Proximus forum 523, Mounting And Commissioning Manual

Page: 96 / 228

Fundamentals
94 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:
● 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. This codec can not be used with an ISDN data
connection.
● G.729A (Level 2): Reduction to approximately 8 kbit/s.
● G.723.1 6.3 (Level 3): Reduction to 6.3 kbit/s.
● 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:
● 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.
4 Limited Defective GSM
> 4 Unacceptable No Connection
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 depreci-
ation Loss 1-2 % 0.5 level depreci-
ation
Quality Levels for Voice Transmission with VoIP
Level Voice Comprehensibility Comparable to