 circuit as the voice path, with digits and characters represented by audio tones. These are the tones that used to be barely discernible on long-distance calls before ring tone is heard. The tones are called multifrequency (MF) tones. They are similar to the tones used to signal between a telephone and a central office switch, which are DTMF tones. Long post dial delay is introduced because of the time taken to out-pulse long strings of digits. Also, CAS is susceptible to fraud, as fraudulent tones can be generated by the caller to make free telephone calls. This type of signaling is common in trunk circuits between a central office and a corporation's private branch exchange (PBX) switch. 7.4.2 ISUP Signaling ISDN User Part is the protocol used between telephone switches in the PSTN for call signaling. It is used over a dedicated packet-switched network that uses Signaling System #7 (SS7) for transport. This signaling method was developed to overcome some of the delay and security problems with CAS. There are examples of ISUP signaling in the call flow examples of Chapter 10. The adop- tion of this out-of-band signaling protocol was the first step taken by telecom- munications carriers away from circuit-switched networks and towards packet-switched networks. The final step will be moving the bearer channels onto a packet-switched network. 7.4.3 ISDN Signaling Integrated Services Digital Network (ISDN) signaling was developed for all- digital telephone connections to the PSTN. The most common types of inter- faces are the basic rate interface (BRI) and the primary rate interface (PRI). A BRI can contain two 64-Kbps B-channels for either voice or data and a 16-Kbps D-channel for signaling. BRI can be used over conventional telephone lines but requires an ISDN telephone or terminal adapter. PRI uses a 1.544-Mbps link called a T-1 or a DS-1, which is divided up into 23 B-channels and one D-channel, with each channel being 64 Kbps. The H.323 protocol, described in Related Protocols Chapter 9, reuses a subset of the 