Los Angeles MTA 900Mhz TRS Usage

[LAflyer]

Hello,

Been playing around listening to the LA County MTA 900Mhz TRS and I am trying to figure out the logic in the bus Talkgroup assignments.

Unlike the supervisor, maintenance and freeway service patrol talkgroups which seem fixed, the bus talkgroups seem to be dynamic, or atleast organized in some manner which I have yet been able to determine.

First I thought they were geographically based as per what we have in the database, however this is incorrect. Same TG can be utilized for conversations across the service area from the Valley one minute, to downtown the next. Matter of fact I listened to a conversation with a single stuck bus over 10 minutes on 3 TGs.

Then I figured they might be assigned per garage divison. I found a list of divisions and bus lines, but again this does not seem to correlate as TG can have activity across divisions.

The only somewhat corrolation I have been able to determine is the radio operators seem to be heard on 2 or 3 TGs per shift. I noted the same operator over a few hours come up repeatedly on the same couple TGs only. But with change of operator shifts, the jumble changes again.


Anyone have any ideas?

[WayneH]

I'm pretty sure it's what's called "Transit Trunking". This is where a handset/data device sits between the driver and whatever type of radio (trunked or conventional) and controls when the bus driver can talk. So basically the bus driver pushes a button to talk. The dispatcher sees this and assigns a channel from the pool (one of those in the group of TG's) and acknowledges the call. The driver gets the cue and starts talking. It works in the same direction for the dispatcher. They pick a channel and send the request to the driver.

The background control can be via the cellular data network or a conventional data repeater.

[MtnBiker2005]

Some more info on this thread about this "Transit Radio" stuff with photos and links.
http://scandiego.com/phpBB3/viewtopic.php?f=1&t=1727

[inigo88]

I believe Wayne is correct. Large transit agencies all over the state love these "Request-To-Talk" (sometimes called "Transit Trunking") systems. There's really nothing else like them outside of public transit (because I assume no other radio users would want those features).

After riding San Diego MTS a lot and observing the system, I wrote up a thread on it here: ScanDiego.com • View topic - MTS/NCTD 800 MHz digital radio system

Basically the driver has a telephone style handset and an MDT. The MDT is an intermediary between the driver and the radio on the bus. If the driver wants to call dispatch, they press RTT (Request to Talk) and they get put into a callback queue. If they have an emergency they can hit PRTT (Priority Request to Talk) and bypass the queue. When the dispatcher answers a 90 second timer is started, and if the conversation exceeds that time the call is automatically dropped. The MDT provides other functionality, like pre-selected text messaging to report delays, wheelchair passengers and mechanical problems, as well as allowing the drivers to automatically log on and log off.

Most transit agencies use analog conventional for these systems, with the first channel carrying the MDT data and the subsequent channels being given generic labels like "Bus Voice 1", "Bus Voice 2", etc (In norcal, Golden Gate Transit, AC Transit, SF MUNI and more can all be found on 480 MHz with this type of set up). Here in San Diego, both MTS and NCTD do the same thing but they use conventional 800 MHz repeaters with digital modulation.

I would not be at all surprised if LA MTA is using this same setup for their buses. If that were the case, each of those talkgroups you identified would simply be arbitrarily labeled as "Bus 1", "Bus 2", etc. It would explain why there's no rhyme or reason to what you're hearing, because the MDT "Request to Talk" system is doing the talkgroup assignments. While the MDTs can use cellular architecture, almost every one of these agencies I've seen has a conventional data channel for the MDT downlink/uplink. It may be worth checking MTA's 900 MHz licensed frequencies and see if you can find one.

Edit: Thanks for the plug Brian!

[LAflyer]

OK thanks for the info guys.

Yes something like this makes sense listening to the system and the seeming randomness. Interesting operating concept.

I do wonder however what explains the fact the the same dispatch operators seem to be repeatedly using same two or three TGs over and over during their shift? Is this because they are purposely selecting the same channel (TG in this case) for their conversations?

Regarding MDT channels, yes the MTA has almost a dozen data channels that we have in the Db. (mix UHF/900Mhz)

I cleaned the Db up yesterday removing the incorrect geographic references and simply renamed the 29 TGs as 'bus operations'.

[inigo88]

It could be that each dispatcher is assigned a certain number of talkgroups, and they just answer the calls in the queue first-come-first-serve on their respective groups?