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VoIP Technology (How VoIP works)

This page is more about the VoIP technology. To know about VoIP phones, its types and pros and cons, go to VoIP Phones.


What is VoIP?

VoIP is the short form of Voice over Internet Protocol. It is also called IP Telephony, Internet telephony or Digital Phone. It utilizes the IP network (Internet or intranets) for telephone conversations. The service it provides is similar as that of normal landline phones but generally offers a cheaper solution. It has some added advantages and as any other service, it faces some challenges too.

How does VoIP work?

  • When you speak at the handset or a mike or a microphone, your voice generates electrical signals inside the gadget. These are analog signals i.e. the voltage level can take up any value within a range.
  • The analog signal is converted to a digital signal using an algorithm implemented by the device you are using. It can be a stand-alone VoIP phone or a softphone running on your PC. If you are using an analog phone, you will need a Telephony Adapter (TA) for this purpose. The digitized voice is arranged in packets (i.e. collection of bytes) and sent over the IP network.
  • The data is channeled through gateways and servers to the destination. If the called number is on the PSTN, the server opens a connection to the PSTN and routes your call there.
  • While going to the PSTN or at the end device of a VoIP connection, the voice is again brought back to its analog form so that it is perceptible to a human ear.
  • During the entire process a protocol like SIP or H.323 is used to control the call (e.g. setting up connection, dialing, disconnecting etc.) and RTP is used for reliable transmission of data packets and maintain Quality of Service.

How voice is coded digitally for voip?

The digitization of analog voice signals is a must to transmit voice over the digital IP network. It can be done in several ways

  • PCM (Pulse Code Modulation) is a simple technique of sampling the sound signal at a fixed rate (8000 times/second) and generate a number corresponding to each sample. It assumes no specific property of the signal. So it works reasonably well with all types of sounds.
  • LPC (Liner Predictive Coding) assumes specific properties of human voice and uses a more complex algorithm to digitize and compress voice data. It works well for sending human utterances offering a low data rate but is not suitable for transmitting music or fax.
  • SBC (Sub Band Coder) uses a different approach of representing sounds in terms of frequencies rather than sampling at regular intervals.
  • Hybrid coders like the CELP (Code Excited Linear Prediction) use a mixture of the techniques to transmit sound of adequate quality.

What is a VoIP phone?

Please refer to the VoIP phones page.

What are the advantages and disadvantages of VoIP?

Please refer to the advantages and disadvantages discussed on the VoIP phones page.

To know about VoIP phones, its types and pros and cons, go to VoIP Phones.

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© 2005 K.Banerjee
Last updated: March 01, 2018