Offsetting these positive factors were a few negative ones: the technology was new, there were few TETRA systems in Brazil at the time, and there was no TETRA-equipped steel plant anywhere in the world (though there are others now, such as in Russia and South Africa). Another was the price. “When you compare it with MPT, for instance, we had to have very strong arguments for our Board to justify this”, Mr Gagno commented.

The next step was to install an experimental TETRA base station so that radio coverage tests could be performed. From these it was clear that a two-site system would be needed, with one base station located on top of the administrative buildings and the other in the industrial area.

With this experience, the team assessed six or seven TETRA suppliers, but had to cross some of them off the list because they could not offer adequate on-the-spot support in Brazil. This was one of the key factors which directed the decision towards the Spanish manufacturer Teltronic, which was already very active in South America. “With Teltronic, we have very good support in Brazil”, Mr Gagno said. “It is very important for us.”

IP connections

Also favouring Teltronic, he added, was the fact that its Nebula TETRA infrastructure is wholly based on ethernet IP. It would therefore be fully compatible with the plant’s extensive Cisco ethernet IT network environment and with IP-based applications. “We decided to have it because of cost reduction and standards based on the Internet, and simplified management because our management system can be any IP point”, he said.

The IT network would also allow call recording via VoIP and would provide convenient and secure communications links – such as between the two radio base station sites. “They are redundant, the links”, Mr Gagno explained. “One is by the corporate network, because it’s ethernet and easier for us to integrate, and the other one is by the telephony network. But today we’ve put in a dedicated optical fibre link for this.”

As for radio channels for the new network, the team could have migrated to 400?MHz, the band most widely supported by TETRA terminal manufacturers and offering the best choice of products. But because the plant already had many wireless remote controls and other systems on 400?MHz, the decision was made to remain in the old 800?MHz band, to avoid any danger of interference to those controls.

With the system installed and operating, the plant now has 2000 Teltronic mobile and handportable radios connected and more than 400 TETRA talkgroups in operation. These include emergency fire and medical teams as well as working groups across the site. 

Among the novel service features for radio users is telephone-style full-duplex voice communication when calling a PABX extension or a mobile phone via the gateway.

Teltronic was able to help in defining the TETRA talkgroups by studying the traffic on the analogue system. During the process of migrating the users from analogue, back-to-back radios provided a temporary bridge between the old system and the new. 

Optimizing the system

Though the TETRA system immediately delivered a significant improvement in communications performance, the need for some adjustments to the system soon emerged, and the communications team has worked through these with Teltronic’s help. “We were not surprised because we had some problems with the EDACS too”, Mr Gagno commented.

Perhaps the biggest change was a decision to upgrade the traffic capacity of the system. The radio site in the industrial area was equipped with four TETRA carriers, providing 15 voice channels plus a control channel. But because of the lesser radio traffic expected in the administrative area, the other site was installed with only two carriers – a control channel plus just seven voice channels.

With experience, though, it was found that calls initiated in the industrial area were sometimes being dropped at busy times if the users roamed across the site, because there might be no free channel available on the other base station to pick up the call. The solution was to add two more TETRA carriers at the smaller station, making the system symmetrical. In a further step, network capacity has since been doubled to 8 + 8 carriers.

End of the beep

A separate problem, solved partly by training the users, arose from a difference in the mode of operation between TETRA and the old EDACS system. Users were in the habit of pressing the push-to-talk button and waiting for a beep, but with TETRA no beep came. “They said, ‘This is not a good system, there is no beep’?”, lamented Antônio Gagno. 

He explained: “You have two ways of trunking – by message or by transmission. In the case of EDACS, we used transmission – so every time you pushed the PTT, you started a new conversation on another channel. With TETRA, it’s different, like MPT. So at the beginning, to make it transparent to the user, we tried to put zero seconds of retention time, so it would work like EDACS. We had to put it at two or three seconds and to train and to explain to all the users this difference.”
(中国集群通信网 | 责任编辑：陈晓亮)