techniques used in secure transmission systems. Simply put, they represent the many rule sets that govern the game. Many countries play football, for example, but the rules are all slightly different. In the United States, players are required to weigh more than a car, yet be able to run faster than one. In Australian rules football, the game is declared forfeit if it fails to produce at least one body part amputation on the field or if at least one player doesn't eat another. They are both football, however. In data communications, the problem is similar: Many protocols out there accomplish the same thing. Data, for example, can be transmitted from one side of the world to the other in a variety of ways including T1, E1, microwave, optical fiber, satellite, coax- ial cable, and even through the water. The end result is identical-the data arrives at its intended destination. Different protocols, however, govern the process in each case. A discussion of protocols would be incomplete without a simultaneous discussion of standards. If protocols are the various sets of rules by which the game is played, standards govern which set of rules will be applied for a particular game. For example, let's assume that we need to move traffic between a PC and a printer. We agree that in order for the PC to be able to transmit a printable file to the printer, both sides must agree on a common representation for the zeroes and ones that make up the transmitted data. They agree, for example (and this is only an example), that they will both rely on a protocol that represents a zero as the absence of voltage and a one as the presence of a 3-volt pulse on the line, as shown in Figure 1-1. Since they agree on the representation, the printer knows when the PC is sending a one and when the PC is sending a zero. Imagine what would happen if they failed to agree on such a sim- ple thing beforehand. If the transmitting PC decides to represent a one Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com