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WDM can be applied to older fibers which were installed in the late 1980s for the lower data-rate PDH systems of the day, but the high dispersion of those fibers requires special correctional techniques if it is to be used at 2.5Gb/s over distances of many hundreds of kilometers.
All submarine cable projects and many terrestrial installations and upgrades are based on WDM. The boldest by far is Project Oxygen - the proposal for a global network of submarine and terrestrial cables to link 74 countries.
One concrete example of WDM being deployed in early 2000 is the Southern Cross link between Australia, New Zealand and the United States. Each of the three fiber pairs carry 16 wavelengths modulated at 2.5Gb/s, so the total capacity of the cable is 120Gb/s. The longest hop is about 7,200Km between Hawaii and New Zealand. Light pulses approximately 80mm long leave New Zealand and are amplified en route with Erbium Doped Fiber Amplifiers. Two cables are laid in different waters to provide fully redundant rings between Hawaii and the US west coast and between Australia, New Zealand, Hawaii and Fiji.
WDM has potential applications in Local and Wide Area Networks as well. One aspect of WDM is the possibility of purely optical switching of datastreams in individual wavelengths - without first converting them into electrical signals. These approaches seem less developed than the main thrust of WDM - which is to dramatically multiply the carrying capacity of fibers in long distance links, whilst replacing regenerators with simple Erbium Doped Fiber Amplifiers.
With Internet traffic poised to grow immensely as residential and business customers adopt permanent, broadband links to the Net, WDM has a crucial role in expanding the capacity of intercapital and international data-links. Demand for WDM is driven primarily by the rising demand for data communications rather than voice, as is evidenced by FLAG Atlantic-1's capacity of 150 million 16Kb/s telephone calls.
Geostationary Satellites for Telecommunications
Geostationary satellites orbit at an altitude of 36,000 km, so that their orbital period is the same as the rotation of the Earth - causing them to be permanently located above a particular part of the equator. These satellites are powered indefinitely by solar energy, however they have a limited lifetime due partly to technological obsolescence and primarily to the limited reserves of compressed propellant in the thrusters, which must occasionally be fired to adjust their orbit. Once the propellant has been used, there is no way of keeping the satellite in its correct location.
Geostationary satellites receive microwave transmissions from ground stations, and retransmit the same signals - either directly or after recovering their binary data - back to the Earth. Thus they are a means of communicating from one point on the Earth to another with little or no regard to distance. Of course the two points must be on the side of the Earth, which is visible to the satellite. In practice, it is best to avoid signal paths close to the horizon, so the area covered by a satellite is substantially less than half the Earth's surface.
Geostationary satellites do not usually perform complex switching or processing operations on the signals they carry. Nor do they communicate with other satellites.
Satellite Beams, Footprints, and Signal Strength
In principle, a relatively broad-beam (30° or so) antennae or multiple antennas on the satellite could transmit and receive to and from the entire visible surface of the Earth - but this would spread the satellite's transmitter power rather thinly, and make its receiver relatively insensitive. Most satellites have their antennae carefully designed to create one or more 'beams' which concentrate the transmitted energy, and the receiver's sensitivity, on particular countries or regions within a country. This means that receiver dishes on the ground can be smaller than would be required with a larger beam. These beams do not have sharp boundaries and the combination of the signal strength and the attenuation of microwaves in the atmosphere, particularly when the satellite is close to the horizon, results in each beam having a geographic 'footprint' - an area where the signal is strongest. These are usually shown with contour maps indicating the transmitted signal strength on the ground, or the minimum size of dish required of the earth station.
The shape of a satellite's footprint cannot be altered, but they usually have very flexible arrangements for switching the receivers and transmitters at various microwave wavelengths between the multiple antennas - to achieve operational flexibility in the five or ten year lifespan of the satellite. The combination of receiver and transmitter is known as a transponder. For instance the B1 and B3 satellites of Optus Communications have 15 transponders and 9 beams covering Australia, New Zealand and Papua New Guinea. Some of the beams have unlikely, but commercially valuable, shapes. For instance the 'banana' beam has two separate footprints. One concentrates on the arc of population from Adelaide, Victoria, Tasmania, and most of the east coast of Australia, and the second part of the beam targets Perth, 3,000 km to the west. The satellite does not have a separate dish for each beam: there is one parabolic dish, and some artfully designed feed-horns at its focal point to sculpt the beams into the required shapes.
Geostationary satellites involve hundreds of millions of dollars investment and considerable risk - particularly in the launch phase (for instance Optus B2 which was unsuccessfully launched). They are generally owned by telecommunications carriers, or consortia of carriers - but increasingly by independent companies.
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