As an open-standard digital radio technology, DMR is immensely popular in the commercial sector, but how does it fare in the mission-critical world, asks Sam Fenwick

Digital mobile radio (DMR) first came onto the scene in 2005, and its Tier III trunked version was launched in 2012. The digital standard, developed by ETSI, has become very popular and widely used, with a focus on business-critical usage. “We see DMR as the first choice when it comes to replacing analogue or early digital systems in the industrial, utility and transportation market,” says chairman of the DMR Association, Leonardo’s Mario Micheli. “We have no interest in directly competing in the public safety market, with P25 in the US or with TETRA for large national public safety communications systems.”

That said, some organisations are using DMR in a mission-critical context. “There are public safety users in the US that consider DMR as a valid and cost-efficient option [providing] all the features they require, but in this case, they do not have interoperability with P25, other than going [over] to an additional analogue channel,” says Micheli. “Here in Italy, the municipality police forces use DMR because they are not in a position to [buy] a TETRA system or because they don’t have [access to] 25kHz channels in the UHF band. [In addition], firefighters are happy with the basic feature set [of DMR].”

“We are seeing an increase in interest in DMR particularly for tactical or incident teams, things like search and rescue or fireground incident response teams, where they want to have good instantaneous communications but they don’t necessarily want to be putting their local team communications across the TETRA network for them,” says Sean Fitzgerald, solutions marketing manager at Motorola Solutions. “It’s nice and easy to deploy; very mobile.”

However, Peter Hudson, head of global TETRA terminals product line at Sepura and former chairman of the DMR Association’s technical working group, says: “If you compare feature sets at the very top level, they [DMR and TETRA] look exactly the same because there’s group calls, individual calls, messaging, etc. When you get down into the lower levels of the protocols, there are some significant differences. DMR was designed to be simple, cheap to implement and easy to deploy, and so it has limited features in terms of prioritisation and pre-emption, the sort of things which are critical for a mission-critical service. [DMR] Tier II is a best-effort system; Tier III is better as it has trunking protocols, but it was never designed to be mission-critical.”

Cost, simplicity, spectrum
Why then are some organisations using DMR for mission-critical applications? “I believe most of all cost,” says Micheli. “P25 is very expensive. End-user terminals, handhelds, are far out of any region that any European customer would consider – $2,000, maybe even more. A handheld public safety TETRA radio is around €500, and a DMR handheld is €250, €300.”

Naturally, the cost of the radio network infrastructure has to be considered as well. Sepura’s Hudson says TETRA has an advantage in high-density urban deployments as it has more capacity per base station, eliminating the need for the combiners and additional RF equipment that would be required to obtain the same capacity using DMR.

However, for low-density rural areas, DMR has the advantage “as it needs less sites than TETRA to provide the same coverage”, explains Micheli, due to its handsets’ higher output power and the potential to use VHF (very high frequency) spectrum, which has different propagation characteristics from the UHF (ultra-high frequency) spectrum that TETRA uses – “you then have VHF; the range is even higher”.

He adds: “[TETRA is] an extremely sophisticated system with a really well-developed feature set [and] is very powerful, but also quite expensive and complex. Simplicity can be a value that is often underestimated.”

Micheli says spectrum is the other major reason why some mission-critical organisations use DMR. “TETRA is available only in the UHF band from 300-900MHz and requires 25kHz channels; their availability can be a problem, and there’s no TETRA system available in VHF.”

Both strength and weakness
One factor that could be described as both a strength and weakness for DMR is its flexibility – in that DMR manufacturers have developed many different proprietary features that act as bolt-ons to the core standardised radio technology; and the ease at which new features can be developed in this way allows DMR to quickly adapt to user requirements. However, according to Hudson, “particularly in DMR Tier III, the level of interoperability is still very low”. This restricts the features that can work over a network consisting of a mix of infrastructure and/or terminals from different suppliers.

