 3 more turn on, will the broadcasting receivers trying to link with the transmitters cause any interference as the channels are full??

According to posted specs though, 120 people can fly at the same time....

[klhoard]

	Quote: Originally Posted by KrisW
	Okay. . give me one. . important. . logical. . all encompassing reason why I should NOT fly 72mhz. . and SHOULD fly 2.4 instead. <snip> Again .. ONE reason to switch.

.
.
Kris,

72Mhz is still working great for me too. I hate to see people still putting XPS in the same league as Futaba and JR, it's not.
.
.
If 72Mhz is working for you, then indeed stick with it. However, when you decide to consider making the jump, I wouldn't recommend basing your decision on the track record of XPS.
.
.

[sweetpea]

	Quote: Originally Posted by jonkoppisch
	Good question about the 13th transmitter. Also, Does the transmitter broadcast any kind of signal before it hears from the receiver? Ie, you turn on the transmitter (not broadcasting), turn on the receiver (broadcasts signal to trans), establishes link then the trans starts to broadcast 2.4 signal? I assume that the receiver is broadcasting a 2.4 signal so.. Another question. If 12 people are flying (the limit and fills the frequencies) and 3 more turn on, will the broadcasting receivers trying to link with the transmitters cause any interference as the channels are full?? According to posted specs though, 120 people can fly at the same time....

I can't remember why you can have 120 but there was a reason stated way back. If it is truly only 12......that does raise some questions at large events for the lucky #13.

From what I recall on several posts and talks with XPS.....the module does not transmit until it hears from the RX. The RX is the brains of the unit

	Quote: Originally Posted by klhoard
	. . Kris, 72Mhz is still working great for me too. I hate to see people still putting XPS in the same league as Futaba and JR, it's not. . . If 72Mhz is working for you, then indeed stick with it. However, when you decide to consider making the jump, I wouldn't recommend basing your decision on the track record of XPS. . .

Correction.......JR has yet to produce a 2.4 unit.........They use Spektrum 2.4 in their radios and Spektrum uses JR radios for their casings. But that is splitting hairs really.

I'm also with you......if your scared, unsure or anything else......stick with 72mhz.....you know the risks there. They have not changed in years.

2.4 is full of new risks that cause the same problem as 72mhz.....crashes. I would say you have to look at all the brands across the board for 2.4........(XPS. Spektrum, Futaba, ASSAN) then once you decide 2.4 is for you, look at specific brands that meet your needs/wants/price.

[jonkoppisch]

	Quote: Originally Posted by jonkoppisch
	Another question. If 12 people are flying (the limit and fills the frequencies) and 3 more turn on, will the broadcasting receivers trying to link with the transmitters cause any interference as the channels are full??

The more I think about this the more curious I am. All 12 channels are full, 3 more people decide to turn on. The receiver has to broadcast on 2.4 to establish the connection with the transmitter?? Something has to broadcast to something!! So now there are 3 more 2.4 sources at close range broadcasting looking for a connection. What frequency do they first broadcast on for the connection? etc.... Could they swamp 3 connected receivers?

Wait. Duh, it finally occured to me that the receiver will (or should) scan the frequencies first. If it sees that they are full then it should simply not broadcast for the connection? It would be interesting to have an oscilloscope (& know what the heck that I'm doing) and check this out...

[XJet]

	Quote: Originally Posted by jonkoppisch
	The more I think about this the more curious I am. All 12 channels are full, 3 more people decide to turn on.

This is one of the cool things about 2.4GHz, you *can* have multiple transmitters and receivers on the same frequency without problems.

Your standard 72MHz system is transmitting constantly -- that's because it has to work with a very narrow bandwidth and thus it needs to send data continuously to get enough information to the receiver at a rate that will allow smooth control.

A system such as XPS or Spektrum has much more bandwidth available to it (several megahertz rather than just 10 kilohertz) which means it can send the necessary data in a fraction the time. Think of it like a thin straw and a thick straw. If you have to suck up a gallon of liquid an hour, you'll have to suck continuously on the thin straw but with a thick straw you can drink the lot in just a few minutes then put your feet up and rest for a while.

Since the 2.4GHz system can send a whole frame of data in about 1/10th the time of your 72MHz system, it can transmit for just 10% of the time -- simply sitting idle for the remainder. This is called the duty-cycle (the percentage of available time that is used in every second)

During this other 90%, other transmitters (on the same frequency) can send their data.

Now all the receivers on the same frequency will receive all transmissions on that frequency but (and here's where "binding comes in") they will ignore the data that doesn't come from the transmitter they're bound to.

The receiver sits there waiting for the next lot of data to arrive. When a packet arrives it looks at it and either says "that's for me, and passes it onto the servos -- or says "not mine" and totally ignores it.

Since XPS appears to have a duty-cycle of about 10%, it means that in theory, you could have up to 10 transmitters all working simultaneously on the same channel. With a total of 12 channels available, that's how JD comes up with his "120 simultaneous users" number.

So what stops two transmitters from sending out their data at the same time and confusing the receivers?

Simple, the XBeePro modules always listen for other transmissions before they begin to broadcast their own signal. If a transmitter finds that the frequency is in use, it goes to sleep for a very short (random) time and then tries again.

This is called Carrier Sense Multiple Access (CSMA) and is a very common way for computer networks to all share a single cable on a large network. It's tried and tested -- but not 100% efficient.

What tends to happen is that as you get more and more transmitters vying for that frequency, they will each increasingly find that it's busy when they go to transmit. As a result, they go back to sleep for a moment and try again later.

The more transmitters are working on a single frequency, the more sleeping that is likely to occur between the time a transmitter is ready to send and the time it finds the channel unused by another.

It is not possible to get 100% utilization of the available frequency because, the more transmitters are contesting for the available time, the less available time there will be so the less likely the are to find a "quiet" period.

In the case of XPS, this situation is worsened a little because not only are all those transmitters trying to send data to their receivers, but those receivers are also trying to send an acknowledgment back to the transmitter (consuming more valuable time).

However, it is *true*, you can run 10 XPS systems on each channel, however there will be significant latency in the response of the system when you start approaching this theoretical maximum. Your servos will lag behind your control inputs and this could make flying very difficult.

I strongly suspect that this is a non-issue however, since I doubt many people would even dare take to the air if there were another 119 models flying :-)

Of course if I were just out to "knock" XPS (which despite the claims of some, I'm not) I'd come up with a reason why this is bad, but it's not -- it's pretty standard stuff for networking and 2.4GHz-based data links. It's nice, it degrades slowly under increasing load rather than failing without warning.

There's no problem here.

[BoneDoc]

XJet,

So what did you do on your testing? So far, you've been telling us of what XPS does NOT do, but not HOW. Curious minds want to know and get to the facts. If you have the time to make counterpoint arguments, surely you'll have the time to at least write about your methods.

I'm no fanboy as I do not own XPS. I have Spektrum right now, and I have issues as well from time to time (2 receivers died).

Please publish the facts.

[jonkoppisch]

Interesting. Thanks! I take it that the xbee modules are from wireless routers and such? What is the range of a typical 2.4 wireless router? Does it have the same strength as the xbee modules being used for rc? If so, with everyone getting wireless routers for their home networks, say 3 or 4 in the area, could this cause any interference? As I understand it, ground clutter would weaken or thin a lot of the signals but as you say, once you're in the air?

Added: Does a wireless router/network etc use more time on the frequency also? So instead of only using 10% it might use 30% or 70%??