Digital trunking: DMR and TETRA
Written by: Sam Fenwick | Published:
Organisations above a certain size are often drawn to trunked systems due to their greater spectral efficiency and greater range of features; image credit: Tait Communications

Land Mobile explores the use of DMR Tier III and the factors that need to be considered when deciding whether it or TETRA is the best fit for your company

Many large organisations need resilient, high-capacity two-way radio networks with wide-area coverage, particularly those in the utilities, petrochemical and aviation industries. Around the world companies rely on TETRA (Terrestrial Trunked Radio) for their mission- critical communications, but in the UK things are a little more restricted. While there are a few private business radio assignments for TETRA such as Heathrow airport, the centre:MK shopping centre in Milton Keynes and some offshore wind farms, Ofcom notes that these have been difficult to accommodate in its spectrum configuration. Part of the issue is that only the emergency services and Arqiva have spectrum in the harmonised bands with the standard channel arrangement. Ofcom’s line is that it assesses requests for TETRA on a case by case basis, but usually with a warning that it is difficult to find assignments in a suitable configuration.

Fortunately, there are other options. One of TETRA’s most important features for those looking to run a network with many users – trunking – took a leap forward in availability when it was implemented in the DMR standard back in 2012.

DMR Tier III is an open standard currently supported by eight equipment manufacturers. Simoco expects that this number will grow as new entrants to DMR typically start by offering Tier II products. Once these are established they then go on to market Tier III products as well. Simoco Group’s technical director Barend Gildenhuys says there are currently eight manufacturers selling Tier III products and 11 manufacturers selling Tier II (only counting those with interoperability (IOP) certificates).

A trunked radio network has a pool of frequencies dynamically and automatically shared between radio users as they initiate calls and messages. This has two main benefits: it more efficiently shares repeater resource among users, and it builds redundancy into the network.

With non-trunked systems a fixed group of users are tied to a channel, so channel resource is under-utilised when these people aren’t active. A trunking system allows such a channel to be dynamically and automatically accessed by a greater pool of users. The resulting efficiencies can be used to improve service levels, increase the size of the user population, and/or decrease the quantity of repeater channels required. If a repeater fails then it gets removed from the set of repeaters being shared between radio users.

According to Gildenhuys this benefit of DMR Tier III appealed to the operator of a high-profile government building, which managed to achieve cost-effective redundancy by adding an extra repeater channel that, through trunking, is accessible to all user groups on the site.

He adds that DMR Tier III “is particularly effective” where frequencies are scarce, or wide area roaming is required with a unified communications platform.

Key features
Aside from trunking, DMR Tier III offers wide area roaming, automatic vehicle location (AVL), multiple call types, and backwards compatibility.

Much of the appeal of wide area roaming comes from the fact that users don’t have to change channels as they move, eliminating the need for them to remember the coverage area of the various channels they operate on, says Gildenhuys.

He notes that DMR Tier III has standard location reporting mechanisms, “which makes AVL applications vendor-independent and attracts software developers to the consolidated market that DMR Tier III offers”.

DMR Tier III supports group calls, individual calls and telephone interconnect calls, as well as short data messages. Gildenhuys says that campus-bound organisations can make great use of these call types, as integrating DMR Tier III into their telephony and building management systems (e.g. fire alarms or access control) creates a unified communications platform to support personnel that have to roam the site. Full duplex terminals are available from Excera and this feature is particularly useful for those operating in areas with poor phone reception.

Excera had its range of DMR handsets and mobile radios on show at Critical Communications World

DMR Tier III’s backwards compatibility with analogue radio technologies means businesses have some flexibility when it comes to migration and can choose to do so on an infrastructure-led or terminal-led basis, Gildenhuys adds. This makes DMR Tier III a particularly good fit for those looking to migrate from analogue MPT1327 systems, which are heavily used in the UK’s utility sector.

