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Internet Phone System : Part 3

Present VoIP Standards

H.323 is an early VoIP standard which was designed for videoconferencing through internet protocol. However, this standard operates at a relatively slow speed because it is specifically designed for video conferencing application according to the ITU but usage has later been extended to VoIP. It supports both Point-to-Point Communications and Multi-Point Conferences. At present, equipment produced by different manufacturers can be interoperated according to this standard.

SIP (Session Initiation Protocol) is the standard for voice and video transfer. In the past, this standard is specifically applied by Instant Messaging software and online games. SIP is designed under IETF (Internet Engineering Task Force) standard which works at Application Layer. This standard is suitable for a network that uses firewall or is using NAT (Network Address Translation) which applies with Private IP or Proxy Server. Generally, SIP is more likely to be used with Soft Phone.

Process of VoIP

  • Conversion to PCM (Pulse Code Modulation) is the process to convert analog voice signal into digital format or PCM.
  • Echo cancellation is the process to isolate and remove unwanted signal energy created ty echoes of the main transmitted signal by using DSP (Digital Signal Processors).
  • Framing is the process to arrange digital voice data into Frame format. During this process, compression will be done to reduce the size of data. Different types of CODEC (Compression-Decompression) shall be applied.
  • Packetization is the process to convert digital signal frame into packet format by adding header to specify sequence number and time stamp.
  • Addressing and Delivery is the process to identify the destination by adding recipient's IP Address before sending out.
  • Conversion to Analog is the process at the destination. Headers and other unnecessary data will be removed from the packet until there is only the frame of voice signal. The data shall be converted back into analog signal.
  • Error Correctin is the process to check for errors and correct them as it is possible that some parts of data maybe distorted or lost during transmission.

ѹ 18 - 06 - 2006  


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