VoIP Voice and Fax Signal Processing

20.3 VoIP VOICE QUALITY CONSIDERATIONS

A VoIP solution supports full-chain voice and fax modules as explained in the previous chapters. Many of these algorithms and modules are based on several standards. In VoIP solutions, the main voice quality goals would be to match or exceed PSTN quality. To achieve this goal, several proprietary solutions are incorporated into voice processing. Major influencing items can be analyzed through the voice quality measuring models like the E-model, PESQ, and perceptions based on subjective listening. In VoIP, some critical areas considered for voice quality contribution are given below [Alexander (2006), Bellamy (1991), ITU-T-G.1020 (2006), ITU-T-G.177 (1999), TIA/EIA-810A (2000), TIA/EIA-116A (2006)]. Some of these aspects are also covered in the previous chapters of this book while presenting an individual operation like codec, echo canceller, VAD/CNG, PLC, jitter buffer, and so on. A summary of the major voice quality influencing operations and improvements is given as follows:

• End-to-end delay reduction
• Packet flow impediments in processing
• Adaptive jitter buffer with utilization of silence zones
• Packet loss concealment
• Echo cancellation
• Voice compression codecs
  • Narrowband coding
  • Wideband coding
  • Transcoding tandem operation of codecs
  • Codecs and congestion
• Country-specific deviations
• Signal transmission characteristics
• Transmission loss planning
• Subscriber line interface circuit (SLIC)–CODEC interfaces and configurations
• Dual-tone multifrequency ...