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Don't get wired, get a radio

作者:利坐运    发布时间:2019-02-28 10:16:07    

By Barry Fox IMAGINE taking your PC into the office and connecting it instantly to the PCs already there, without the need to unravel any wiring or even plug it in. The Institute of Electrical and Electronics Engineers in the US is close to finalising a radio network standard that will make this possible, and Europe is expected to adopt the same standard. Present office networks link computers using copper wire. PCs can also communicate via radio cellphones, but users must pay the high cellphone network charges for time spent online. In comparison, the new system will be very cheap to run. The new standard, called IEEE 802.11, uses a frequency band of between 2.4 and 2.5 gigahertz, which is normally set aside for industrial, scientific and medical (ISM) equipment, such as microwave ovens and heaters. The IEEE hopes to finalise the standard before the end of the year. This will allow the transceivers that will pass data between computers to work together seamlessly, even if they are made by different manufacturers. Until recently the ISM band was considered to be too cluttered with interference from devices such as microwave ovens to carry computer data reliably. But the IEEE system takes advantage of technology developed by the military to prevent an enemy from deliberately jamming radio communications. The computer connects to a transceiver that continually monitors the ISM band for interference and transmits on a clear frequency. If that frequency starts to suffer from interference, the transmitter hops through a list of alternative frequencies until it finds a clear one. At the same time the receiving computer hops through a matching list until the connection is re-established. To avoid the chaos of all the computers in a network trying to send data at the same time, the wireless link uses a technique called collision avoidance. When a computer needs to send or receive data, it first sends out a request signal. This tells the other computers in the network to remain silent until its data has been transmitted. Other computers can then request radio silence to send their data. The information is sent at around 2 megabits per second, so a full floppy disc takes only around 5 seconds to transmit. Most transmissions are much shorter, so network users are unlikely to notice a delay. The transmitters have a maximum range of 100 metres inside a building and can be organised into a work group of up to 30 stations. The short range reduces the risk of interference between neighbouring offices. User passwords will ensure that PCs only respond to network commands when authorised to do so, in the same way that wired networks prevent unauthorised access. Philips’s semiconductors division in Sunnyvale, California, is developing chips that computer manufacturers will be able to build into add-on transceivers, or incorporate into the internal circuit boards of their PCs. Philips expects mass-production to drive the price of transceivers down to $200. The European Telecommunications Standards Institute is developing an alternative European system called Hiperlans. This will carry more data than the IEEE’s system, in a higher frequency band of between 5 and 17 gigahertz, but will not be ready this year. Once the IEEE has set its standard the European computer industry is expected to adopt it alongside Hiperlans. This should be relatively straightforward, because a narrow,

 

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