Aastra Telecom 800 manual Isdn

Voice over IP (VoIP) Fundamentals

The following comparison provides a guide to voice quality with specific quality levels:

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

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.

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