“Some organisations may consider moving to pseudo-trunking solutions but they need to be mindful that they will not give access to call queuing and call priority,” says Andrew Trickett, enterprise account manager at service provider Pennine Telecom. “[DMR] Tier III does, and is the natural fit for those upgrading existing analogue MPT systems.”

Trickett adds that both call queuing and call priority are “essential features for mission- critical organisations, because as their names suggest they provide the ability to prioritise calls to individual radio users or user groups.”

He says that as each radio manufacturer offers their own range of features it is important to seek independent expert advice, to determine which best fits your organisation’s requirements.

Trickett points out that when setting up a DMR Tier III network it’s vital to fully understand the organisation’s working practices and challenges. For example, who are the user groups and how do they need to interact? Who needs priority privileges? Pennine recommends that the structure of the radio groups mirrors the organisation to ensure reliable communications between teams and enable effective management of unplanned incidents.

Gildenhuys is of a similar opinion. He says that the best advice to anyone rolling out a DMR Tier III network is to adopt a solutions- orientated design approach, which models the use cases first and then assembles a suite of services to fulfil each of them. Only once that has been done does it start to analyse the demands these will place on coverage, capacity and the fleet map.

Market movements
The reach of DMR Tier III can easily be extended with gateways to allow smartphone users to communicate with DMR portable users via systems such as Motorola Solutions’ WAVE, Tait’s UnifyVoice and Tassata’s T.Bridge. [See DMR and LTE: a bridge over troubled waters in our November 2015 issue – Ed].

Back in March Simoco Group and SLA Corporation announced that they have integrated SLA’s Enterprise Secure Chat (ESChat) technology with Simoco’s DMR Tier III radio system using the DMR application interface specification. This provides secure interoperability for Simoco’s DMR customers with commercial 3G, LTE and Wi-Fi networks using ESChat’s push- to-talk over cellular functionality.

In the same month Zetron and Tait said they have integrated Zetron’s MAX Dispatch system with Tait technology to allow users to track vehicles and workers using DMR Tier III equipment. Resources with GPS-equipped DMR radios will appear on a map as icons.

The DMR Tier III standard is also evolving, having undergone a series of enhancements over the last few years driven by equipment manufacturers’ requirements. According to an ETSI spokesperson the latest revision was in February and a further revision is being discussed, with approval expected in Autumn 2016 and publication to follow shortly afterwards.

DMR Tier III gained additional market presence when Motorola Solutions unveiled MOTOTRBO Capacity Max, its first DMR Tier III-compliant solution in November 2015. Additionally, Pennine Telecom is set to install the first Capacity Max system in the UK for BAE Systems’ submarines business at Barrow-in-Furness.

However, the DMR ecosystem is mourning Sepura Group’s decision to withdraw from DMR “in order to concentrate its resources on the more attractive opportunities for TETRA in the transportation and North American markets”.

Fortunately, the technology received a boost with the recent news that CML Microcircuits and Etherstack are collaborating in an attempt to lower the barrier to building “high quality, low cost DMR equipment”. Etherstack will offer its DMR Tier II and Tier III Protocol Stack software integrated with DMR products from CML. This arrangement was demonstrated at Critical Communications World in Amsterdam back in June.

“This solution will allow new manufacturers to enter the DMR market quickly with low risk, and facilitate existing manufacturers to implement DMR Tier III quickly,” said Malcolm Lyman, marketing manager at CML Microcircuits, in a statement announcing the collaboration.

How do DMR and TETRA compare?
The DMR Association is not the only useful authority if you’re a critical communications organisation looking for additional support. Phil Kidner, CEO of the TCCA says: “With our new strategy we are willing to support any critical communication users regardless of their choice of technology. If a critical communications user has chosen to do DMR Tier III that’s their decision and we’re happy to support them in any way we can.

“It is for the users to decide what they want,” he continues. “There are 44 million PMR users in the world. Many of them, probably two-thirds, are analogue users and the rest [use] four to five different standards. And these standards have one thing in common; they’ve all been developed to meet critical communication PMR users’ requirements.

