Aastra Telecom 800 - Manual (Page 66)

Voice over IP (VoIP) Fundamentals
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The following comparison provides a guide to voice quality with specific quality
levels:
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 u-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.
■ G.729A (Level 2): Reduction to approximately 8 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 <= 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.
Note: Measurements made with “ping” are round-trip prop-
agation delays. Divide the maximum value displayed by two.
Quality Levels for Voice Transmission with VoIP
Level Voice Comprehensibility Comparable to
1 Very Good ISDN
2 Good DECT
3 Satisfactory GSM
4 Limited Defective GSM
> 4 Unacceptable No Connection