Open standard DMR Tier III trunking

The open standard nature of DMR Tier III trunking has driven its emergence, ongoing development and adoption across global markets.

The majority of radio systems are privately owned and operated, representing a significant investment not only in terms of the radio system components but the necessary infrastructure to support the system, such as site accommodation, radio masts and backhaul services. Since much of the investment required involves upfront capital expenditure, rather than ongoing operational costs, the cost/benefit analysis will usually rely on a long-term commitment to the chosen system to ensure that the costs are spread over many years.

Since the key to the financial case for a radio system often rests with amortising the capital cost, then the equally relevant second consideration is to minimise the ongoing operational costs of the system such that the long-term cost of ownership is justifiable.

The graph below shows how the cost of installing a private mobile radio system diminishes over time. It is typical for such systems to be purchased for a planned 10-year lifespan with systems often being used well beyond this period. In addition to the financial case, and with the alternative to a privately owned radio system often coming from public cellular networks, it should also be noted that radio is a business tool. The business purpose of mobile radio is to ensure effective voice and data communications to maximise efficiency and promote worker safety. This means that the success of a radio system is judged by its ability to deliver a reliable service to its users rather than providing a regular refresh of handsets as is often expected by smartphone users.

The need to spread the cost of the radio system over a significant time period can make a managed service program an attractive option for end-user organisations. A managed service approach offers a much lower upfront cost and reduces the long-term investment for the organisation. However, whether the purchaser of the PMR/LMR system is an end-user organisation or the organisation intending to provide a managed service, both are making a long-term commitment to the system that they select.

Some key drivers for PMR/LMR are those aspects that public cellular services fail to deliver. In summary, the unique aspects of PMR/LMR make it an essential communications solution for many organisations but the upfront costs usually mean that it is a long-term commitment. So what protects the purchaser over the long term?

Open standards

Definitions of what constitutes an ‘open standard’ can be subtly different. For the purposes of this article, it is taken to mean a published specification that anyone with the required level of expertise and resources is able to implement to realise products that conform to that specification. In this context, access to an open standard is not necessarily without monetary cost through either purchase of the specification, membership of an organisation or the payment of royalty fees for licensing of intellectual property.

The general availability of an open standard is the first and critical step in establishing its success. It means that more than one organisation is able to realise products that are designed to work to a common standard. The other two main factors in the success of an open standard are tightly related. Once a standard is openly available, then it requires a critical mass of organisations to adopt the standard in order to produce a range of products to service the market needs. Linked to this is the market demand that is required to justify organisations investing in the development of products.

These two factors are like ‘the chicken and the egg’; organisations require market demand to justify development spend and market demand cannot easily be proven without the availability of products.

An interesting example of the battle of competing standards comes from the commercial world in the form of Betamax vs VHS format video recording. The popular view of this is that Betamax was the superior technology but was dominated by a single supplier, Sony, whereas more VHS products became available from more vendors which then provided choice to the consumer. In fact, there is still debate over why VHS came out on top but the point here is that the open standard of VHS enabled many manufacturers to produce products to meet a growing market demand.

Unlike the Betamax/VHS standards war above, PMR/LMR standards are not designed for a consumer market but for a business-to-business professional market. While this difference doesn’t fundamentally change the reasons why a standard is successful, it does bring with it some additional factors that need to also be satisfied for the standard to gain a following. It is the longevity of the technology which can provide comfort to those choosing it in terms of the long-term cost of ownership.

Before we concentrate on DMR Tier III Trunking, it is important to consider how this fits into a space that is crowded with a number of other digital standards. With over 90% of the worldwide installed base of PMR/LMR currently being analog, it might seem remiss to gloss over analog standards. However, the high figure of legacy technologies is more to do with the extended life cycle of radio systems than with any reticence to migrate to digital. In fact, most new radio systems being specified are now digital.

