utilize the older Channel Associated Signaling (CAS) system, in which the signaling information about a call, such as the number that is called and the caller ID, is incorporated into the same channel as the media, or a Primary Rate Interface (PRI) that uses a common signal- ing channel for all voice channels in that T1. A T1 PRI multiplex, 23 audio channels, called B channels, and a single signaling channel, called a D channel, into a single connec- tion are referred to as 23B+D. In Europe and most of the rest of the world, the primary digital connection is an E1 that contains 30 audio channels and one signaling channel, known as 30B+D. There is also a standard in Japan that is referred to as a J1 interface. State-of-the-art voice gateways, such as Cisco Voice Gateways, support modular interface cards that can support many different types of interfaces. When making the transition to SIP trunking, you need to know these different solutions because much of the complexi- ty involved in the solution is mapping the TDM signaling to SIP. Transitioning to SIP trunking can be analogous to outsourcing your TDM gateways to the service provider. Finally, Signaling System 7 (SS7) is another form of signaling used primarily by carriers and has separate messages that can be mapped to SIP in a SIP-T protocol. Supplementary Services The term supplementary services is often used without a good explanation of what it means. Generally, supplementary services is a blanket term that covers any change that occurs in a voice call outside of most basic SETUP, RING, TALK, and DISCONNECT that occurs. The best example of a supplementary service is putting a call on HOLD SO that the person initiating the hold no longer transmits audio to the person ON HOLD. This basic feature also requires that a call can be resumed when it is resumed from hold. This means that while a call is on hold, the receiver does not hear the caller. When taken off hold, the receiver transitions back to a standard call in which the caller and called party can talk. 