oo, can be represented either analogically or digitally. For example, the broadband access technology known as DSL is not digital at all: There are analog modems at each end of the line, which means that analog signaling techniques are used to represent the digital data that is being transmitted over the local loop. Combining Signaling Techniques for Higher Bit Rates Let's assume that we are operating in an analog network. Under the standard rules of the analog road, one signaling event represents one bit. For example, a high-amplitude signal represents a one, and a low- amplitude signal represents a zero. But what happens if we want to increase our bit rate? One way is to simply signal faster. Unfortunately, the rules of physics limit the degree to which we can do that. In the 1920s, a senior researcher at Bell Laboratories, who has now become something of a legend in the field of communications, came to the real- ization that the bandwidth of the channel over which the information is being transmitted has a direct bearing on the speed at which signaling can be accomplished across that channel. According to Harry Nyquist, the broader the channel, the faster the signaling rate can be. In fact, put another way, the signaling rate can never be faster than two times the highest frequency that a given channel can accommodate. Unfortunately, the telephone local loop was historically engineered to support the lim- ited bandwidth requirements of voice transmission. The traditional voice network was engineered to deliver 4 KHz of bandwidth to each local loop,2 which means that the fastest signaling rate achievable over a tele- One way in which this was done was through the use of load coils. Load coils are electrical traps that tune the local loop to a particular frequency range, only allowing certain frequen- cies to be carried. This created a problem later for digital technologies, as we will discuss. Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGr