r's network. Outside of the United States, these networks are usually in other countries; within the United States, they might be in other states. Roaming is important because no one carrier has coverage everywhere. People with GSM service often obtain com- patible Subscriber Identity Module (SIM) cards for countries to which they frequently travel in order to save on roaming fees. People who travel tend to be high-end custom- ers who use more mobile services. Thus, roamers from other networks are profitable sources of revenue worldwide. Agreements among carriers are required for every region in which roaming is enabled. Roaming agreements spell out costs, billing, and payment terms. To illustrate the complexity of roaming arrangements, most carriers have agreements with 200 to 250 other carriers. Some carriers use brokers that already have agreements worldwide and share revenue with the broker. Thus, calls made and received while roaming are more expensive than those in the subscriber's home territory; with the additional costs covering the fees imposed by the other network. Once the agreement has been signed and the service tested, roaming is activated. Carriers lease high-speed links to other providers such as AT&T, France Telecom, and Belgacom (in Belgium), all of which have an international presence. These links carry the actual voice and data traffic. Signaling links are also established to perform func- tions such as the handshake between the handset and the user's home carrier. The handshake verifies that users are legitimate customers of the originating network and have roaming privileges. Because parts of the world use incompatible types of Sig- naling System 7 or Session Initiation Protocol (SIP), gateways are used to translate between signaling types. Second-, Third-, and Fourth-Generation Digital Networks CDMA Service CDMA divides the spectrum by associating a unique code with each call. This is known as a spread spectrum technology. Each conversation transmitted is spread over a 1.25MHz frequency ch