In communications, a signal is any measurable difference that can be used to convey information. Some typical signals used in networking include:
Signals can be interpreted in either an analog or a digital manner. In analog signalling small differences matter and the entire carrier wave represents (i.e. is analogous to) the original communication. In digital signalling, small differences are overlooked, and only exaggerated differences are noted. These differences are interpreted such that the signal is always received as one member of an on/off, yes/no, high/low, load/soft, or some other binary pair. Digital signalling thus transmits only binary digits, called bits.
The opposite of signal is noise. Noise arises from some distortion in the carrier wave that obscures the intended signal. One key measure of network performance at the physical layer is signal/noise ratio.