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Digital Radio Reprint RadioResourceQuarter 3 2012 RRImag.com INTERNATIONAL THE GLOBAL INFORMATION RESOURCE FOR MISSION-CRITICAL COMMUNICATIONS NXDN: A Digital Trunking Option Photos courtesy TGS Photos courtesy Users around the world, including a Turkish airport ground services firm and a U.S. public-safety agency, are deploying digital technology for mission-critical communications. By Jens Toobe Much has been written about the technologies is also extensive, and DMR and its early predecessor can migration to digital trunked radio sys- includes Project 25 (P25), TETRA, be traced back to the mid-2000s. The tems from an established analog envi- Digital Mobile Radio (DMR), digital DMR standard was ratified in 2005, Mronment. However, the focus should Private Mobile Radio (dPMR) and and currently there are about a dozen be which technology can and should NXDN. Differentiators such as active core and equipment manufac- be used to provide the best system per- FDMA- or TDMA-based system turers. DMR is also an open standard. formance and resilience for a given access and respective spectral efficien- By the end of the second quarter, application and how that goal is cy properties further divide the group. NXDN standards and technical docu- achieved. Of course, there is no simple mentation will be opened to the pub- answer to this question. Digital Standards lic domain as announced by the Radio industry experts are unani- TETRA has been operating since NXDN Forum in February. The mous in agreeing to a, “It depends” 1990, and about 30 companies manu- NXDN Forum will continue to oper- viewpoint. The list of contributing fac- facture TETRA core products and ate and accept new members after tors is long, including the application, radios. More companies are involved opening the technical specifications user requirements, deployed environ- in providing applications, solutions, and will maintain the specification ment, user preferences, associated accessories and services. TETRA is and related interoperability testing. costs and more. The list of digital an open standard. Currently, 19 international radio and test equipment manufacturers support channels are occupied, and it provides voice channel resources available for the technology with some members pre-emptive call handling to ensure emergency call-handling purposes. designing and manufacturing radio radio access in case of emergencies. TETRA and GSM systems also use equipment since 2007. Support for the control channel (or slot) concept. the different operating modes such as NXDN Basics NXDN trunking was developed digital conventional and digital trunk- While other technologies may not based on the experience gained from ing is high, with the number of sup- need RF combining systems in small working with MPT 1327 systems. porting manufacturers steadily channel number deployments, the The standard combines a proven increasing. technologies are vulnerable to a criti- infrastructure concept with digital cal component, endangering the signal processing (DSP), RF and Trunking Fundamentals whole system in case of a failure. A vocoder technology, creating a high- A trunked radio system is funda- failure of one repeater could leave the performance digital, 6.25-kilohertz- mentally different from a conventional complete radio population without capable, spectrally efficient, secure two-way radio system in that it pools means of wide-area communications. and reliable digital radio system. all available channels and allocates NXDN trunking has followed a dif- NXDN supports true mixed-mode capacity as required. A conventional ferent route. operation. While a user can select, via radio system employs a dedicated or The philosophy behind NXDN appropriate radio programming, the fixed channel for each individual can be traced to the MPT 1327 preferential mode, the radio terminals group of users, and when channel standard. MPT 1327, established by and repeaters are inherently capable capacity is reached, the user will have various companies in Europe and of working in analog or digital mode to wait until it is freed. Put simply, a adopted throughout the world, was a without user intervention. For exam- conventional system is limited by the dominant radio system concept and ple, if an analog radio population number of users calling on its capacity at the same time, while a trunked sys- tem allows virtually unlimited user NXDN trunking was developed based on groups and provides the most efficient the experience gained from working with use of the radio frequencies and chan- nels allocated to a system. MPT 1327 systems. When a user places a call on a trunked system, a channel is allocated standard for more than 20 years in needs to be replaced, a soft migration to all participants, and once the call is mission-critical installations ranging strategy may be advantageous. This completed, the channel is returned to from airports, utilities, public-safety means that new digital radios must be the pool for other users. This sharing agencies, public transport, and busi- able to fit into the existing radio of the channel