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SIP Gateways
A SIP gateway is an application that interfaces a SIP network to a network utilizing another signaling protocol. In terms of the SIP protocol, a gateway is just a special type of user agent, where the user agent acts on behalf of another protocol rather than a human. A gateway terminates the signaling path and can also terminate the media path, although this is not always the case. For example, a SIP to H.323 gateway terminates the SIP signaling path and converts the signaling to H.323, but the SIP user agent and H.323 terminal can exchange RTP media information directly with each other without going through the gateway. An example of this is described in [H.323 to SIP Call]
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 [PSTN Protocols]. A PSTN gateway also converts 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 [SIP to PSTN Call Through Gateway] and 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 .
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 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 corporate 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 the media connection. This decomposition is transparent to SIP, and the protocols used to decompose a gateway are not described in this book.
Another difference between a user agent and a gateway is the number of users supported. While a user agent typically supports a single user (although perhaps with multiple lines), a gateway can support hundreds or thousands of users. A PSTN gateway could support a large corporate customer, or an entire geographic area. As a result, a gateway does not REGISTER every user it supports in the same way that a user agent might. Instead, a non-SIP protocol can be used to inform proxies about gateways and assist in routing. One protocol that has been proposed for this is the Telephony Routing over IP (TRIP) protocol [6], which allows an interdomain routing table of gateways to be developed. Another protocol called Telephony Gateway Registration Protocol (TGREP) [7] has also been developed to allow a gateway to register with a proxy server within a domain. 
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