Some Standards Used in IP PBX Software

-G.711: an international standard used for encoding (packetizing) telephone voice at either 56kbs or 64 kbps - this is uncompressed digitized voice.
-G.723: a protocol for compressing voice to 6.4 kbs or 5.3 kbs. The compression quality is very good with voice quality as good as normal telephone voice quality. It is supported by virtually all IP telephone equipment.
-H.323: Signaling & telephone services protocol for the transmission of IP packets representing any combination of voice, video and data. H.323 is designed for operation over existing IP networks. Includes facilities call setup signaling and media control. Allows VoIP equipment to interoperate.
-SIP (Session Initiation Protocol): A signaling & telephone services protocol similar to, but simpler than, H.323.