“Maybe not DMR; DMR isn’t actually made for critical communications because it came out of ETSI and ETSI did TETRA for critical communications and DMR for lower end,” concedes Kidner. “But I’m aware that developments in DMR have made it attractive for some [critical communications] users.”

“All of the standards have pluses and minuses. If you look at what’s happening in the market today at the Dutch national critical communications network, the Finnish one, the Belgian one, and so on, they are replacing them with TETRA because it appears to best meet their requirements for national critical communications,” he points out. “We’re still selling TETRA to many different sectors because the users in those sectors recognise it as the best for them. What we’ve done is try to make a technical comparison between the two technologies so users can make an informed decision.” [See the TCCA’s TETRA versus LTE whitepaper – Ed]

For those in regions where 25 kHz channels are easier to come by, some of the key points to consider when selecting a technology are summarised in the table on page 19. TETRA has the advantage in terms of data rate and is bolstered by TETRA Enhanced Data Service (TEDS), which allows channel bandwidths of up to 150 kHz and a data transfer rate of up to 538 kbps. That said, a lot can be done with DMR in this regard [See Vaughan O’Grady’s Tier III for IoT feature in our June 2016 issue – Ed].

TETRA also provides superior capacity on a per base station basis, which is good from a cost and energy efficiency perspective. However, this advantage is eroded where wide coverage and low capacity is preferred as DMR can operate at lower frequencies, resulting in better propagation over long distances.

The situation is a bit more complex as support for VHF was introduced to the TETRA standard in 2013 and Etelm unveiled the first VHF TETRA base station at the end of 2013. However, a quick look at some of the latest TETRA portable radios such as Motorola Solutions’ ST7000, Sepura’s SC20 series and Hytera’s PT790 Ex shows that they don’t support VHF.

DMR makes the transition to digital easier for those looking to upgrade from legacy analogue systems, mainly because it can use the same frequencies and therefore RF combining equipment and power supplies can be reused. However, as the TCCA points out in its TETRA versus DMR whitepaper, initial cost savings are outweighed on a total cost of ownership basis because new equipment has superior power efficiency. DMR can also provide analogue fall-back and it is possible to communicate with analogue users without the need for a gateway.

TETRA users (like their DMR Tier III equivalents) can communicate with LTE users thanks to gateways, such as Airbus Defence and Space’s Tactilon Suite, and it is also possible to use gateways to communicate with analogue users.

An issue that should be considered is the extent to which DMR Tier III’s non-core features are proprietary and whether this means handsets will have to be purchased from a single supplier after the network has been rolled out. The DMR Association is well aware of this issue and works to identify useful new features, develop them and include them in new ETSI releases. If in doubt, the interoperability certificates for any DMR product will show the mandatory and optional features that have been shown to be interoperable.

As with any major purchase, it helps to ask around. Our sister publication TETRA Today is full of features looking at TETRA use in specific sectors. There are also a host of DMR Tier III installation case studies available on the major vendors’ websites. Regardless of the standardised digital trunking technology you decide to use, you’ll benefit from the overlap between the two technologies because competition works to drive down costs and creates something of an arms race when it comes to features and interoperability.

Factors to consider when choosing between TETRA and DMR Tier III

Spectrum availability
Do you have access to 25 kHz channels in the frequency ranges supported by TETRA?

Coverage and capacity
How much geographical coverage doyou require? How many users does the system need to accommodate and how much airtime are you expecting per user? High coverage with low usage plays to DMR’s strengths. With small to medium size sites, you may find it cheaper to usea single TETRA base station with coverage supplemented using repeaters if neccessary.

Data
What are your data requirements? Do you need flexibility and/or future-proofing?

Legacy systems
Are you migrating from a trunked analogue network? How old is the ancillary equipment?

Mission-critical or business-critical?
TETRA is designed with public safety use in mind, while determining whether TETRA or DMR is the best fit for business-critical use needs to be done on a case-by case basis.

Further reading


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