. B ANDWIDTH OF T RANSMISSION M EDIUM Similar to message signals, different types of transmission media offer different bandwidths. The commonly used transmission media are wire, free space and fiber optic cable. Coaxial cable is a widely used wire medium, which offers a bandwidth of approximately MHz.
Such cables are normally operated below GHz. Communication through free space using radio waves takes place over a very wide range of frequencies: from a few hundreds of kHz to a few GHz. This range of frequencies is further subdivided and allocated for various services as indicated in Table . .
Optical communication using fibers is performed in the frequency range of THz to THz (microwaves to ultraviolet). An optical fiber can offer a transmission bandwidth in excess of GHz. Spectrum allocations are arrived at by an international agreement. The International Telecommunication Union (ITU) administers the present system of frequency allocations.
FIGURE . Approximation of a rectangular wave in terms of a fundamental sine wave and its harmonics.