th T.120 and H.323 must begin the call with H.323 to standardize and simplify the process. Tunneling-Enables Q.931 channel to carry H.245 Protocol Data Units (PDUs). User Input Indication (PDU)-Enables more inclusive DTMF signaling by providing new structures, such as length of tones. Standards for Videoconferencing H.324 The formal name of H.324 is "Terminal for Low Bit Rate Multimedia Communication." It was approved in May 1996 as the first standard to provide point- to-point video and audio compression over analog telephone lines (POTS). A shortcoming of H.320 is its requirement that customers use similar devices at all endpoints. H.324 allows users to interoperate across diverse endpoints (IP, ATM, ISDN, POTS, or wireless telephony). In July 1996, sixteen vendors participated in the first formal H.324 interoperability test. More than a dozen participated in subsequent testing. Whereas H.320 is designed for use with multipoint control units (MCU), H.324 allows users to choose what party they want to see on their monitor through a feature called continuous presence. Continuous Presence allows images to be stacked; it is useful in videoconferences in which one camera cannot adequately capture an entire group at one end. H.320 suffered from incompatibilities between systems that were only technically H.320 compliant. In contrast, H.324 allows users to share video, voice, and data simultaneously over high-speed (28.8 Kbps or faster) modem connections using the application interface, V.80. Within the V.80 standard, voice call first allows users with V.8 bis enabled modems to add video at any time during an audio only call. V.80 also allows the transmission of still photographs when a signal is inadequate to transmit motion video. H.324 incorporates both DCT and pulse code technology to provide enhanced video performance at low bit rate transmission. The ITU-T updated the H.261 standard to H.263; H.263 is H.324's video algorithm. H.223 The formal name for H.223 is "Multiplexing Protocol for Low Bit Rate Multimedia C