igital circuit rented from AT&T (it's their marketing term) or the local telephone company. The same type of service is available from other long distance carriers. One source of con- fusion in the telecommunications industry is that different carriers (long distance and local) give different names to the same types of services. DDS can be subscribed to only as one fixed speed at a time up to 56 Kbps. DDS circuits (typically delivered in pairs: one send and one receive) are usually installed along with two separate dial tone voice-grade lines with modems known as dial back-up lines that are used if the DDS lines fail. More recently, BRI ISDN lines are used for the dial back up, each providing twice the bandwidth of the conventional voice-grade telephone line. Although many of these lines are in use, they are considered to be slow by today's standards. Asynchronous Transfer Mode (ATM) ATM (Asynchronous Transfer Mode) is used for transmit- ting very high volumes of information over a circuit at very high speeds. One ATM circuit can handle 155 megabits per second. This is a digital service and uses fiber optic cable. You may also hear the term. SONET, which stands for Synchronous Optical Network, which is necessary for proper and reliable transmission and reception of ATM. SONET is 232 a The Telecom Handbook CHAPTER 10 an optical interface standard that enables transmission prod- ucts from different vendors to network with each other. The service delivered is referred to as the SONET ring. See The Telecom Tutorials by Jane Laino for help on or- dering telecommunications lines: 1-800-LIBRARY or www.cmpbooks.com. The Telecom Handbook - 233 Taylor & Francis Taylor & Francis Group http://taylorandfrancis.com CHAPTER 11 Transmission Transmission is a complex subject. Here we introduce some basic concepts. Transmission has to do with getting information from one place to another, whether it's across the office or thou- sands of miles away. The connection between the two points is referred to as the circuit. The connection