Dick, You have pretty much described the primary feature of the IMTS (Improved Mobile Telephone System). Although there were 11 channels available, even large cities normally had no more than seven or eight assigned. One vacant channel was always transmitting an "idle tone", which all IMTS telephones would seek out and park on. When a base-to-mobile call was initiated, each mobile radio would listen for its unique ID number. When a mismatch occurred, each mobile unit would disengage from that channel and seek an idle tone on another vacant channel. The central office equipment would establish the idle tone on another vacant channel, to capture all of the mismatched mobile units. Obviously, every mobile unit except the one that was called will quickly be monitoring the new idle channel. The called mobile transmits an acknowledge tone, and the CO then connects the caller and called parties together.
It's interesting to consider that the analog 800 MHz cellular telephone systems that are still in use today use exactly the same process. Both the Block A and Block B cellular frequency plans include 21 control channels: Block A is 879.390 to 879.990 MHz at 30 kHz intervals, and Block B is 880.020 to 880.620 MHz at 30 kHz intervals. Each cell site transmits a continuous data stream on its assigned control channel, and all mobile phones monitor this channel. The data stream tells each phone where to go when a call comes in, where to go when placing a call, and what control channel to go to when the signal drops below a specified level. Although most carriers are moving to digital systems (CDMA, PCS, GSM, etc.), the analog 800 MHz systems will be around for a few more years to serve the dwindling group of hangers-on. 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Dick Sent: Sunday, March 26, 2006 8:44 AM To: [email protected] Subject: Re: [Repeater-Builder] AGT = IMTS ? Hey, Eric: I had a full-blown duplex UHF IMTS way back when and it, too, had a Secode system. It was about as automatic as one could get back then. In full auto, it would locate an unused channel, lock on it and let you know that it was time to dial. You could also go semi-auto and poke around the channels until you fell on an open one. As I recall, an open channel was indicated by a tone transmitted continuously on the open channel. When the IMTS mobile captured the channel, the tone went away and the Secode stuff did its thing. This IMTS phone was built into an attache case. The RCVR had no squelch, but the RCVR audio was turned off until the radio captured a channel. 73, Dick W1NMZ ----- Original Message ----- From: Eric Lemmon To: [email protected] Sent: 25 March, 2006 21:24 Subject: RE: [Repeater-Builder] AGT = IMTS ? Skipp, I'm just grasping at straws. The IMTS is a lot more complex than a simple duplex system that might have been used internally by AGT rather than by subscribers. Back in the late 60's, I used a half-duplex GE Pacer (!) radio in an MTS system for a couple of years after IMTS became available. It would have been better to use a full-duplex radio, but this is what I had at the time. The Pacer was an all-tube radio, except for the transistorized power supply. It had a mechanical decoder made by Secode. To make a call, I had to key the handset for a few seconds, and wait for the mobile operator to respond. That was not much more sophisticated than a magneto telephone! I eventually got kicked off the system because there were only a handful of people using the manual MTS- everyone was scrambling to get on the IMTS bandwagon. Ah, yes... Those were the good old days! 73, Eric Lemmon WB6FLY -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of skipp025 Sent: Saturday, March 25, 2006 9:01 PM To: [email protected] Subject: [Repeater-Builder] AGT = IMTS ? Would the Alberta Gov Telephones system be something like an IMTS (system) operation? Cute to think Motorola came out with something later than the Pulsar for IMTS... or was there a UHF/VHF Privacy Plus IMTS Phone? skipp > "Eric Lemmon" <[EMAIL PROTECTED]> wrote: > I suspect that your full-duplex radio was used in a mobile > telephone system by Alberta Government Telephones (AGT), > which later changed its name to TELUS. If there's no > selective call decoder in it, or loose wiring suggesting > that one was connected, it might have used multiple PL tones for > simple selection. Just a thought... > > 73, Eric Lemmon WB6FLY Yahoo! Groups Links Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/Repeater-Builder/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
