Short-duration pulses, like the one in Example 39.3, are used to transmit digital information. Digital signals are sent over a phone line by brief tone pulses, over satellite links by brief radio pulses like the one in the example, and through optical fibers by brief laser-light pulses. Regardless of the type of wave and the medium through which it travels, any wave pulse must obey the fundamental relationship .
Sending data at a higher rate (i.e., more pulses per second) requires that the pulse duration be shorter. But a shorter-duration pulse must be created by the superposition of a larger range of frequencies. Thus the medium through which a shorter-duration pulse travels must be physically able to transmit the full range of frequencies.
The range of frequencies that can be transmitted through a medium is called the bandwidth of the medium. The shortest possible pulse that can be transmitted through a medium is
A pulse shorter than this would require a larger range of frequencies than the medium can support.
The concept of bandwidth is extremely important in digital communications. A higher bandwidth permits the transmission of shorter pulses and allows a higher data rate. A standard telephone line does not have a very high bandwidth; it is limited to sending data at the rate of no more than 50,000 pulses per second. A pulse can’t be sent over a phone line simply because the phone line won’t transmit the range of frequencies that would be needed.
An optical fiber is a high-bandwidth medium. A fiber has a bandwidth and thus can transmit laser-light pulses with duration . As a result, more than per second can be sent along an optical fiber, which is why optical-fiber networks now form the backbone of the Internet.