Public safety digital

TETRA became available in the 1990s and was a standard specifically designed to meet the needs of public safety organisations in Europe. TETRA was released as an open ETSI standard and has been very successful in meeting the needs of its original target market, and has also found its way into other vertical markets and in regions outside of Europe. The availability of an open standard together with a large addressable European public safety market, which had no alternative digital PMR/LMR technologies to select from, attracted a healthy number of manufacturers in developing products. The emergence of a digital alternative to analog PMR/LMR then generated a non-public safety market for TETRA together with adoption of the standard outside of Europe.

During the same time period as the emergence of TETRA, APCO-25 was developed to meet the needs of federal, state/province and local public safety agencies in North America. Again it was successful in meeting its design goals and, like TETRA, having gained the support of several manufacturers it subsequently found applications outside of North America and markets outside of public safety.

While TETRA and P25 were deployed for some non-public safety markets, the price point of such systems was significantly higher than comparable analog systems. Therefore, although many customers saw the benefits of digital technologies, the high-end offerings of TETRA and P25 didn’t fit within budget constraints. Consequently, there was a demand for systems offering the benefits of digital but at a price point comparable with analog systems. This was a driver for the emergence of a number of low-cost digital technologies.

Digital Mobile Radio (DMR) is a 2-slot TDMA (time division multiple access) technology operating within 12.5 kHz channels. It was specifically designed to offer low-cost digital solutions to provide a digital alternative for the existing analog installed base. One of the key features of DMR is the fact that it operates in the existing analog 12.5 kHz spectrum but provides two usable timeslots or logical channels, making it twice as frequency efficient as the analog systems that it is designed to replace.

The DMR ETSI standard defines both conventional and trunked operation and is not limited in terms of PMR/LMR frequency bands. For existing analog users, this enables migration to digital using existing spectrum and regardless of system type.

dPMR (digital private mobile radio) is an FDMA (frequency division multiple access) technology operating within 6.25 kHz channels. dPMR is a technology which competes directly with DMR above, both being ETSI standards and designed to offer low-cost digital solutions across the PMR/LMR frequency bands and in conventional and trunked modes of operation.

NXDN is similar to dPMR above in that it is an FDMA technology operating in 6.25 kHz channels and offering a low-cost digital solution. However, rather than an open standard, this is a proprietary technology developed jointly by Icom Incorporated and Kenwood Corporation.

PDT (Professional Digital Trunking) is very similar to DMR, 2-slot TDMA working in 12.5 kHz channels but has some differences in the Chinese standard that was driven by the Information and Telecommunication Bureau of the Chinese Ministry of Public Security.

Like NXDN, e-DMR is a proprietary technology, this time from a single manufacturer, Detracom. Similar to DMR in that it is a TDMA technology operating in 12.5 kHz channels, e-DMR provides us with a good example of a company that has designed its own protocol and realised products outside of any standards-based framework.

DMR Tier III trunking

Digital Mobile Radio (DMR) is a digital radio standard specified for professional mobile radio (PMR) users developed by the European Telecommunications Standards Institute (ETSI), and first ratified in 2005. The standard is designed to operate within the existing 12.5 kHz channel spacing used in licensed land mobile frequency bands globally and to meet future regulatory requirements for 6.25 kHz channel equivalence. The primary goal is to specify affordable digital systems with low complexity. DMR provides voice, data and other supplementary services. Today, products designed to its specifications are sold in all regions of the world.

In addition to DMR Tier III trunking, the DMR protocol covers unlicensed (Tier I) and licensed conventional (Tier II) modes of operation, both of which are suited to deployments involving a small number of radio sites.

DMR Tier III trunking features a control channel on each radio site and allocates traffic channels on demand, making it very frequency efficient and enabling a large number of users to share a relatively small number of channels. Radio sites can easily be interconnected, usually using IP connections, making it possible to deploy systems ranging from a single site to hundreds of sites spread over a large geographical area. Clearly, it is the larger systems that present the most significant financial commitment and therefore where the long-term costs become an important consideration.

Included in the DMR standard is the facility for implementers to provide ‘manufacturer extensions’. This enables manufacturers to provide proprietary features but within the framework of the DMR air interface definition, enabling manufacturers to complement the standard set of DMR call functions with manufacturer-specific facilities.

