From RFC 2068 Hypertext Transfer Protocol -- HTTP/1.1:
10.4.5 404 Not Found
The server has not found anything matching the Request-URI. No indication is given of whether the condition is temporary or permanent.
If the server does not wish to make this information available to the client, the status code 403 (Forbidden) can be used instead. The 410 (Gone) status code SHOULD be used if the server knows, through some internally configurable mechanism, that an old resource is permanently unavailable and has no forwarding address.
A high-speed wireless technology that is fundamentally different from existing radio frequency technology.
For radios today, picture a guy watering his lawn with a garden hose and moving the hose up and down in a smooth vertical motion. You can see a continuous stream of water in an undulating wave. Nearly all radios, cell phones, wireless LANs and so on are like that: a continuous signal that's overlaid with information by using one of several modulation techniques.
Now picture the same guy watering his lawn with a swiveling sprinkler that shoots many, fast, short pulses of water. That's typically what UWB is like: millions of very short, very fast, precisely timed bursts or pulses of energy, measured in nanoseconds and covering a very wide area. By varying the pulse timing according to a complex code, a pulse can represent either a zero or a one: the basis of digital communications. UWB energy pulses operate in the same frequency spectrum as electronic "noise" emitted by Pentium II chips, TV monitors, electric razors, automobile ignitions and fans. This is a huge swath of the spectrum, regulated only by FCC rules on how much power these devices can use.
This difference in transmission methods has several big implications.
For one thing, because UWB pulses don't actually use a traditional radio signal, called a carrier, UWB transmissions don't take up any of the radio spectrum. Spectrum is limited, and demand for it is growing fast. That's one reason for the FCC interest: UWB would allow a whole new class, and volume, of voice and data communications that, in effect, wouldn't take up any more "space" in the crowded radio spectrum.
Second, and partly s a result of the fact that UWB doesn't use a traditional radio signal, UWB transmitters and receivers will be much simpler to build, run and maintain than those in use today, says Douglas Cummings, an engineering scientist at the Applied Research Laboratories at the University of Texas at Austin. "For UWB, you don't need complex radio frequency converters and modulators and so on," he says. "I only need a digital method to construct the pulses and modulate them. This can all go on a single chip. One vendor already does this on a chip the size of a penny."
Third, because UWB operates in the electronic "noise" area of the spectrum, it requires little power. "These systems can use 50 to 70 milliwatts of power," says Adrian Jennings, technologist with Time Domain in Huntsville, Ala., one of the pioneer vendors in UWB. "That is one ten-thousandth the power of a cell phone." The low power limits the range, but there are features of pulse transmission and some tuning techniques that can, in effect, extend or maintain the range.
In addition, low power and the characteristic wide spread of the pulses means the pulses don't use up already crowded chunks of the radio spectrum, today occupied by 802.11b wireless LANs and Bluetooth devices.
Despite the low power, UWB also has greater capacity - higher bandwidth for more users - compared with these other technologies. In early August, Time Domain began testing its just-fabricated, second-generation UWB chipset using silicon germanium technology created by IBM. The new chipset can reach 40M bit/sec, compared with just 2.5M bit/sec for the first chipset two years ago, Jennings says. Another start-up, Fantasma Networks, which Pulse-Link acquired in May, claims to have reached 60M bit/sec.
Finally, UWB promises to be highly secure. It's very difficult to filter a pulse signal out of the flood of background electronic noise, and vendors such as Time Domain are encrypting the zeros and ones being transmitted by the pulses.
