Aastra OpenCom 131 User Guide - Page 112
G.711, G.729A, G.723.1 6.3, Quality Levels for Voice Transmission with VoIP, Level
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Voice over IP (VoIP) Fundamentals Quality Levels for Voice Transmission with VoIP Level Voice Comprehensibility 3 Satisfactory 4 Limited > 4 Unacceptable Comparable to 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 or µ-Law (Level 1, uncompressed): The audio data of a PCM channel (64 kbit/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 kbit/s. ■ G.723.1 6.3 (Level 3): Reduction to 6.3 kbit/s. ■ G.723.1 5.3 (Level 3): Reduction to 5.3 kbit/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" programme. Note: Measurements made with "ping" are round-trip propagation delays. Divide the maximum value displayed by two. 110