“So we decided to introduce TETRA technology because it is an open standard”, Christo concludes. “And it’s also digital.”

It was to be a landmark project, not just for Sasol but for South Africa, because it meant moving into the 410–430 MHz frequency range. This is a main TETRA band, but no channel plan then existed for its use in South Africa.

To meet the need for ultimate reliability, the team built extensive back-up features into their system architecture. The fibre and microwave rings which interconnect the radio sites provide dual paths to each one, so that service can be maintained even if the ring is breached at some point. “You get more redundancy”, Christo explains. “So that’s why we also decided, let’s have more than one ring and bolt in another layer of redundancy into the system.”

Furthermore, the all-important IP switches, the twin hubs of the network, are duplicated on sites separated by some 20 kilometres, as a further insurance against catastrophe. “I’ve come across a lot of systems that have the redundancy but it’s in the same rack”, says John Rush. “You could call it redundancy – but what happens if the power goes, or an explosion takes out that site? That was really important to me when we sought the new system. I believe there are other manufacturers now that can offer geographical separation, but Rohde & Schwarz were one of the first to offer it.”

Building the network

Preparing the ground for the new system was a substantial task for Christo Kriek and his colleagues. An important step was to obtain access to the plant’s LAN – a central nervous system which supports business activities as well as factory operation – to establish a backbone for the TETRA radio network. In addition, it was necessary to integrate the IP addressing of the radio system within the overall Sasol IP environment.

Outside in the plant, there is a high level of noise and a bustle of activity, and the pungent tang of syngas in the air contrasts with the sweetish smells of alcohols – but in the radio workshops, all is quiet. “This is what we call our TETRA Operating Centre, TOC for short”, says John Rush, leading a tour of the facilities. But he admits: “This system runs by itself! We hardly ever go in here unless we need to update subscribers.”

Currently, 20 frequencies are in use at the four radio sites within the factory area, with five TETRA carriers at each one, and they supply full handportable coverage. Traffic on the system regularly peaks close to 40 000 calls in a day. “And we’re still not hitting queues”, John continues, with pride. “Previously, with 50 channels on the MPT system, we were lucky if we got 20 000 calls.”

On the outside sites, only one carrier was initially installed but they have since been expanded. “I have two types of scenario”, John explains. “Inside the factory we are looking for capacity and not coverage. On the outside, we’re looking for coverage, not capacity.”

On top of all that, he is looking for resilience. Sitting at a bank of screens, John demonstrates the diverse routeing in the system by commanding one side of a microwave ring to shut down. The traffic promptly switches to the other route around the ring, reconnecting the isolated base stations automatically.

Further resilience is provided by deliberate coverage overlaps between the TETRA radio sites within the plant. “Now I’ve actually shut down this one”, John explains, clicking his mouse. “The population will move to the appropriate ones, and the user is oblivious to it.”

This multiple coverage has put an end to the serious communication breakdowns that could occur with the old analogue system if a site failed during an emergency at the plant. “Every time we had an emergency, the Number One complaint was communications”, John recalls. “But I’m proud to say, with the TETRA system we have, we don’t feature on the complaints list any more. It really is magic. It really has made a difference!”

Christo cites one serious incident which occurred while the main radio site serving the emergency services was out of action because of a power fault. “All these people went out to the incident scene and their radios logged on to the alternative towers, and they didn’t even know”, he says. “Throughout the entire incident, not the plant, not the emergency services, knew that one of the sites in the plant was down. That’s what I call the resilience of TETRA.

“The response time for the fire brigade was much faster, owing to better communication with the TETRA system. As a result, equipment of great value was saved. But most importantly, people’s lives were saved. This is what the TETRA system investment is all about.
(中国集群通信网 | 责任编辑：陈晓亮)