media information directly with each other without going through the gateway. An example of this is described in Section 10.6. A SIP to Public Switched Telephone Network (PSTN) gateway terminates both the signaling and media paths. SIP can be translated into, or interwork with, common PSTN protocols such as Integrated Services Digital Network (ISDN), ISDN User Part (ISUP), and other Circuit Associated Signaling (CAS) protocols, which are briefly described in Section 7.4. A PSTN gateway also con- verts the RTP media stream in the IP network into a standard telephony trunk or line. The conversion of signaling and media paths allows calling to and from the PSTN using SIP. Examples of these gateways are described in Sections 10.3 and 10.4. Figure 3.1 shows a SIP network connected via gateways with the PSTN and a H.323 network. There is work underway to standardize the SIP/H.323 interworking function [5]. In the figure, the SIP network, PSTN network, and H.323 networks are shown as clouds, which obscure the underlying details. Shown connecting to the SIP cloud are SIP IP telephones, SIP-enabled PCs, and corporate SIP gateways with attached telephones. The clouds are connected by gateways. Shown Black phone Corporate Corporate SIP gateway Black phones Black phones SIP network SIP/PSTN gateway phone SIP/H.323 H.323 enabled gateway network SIP phone H.323 terminal H.323 enabled Figure 3.1 SIP network with gateways. SIP Clients and Servers attached to the H.323 network are H.323 terminals and H.323-enabled PCs. The PSTN cloud connects to ordinary analog black telephones (so-called because of the original color of their shell), digital ISDN telephones, and corpo- rate private branch exchanges (PBXs). PBXs connect to the PSTN using shared trunks and provide line interfaces for either analog or digital telephones. Gateways are sometimes decomposed into a media gateway (MG) and a media gateway controller (MGC). An MGC is sometimes called a call agent because it manages call control protocols (signaling), while the MG manages 