11.1.2 ISDN Signaling

Integrated services digital network (ISDN) signaling was developed in the 1980s for all digital telephone connections to the PSTN. The most common types of interfaces 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 was designed as a replacement for conventional telephone lines but requires an ISDN telephone or terminal adapter. PRI was designed for higher volume applications such as PBX trunks. In North America, 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. In Europe and much of the rest of the world, it uses a 2.048 Mbps with 30 B-channels and one D channel. The H.323 protocol, described later in this chapter, reuses a subset of the ISDN Q.931 signaling protocol used over the D-channel.

11.1.3 ISUP Signaling

ISDN user part (ISUP) is the part of the signaling system no. 7 (SS7) protocol stack used between telephone switches in the PSTN for call signaling. SS7 is a dedicated packet-switched network used all over the world in the PSTN. 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 17. The adoption of this out-of-band signaling protocol was the first step taken by telecommunications carriers away from circuit-switched networks and towards packet-switched networks. The final step will likely be moving the bearer channels onto a packet-switched network as providers move towards an all-VoIP network using SIP.

11.2 SIP for Telephones

SIP for telephones (SIP-T) is a framework for SIP interworking with the PSTN [2]. It includes two approaches: translation and encapsulation. Translation is the direct mapping between PSTN protocols and SIP. The mapping between common PSTN protocols such as ISUP [3], Q.SIG [4], and others has been defined. Examples of SIP interworking with PSTN protocols including ISDN and CAS are in the SIP PSTN call flows document [5]. In this approach, as much of the information that is common to each protocol are mapped between them, with the remaining values being set to configurable defaults. A SIP call from a PSTN gateway is indistinguishable from a SIP call from a native device, and is handled such by the protocol. However, since not every single parameter in a PSTN signaling message has a counterpart (or has any meaning) in SIP, some information is lost if the call routes back to a PSTN termination point.