world congress
Connecting differing radio networks across borders
H
aving four offi cial languages, plus a fi fth on its coins and postage stamps, Switzerland is the country to go to
for interpreters and translation – and on the Siemens stand at TWC, the company’s Swiss engineering team was presenting its inter-system interfacing technology for radio networks. In Switzerland, Siemens has built a
nationwide public safety network based on Tetrapol digital technology – but neighbouring Germany has instead chosen TETRA for its public safety system, BOS-net. And now Siemens has been asked by customers for a means of interworking between the two systems, to help in handling cross-border incidents and operations. “What we are off ering here is a product
called the PMR Gateway”, said Patrick Mächler, project manager. “What we’ve built now is a solution which is based on the dispatch interfaces of the diff erent PMR systems.” In each of the two countries, he explained,
the gateway plugs into the relevant talk group via a dispatcher interface. T e speech or data is decoded there according to the national encryption standard and then re- encrypted for forwarding over the cross-border communication link. At the far end it is decoded and passed onward to the other network for re- encryption according to its own standard. “So the idea behind this is the crypto keys
never leave the country, which is important, especially for the BOS-net people”, Mr Mächler explained. “We directly re-encrypt each session uniquely, so even between the sessions there is no possibility to get the information.”
Interrupting police
T e interface also uses priority status assignments to ensure that local rules are satisfi ed. “German law doesn’t allow the foreign dispatchers interrupting German policemen”, he continued. “So
it’s possible then to select the highest priority allowed for a foreign speaker. You can reduce this person to normal priority or you keep it up. For Switzerland, it’s not a problem: they say, OK, if the German dispatcher is handling the incident, then he’s allowed, like a normal dispatcher in Switzerland, to talk and to interrupt a normal handset. “What we can do as well is we can talk to
multiple countries. For example, in Basel there is a combination with Germany, France and Switzerland. And then we prevent loops, because it could happen if you confi gure it wrong that you have some voice loops. So a call in Germany comes in, is sent over to France, talked again, comes in and goes back endlessly in circles. We detect this and prevent voice loops.” Because of the loose coupling between
the networks, the gateway cannot always relay push-to-talk requests directly; it uses a store-and-forward procedure to handle them. “We buff er the voice, like recording the voice until the push-to-talk on the other network is granted, and then we play it”, said Mr Mächler. “If it takes longer than 30 seconds, we play an announcement – ‘delayed’ – and then play it. So the person knows it is already quite old. And we have an upper limit of maybe 60 seconds where we directly drop it. And we say, ‘OK, you’ve lost something’.” Certain other network functions are lost too,
such as caller identity indications, because of diff ering number plans on the two systems. But besides linking TETRA and Tetrapol networks, the gateway can link TETRA to TETRA, Tetrapol to Tetrapol, and also P25 systems.
Damm engineers connect the service box to the radio unit (not seen) and the new base station is ready to be powered up
From zero to
TETRA in just 30minutes
I
n a sideshow presented on the Damm stand, engineers from the company showed how a TETRA base station can be installed and commissioned in 30 minutes, using their lightweight two-box radio package.
From Siemens, an inter- network link for cross- border communication. In the absence of a more
comprehensive, standardized network interface specifi cation, Siemens employs the dispatcher interface
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In the event, the work took very much less time than that, since it was merely a matter taking the units out of their cardboard boxes, bolting the radio unit and the service box to a suitable mast, connecting the power and antenna cables, and plugging in the all-important ethernet network cable. Through this, the central network management system downloaded all the necessary subscriber and confi guration data and integrated the new station automatically with neighbouring cells in the existing network.
Damm comments that its out-of- the-box technology makes enjoying the benefi ts of a class-leading TETRA communications system as easy as installing a new PC.
TE TRA TODAY Issue 4 2011
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