Drowning in wireless spaghetti
Published: 01 Nov 2005 13:00 GMT
Negotiating the world of wideband wireless data is like swimming around in alphabetti spaghetti, only somewhat less nutritious.
Not only do many of the standards have multiple names and multiple incarnations, but the things that really matter — how much they cost, how fast they go and where they work — are almost never specified. However, some context can help you start to make sense of the current, rapidly changing state of the art.
Everything has evolved from the simplest kind of radio communication, where a radio station is transmitting a message to a listener. A single frequency is used for a single purpose: simplex transmission.
Film stars
If the listener wants to talk back, though — a process known as duplex — there are two basic ways to do so. Either the original transmitter stops and listens on the same frequency for a reply, or the listener talks on a different frequency. The first choice sees one frequency being used for different transmissions at different times — time domain duplex (TDD) — while the second is called frequency domain duplex (FDD). Both ideas were well established seventy years ago.
A more complex system evolved from ideas first mooted during World War II by, among others, film star Hedy Lamarr. Here, transmitter and receiver hop around a band swapping small chunks of information at each stop before moving on to a new frequency. Provided both are properly synchronised and know where each other is moving to next, they can pass complete messages while many other stations are doing exactly the same thing in the same band but to different sequences. There'll be occasional mutual interference when by chance two conversations alight on the same spot at the same time, but with a well-constructed coding scheme these errors can be made statistically insignificant.
World War II
In wartime and other hostile environments, the advantages of this idea is that without the right sequence or code a station is practically impossible to monitor or jam: in the civilian world, the system makes very efficient use of a band and is particularly forgiving of mild overload. This is known as Code Division Multiple Access (CDMA), but it's also known as spread spectrum: a company called Qualcomm holds many patents for this as applied to mobile phones, but the technique's been in use since the 60s and is a key part of 802.11, GPRS and other systems.
The first generation of mobile phone was based around FDD, with a pair of analogue channels per conversation. This was the case around the world, although...
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