Multiple data transmissions can travel over copper by chopping the available bandwidth into channels. Each channel is then allowed to send data for a certain amount of time, transmitting in a round-robin fashion and giving everyone an equal chance. This method is called TDM (Time Division Multiplexing). Another transmission method is FDM (Frequency Division Multiplexing), which allows multiple transmissions by requiring each one to operate within a given frequency range. TV and radio signals are examples of FDM.
Most fiber systems combine these two methods, letting multiple frequencies carry multiple channels of data. Tunable laser diodes are used to create this WDM (Wavelength Division Multiplexing) combination. WDM is just one of three classifications for combining multiple wavelengths, or lambdas, onto a single fiber. WDM actually sits in the middle and defines the means of combining frequencies that are 10 nm apart from one another. DWDM (Dense Wavelength Division Multiplexing) is the far end of the spectrum, defining wave separation that is no more than 1 nm, with some systems running as close as 0.1 nm--a hundredfold increase over WDM. Because of this tightness in spectrum, systems using DWDM tend to be very expensive.
At the opposite end of the cost spectrum are new systems that are being designed around CWDM (Coarse Wavelength Division Multiplexing). The individual light frequencies are at least 20 nm apart, with some spaced as far as 35 nm apart. CWDM systems are being used in the metro area and even on LANs, where cost is a factor. CWDM is also not constrained to any one portion of the spectrum and operates between 1300 nm and 1600 nm, while DWDM systems usually operate above 1500 nm.
Going the Distance
Getting light pulses from Point A to Point B is the purpose of the exercise, but doing so involves some creativity. While light in fiber travels at about 200,000 kilometers per second, no light source can actually travel that far and still be interpreted as individual 1s and 0s. This is not just because of dispersion; photons also can be absorbed by the cladding that is used to surround the fiber core, decreasing the number that reach the receiver. Because the receiver counts the photons that reach it in a given period, with anything above a certain number considered a 1 and anything below considered a 0, fewer photons could result in false 0s.
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