capacity increases the ness and industry applications. The infrastructure and emulate analog sig- availability of the system to all radio standard was first published in 1988 naling systems without the need to users, maximizing availability, espe- by the British Radio Communication add extra option boards to each radio, cially important at times of peak use. Agency, now called OFCOM. MPT which would increase the overall A trunked radio system is config- 1327 wasn’t a European Telecommu- costs and delay deployment. ured on the basis that with any given nications Standards Institute (ETSI) Some industry insiders have sug- number of users, not all will require standard. gested that NXDN trunking has not channel access at the same time. An MPT 1327 system uses a con- been deployed in important installa- Fewer individual radio channels are trol channel to manage radio access, a tions. However, several companies required, providing a number of bene- major contributor to its widespread have successfully implemented a fits including savings in the cost of adoption. A control channel-based large number of mission-critical channel licenses, the ability to accom- system offers the ability to control NXDN installations worldwide, modate more users and user groups, radio call requests based on system including at the G8 and G20 Summits the flexibility to configure access infrastructure capability and pro- in Toronto, a public-safety system at between user groups, increased secu- grammed parameters. It does not Christian County, Kentucky, USA, rity against eavesdropping and overly rely on any intelligence in the and for the German state of Hessen’s increased availability of the system at radio itself to find the next rest highway and road traffic manage- all times. channel, because it can still deliver ment. NXDN is the recognized stan- The control channel is a vital messages to radios, even if all voice dard for the railroad industry in North instrument in the management of a channels are occupied and provide America. trunked radio system, ensuring seam- resilience. Emergency calls can be To further illustrate the case for less operation in all situations. It can treated with maximum efficiency and FDMA 6.25-kilohertz control channel be used to transmit small data mes- importance by allowing the clear- trunking, the first case study is sages between radios even if all other down of calls in progress to make indicative of the type of mission- radio equipment to operate on the existing infrastructure. After a comprehensive study established the core needs of TGS and the operational requirements at the airports and an extensive review of the existing system and infrastruc- ture, an NXDN 6.25-kilohertz digital trunked radio system was selected. Contributing factors for the project award included the radio coverage that would be provided relative to the site required to provide such cover- age, and extended capabilities, scala- bility and resilience. NXDN trunked radio systems are now deployed in all six airports and demand for more equipment is likely to increase to The Turkish Ground Services (TGS) uses NXDN technology in six airports in Turkey. more than 3,000 radios and 80 chan- nels by 2013. critical applications suited to NXDN ■ Insufficient number of channels trunking and mission-critical deploy- ■ Poor coverage Christian County, Kentucky ments in which a trunked two-way ■ Poor voice quality In Christian County, Kentucky, radio system is the preferred option. ■ No built-in redundancy various county departments used their own radio systems, which brought Turkish Ground Services A number of technology alterna- about issues of interoperability Turkey, situated between Europe tives, including TETRA and DMR, between the departments. Some and Asia, has a population of around were considered. TETRA requires a cross-department calls had to be rout- 75 million and is about the size of the minimum of one carrier, which ed through various dispatchers to U.S. state of Texas. Turkey is a lead- includes one control channel slot and reach their intended destination, ing tourist destination, attracting three traffic channel slots. DMR resulting in wasted time and some- close to 31 million visitors in 2011. requires one carrier as a minimum, times loss of crucial information. The bulk of tourists arrive by air, so providing two voice channels. But in The need for a homogenous and there is a heavy reliance on the both cases there is no further system efficient communications system was smooth and effective operation of its redundancy built in, should the single one of the driving forces for the Chris- major airports to provide seamless carrier base stations fail. NXDN tian County officials to start looking at and secure operations for airlines, trunking uses one base station per a new radio system. Officials from staff, airport employees and tourists carrier with redundancy built into the Christian County and the city of Hop- in transit. multicarrier trunked radio system kinsville formed a committee consist- Operating in six of the busiest air- installations.
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