“We’ve currently got 1400 subscribers, but we are still busy with the rollout”, says chief technician Anton Redelinghuys, in the network control room. “There’s another 100 radios that are part of the project that we still need to rollout. We’ve already got a new customer that has ordered another 144 radios. So it is very likely that we are going to end up somewhere in the vicinity of 2000 radios.” 

Most of the radios are Motorola handportables, but in addition many Motorola vehicle radios are used as fixed terminals in various plant control rooms. As at Secunda, the network infrastructure is built around Rohde & Schwarz (Hytera) equipment, though it is a slightly later hardware version. The two switch sites are at separated locations, linked by dual, redundant fibre, and there are three radio sites within the plant.

On the control desk in front of us, a long row of screens provides views of this system for control and monitoring, with management displays for the microwave network, a Siemens dispatcher terminal with automatic vehicle location, a voice recorder client, and the three Rohde & Schwarz network management computers. “On the other side, I’ve plugged in a laptop to give Rohde & Schwarz remote access to the one I am currently using, with a remote desktop”, Anton explains. “From all three machines you can do everything.”

The radio system is currently licensed for 20 frequencies, but for future expansion the R&S switches could be upgraded to 60 frequencies without any hardware change. Some 170 talk groups have been defined.

Learning curve

For radio users at Sasolburg, who have upgraded direct from conventional analogue radio to TETRA without passing through an MPT trunking experience, the new network has been a big step, although not a steep one.

“Most of the users are accustomed to GSM phones, so the functionality is natural to them”, Anton comments, describing one of his departmental rollout projects. “My training to them was five minutes. ‘Press the PTT, look at the screen if you want to. Here’s the radio, play around with it, tell me if there’s problems’. They took the radios and they carried on. We haven’t had any major problems with users complaining that the radios are too difficult.”

One TETRA feature which has been especially welcomed by users is the ability to make duplex calls, radio-to-telephone or radio-to-radio. This facility is very practical for workers carrying out tasks which require both hands. “With a remote microphone they can make a duplex call and they can carry on doing things”, Anton says. “They don’t have to keep pressing PTT all the time.”

Others using duplex calling are field security workers: if someone threatens them, they can carry on a conversation with the intruder while allowing their control room to listen in. The security department is also keen on using the system’s positioning and tracking capability, to ensure that the guards follow their prescribed route, and to help in rescuing them in the event of trouble. 

Anton has already activated the GPS functionality on all the radios. They report their position every five minutes or more frequently if the radio moves more than 200 metres, and also at switch-on and switch-off.

Intensive use of AVL can congest a network with position reports, but John Rush points out that the Rohde & Schwarz technology allows the use of multiple control channels, and could thus provide increased capacity for this traffic if needed.

“Dynamic geo-fencing is another functionality that security seriously needs”, Anton adds. “It would enable them, in case of an emergency in a specific area, to select that specific area and communicate to people in that area only. It wouldn’t matter which group a person belongs to, or which high site they are registered. As long as they are in that area, they would receive the communication or warning.”

Group calling

Looking back over the progress of the rollout, Anton feels that acceptance of the system was quite good. “I actually expected more problems than we got”, he says.

“With the conventional system, there were only a certain amount of channels available – and even with 72 channels, there were two or three plants that worked on the same channel. It created a couple of problems in the past – for example, a certain control room would tell its operator to close a valve and an operator from another plant would hear the message.

“Now we basically give them all their own groups, so they are quite happy. There’s places where we’ve given two plants separate groups, but in an emergency there’s an overall group so that they can speak to their neighbours as well. Functionalities like that weren’t available with analogue. I think that’s the biggest reason why the users have accepted it better than the analogue system.”
(中国集群通信网 | 责任编辑：陈晓亮)