Aastra OpenCom X320 User Guide - Page 100
G.711 A-Law, G.729A, G.723.1 6.3, Quality Levels for Voice Transmission with VoIP, Level
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Voice over IP (VoIP) Fundamentals The following comparison provides a guide to voice quality with specific quality levels: Quality Levels for Voice Transmission with VoIP Level Voice Comprehensibility 1 Very Good 2 Good 3 Satisfactory 4 Limited > 4 Unacceptable Comparable to ISDN DECT GSM Defective GSM 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. ■ G.729A (Level 2): Reduction to approximately 8 kilobit/s. ■ G.723.1 6.3 (Level 3): Reduction to 6.3 kilobit/s. ■ G.723.1 5.3 (Level 3): Reduction to 5.3 kilobit/s. Unfavourable packet length selection may reduce voice quality. The duration of the recording and not the data packet's byte count is relevant in making this selection: ■ Duration 60 ms: two quality-levels depreciation The achievable voice quality also depends on the packet propagation delay and the packet loss between the terminals involved. These parameters can be determined using the "ping" program. 98