I’m curious how adding a trunked repeater system to the amateur community would be a benefit ? Beyond the cool factor of actually getting it to work, how would it be better than what already exists with RF and VOIP-linked systems?
And doesn’t a trunked system require several frequencies ? So you be required to coordinate those frequencies through your regional coordination council, and that might be a major PITA ? I have heard this idea floated in the past, and I am just curious about the ”how” and “why” when it comes to amateur radio. Thanks for any explanation.
The major downfall to DMR (as built by the amateur radio community) is how it was built. Originally, the setup called for a local timeslot (not linked to anything) and then a linked time-slot which carries multiple talkgroups. The downside to this, if a local timeslot is busy a user at the remote site has no way of knowing and the traffic to the remote site is simply never received. Creates missed traffic all over the place especially when multiple local users are fighting for different talk groups on the same timeslot. This is not an issue limited to amateur radio but it is most prevailiant with amateur radio (that being said if a commercial customer has more than three talk groups in use for a standalone DMR repeater they really need to look into a small trunking system).
The quick solution is to convert the system to a pseudo trunking format such as Motorola's Capacity Plus or Hytera's XPT. We call this pseudo trunking because it is not true control to the system. Timeslot assignment is based purely on the concept of next available timeslot. Essentially, if someone is actively using timeslot 1 the system will automatically direct the next incoming call to timeslot 2 (regardless of what the TG is). If all timeslots are busy, then the users will get a notification when they try to key up but will receive a notification when a timeslot is available. This also scales to multiple repeaters at a site. DMRA really screwed up when they failed to define a pseudo-trunking standard. Pseudo-trunking came about partially because it took DMRA nearly 5 years to created the Tier III standard (by that time Motorola and Hytera had both created pseudo-trunking formats and Trident Micro systems (the guys behind LTR Passport) had created Connect Plus trunking (and would later be bought out by Motorola). One of the big issues with pseudo-trunking though is that you can still have some network hangups due to not having enough resources or a site trying to pass more remote talk groups than it has capacity for (which is where control based trunking comes in).
Control based trunking requires a controller...typically multiple. The advantage to control based trunking is that is smart. A zone controller watches a group of sites. Site controllers then control the repeaters at the sites. When a user at a site selects a talkgroup, a request for affiliation is sent to the site controller. The site controller checks with the zone controller to see if that user can use that talkgroup and if yes will tell the site controller what other sites that talkgroup has affiliations (so the traffic can be routed appropriately). The benefit to this, traffic doesn't pass to sites where there are no active affiliations to a talk group (i.e. you don't key up repeaters where no one is listening to the talkgroup). Very efficient but can also be quite costly (Simoco's Xd is the cheapest Tier III solution at the moment as the function of the Zone Controller and Site Controllers are built into the Xd repeaters).
Now, DMR Tier III can operate on a single channel trunk (one timeslot for control, one for vocie). P25 can also operate on a single channel trunk (control channel will drop for a call).
