Independent tests prove lack of frequency hopping with XPS

[Flyfast1]

Perhaps better than any other thread I've read, these last few posts have explained some of the limitations of using a single channel for communications in an environment where the channel needs to be changed. It does seem to be more limited than using two channels like Spektrum or frequency hopping like Futaba. That's too bad because I have a significant investment in XPS equipment which I will continue to use, but at some point I may decide to go a different route.

-Ed B.

[Wojcigitty]

Could JD have used TWO XBPro modules in concert on different channels, with some circuitry on the receiver side to decide which one to use? That way, if one is knocked down, the system can continue to function while the other set takes all the time it needs to find a new frequency. Of course, that would have probably ruined his price advantage...

[aviti]

	Quote: Originally Posted by Wojcigitty
	Could JD have used TWO XBPro modules in concert on different channels, with some circuitry on the receiver side to decide which one to use? That way, if one is knocked down, the system can continue to function while the other set takes all the time it needs to find a new frequency. Of course, that would have probably ruined his price advantage...

This won't be any help accoring to Jim Drew's logic. He maintains that any interference strong enough to affect his one channel will swamp the entire 2.4 band...which is also the reason he says his system is not designed to hop when the noise level increases suddenly. Do I agree with this logic? Well let's just say it doesn't make sense to me but I'm not an RF engineer.

You could just but a Spektrum system if you want to transmit on 2 channels...

[XJet]

	Quote: Originally Posted by Wojcigitty
	Could JD have used TWO XBPro modules in concert on different channels, with some circuitry on the receiver side to decide which one to use?

There is a problem with that...

The transmitter in the XBeePro module is relatively powerful (100mW ERP) and having two modules that close to each other (even if on different channels) would likely cause issues related to one receiver affecting the other when it was transmitting its ACK packet back.

The situation would be even worse at the transmitter end where, in order to work properly, you'd also need two modules operating in *very* close proximity to each other.

This dual receiver model works okay with other 2.4GHz systems because the receivers are not also transmitters -- but imagine what happens when one receiver is busy transmitting an acknowledgment while the other is trying to receive a data packet. Even if they're on different frequencies, that powerful transmitter right next to a sensitive receiver has the potential to cause problems -- just like bringing a working GSM mobile phone near to an AM radio can produce a loud buzz -- even though they're hundreds of megahertz apart in frequency.

Once again, it's the reliable transport layer of the XBeePro module that becomes a handicap rather than an asset in this situation.

The larger the blocks you use to build something the less time it takes and the easier it is. However, the price you pay is a significant loss of control over the low-level details -- and that's the trade-off that XPS has made in opting for the XBeePro modules rather than the lower-level chipsets that other vendors are using.

[E_ferret]

	Quote: Originally Posted by Daemon
	If the Tx loses ACKs for some interval (Tx module isn't programmable, so who knows if it'll match the failsafe interval), it hops to backup. ian

Hi Daemon,
A hop based on tx/rx timeouts is feasible but like you describe, very time consuming.
The coms scenario seems to be very basic, transmit, wait for ack, if timeout then retransmit x number of times during frame period. If Nack retransmit.
I can see no reason why a retransmit reason code cannot be included in the data packet . That way the rx becomes aware that the tx did not get the ack/nak back and that the link is frail, before a link timeout period expires.
eg. reason code 0 = good packet, original transmit
reason code 1 = you send me an 'not acknowledge'
reason code 2 = ack/nak timeout occured for the first time
reason code 3 = ack/nak timeout occured for the second time
etc...
this should put a flea in the ear of the rx.

[zoomer260]

So many on this thread are so much more knowledgable about radio than others (ME). But it has been mostly kept in laymans terms that a simple electrician can grasp the "points".
I've really enjoyed this thread not because of XPS hopping or not, who cares at this point ? But because I've learned more about 2.4 than I ever would have had there not been XPS. Thanks XPS !

I'm a guy that likes to understand the "logic" used in creating a new system and why it came about and then can it be trusted.

Had this thread not been started I would know next to nothing about 2.4.
Thanks XJet !!

I wonder if most viewing this thread have come to believe (as I have) that a single channel system operating in the 2.4 range is not the way to go even if there were no alternatives in that range ? I mean, am I getting it ?

[KrisW]

	Quote: Originally Posted by zoomer260
	So many on this thread are so much more knowledgable about radio than others (ME). But it has been mostly kept in laymans terms that a simple electrician can grasp the "points". I've really enjoyed this thread not because of XPS hopping or not, who cares at this point ? But because I've learned more about 2.4 than I ever would have had there not been XPS. Thanks XPS ! I'm a guy that likes to understand the "logic" used in creating a new system and why it came about and then can it be trusted. Had this thread not been started I would know next to nothing about 2.4. Thanks XJet !! I wonder if most viewing this thread have come to believe (as I have) that a single channel system operating in the 2.4 range is not the way to go even if there were no alternatives in that range ? I mean, am I getting it ?

The more channels the merrier, IMHO. I wish Spektrum had a firmware update that allowed the user to manually switch channels in mid flight. It would be simple to initiate this using the Bind button and a double push for the function.

[zoomer260]

	Quote: Originally Posted by KrisW
	The more channels the merrier, IMHO. I wish Spektrum had a firmware update that allowed the user to manually switch channels in mid flight. It would be simple to initiate this using the Bind button and a double push for the function.