One feature of relevance to mission-critical users is in-call handover, which Micheli says is very strong in TETRA. It allows a user to move from one cell to another while making a call without noticing any interruption. While in DMR, Fitzgerald says “some of the solutions available” will allow a seamless handover, both Micheli and Hudson say that it doesn’t really exist in the DMR standard, with Micheli describing it as “call re-establishment”, rather than a handover. He adds that while standardising this feature is on the DMR Association’s [to do] list, it isn’t the “highest priority at the moment. We are not going for a seamless solution like in TETRA because that would charge additional signalling traffic over the network; we are looking for a simpler [option] and we would tolerate, let’s say, a time-out of half a second, which is good for business-critical operations but it is not [ideal] for public safety.”

Returning to interoperability in DMR, Sepura’s Hudson says: “In Tier III, the interoperability tests are still quite basic in the coverage of the developed feature set. That hasn’t changed over the past few years. As DMR technology is aimed more at a commercial user base, there was a somewhat justifiable fear that it was going to become an expensive and onerous process to undertake, but equally this limits the range of suppliers that are fully interoperable.”

However, Micheli says: “One of the major work items in our technical group is to extend the IOP certification process [across] all the additional functions that we have developed. So, our goal is to cover most, if not all, of the functions that are available in DMR with the IOP certification process, but we [currently] cover all the basic functions.”

He adds that the association is looking to release a significant package, which he predicts will come out at the end of this year, that will cover additional functions, and that the process to standardise these will go on for as long as new functions are developed for DMR.

He explains that the association had to opt to standardise a significant number of functions in one go, as an interoperability test session requires manufacturers to convene from all over the world – “it’s not [worthwhile] to do it for just one or two simple functions, so for this reason it will be a significant package”.

What about data?
While, as several contributors to this magazine have said on numerous occasions, “voice is king”, there is no getting around the fact that data services such as AVL and GPS location data are playing a growing role in public safety. With this in mind, how do TETRA and DMR compare?

“TETRA’s data capabilities are more mature, [because] they’ve been built up over a period of time based upon end-user input; and so you have features such as concurrent voice and messaging, so nothing is missed or delayed, a transport layer on the SDS messages, so you have acknowledgements and read-receipts – there’s a much richer set of things you can do with the messaging service,” says Sepura’s Hudson. “That lends itself to enable users to move forwards with the technology, not just using it as a typical voice service but as a service that is more integrated into their business needs as well, whether it be public safety or commercial users, and that’s quite a clear differentiator between TETRA and DMR; DMR has text services – they work well, but they are not as developed or as comprehensive as those of TETRA.”

“TETRA does have a higher data-handling capability, but it depends partly on the implementation,” says Motorola Solutions’ Fitzgerald. “In a MOTOTRBO [Motorola Solutions’ implementation of DMR] network, we can define what we call data revert channels: they are channels that will be dedicated on the system to look after data. A lot of the data [traffic] we are seeing is location-based – GPS live updates – and we’ve defined some very efficient functionality within MOTOTRBO to deal with that.

“We are seeing an increase in the use of data as people use applications more to improve productivity, which drives the requirement for data; but at the end of the day, if people have a very high data requirement, then two-way radio as a narrowband device is not really the best solution for that.” In which case he argues that a broadband-based system would be the better approach. “Where you have more voice applications with data [providing added] value, that’s really where two-way radios come into their own.”

As might be expected, given the importance of auxiliary equipment such as power generators, redundant backhaul and geologically redundant switches to the availability and resilience of a radio network, there is little difference between the two technologies, as Fitzgerald explains. “For both our TETRA and MOTOTRBO solutions you can build equivalent layers of redundancy and back-up, so if I were to compare our Capacity Max, our Tier III solution, versus a TETRA solution, we could have equivalent levels of resilience, although they wouldn’t necessarily be achieved in the same way. With TETRA we have distributed architecture; with MOTOTRBO it’s more centralised – so you would achieve things in a different way, but [you get] the same kind of end result.”

