adband DCSs-Broadband DCSs interface at the various SDH/SONET signal levels as well as the legacy DS-3/E-3 levels, but they then switch at the optical carrier levels. They can make cross-connections at DS-3/E-3, OC-1, and concatenated levels (i.e., where several frames of an OC-1 are combined). Generally, a broadband DCS is used as an SDH/SONET hub that grooms the optical carrier levels for broadband restoration purposes or for routing traffic. Signaling Systems The signaling system is the nervous system of the network. A great deal of informa- tion needs to pass back and forth between the network elements in the completion of a call and also in the servicing of specialized features. Four main types of signals handle this passing of information: Supervisory signals-Supervisory signals handle the on-hook/off-hook condition. For instance, when you lift a telephone handset (i.e., go off- hook), a signal tells the local exchange that you want a dial tone, and if you exist in the database as an authenticated user, you are then delivered that service; when you hang up (i.e., go back on-hook), you send a notice that Chapter 4 The PSTN says you want to remove the service. A network is always monitoring for these supervisory signals to determine when someone needs to activate or deactivate service. Address signals-Address signals have to do with the number dialed, which essentially consists of country codes, city codes, area codes, prefixes, and the subscriber number. This string of digits, referred to as the tele- phone number, is, in effect, a routing instruction to the network hierarchy. Information signals-Information signals are associated with activating and delivering various enhanced features. For instance, a call-waiting tone is an information signal, and pressing *72 on your phone might send an information signal that tells your local exchange to forward your calls. Alerting signals-Alerting signals are the ringing tones, the busy tones, and any specific busy alerts used to indicate network congestion or unavail