tandards for power control, and others. In contrast, the CDMA standards used in cellular telephony specify only the air in- terface. They define how voice signals, user data, and control signals will operate be- tween the base station and the user terminal. The backhaul network from base station to MSC is managed by independent standards, such as ANSI-41. For complicated systems, it is valuable to break a single interface standard down even further in several ways. The best way to subdivide a standard sometimes depends on the nature of the tech- nology. For example, in CDMA, we have subcomponents, including voice coding, the for- ward link, the reverse link, and power control, for each of the links. A section of any given CDMA standard (cdmaOne, cdma2000, W-CDMA, or TD-SCDMA) will be devoted to each of these subcomponents. This makes standards flexible and easy to improve. For example, a change to the re- verse link would not necessarily affect the forward link. As standards evolve, new ser- vices can be supported through the standardizing of new components. In the 3G initia- tive, the addition of standards that support high-speed data transmission is a major area of growth and change. Another way of dividing a standard is to divide it into layers. The most common method of layering telecommunications and networking systems is the Open System Interconnect (OSI) model. The OSI model is, in a sense, a standard for creating stan- dards. It allows the possibility of having parts of an interface specification independent from one another. For example, the Internet uses the Transmission Control Protocol (TCP) over the Internet Protocol (IP). Voice over IP (VoIP) systems transmit voice over the same Internet Protocol but do not use TCP. 9.1.3 The difference between standards and specifications A standard is not, and should not be, a complete specification. There are two important differences between standards and specifications. First of all, a standard defines an in- terface, whereas a specification defines a component