[Asterisk-Users] Re: FRS over *

[David Josephson]

On the various technical issues raised here (OK, we posted the rules, we 
won't discuss the legality anymore) I think there is only one main 
obstacle to using FRS radios for extensions on *. They are simplex 
(push-to-talk, release-to-listen). The protocol for dealing with 
voice-activated-switching (VOX) has been used in ham and public safety 
simplex autopatches but it's really tricky to get right. It would be 
much easier if you used GMRS or business band radios programmed to 
transmit on one frequency and listen on another, that way the base can 
be set up with one pair of frequencies and the portables on the opposite 
split. Yes, you need two antennas or a duplexer, but this would be a 
better way to do it than trying VOX and carrier interruptions to make it 
work simplex.

Some of the cheap FRS radios have surprisingly good RF performance, I 
wouldn't be too put off by hams telling you they will self-destruct. 
Yes, they are programmed to time out after a few seconds or a minute of 
talking, but you will need to make your transmissions brief anyway so 
you can hear what the other people on the channel are saying. They 
aren't as fragile as they look.

FRS is also narrow band (3 kHz deviation, 11.25 kHz channel bandwidth). 
I don't think you would be able to get more than about 1200 bps of data 
over this channel. Garmin does transmit GPS data over GMRS at that rate.

The modern (last 10 years or more) commercial Motorola radios don't need 
a special programmer, just an interface box and PC software (which can 
be expensive). But only a few of these have the narrow band mode needed 
for compatibility with FRS (and none are legal for FRS as they are not 
type-accepted as such). Nearly all are OK for GMRS though. These were 
very expensive radios though, and while you might get them on eBay for 
$10, the batteries will be much more than that.

Only one brand of cheap GMRS radios that I've seen (Garmin) has the 
duplex mode that allows use with repeaters and duplex base stations. I 
think this is essential for successful integration with a phone system.

My recommendation would be to use a duplex base station radio on a low 
power business band channel pair. Any of the low power UHF repeaters 
would be OK for this (the repeater logic is all handled by app_rpt). You 
can get a license for the itinerant frequencies that costs just a little 
more than a GMRS license, and be able to use real portable and mobile 
radios and real antennas. There are plenty of these available with dtmf 
pads so you could have full control of your * switch.
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