Simulcasting Issues

[jtlmd]

How is it decided which kinds of traffic get simulcasted on ARMER? It is pretty obvious that the various State Patrol talkgroups' traffic is simulcasted, at least from my survey of various sites (e.g. Ramsey County, Hennepin East, etc.). I also have noted that the various TAC talkgroups (e.g. LTAC, PTAC, ATAC, etc.) appear also to be simulcasted from a number of the sites around the Twin Cities.

(1) Is there any published account of just what talkgroup traffic is/is not simulcasted on the sites that are listed as "Simulcast" ?

(2) Does the "brains" of the ARMER system know who is using which TAC and when ? The other day I was listening on the Washington County site and heard a Sherburne County officer talking to one of the State Patrol choppers on LTAC-1--they were searching along the river up there. If an officer in St. Paul, for example, had attempted to use LTAC-1 at that time, what would the system do in response ? It is my presumption that he/she would have been "plugged in" to the conversation ongoing between Sherburne County and the State Patrol's helicopter.

Thanks.

K5KKV
St Paul

 

[n5ims]

There may be some confusion over simulcast sites. A simulcast site is a single "site" that uses multiple towers, all with the same frequencies, that broadcast the same signal over all of the towers. This increases coverage in buildings, etc. that wouldn't get good signal from just a single tower. These signals use GPS based timing to ensure that the signals remain in sync as to appear that they came from a single tower. What is sent out on frequency 4 from the simulcast site goes out on frequency 4 on all towers on that site.

Since this is a linked system, the system knows which site (simulcast or not) each user is on and what talkgroup they're monitoring. If "Bob" is on talkgroup 5000 and in the coverage for site 12, all traffic for talkgroup 5000 is broadcast on site 12 so Bob can hear it. If "Joe" is on site 394 that's 300 miles away from site 12, he can still talk to Bob on talkgroup 5000. They system will get Joe's signal from site 394 and transmit it over site 12 for Bob. Bob can then answer over site 12 and the transmission will also be sent out on site 394 for Joe. Now if Bob's moving and moves from site 12's coverage area to site 23's coverage area, talkgroup 5000 will move with him as his radio switches sites.

In answer to your question 2, which I glossed over with the above discussion, yes, the "brains" know every station that's on the system, which site they're connected to, and which talkgroup they're on. In your example, the talkgroup for LTAC-1 (decimal 504 if I read it right) would be available system wide to all users that dialed up LTAC-1. That officer in St. Paul on LTAC-1 could talk to the helicopter in Minneapolis with assistance from a supervisor on Lake Benton and being monitored by their boss in Duluth. To all of those folks it would appear that they were all using a single repeater called LTAC-1 that had a huge coverage area. In reality, they may be on 4 different sites, using 4 different frequency pairs. Please note that if nobody using a site is on LTAC-1, that site will not broadcast that talk group. Only sites that have folks that are currently on (or more accurately associated to) LTAC-1 will broadcast that traffic. This is why a scanner may not pick up LTAC-1 on one site, but will if they move to another site's control frequency.

There's a bunch more to this, but hopefully this helps make it more clear what's going on.

 

[stmills]

Not all ARMER talkgroups have access to all towers. They all have a home system and are allowed to roam onto others. For example Mpls PD talkgroups are home on MPLS and they split roaming between Metro tower 101 and Hennepin East tower 201.Most agencies are setting up Roaming talkgroups for use by there officers if they leave the home coverage area so they can still talk to dispatch. IF you go to MN Metroscan Homepage and look in the talkgroup section you can see what towers some of the talkgroups are allowed access to.

 

[jtlmd]

thanks

Thanks Steve. Very informative answers.

K5KKV