Aastra OpenCom 510 User Manual page 114

Voice over IP (VoIP)
Quality Levels for Voice Transmission with VoIP
Level Voice Comprehensibility
3 Satisfactory
4 Limited
> 4 Unacceptable
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 <= 30 ms: optimal transmission
■
Duration 40 - 60 ms: one quality-level depreciation
■
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 deter-
mined using the "ping" program.
112
Comparable to
GSM
Defective GSM
No Connection
Fundamentals