Talkgroup Number: 310, 311, & 312
Use: see below
Routing: DMR-MARC & Brandmeister
TAC talkgroups are a project that provides a series of talkgroup that can be used by 2 or more hams (on 2 or more repeaters) without necessarily busying out hundreds of repeaters and/or multiple DMR networks. Consider it a routing method somewhat similar to the TMR radio system, a PSTN trunk line or Tactical or a "go-to, on-demand channel" to be used after making contact on a wide area talkgroup such as North America, text message or by way of a schedule.
TAC 310, 311, 312 are not calling talkgroups per se, but destination talkgroups. Consider it like an old-fashioned party-line telephone, if you don't pick-up the handset (PTT), you do not hear it, you are not heard nor are you impacted by the conversation and likely more importantly, TAC does not load your networked timeslot. It is completely passive or benign to your repeater and the network unless you hit the PTT. This approach puts you or your local users in complete control of your local repeater's timeslot.
Why call them TAC's? Tac is short for tactical, a term used in public service agencies for unit to unit operations or communication so that the main dispatch frequency is not tied up for lengthy conversations. The idea is the same for DMR; that is, to provide talkgroups that do not tie up wide area talkgroups which have hundreds of repeaters connected. This approach enables a few hams to chat without that negative impact to the main talkgroups while using the fewest possible repeaters to make the conversation possible.
Examples on how it works from K6DGR:
Two users on DMR-MARC WorldWide Talkgroup begin a 15 minute conversation about the weather in Berlin and Danville. When they key up they are doing so on *hundreds* of worldwide repeaters and fully occupying a timeslot on each of the hundreds of repeaters for the entire duration of the conversation. Minutes of repeater time worldwide used is 15 * n where n is the number of repeaters. For sake of argument if there are 400 repeaters that equates to 6000 repeater minutes consumed worldwide for that weather conversation.
Two users, one in Berlin and one in Danville meet on DMR-MARC WorldWide and decide they want to chat further about the weather. They agree to meet over on TAC310 to continue their conversation taking one minute to decide to do so. Danville user switches to TAC310 TG and mashes the PTT and Berlin user does the same. They continue their discussion about the weather on TAC310. Anyone else listening that is *also* interested in the weather in Berlin and Danville can also silently switch to TAC310 and mash her PTT thus joining the party line as a full and equal member. Once they did this the WorldWide TG was freed up for anyone else to use, and hundreds or perhaps thousands of subscriber radios were spared from having to monitor a conversation about the weather in Berlin and Danville. So how does this example add up?
Initial meetup QSO: 1 * n = 1 * 400 = 400 repeater minutes of time used
TAC 310 14 min conversation w/ two participants and one lurker : 14 * 3 = 42 repeater minutes
Total repeater minutes worldwide used for the same exact conversation = 442
So in this example you've reduced the worldwide timeslot and repeater usage from 6,000 minutes to 442. That's a 1,357% reduction in repeater minutes used!
So this pattern, which is very similar to a simplex conversation where the conversation starts on the national calling channel and then transitions to another simplex channel to free up the calling channel is is far more efficient. You are not keying up hundreds of repeaters for conversations that likely have a very limited interested audience. It also provides an interesting way to have a form of point to point (but not private) calls which to date has not been possible on DMR networks which are hard coded to have certain TG's available.