T-Comfort Comfort Pro S, Mounting And Commissioning Manual

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Fundamentals Voice over IP (VoIP)
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:
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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.
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G.729A (Level 2): Reduction to approximately 8 kbit/s.
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G.723.1 6.3 (Level 3): Reduction to 6.3 kbit/s.
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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:
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Duration <= 30 ms: optimal transmission
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Duration 40 - 60 ms: one quality-level depreciation
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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.
N
Note: Measurements made with “ping” are round-trip propagation delays. Divide the
maximum value displayed by two.
Packet Propagation Delay and Packet Loss
Value Quality Level Value Quality Level
Propagation delay
< 50 ms
Optimal Loss < 1 % Optimal
Propagation delay
50-100 ms
0.5 level depreciation Loss 1-2 % 0.5 level depreciation
Propagation delay
100-150 ms
1 level depreciation Loss 2-3 % 1 level depreciation