Micheli has a final point to make: the benefits of using an open standard rather than proprietary technology. “The lifetime of a PMR system is [around] 20 years. During that time, the terminals need to be changed several times, as the average life of a terminal, if you take really good care of it, is seven years – otherwise it becomes four years. If you buy an open-standard system like TETRA or DMR, you don’t have the problem of being tied to a single manufacturer.”

A case study from Croatia
Let’s move our scene to Croatia, where CRONECT, a DMR Tier III network operated by OiV, is serving a wide range of users, including public safety, emergency and SAR teams, utility companies (electricity and water distribution companies), public services and transport companies.

According to Tigran Vržina, OiV’s technical principal consultant, DMR Tier III was selected as the technology of choice for the CRONECT network because of its “technical advantages and lower price compared with other similar technologies. In addition, several customers that we planned to include in the project had already some experience with DMR Tier II technology.”

Hytera Mobilfunk in Germany, together with local partner Markoja in Croatia, won the contract for the design, delivery, installation and commissioning of the network following an international public tender, which took place in 2015. Today, the network, which is in its first phase of its roll-out, provides coverage for more than 73 per cent of Croatia’s territory for handheld terminals and more than 87 per cent for mobile terminals, and includes a redundant network operational control centre system with dispatch, AVL and voice recording services. CRONECT is split into four regions, with one redundant mobile switching office for each, and customers can seamlessly roam between regions or across the entire network.

Vržina adds that the CRONECT network has been designed so it is easy to expand its coverage and increase the number of channels it can provide to users. In co-operation with the Croatian Regulatory Authority for Network Industries (HAKOM), the necessary number of frequency channels has been secured for such upgrade. He explains that the biggest challenge for the project was providing enough frequency channels and designing the network in such a way that it allows easy extension of coverage and the number of communication channels.

Over the past year, OiV conducted a series of tests with potential customers, and the network went live in Q3 2017, at which point the first commercial contracts were also signed.

“Our users mostly use voice services along with dispatch, AVL and voice recording and playback, but we also offer data services. For example, we have a customer that uses CRONECT for water management and control,” says Vržina. “As for the benefits of CRONECT, our customers highlight its good coverage and signal and audio quality, robustness and communication security, advanced functionalities and the customer care provided by OiV.”

He adds that several fire brigades in different counties currently use CRONECT across Croatia, most recently in the continental part as their only available communication option in a flooded area. Vržina highlights the fact it is listed as one of the networks that mission-critical users can use when carrying out civic protection measures and activities during major accidents and catastrophes. In addition, OiV is part of a state activity programme for the implementation of special fire protection measures, and it recently signed a contract for a fire protection video surveillance system that will synergise with the CRONECT network resources in the costal part of Croatia.

Vržina says OiV is currently negotiating the use of the network for a wider number of different mission-critical users, and that the major natural disasters that have hit the Republic of Croatia over the past years (such as fires, floods, etc) have shown that state administration bodies, public services and other bodies related to public authorities participating in the civil protection system do not have an adequate professional radio communications network available that could respond to all requests in such circumstances.

“Our plans for the next phases of the project are to expand the CRONECT network’s coverage, to educate current and future customers as to how they can transition from conventional analogue/digital systems to the digital trunking system that CRONECT provides, and to enable various emergency organisations and services to use the full potential of a digital trunking network and to be connected in the critical situations,” Vržina says.

We have seen that while DMR wasn’t intended for mission-critical use, the development of the standard with the introduction of Tier III, its cost-effectiveness, flexible use of spectrum and ability to provide wide-area coverage mean there is a role for it to play in public safety comms.

While interoperability remains an issue for those seeking the flexibility that comes with using a multi-vendor network, it is encouraging to hear that the DMR Association is looking to address this. Finally, with many manufacturers offering both DMR and TETRA/P25, organisations have plenty of choice, allowing them to select the technology that best meets their particular requirements.

Author: Sam Fenwick