Aastra OpenCom 510 User Guide - Page 120
Voice Quality, Quality Levels for Voice Transmission with VoIP
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Voice over IP (VoIP) Fundamentals Required bandwidth (kilobit/s) with respect to Packet Length and Codec Packet Length (ms) 60 70 80 G.711 (not compressed) G.729A G.723.1 c. 6.3 kilobit/s 6.3 kilobit/s 33.6 30.4 31.09 29.2 G.723.1 5.3 kilobit/s 28.3 Note: To ensure SIP compatiblility, the older system telephones OpenPhone 63 IP and OpenPhone 65 IP does not support the G.723 codec any more. 11.1.3 Voice Quality The achievable voice quality depends on various factors. It is possible to optimise voice-data transmission on an existing network using the available configuration settings. Measuring the network quality may also help. The following comparison provides a guide to voice quality with specific quality levels: Quality Levels for Voice Transmission with VoIP Level Voice Comprehensibility Comparable to 1 Very Good 2 Good 3 Satisfactory ISDN DECT GSM 4 Limited Defective GSM > 4 Unacceptable No Connection When a call is set up, the terminals involved negotiate the voice-data compression ("codec") that will be used. This is the first factor that determines the achievable quality level: ■ G.711 A-Law (Level 1, uncompressed): The audio data of a PCM channel (64 kilobit/s) is adopted one-to-one. Every VoIP terminal must support this codec. This codec can not be used with an ISDN data connection. 118