Digital devices communicate through two symbols which can be conveniently called 1 and 0. Any information can be decomposed into a binary signal, that is a signal consisting of only 1's and 0's arranged in some order. For example, "5" might be written as "101" in binary, and "cat" might be written as "011000110110000101110100". Although this seems very hard to read as humans, machines are only able to read (and write) in binary. It is clear that words can be very long in binary, but they only consist of two symbols so they are ultimately very simple.
We transmit binary signals daily through calling or texting a friend and, of course, typing on a computer. For a digital telephone, the binary signal might be pulses of light (1 = pulse of light, 0 = no pulse), which the receiver converts back to audible signals so that we may carry on a conversation. The transmission methods can include light pulses, voltage levels (e.g. 5V represents 1, 0V represents 0), varying current levels and sound pulses/intensities. The transmission mediums can include various types of cables (such as coaxial or fiber optic), liquids and gases or even empty space (radio, satellite, etc).
Encoding
Encoding refers to how the signal is represented and is a very important factor in how well the receiver interprets the signal. The criteria for signal encoding includes:
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Data transfer rate: A higher data transfer rate means more efficient communication signal, but can also increase the rate of error.
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Signal to noise ratio: High signal to noise ratio decreases the rate of error.
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Ease of clocking: How easy it is to determine the start and end of each bit and thus for the receiver to synchronize with the signal.
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Bandwidth: Bandwidth is the maximum feasible data transfer rate.
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