I've scanned the MTS (San Diego Transit)/NCTD digital 800 MHz frequencies off and on for a while now, and was always interested to learn more about how the system worked. Like most transit radio systems, it uses a "Request to Talk" (RTT) feature where the driver has to push a button, wait for the dispatcher to acknowledge their call request and automatically assign their radio a voice channel before they can start talking. In a sense it operates kind of like a trunking system, only with a real person in the dispatch center (not a site controller computer) responding to the call request.
The MTS and NCTD frequencies can be found here: http://www.radioreference.com/apps/db/? ... #cid-11160
I've been riding MTS a lot lately, and was really surprised to see that the drivers have no direct access to their radio. In fact the only evidence of a radio at all is an easy to miss telephone style handset hanging behind and to the left of the driver. Instead, the radio system (along with GPS Automatic Vehicle Location, farebox integration and a host of other data and messaging features) is integrated into the gray "Orbital SmartMDT" Mobile Data Terminal which hangs from the ceiling above and to the right of the driver's head.
When riding on the North County Transit "Breeze" buses I noticed the same set-up, same Orbital SmartMDT model, and same P25 NAC of 339 on their sole 800 MHz voice frequency.
It turns out that back in 2003 MTS and NCTD teamed up and contracted with Motorola (who in turn hired Orbital and other companies) to build an integrated GPS, data and countywide simulcast voice communications system called the "Regional Transportation Management System" (RTMS). MTS and NCTD still operate on separate frequencies (and I believe separate dispatch centers - NCTD still IDs as "Control 2"), but the combined RTMS effort explains why they share the same NAC and use identical Orbital SmartMDT equipment (and the same digital protocol on their MDT frequencies).http://sandiegohealth.org/sandag/sandag ... 8_9290.pdfhttp://www.orbital.com/NewsInfo/release.asp?prid=437
That explains why things are the way they are, but I was still curious how exactly the radio system aspect of the integrated MDTs worked.
I was able to find this photo of the same Orbital SmartMDT model (this one taken from AC tranist in Alameda County):
and I was also able to find a closeup of the unit itself from an eBay auction:
Operation of the integrated radio system looks to be extremely simple. The driver doesn't select the channel they talk on. They simply pick up the telephone handset and push one of two buttons on the MDT. The two buttons on the top right of the unit say "RTT" and "PRTT", which stand for "Request to Talk" and "Priority Request to Talk" respectively. This likely sends a signal to put the driver's call request into a waiting queue in the dispatch center CAD, and likely sounds a talk permit tone when the dispatcher answers the call (since usually the driver just starts talking).
Even though MTS has four available conventional frequencies for voice (plus the fifth MDT frequency), there's no real way to label them since each "Request To Talk" call is automatically assigned an available voice frequency when it's answered by the dispatcher. In essence there's a pool of voice frequencies (like on a trunked system) and each transmission on the system is like a private-call on between the driver and the dispatcher. I also heard one of the MTS dispatchers mention today that they have 90 seconds to carry out the call before it times out and gets dropped, and the voice frequency goes back into the pool. So for one call they had to RTT twice. Since the radio system control is integrated into the MDT, I wouldn't be surprised if there's an ASTRO Spectra or XTL5000 hidden in the equipment cabinet behind the driver that's being controlled via data cable from the Orbital SmartMDT unit - which tells it which voice channel to switch to when it gets the channel grant over the MDT frequency.
I'm curious what kind of MDT signalling protocol they use - whether it's proprietary to the Orbital SmartMDT or more common (I believe San Francisco Muni and AC Transit use the same format, but AC Transit has the same model MDT and SF might too). I wonder if the packets are encrypted?
Anyway, this is probably not that interesting to 99% of you, but it cleared up the mystery for me of how the system works.