This facility has the advantage of enabling customers to request specific functionality to support their business operation needs and also enables manufacturers to provide innovative features that differentiate their solutions from others implementing the same standard.

The obvious disadvantages are that interoperability can only be possible for those features that are fully defined by the standard and that customers using manufacturer extensions are effectively locked in to a single manufacturer solution rather than enjoying the vendor choice that a standard enables.

Interoperability

An important aspect of any open standard is a mechanism to prove conformance of products to that standard. A robust interoperability process tests that products from different manufacturers have implemented the standard in a consistent manner and provides evidence to purchasers that products from different manufacturers will work together.

This is a very important facet to the success of any standard as interoperability of products from a healthy choice of manufacturers keeps market prices competitive.

However, interoperability is a complex area and it is difficult to reach a good compromise between the conflicting goals of robustness and cost. The lowest cost solution is to give responsibility to the individual manufacturers to ensure that their products conform to the standard. However, without any formal testing or independent audit of the conformance testing, this clearly isn’t a robust approach and wouldn’t provide customers with any level of confidence that products from different manufacturers would, in fact, interoperate with each other. An ultimately robust process would see each product from a manufacturer independently tested with every other available product to prove conformance. While robust, a significant number of manufacturers and products would quickly result in a very high number of permutations of product testing, not to mention the costs of independent testing and the ongoing requirements of retesting for any product modifications.

The DMR Association has struck a balance between robustness and cost with its interoperability process, which focuses on testing conformance of products against the published standard that describes the over-air signalling. The DMRA facilitates testing between a terminal manufacturer and an infrastructure manufacturer, but the actual testing is carried out by the two parties involved, against a standard test specification. Test results and logs of all messages sent over-air are recorded during the testing and then are inspected by one or more independent third parties during a detailed review meeting. Only after the independent third party (parties) is satisfied that the equipment under test has conformed to the open standard specification is an interoperability certificate issued. This approach of ensuring conformance with the standard, rather than testing all combinations of manufacturers’ products, results in a low-cost but very robust approach to interoperability.

Ongoing standards development

As highlighted above, the DMR standard has the facility for manufacturer extensions that can be used to implement non-standard features within the framework of DMR. While this facility can be useful, extensive use of it would call into question whether DMR was a standard that delivered interoperability and thus vendor choice or whether it resulted in proprietary solutions rather than following an open standard.

The answer to this lies in the work of the DMR Association. The DMRA has a technical working group that is made up of competing manufacturers who closely collaborate to ensure that the standard succeeds. Where manufacturers have developed proprietary features which are believed to have wide market appeal or manufacturers identify useful features which the standard doesn’t specify, these are debated in the technical working group and then worked on to further develop the standard to the benefit of all manufacturers and the customers who choose DMR technology.

One example of this is text messaging, which could be achieved using a variety of DMR data services specified by the standard. However, while this provided flexibility for manufacturers and customers alike, it didn’t deliver interoperability and resulted in DMR terminals from different manufacturers which were incompatible when sending text messages. The technical working group resolved this by agreeing on a mandatory method for text and updating the open standard.

The DMRA technical working group is further developing the standard to meet future market demands by identifying important new features and ensuring these are developed and included in new releases of the ETSI standards. The DMRA is very careful not to lose sight of the original goals of DMR in being a low-cost digital technology and is therefore careful that the ongoing development work doesn’t increase the costs for its member manufacturers through a large development burden.

At the time of writing, the DMR Association has the backing of a membership of 56 member companies, 29 of which are manufacturers. This critical mass shows potential purchasers of DMR that there is manufacturer support of the standard and that significant competition exists to drive prices to the lowest possible point. DMR is therefore the VHS of the low-cost digital world in this respect.

Open standards are critical to providing long-term support and stability to customers. They provide a level playing field where the protocols and specifications are widely available to a large potential group of suppliers. The adoption of the standard by a critical mass ensures its long-term future over other similar competing technologies that have lower levels of support by offering the market vendor choice and maintaining low costs