ISDN Q.931 signaling protocol used over the D-channel. 7.5 SIP for Telephones SIP for Telephones (SIP-T) is a framework for SIP interworking with the PSTN [13]. It includes two approaches: translation and encapsulation. Both approaches will be discussed. Translation is the direct mapping between PSTN protocols and SIP. The mapping between common PSTN protocols such as ISUP [14], Q.SIG [15], and others have been defined. Examples of SIP interworking with PSTN proto- cols including ISDN and CAS are in the SIP PSTN Call Flows document [16]. 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. In this approach, a SIP call from a PSTN gateway is indistinguishable from a SIP call from a native device, and is handled such by the protocol. How- ever, since not every single parameter in a PSTN signaling message has a coun- terpart (or has any meaning) in SIP, some information is lost if the call routes back to a PSTN termination point. Encapsulation is another approach that is only useful for SIP/PSTN gate- ways. Using this approach, first, PSTN-to-SIP translation is done to construct the appropriate SIP message, then the PSTN protocol message is encapsulated and included with the SIP message as a message body. If the SIP message is received by another SIP/PSTN gateway, the resulting PSTN signaling message is constructed from both the SIP message and the encapsulated PSTN message that was received by the other gateway. This approach offers the possibility of transparency (i.e., no loss of PSTN information as a call is carried across a SIP network). However, this only works in a network in which only one variation of PSTN protocol is used. Unlike Internet protocols, PSTN protocols vary by region and are not compatible without a special type of PSTN switch, which converts one message format to another. There are many dozens of protocol variants used throughout the world. Another disadvantage of encapsulation is tha