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“Usually we have five digits for normal numbers, but special users have three-digit numbers. We also have 113 for our security department and 111 for the air traffic department.” Critical processesWhen the main TETRA network began operating in October 2007, one of the first user groups to use it was naturally the cleaners. With some reprogramming of their radios and new IDs, they were soon connected to the new system. But Wondra cautions against any suggestion that users such as cleaners are a non-critical group that is safe to experiment on. “It’s dangerous to say this user group is important and that group is not so important”, he says, firmly. “Cleaning is always critical. It’s a very important process, and if cleaners don’t come at the right time, the aircraft will be delayed. All these processes are very important.” Duplication and reliability The central Dimetra IP TETRA switch is located on one of the airport’s two secure data centres, each a building within a building containing a double avenue of server racks in a climate-controlled environment. Servers for specific applications are distributed between the two data centres, but are completely mirrored, and every week they are switched across to the other centre to ensure reliability and continuity of service. The TETRA switch is not doubled up in this way. But Michael Zaddach does not regard this as a point of weakness. “We have other communications systems as a fallback to the digital radio system”, he says. “Many people have also mobile telephones, and many people have data communication devices, for example. So we can use other systems as a fallback if the system completely crashes.” But in addition, the TETRA system still remains usable even if the central switch is lost, because each of the TETRA base stations can act independently as a talk-through repeater. This will enable users to communicate within their own part of the airport. “The processes that are running, on an aircraft, for example, are running on one base station”, Zaddach explains. “So the operation is not so much affected if the central system fails. It is a problem, for sure, but operation can continue.” (中國(guó)集群通信網(wǎng) | 責(zé)任編輯：陳曉亮) |