Digital
FDMA or TDMA? T
“One of the issues in this country is that,
im Sunderland, of Servicom, wondered aloud what was the advantage of the
time-slotted DMR system over rival FDMA technologies (an example of these is the narrow-band ETSI dPMR system, which uses a 6·25 kHz channel but can fi t two of them into a standard 12·5 kHz business radio as- signment; in this way, it can be as effi cient as DMR in effi ciency of spectrum use). “I’m just intrigued to know what would make a customer go for one over the other”, he said. “The key one is you are only using one
repeater for two talk paths, which reduces the cost per channel in terms of hardware”, explained Tom Mockridge, of the DMR Association. “It reduces the amount of space you need to have the equipment in, reduces the amount of maintenance. That’s probably the Number One. And then the Number Two will be the ability to do things like reverse channel signalling; you can’t do those things on FDMA. Everything is a trade-off – but those are probably two killer reasons.” With reverse channel signalling, Jamie
Bishop pointed out, you could have features such as a GPS location report every time the PTT is pressed, without the need for an extra channel. “If you look at other radio technologies,
like GSM”, Tom Mockridge went on, “by breaking down your spectrum in timeslots, you use so much less kit when you’re using bigger systems. The advantages are quite clear. “If you look at the way the 6¼ stuff
originated, it originated in Japan where it was just radio-to-radio communications without infrastructure. In very simple systems, the advantages of TDMA are less obvious; the market will decide. But certainly as you get to more complicated systems, TDMA seems to make even more sense.”
“From an Arqiva point of view”, said John
Mills, “a large amount of our experience is public safety, and obviously Tetra – like this issue with what you do when you get out of handportable coverage of the network. Well, the standard Airwave solution is that every dual-manned ambulance in the country has not a radio but a gateway. T e Tetra radio is a Tetra gateway. “You get out, you always put your handporta-
ble on, your handportable goes into DMO [Di- rect Mode], and you’ve got a DMO link from your handportable to your gateway, adapting to the network. And of course you’ve got end-to- end encryption over the whole thing.
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for the police, for instance, you couldn’t use that Wi-Fi connection for data because it doesn’t meet the encryption standards.... You could say, ‘this bit’s encrypted with this and that’ – but that breaks the rules that you never double-encrypt anything and you never trans- mit things the same with two diff erent encryp- tions on. So you can’t go down that route.”
The app revolution After a break for coff ee, the panel returned to the concept of apps – in particular, for the user who wants to move his mobile phone apps (automatic vehicle location, for exam- ple) across to a DMR system, to avoid high data charges. “To us”, said Adrian Grilli, “call charges are
one of the big issues in going to a private data network, because it’s costing us a fortune on public networks. So, what about these apps, and how easy is it going to be to migrate them from existing platforms on to these newplat- forms?” John Mills, of Arqiva, highlighted the multi-bearer radio gateways widely used by the emergency services in their vehicles. “T e multi-bearer modem is out there from a large number of manufacturers”, he said. “It’s al- most bearer-independent. As long as you have a defi ned interface, you’re just sending data – you are just sending noughts and ones.” “T e question is if you are going to have
some sort of middleware that actually does conversion”, David Taylor said. “If you take applications, say, from your smartphone, it may be just standard TCP/IP: you’re not go- ing to work very effi ciently over a radio chan- nel, especially a narrowband radio channel. “So just as Airwave looked at companies
who did compression, there’s people who do middleware which will connect to your data terminal and choose your bearer method, de- pending on the data. “T at becomes eff ectively an applica-
tion in its own right – an application that understands the radio channel, that under- stands GPS.”
Legacy technology “T e other thing to bear in mind is with legacy”, said Jamie Bishop. “For DMR Tier III in particular, you are talking about often quite large systems, so the application that the customer is running is often a system which could be 15 years old and customer-specifi c, because it has provided a large enough op- portunity to develop a specifi c application.... T ey are not necessarily going to throw that out for the new radio system. It’s part of their working routine, and that training cost to change the behaviour of their employees,
John Mills is with the Government, Mobile and Enterprise (GME) business of Arqiva, the infrastructure company. “I work for product and technology within GME, mainly as pre-sales engineer, but I seem to be getting pulled into lots of other areas”, he says. “My background is in PMR – I started in Philips Telecom”
probably in some cases, would outweigh the cost of the system itself.” “Once it’s that sort of age, it tends to be
very stable”, agreed John Mills. “But the peo- ple who developed it have retired, and the company’s gone and nobody knows how it works any more, because nobody has had to look at it, because it is stable.... If you sudden- ly come along and say, ‘Here’s our new radio scheme’, that’s going to be a major problem for some users!” “It’s really, really crucial for vendors to sup- port legacy interfaces”, Jamie Bishop said.
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Tim Cull, an independent technical consultant, represented the voice of the Federation of Communication Services and its members in the radiocommunications industry. In recent years he has worked on a number of projects for the FCS. He began his career with Marconi. “I think there’s so many reasons why DMR is very attractive in many applications”, he says
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