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Related Protocols

This chapter will introduce some related protocols to SIP. Telephony protocols from the PSTN will be discussed such as circuit associated signaling, ISDN, and ISUP. Media gateway control protocols such as MGCP and H.248 will be introduced. Finally, H.323 will be discussed. Note that Jabber and Jingle are covered in Chapter 8.

11.1 PSTN Protocols

Three types of PSTN signaling protocols are mentioned in this text: channel associated signaling (CAS), integrated services digital network (ISDN), and ISDN user part (ISUP). They will be briefly introduced and explained. How these protocols work in the PSTN today are covered in other references [1].

11.1.1 Circuit Associated Signaling

Circuit associated signaling (CAS), also known as channel associated signaling is a legacy technology still used in the PSTN today. The signaling information uses the same audio circuit as the voice path, with digits and characters represented by audio tones. These are the tones that used to be barely discernible at the beginning of some long-distance calls before the ringtone was heard. The tones are called multifrequency (MF) tones. They are somewhat similar to the tones used to signal between a telephone and a central office switch, which are known as dualtone multifrequency (DTMF) tones. Long postdial delay can be introduced because of the time taken to outpulse long strings of digits. Also, CAS was susceptible to fraud, as fraudulent tones could be generated by the caller to make free telephone calls. This type of signaling is still used in trunk circuits between a central office and a corporation’s private branch exchange (PBX) switch. DTMF signaling is still commonly used in the PSTN.

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