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Fundamentals 109
Voice over IP (VoIP)
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 fac-
tor that determines the achievable quality level:
z
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.
z
G.729A (Level 2): Reduction to approximately 8 kbit/s.
z
G.723.1 6.3 (Level 3): Reduction to 6.3 kbit/s.
z
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:
z
Duration <= 30 ms: optimal transmission
z
Duration 40 - 60 ms: one quality-level depreciation
z
Duration > 60 ms: two quality-levels depreciation
The achievable voice quality also depends on the packet propaga-
tion delay and the packet loss between the terminals involved.
These parameters can be determined using the “ping” pro-
gramme.
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