You know when I got into computers the guy that set me up, advised me etc. was doing computers since 'Nam and loads up THREE credit cards at the "show" each year with stuff that isn't even in production yet. He is in school forever (literally) on CAD and other programs and he is one of the guys the big boys send BETA copies to find the problems with them. He does not write programming just finds what can make it quit working and suggests features that should be intergrated and those that are crap.

Are there any guys out there that have this kind of relationship with the software design on RC radio systems ?
Seems to me that they would at least ask their sponsored pilots for imput such as your suggestion to change channels on the fly.

[dick hanson]

	Quote: Originally Posted by zoomer260
	You know when I got into computers the guy that set me up, advised me etc. was doing computers since 'Nam and loads up THREE credit cards at the "show" each year with stuff that isn't even in production yet. He is in school forever (literally) on CAD and other programs and he is one of the guys the big boys send BETA copies to find the problems with them. He does not write programming just finds what can make it quit working and suggests features that should be intergrated and those that are crap. Are there any guys out there that have this kind of relationship with the software design on RC radio systems ? Seems to me that they would at least ask their sponsored pilots for imput such as your suggestion to change channels on the fly.

The theory on how 2.4 works --for our use -certainly runs the gamut on how it might work and how it really works -- all part of thr fun of discussing it . In suggesting that the Spektrum (DSM2) might be able to change channels whilst flying - suggests a little more looking into the DSM2 is in order:
basically you can't do a "new set of channels unles the system is at low power - and the search for new channels shows a solid set of available ones -also the code for Model Match would be gone - maybe I Missed the objective of th conversation -in that case - please ignore my comments

[Flyfast1]

	Quote: Originally Posted by zoomer260
	I wonder if most viewing this thread have come to believe (as I have) that a single channel system operating in the 2.4 range is not the way to go even if there were no alternatives in that range ? I mean, am I getting it ?

I believe that the XPS system is superior to conventional 72MHz systems for several reasons. First, upon startup the system does scan the available channels and pick a clear one. With 72MHz, one has no way of knowing how noisy the channel is, other than doing a range check. Second, the transmitter is bound to the receiver, which eliminates the problems associated with someone else turning on a transmitter on the same channel. This second reason is important to me. I fly quarter scale warbirds that are not only expensive, but take forever to build. Even if money is no issue, you just can't go buy a replacement. Many of them truly are one of a kind planes. This has always concerned me at large warbird fly ins. Most of these events have excellent transmitter impound procedures and the pilots are very careful and respectful of others. It's when I walk by a trailer full of planes and see five transmitters sitting there that I get nervous. Third, the XPS system does have the ability to retransmit frame data several times before the next frame, which should, at least in theory, reduce the number of bad frames. Fourth, the system does provide two-way communications, which at some point should allow the transmission of telemetry information back to the transmitter. This has yet to be implemented, but I really hope it does.

So, as we have seen, the XPS system has its limitations and I really hope they find a way to address the channel change situation. It would also be great to have a model match feature. But, I think it's a lot better than conventional 72MHz systems and if I could only have this 2.4 GHz system, I would take it any day. I am a current XPS user and have been very satisfied with the system. I was an early adopter and have made a significant investment, so I will continue to use the system, but I am keeping my eye on the other offerings.

Yes, this thread has been excellent and I am thankful for the contributions of XJet and others.

-Ed B.

[Daemon]

	Quote: Originally Posted by dick hanson
	The theory on how 2.4 works --for our use -certainly runs the gamut on how it might work and how it really works -- all part of thr fun of discussing it . In suggesting that the Spektrum (DSM2) might be able to change channels whilst flying - suggests a little more looking into the DSM2 is in order: basically you can't do a "new set of channels unles the system is at low power - and the search for new channels shows a solid set of available ones -also the code for Model Match would be gone - maybe I Missed the objective of th conversation -in that case - please ignore my comments

For whatever it's worth, Spektrum *can* change channels in flight. It's easy. Switch
the Tx off and back on. It'll scan for new channels every time. The Rx will lose the link for
about 1 second, and then start scanning for whatever new channels the Tx is on
and re-link, which usually takes less than 2 seconds on anything but the AR6100 Rx
(which based on my testing may simply fall from the sky, before link is re-established )

ian

[erichevy]

	Quote: Originally Posted by Daemon
	For whatever it's worth, Spektrum *can* change channels in flight. It's easy. Switch the Tx off and back on. It'll scan for new channels every time. The Rx will lose the link for about 1 second, and then start scanning for whatever new channels the Tx is on and re-link, which usually takes less than 2 seconds on anything but the AR6100 Rx (which based on my testing may simply fall from the sky, before link is re-established ) ian

This is not my expierience!!!!
If I turn my radio off/on too quick it can take up to 10 15 minutes to re-bind!!
That's with my Futaba 9C with spektrum 8 channel module.
The led in the module will keep blinking when switched too quick! I've try'd that many times.
It will only (most of the time)succeed if you turn the radio off for around five seconds before turning on again. Then it will rebind as normal.
And sometimes 5 seconds is too quick to turn on, again.... it CAN take a very long time then to re-bind.
Be very carefull with this, don't try this while being airborn!!!

Eric.