Having four official languages, plus a fifth 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 offering 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 different PMR systems.”

In each of the two countries, he explained, the gateway plugs into the relevant talk group via a dispatcher interface. The 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

The interface also uses priority status assignments to ensure that local rules are satisfied. “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 configure 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 buffer 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 differing 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.