This Recommendation describes terminals and other entities that provide multimedia communications services over Packet Based Networks (PBN) which may not provide a guaranteed Quality of Service. H.323 entities may provide real-time audio, video and/or data communications. Support for audio is mandatory, while data and video are optional, but if supported, the ability to use a specified common mode of operation is required, so that all terminals supporting that media type can interwork. The packet based network over which H.323 entities communicate may be a point-to-point connection, a single network segment, or an internetwork having multiple segments with complex topologies. H.323 entities may be used in point-to-point, multipoint, or broadcast (as described in ITU-T Rec. H.332) configurations. They may interwork with H.310 terminals on B-ISDN, H.320 terminals on N-ISDN, H.321 terminals on B-ISDN, H.322 terminals on Guaranteed Quality of Service LANs, H.324 terminals on GSTN and wireless networks, V.70 terminals on GSTN, and voice terminals on GSTN or ISDN through the use of Gateways. H.323 entities may be integrated into personal computers or implemented in stand-alone devices such as videotelephones. Note that the title of H.323 (1996) was Visual telephone systems and equipment for local area networks which provide a non-guaranteed quality of service. The title changed in Version 2 to be consistent with its expanded scope. Products claiming compliance with Version 1 of H.323 shall comply with all of the mandatory requirements of H.323 (1996) which references ITU-T Rec. H.225.0 (1996) and H.245 (1997). Version 1 products shall be identified by H.225.0 messages containing a protocolIdentifier= {itu-t (0) recommendation (0) h (8) 2250 version (0) 1} and H.245 messages containing a protocolIdentifier= {itu-t (0) recommendation (0) h (8) 245 version (0) 2}. Products claiming compliance with Version 2 of H.323 shall comply with all of the mandatory requirements of this Recommendation, H.323 (1998), which references ITU-T Rec. H.225.0 (1998) and H.245 (1998 or later). Version 2 products shall be identified by H.225.0 messages containing a protocolIdentifier= {itu-t (0) recommendation (0) h (8) 2250 version (0) 2} and H.245 messages containing a protocolIdentifier= {itu-t (0) recommendation (0) h (8) 245 version (0) x}, where x is 3 or higher. Products claiming compliance with Version 3 of H.323 shall comply with all of the mandatory requirements of this Recommendation, H.323 (1999), which references ITU-T Rec. H.225.0 (1999) and H.245 (1999 or later). Version 3 products shall be identified by H.225.0 messages containing a protocolIdentifier = {itu-t (0) recommendation (0) h (8) 2250 version (0) 3} and H.245 messages containing a protocolIdentifier = {itu-t (0) recommendation (0) h (8) 245 version (0) x}, where x is 5 or higher. Products claiming compliance with Version 4 of H.323 shall comply with all of the mandatory requirements of this Recommendation, H.323 (2000), which references ITU-T Rec. H.225.0 (2000) and H.245 (2000 or later). Version 4 products shall be identified by H.225.0 messages containing a protocolIdentifier = {itu-t (0) recommendation (0) h (8) 2250 version (0) 4} and H.245 messages containing a protocolIdentifier = {itu-t (0) recommendation (0) h (8) 245 version (0) x}, where x is 7 or higher.