Internet Telephony

The term “Internet Telephony” has been broadly applied to a family of applications where real-time voice communication occurs over a packet network using Internet protocol.
Internet Telephony has started being widely used in the developed countries for its long distance call charge.
In traditional circuit switched network each call requires a fixed amount of bandwidth and a dedicated circuit is allocated between the two ends of the call. So, unused bandwidth due to a pause in conversation can not be recovered by the network. In IP based network, packets are routed individually over the network and if there is a pause in the conversation of an Internet Telephony call, no packets are generated and packets of other applications/users can be sent in this void. This statistical sharing of a potentially constrained resource-bandwidth leads one to believe at first thought that Internet Telephony is more efficient and, hence, less expensive than traditional circuit switched telephony.
The first documented Internet telephony experiments where conducted on the ARPANET by researchers at MIT in the mid 1970s.
By 1993 the computing power to compress to 14.4 kbps (previous 64 or 56 kbps) or below and modems to connect at that rate become affordable, and the first commercial Internet phone applications appeared.
More recently, all of the major desktop software and hardware vendors (Netscape, IBM, Intel and Microsoft) have available ‘free’ I-phones, bundled with their operating systems or World Wide Web browsers.
An Internet phone requires several elements to work. These are:
  • An audio input/output device (a sound card) and transducers (speaker and microphone).
  • A LAN or dial-up connection to the Internet.
  • An audio codec (software or hardware) capable of compressing speech from the sample rate of the sound card (typically 8k 16-bit samples per second= 128 kbps) to a lower rate that will fit the modem capacity.
  • A processor of sufficient power to run the audio codec in real-time.
  • A protocol stack for the delivery of real-time data over a packet network.
  • A signaling protocol to permit the establishment, maintenance, and clearing of calls.
Classifications of the Internet Telephony Networks
There are three basic networks:
     In this scenario, both calling and called subscribers are using computers attached with an IP network as terminals.
PC-to-Phone and Phone-to-PC
     In this scenario, one subscriber is using a computer for voice and other uses a phone on a PSTN/ISDN/GSM or other network. A gateway on one edge of the IP network translates IP to voice and takes care of the signaling between the two networks.
     In this scenario, both the subscribers are using conventional phones over IP network for long distance connection. Gateways at both ends translate between networks.
Comparison between different Telephony networks
Consumer equipment required
Caller and recipient both need multimedia PC. Can use notebook PC or PDA with cellular modem.
Multimedia PC at one end and phone at the other end.
Phone at the both ends.
Internet connection required
Caller and recipient both need Internet connection. Can be dial-up via modem or office local-area network(LAN)
Internet connection required only at one end. Gateways ate the other end provide the connection.
Gateways at each end provide the net connection.
Regulatory Issues
No PSTN breakout at all. Regulators may be powerless to obstruct.
Breakout into PSTN necessary at one end at least. Regulator controlling PSTN side may obstruct.
Breakout at both ends, liable to obstruction in countries where existing resale sensitivities

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