Proximus Forum 500 Series, Руководство по монтажу и пусконаладке

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Fundamentals
Voice over IP (VoIP) 157
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 ?-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 determined using the “ping” programme.
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
Propagation
delay
150-200 ms 2 level
depreciation Loss 3-4 % 2 level
depreciation