Aastra OpenCom 510 User Manual

Also see for OpenCom 510: User guide Quick reference guide Mounting and Commissioning User Guide Quick reference guide Short installation guide

Contents

Aastra OpenCom 510 User Manual Manual pdf 114 page image

Voice over IP (VoIP) Fundamentals112When 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 achievablequality level:■ G.711 A-Law (Level 1, uncompressed): The audio data of a PCM channel(64 kilobit/s) is adopted one-to-one. Every VoIP terminal must support thiscodec. This codec can not be used with an ISDN data connection.■ G.729A (Level 2): Reduction to approximately 8 kilobit/s.■ G.723.1 6.3 (Level 3): Reduction to 6.3 kilobit/s.■ G.723.1 5.3 (Level 3): Reduction to 5.3 kilobit/s.Unfavourable packet length selection may reduce voice quality. The duration ofthe recording and not the data packet’s byte count is relevant in making thisselection:■ Duration <= 30 ms: optimal transmission■ Duration 40 - 60 ms: one quality-level depreciation■ Duration > 60 ms: two quality-levels depreciationThe achievable voice quality also depends on the packet propagation delay andthe packet loss between the terminals involved. These parameters can be deter-mined using the “ping” program.3 Satisfactory GSM4 Limited Defective GSM> 4 Unacceptable No ConnectionQuality Levels for Voice Transmission with VoIPLevel Voice Comprehensibility Comparable to