Digital Trunking
What is Trunking
In two-way radio, a digital trunking system is one that allocates frequency resource to a group of radio users, as opposed to these groups of users always being fixed to specific frequency resource. Sounds complicated, right? So let’s remove the jargon to look at what digital trunking means in real terms and how a trunking radio system can benefit your organisation.

Firstly, it’s important to understand there are different types and levels of digital trunking in the two-way radio world. Legacy analogue MPT1327 systems and TETRA solutions are just two disparate examples of trunked radio networks. Neither example will be relevant for the majority of UK radio users but it’s worth noting that the trunked radio family extends beyond what we’ll overview here. Most of our UK clients take advantage of DMR (Digital Mobile Radio) trunking solutions. These networks can be found in every market sector from aviation to education and it’s these variants of DMR trunking we’ll take a closer look at now.
The most common types of DMR (digital) trunked radio systems in the UK today are sometimes unofficially grouped together and referred to as “DMR Tier 2.5”. They are called this because they are a level above DMR Tier II “conventional” systems – they offer digital trunking. However, they don’t come close to meeting the requirements to be classed as true ETSI Standard DMR Tier III trunking networks. Examples of DMR Tier 2.5 solutions include MOTOTRBO™ Capacity Plus from Motorola Solutions as well as Hytera’s Psuedo Trunking and XPT offerings.
How does trunking work?
“DMR Tier 2.5” digital trunking solutions are popular because, for many, they offer the best of both worlds – mixing the flexible enhancements of trunking with low costs of a conventional digital (Tier 2) system. Let’s see how they achieve this…
Imagine you oversee the communications for a large manufacturing site with two radio user groups, “Security” and “Maintenance”. Each group utilises their own radio channel or “talk-group”. Your organisation currently has a conventional/Tier II digital (DMR) radio system and each user group’s radio channel is fixed to a time-slot on a your MOTOTRBO™ SLR8000 digital repeater. As digital repeaters provide two time-slots each, “Security” are fixed to repeater time-slot 1 (T1) for their radio communications and “Maintenance” are fixed to repeater time-slot 2 (T2). All making sense so far?
You’ve noticed that the Security team only communicate via radio a couple of times a day, whereas the Maintenance radios are always in use. The Maintenance Supervisors have the ability to make one-to-one calls between each other, separate from the Maintenance talk-group. Given that Maintenance only have access to the one time-slot on your existing repeater, you know that whenever the Maintenance Supervisors are making one-to-one calls, the rest of the Maintenance group can’t speak and vice versa. This is causing some problems for the Maintenance Team who are regularly finding their radios emit a “busy” tone when they try to initiate a radio call, due to their time-slot already being in use. Meanwhile the time-slot for Security sits empty and unused 99% of the time.
You think about purchasing an extra repeater to give the Maintenance Team another talk-group or two. But wouldn’t it make much more sense to allow the Maintenance radio users to utilise the existing Security time-slot when it’s not busy rather than spending thousands of pounds purchasing another repeater, alongside the relevant Ofcom licence, antenna and associated ancillaries? Of course it would! This is where trunking comes in! Your existing MOTOTRBO™ SLR8000 repeater can be upgraded to digital trunking mode by merely adding a Capacity Plus licence key and your MOTOTRBO™ DP4601e radios are compatible with trunked operation. By trunking your repeater’s time-slot resources, when a Maintenance Supervisor makes a one-to-one call to another Supervisor, it’s automatically allocated an available time-slot on the site’s repeater. In the example to the left, the call has been allocated to time-slot 1. All other Maintenance and Security radio users sit on a “rest” channel for the time being.
Whilst the Maintenance Supervisors continue their one-to-one radio call, another member of the Maintenance Team wants to make a group call to the rest of the Maintenance Group. In this scenario, this is automatically allocated to time-slot 2 on the repeater (as time-slot 1 is already in use for the one-to-one call). Once either the one-to-one or group radio calls have ended, that time-slot is available again. It can be used for further group or one-to-one radio calls by either Maintenance or Security radio users.
You know straight away that trunking your radio system has saved your Factory money. Your system now intelligently spreads radio calls over the two repeater timeslot resources so you didn’t need to spend money buying another repeater. If your system is still not busy, additional groups can continue to be added to the existing set-up. After hearing about how good the radio system is from the Maintenance and Security users, the site’s Cleaners have asked to come on board. A second user group, “Maintenance 2” has also been deployed as the Maintenance Team found it helpful to operate as two separate groups when they purchased even more radios.
Managers from different Teams have expressed the desire to make cross-group one-to-one calls. For example, the Security Manager wishes to contact the Cleaning Manager without the conversation being overheard by the wider Cleaning or Security Teams. This can be easily achieved thanks to your digital trunking network. The Security Manager doesn’t even need to know which group (channel) the Cleaning Manager is set to in order to make that one-to-one call to them. The Security Manager simply dials the Cleaning Manager’s radio ID or selects them from his radio’s contact list. As long as there is sufficient capacity on the system, the trunking infrastructure sets up the one-to-one call seamlessly.
For the safety and efficiency benefits offered, you have now decided to deploy a TRBOnet PLUS software solution alongside your digital trunked radio system. Data associated with TRBOnet PLUS software such as radio registration and GPS location is also trunked. You work with DTS thoroughly prior to the purchase of TRBOnet PLUS to assess what impact adding data features and extra talk-groups onto your trunked radio system will have. DTS recommends the deployment of TRBOnet WATCH software to continually check how busy your system is as part of this assessment.
