COMMUNICATIONS

SELEX Communications

(UK)

RoIP (Radio Over IP) Radio Trunking Network

Case Communications provides SELEX Communications with Radio Over IP equipment using the Case Communications Viper

The diagram above provides a simplified overview of the Radio Trunked Network, showing a 'Head-End' master switch communicating with remote Base Station sites.

The challenge

When SELEX Communications, a Finmeccanica company, won a contract to provide a Radio Trunked Network in Romania, it spoke to Case Communications about using the Case's KMX technology, which the two companies had so successfully used in their Radio Trunked Network previously.

Their customer liked the Time Division Multiplexing technology but asked if it was possible to provide a technology that was more 'future-proof' and also highly cost-effective.

The solution

Case Communications saw their Viper (Voice Over IP Router) as being the ideal platform for connecting the Radio Base stations to the main switch site, as it was both future-proof, highly flexible and cost-effective.

A Radio Trunked Network has different requirements to VoIP (Voice Over-IP), which meant the standard Viper routers would require several changes to make the system operate successfully with Radio. Some of these changes are:

• Radio has no call 'set up' or 'tear down'; the channels are 'always-on'.
• The network needed to support FFSK (Fast Frequency Shift Key) technology.
• Base Station to the main switch communications was via a continuous HDLC stream.

Case Communications developed software to handle the 'always-on' requirement and used the Viper G.729 compression to compress the radio channel down to 9Kbps. The FFSK channel was supported using 32Kbps ADPC.

A Case Communications Multi-Access Router (a router designed to handle non-IP protocols over IP) was used to transport an HDLC stream between the main switch site and each of the base stations.

Initial testing in the SELEX Communications laboratories proved the technology, and ironed out any problems. One major change was that the equipment was to be installed in remote mountain sites and where the power supply was 24 volts DC and this meant every component of the network had to be developed to support 24 Volts DC.

One other key difference between RoIP and VoIP is 'silence suppression'. While both systems save bandwidth using silence suppression, its usual to output a sample of background sounds, so the VoIP telephone user knows the circuit is still open, and the person at the remote site has not 'hung up'.

In RoIP this background sound becomes amplified by the radio system, especially when operating in broadcast mode, where one channel is copied to multiple other channels, therefore the Viper silence suppression was enabled, but the background sound generation was set to complete silence.

Having proved the technology in the laboratories the entire system was shipped to Romania and installed in the mountains.

The installation went very smoothly and voice quality was comparable to the original KMX TDM system, but at a much lower cost. Typical bandwidth to each site is 59Kbps, with 2 x G.729 channels, 1 x 32Kbps ADPCM channel and 9600bps for the HDLC stream. Quality of Service mechanisms provide automatic fragmentation and give high priority traffic priority over less time critical traffic.

The new network continues to roll out and provides a highly cost effective and reliable transport mechanism to support the Romanian Radio Trunked Network.

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