Talkgroups – TAC’s

Why call them TAC’s?  TAC is short for tactical, a term used in law enforcement and other public service agencies for unit to unit operations or communications made off the main dispatch frequency so not to tie up the main channel with lengthy or special purpose conversations.  The idea is the same for DMR; that is, to provide talkgroups that do not tie up the wide area talkgroups which have many 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.

TAC talkgroups are a specialized set of talkgroups that provides a more efficient use of repeaters and networking.  The technique enables 2 or more hams (on 2 or more repeaters) to converse without necessarily busying out scores of repeaters and/or multiple IPSC networks.  It is much like Local 1 or Local 2 but using needed more than the single repeater for 2 or more hams. Consider it a routing method somewhat similar to STARnet, 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 PNW 1 or 2, WA 1 or 2, text message or by way of a schedule.  It is not a calling talkgroup per se but a destination talkgroup in most cases.

To use a TAC (TAC 1, 2 or 3 on PNWDigital), the interested hams typically make contact on a wide area calling or QSO talkgroup (WA 1 or WA 2 for example), agree to switch to a TAC and clear. After switching to the designated TAC talkgroups, both or all hams MUST kerchunk their local repeaters. The kerchunk activates or patches that TAC to connect to a waiting “trunk line” so that those repeaters are then connected together. This technique leaves ALL other repeaters free for use on either or both timeslots and is a very efficient method to allow for many simultaneous QSO’s.

TAC’s are destination talkgroups by design. But some outside of PNWDigital are now used for full time QSO such as TAC 310.  TAC’s should be considered similar to an old-fashioned MA-Bell 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, a TAC QSO does not load your your repeaters networked timeslot.  It is completely passive or benign to your repeater or IPSC network unless or until you hit the pickle (PTT).  This approach puts you or your local users in complete control of your local repeater’s timeslot without unwanted or untimely network traffic as many wider area talkgroups tend to do.

All PNW DMR managers/repeaters/networks now have the TAC series talkgroups enabled and available for use.  But not all TAC’s are available everywhere and some have very specific purposes.  Consult the talkgroup list, talkgroup detail and the Matrix for specific purpose and availability.

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Legacy Information from TRBO-6/DCI Network Days

Initial implementation will be via c-Bridge CC’s.  DCI has added a second c-Bridge to act as a part of the “Buffer Bridge BackBone” project, also known as QB.   We then enabled CPM’s for these QB c-Bridges as well as for the X Box 15’s, 30’s & 50’s if desired.  c-Bridge admins and repeater owners may wish to explore the value of switching from your CC to a CPM on the QB Backbone.

The c-Bridge development team has a different and far more elegant approach on the drawing board to reduce the number of impacted repeaters for Point-to-Point routing but the timeline is such that they have suggested that I go forward with this approach for now.  Certainly it would be a simple process to “unwind” TAC if/when their TG routing native to the c-Bridge, is rolled out. 

Both projects are in the interest of the entire ham community to explore a more efficient  and dynamic use of repeater air and timeslot time.

The TAC talkgroup project was was created on February 2, 2014 after first proposed on January  21st.  TAC 311, the first expansion TG modeled on the innovation of TAC 310, began operation on NATS on 8-16-2014.  TAC 1 created also on NATS is the first international, any language TAC talkgroup.

Benefits and Goals:

• Provide maximum North America coverage with the minimum of repeater loading
• Provide a universal “channel” anyone anywhere can use to off-load the traffic from the primary talkgroups
• Simple scaling for growth as HAM DMR expands its base of repeaters and users by adding 311, 312, etc.
• No loading of traffic onto timeslots unless there is an active local user who activates the talkgroup
• Passive routing of talkgroups only to repeaters/IPSC networks much like D-STARS repeater to repeater routing approach
• Wider availability and display of the VU meter for testing and network integrity  (now active on “TAC 310”)

Requisites:

• c-Bridge using Super Groups and PTT controls, a repeater hosted by DCI or IPSC network on DCI
• Expanded use of CPM’s or buffer bridges as well as the traditional c-Bridge Conference Connections
• SmPTT, TRBOnet and other bridging solutions can participate as end users or with bridging tools
• Dedicated talkgroup ID; possibly 310-316 as open currently on the DMR/GSM TG Plan
• Web of c-Bridge Conference Connections (or CPM’s for non c-Bridge partners) in a hub/spoke and token ring configurations

Proposal:

DCI already has this project running internally on it’s cBridge.  It is known as Local Net 2 and it used extensively by some of the IPSC networks on the DCI c-Bridge.  It only lights up the repeaters or IPSC networks that are active.  All other repeaters and IPSC networks are not involved but they can join in at any time via PTT. 

DCI also has the Bridge talkgroup which is similar to this TAC idea but yet, very different.  The Bridge designed to be full time and will key up all networks and repeaters when active, while a TAC approach will only light up select repeaters or IPSC networks.

How it works:

• Check out Push-to-Talk for the basics on PTT
• The TAC project will encourage ALL repeaters/IPSC networks to be on PTT by default
• Anyone can leave TAC on full or extended time if they wish to “listen in on the party-line”
• Standard 2 minute off or idle timer and TAC drops off or goes silent  (with 2 minute hold-off timers encouraged)

Negative Considerations:

• It’s another talkgroup to add to Digital contacts, RX Groups and/or, Scan Lists
• More complex programming of the c-Bridge
• User education

Scaling for Growth:

• Simply add another TG and Conference Connections for those c-Bridges that desire the expansion
• If TAC 310 is chosen, then TAC 311, TAC 312 would logically follow
• Use of buffer bridges and CPM’s to cut down on the licensing allotments of managers and CC’s now part of life with the new X Box c-Bridges

TAC – Name and ID:

• Suggestion for names and talkgroup ID’s; TAC using ID 310 for now (310 being 1 of 6 USA country codes permitted under the DMR/GSM/MARC talkgroup scheme)
• Possibilities are: Net 310, DMR 310, USA 310
• If not 310-316, then in statewides, unassigned are: 3103, 3107, 3114, 3143, 3152, 3157-3159, ex: USA 3157
• If those are unacceptable then 3186 is the next TG ID in the DCI block of issued ID’s.
• Further discussion needed  (but didn’t happen so TG ID 310 was chosen)

Don, VA3XPR’s Press release on TAC 310, March 2, 2014  (image)

Below is a thoughtful analysis of how TAC 310 or any end-to-end fully PTT talkgroup can function as well as a comparison to a more typical wide area talkgroup.  Lifted from a post in a DMR Yahoo Group, authored by Glen Rempe, K6DGR  (used with permission)

Example 1: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.  Example 2: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.  And there is nothing to lose by implementing it except some admin time to set it up!  If no one uses TAC310 by mashing the PTT on that channel then nothing changes from status quo.  But if TAC310 is implemented on *all* of the DMR repeaters worldwide the potential efficiency gain is really stunning.  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.

First Published: September 27, 2021 Last Updated: 3 years ago by Mike – NO7RF

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