Through the helpful statistics, graphs and analytics provided by WATCH software, you can see that now is the perfect time to invest in an extra repeater to increase your radio solution’s capacity. By adding an extra MOTOTRBO™ SLR8000 repeater, you go from having two time-slots to four time-slots, meaning four concurrent radio calls (or data paths). Your radio users now can’t live without the radio system and another benefit of utilising multiple repeaters in digital trunking mode is that you increase your system resilience. Should one of your repeaters fail, the other will continue to function and, effectively “take the load”. All your radio talk-groups can still be used and your system will stay functional, albeit with reduced capacity. When you think back to the conventional system that fixed a talk-group to a time-slot, you know that the loss of one repeater would have meant critical user groups would have been without radio communications, even if you had purchased that second repeater from the outset.
As your site grows, your radio system expands. With a MOTOTRBO™ Capacity Plus system, up to 8 repeaters can be trunked together on a single site for voice and data, providing capacity for up to 16 concurrent radio calls. When looking to deploy capacity Plus networks, DTS work on the estimate that each repeater can handle about 200 radio users at once. This means that your Capacity Plus single site system can grow to support up-to 1600 radio users, which will satisfy your organisations radio communications requirements for years to come. On top of this, as you add even more data features like indoor positioning, you can allocate up-to four additional repeaters solely for the handling of data. These are known as “dedicated data revert” repeaters.
This is just one example of an existing client of ours. It demonstrates how they went from a small single-repeater system to a solution with hundreds of radios and dozens of advanced safety-driven features over the course of 18 months.
When our client opened a second manufacturing facility 15 miles away from their main site, we were there to help again. We deployed a new repeater site and linked it to their head office via IP, allowing cross-facility communication and creating a Capacity Plus Multi-Site network. Up-to 15 Capacity Plus sites can be linked in this manner to extend radio coverage or connect remote sites. We touch more on the benefits of multi-site systems HERE.
We’ve covered Capacity Plus and touched on other forms of DMR “Tier 2.5” trunking. So why would you ever need a higher-level DMR Tier III solution?
For many organisations, DMR Tier 2.5 is perfectly designed to meet their radio communication needs, as we saw in the example above. But for those relying on their radio system for large, mission-critical operations like airports, power stations and ports, a higher tier of radio network may be required.
DTS tend to recommend a DMR Tier III solution, such as MOTOTRBO™ Capacity Max instead of DMR Tier 2.5 in the following circumstances:
- Massive Coverage Requirements: If you need more than 15 radio sites to meet your needs, Capacity Max is the solution for you. It offers up to 250 sites on a single radio network.
- Massive User Requirements: For organisations with more than 1600 radio users in a single location, Capacity Max offers up-to 29 voice and data repeaters per site. A further 12 dedicated data repeaters can be added on top of this for every single site in the network giving you unmatched capacity for up-to 3000 radio users per site alongside support for data-heavy operations.
- Increased Resiliency: While DMR Tier 2.5 systems offer a generous amount of fail-over, MOTOTRBO™ Capacity Max networks provide multi-level resiliency and can be run with no single points of failure in the majority of cases. This makes Capacity Max the intelligent choice for organisations where radio downtime is simply not an option.
- True Priority Calling & Pre-emption: With DMR Tier 2.5, initiated radio calls access the system on a first-come, first-served There is no priority on the system. You can create the illusion of priority levels via functionality like transmit interrupt but this can prove clunky. With a DMR Tier III solution like MOTOTRBO™ Capacity Max, you can set true priority levels for your radio talkgroups, allowing less important conversations to be automatically superseded by critical ones. This is a useful feature during day-to-day operations but, in a crisis, priority call levels mean there is always space for users that need it. In the most extreme cases, this can be the difference between life and death.
- Dynamic Site Light-Up: For organisations operating different groups at different sites, MOTOTRBO™ Capacity Max analyses whether repeater site resource needs to be allocated during specific group calls by knowing which radio users are using which group on each site. For example, a large city council deploys an 8 site radio system to support refuge collection, parking wardens, street cleaning and 10 other teams, each with their own radio talkgroup. There are two repeaters at each site. As each council radio user could be anywhere within the city at any given time, with a DMR Tier 2.5 solution, every group call would have to be rebroadcast at every single site. This results in wasted resource as, most of the time, sites are rebroadcasting a call with no radio users listening to that group. It also means there’s only the capacity for 4 concurrent radio calls across the entire network. With Capacity Max, the infrastructure checks whether a radio user connected to a site is set to a given talkgroup before rebroadcasting the call on that repeater site. If no one’s listening, it won’t “light up”. With Capacity Max’s intelligence, the council can best make use of its infrastructure.
In many cases, infrastructure from DMR Tier 2.5 systems can be upgraded for use as part of a DMR Tier III system. For example, you can utilise Motorola Solutions SLR5500 or SLR8000 repeaters as part of analogue, digital (DMR Tier II) conventional, Capacity Plus Single Site, Capacity Plus Multi Site or Capacity Max networks. DTS work alongside your organisation from the outset to ensure all aspects of your system are right for you now and are future-proofed as your company evolves and expands.
The image below helpfully sums up the differences between conventional DMR, IP Site Connect, DMR Tier 2.5 (Capacity Plus) and DMR Tier III (Capacity Max/Connect Plus) systems.
Now you understand a bit more about trunking solutions, we’d be delighted to discuss your requirements. We offer free onsite consultancy and site survey visits which subsequently allow us to compose a quotation offer that takes into account your needs and your budget. We are more than happy to provide multiple options as well as indicative costs for future upgrades and ongoing maintenance.
If you want your new trunked radio system to be packed with useful features, proactively overseen by experts and with costs spread across the length of a 3-year or 5-year term, ask about our Managed Service contracts. More info on Managed Services from DTS can be found here.