 which defines how voice, video, and data traffic should be transported over IP-based LANs. It also incorporates the ITU- T Rec. T.120 [21] data-conferencing standard. The H.323 recommendation is based on RTP/RTCP (real-time protocol/real-time control protocol) for managing audio and video signals. What sets H.323 apart is that it addresses core Internet applications by defining how delay-sensitive traffic such as voice and video get priority transport to ensure real-time communication service over the Internet. A related protocol is ITU-T Rec. H.324 [14] specification, which defines the transport of voice, data, and video over regular telephone networks. Another related protocol is ITU-T Rec. H.320 [15], which covers the transport of voice, video, and data over the integrated services digital network (ISDN). H.323 deals with three basic functional elements of VoIP. These are: Media gateway Media gateway controller (MGC) (in some settings this is called the gatekeeper) Signaling gateway H.323 is an umbrella protocol covering the following: H.225 [16], which covers the setup of multimedia channels H.245 [17], which deals with the setup of single-channel medium The standard H.323 [3] prefers the use of the term gatekeeper (versus media gateway controller). Some of the more important responsibilities of a gatekeeper are: Security. It authenticates users of the H.323 network. It performs address translation between Internet addresses and ITU-T Rec. E.164 [18] addresses. It polices the capacity of the network in question, whether that network can accept another call. H.323 determines call routing, to route through a gateway or be sent directly to the destination. It keeps track of the network's bit rate capacity. H.323 assumes that the transmission medium is a LAN that does not provide guaranteed delivery of packets. In the ITU H.323 standard we will find the term entity. An entity carries out a function. For example, a terminal is an endpoint on a LAN that can support real-time communications with another entity on t