Bandwidth Requirements for Signals
The ratio at which information can be transmitted depends on the bandwidth of the transmission media.
Transmission rates for data communications seem to follow a corollary of Parkinson's Lawthat is, data rates increase to fill the bandwidth available. A good example is the introduction of low-cost modems operating at 14400 and 28800bps for personal computers, which are 48 and 96 times faster than the rate of 300bps that was used for a long time. Large-scale integrated circuits made possible the remarkable increases in performance and decreases in price of these modems.
The 300bps full-duplex modem signals use two bands of frequencies, each occupying about 300Hz. Thus, the total 600Hz used out of the 3000Hz available bandwidth is inefficient. The 1200bps modems also are full-duplex and use most of the available bandwidth. Therefore, four times as much information can be sent in the same channel in a given time period. Modern modems operating at or above 9600bps use a sophisticated echo-canceling technique that enables both transmit and receive signals to flow on a common wire pair. Through sophisticated signal-processing techniques, modems can even carry up to 33600bps using an analog signal over a voice channel. A relatively new type of modem that permits only one analog-to-digital conversion is capable of supporting an operating rate of 56000bps. However, the use of this modem at that operating rate requires one end of the link to be directly connected to the communications carrier's digital network. In addition, the 56Kbps data rate is unidirectional, with the maximum modem rate being 33600bps in the opposite direction. This new modem is referred to as a V.90 modem. A more recent modem referred to as a V.92 modem permits data transmission up to approximately 44000bps in the uplink direction. Both V.90 and V.92 modems are discussed in Chapter 5, "Synchronous Modems, Digital Transmission, and Service Units."
Transmission of signals in binary form can require considerably more bandwidth than an equivalent analog signal. For example, the transmission of 24 analog voice channels requires about 96KHz (24 x 4KHz). Transmission of these same 24 voice channels in digital form using the standard T1 time division multiplexing format requires about 776KHz, or about eight times as much bandwidth (776/96). The advantages gained by sending the signals as binary data, however, more than offset the requirement for greater bandwidth. The primary advantage of digital signaling is the ability to regenerate pulses, which means that new pulses can replace ones with distortion. This results in a lower error rate than an analog signal because the latter uses amplifiers, which increase the signal as well as any prior distortion to the signal. (For a discussion of the trade-offs and advantages, refer to Understanding Telephone Electronics, by Stephen J. Bigelow.)