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Types of VoIP And Packet Switched Networks

It is necessary to look at the traditional telephone system. The Plain Old Telephone System is referred to by the acronym POTS. It used a telephone which converts your voice to an electrical signal that is transferred through the medium (wires) and is reconverted from the electric signal to voice at the receivers end. The direction of your voice signal to a particular person is achieved by switches at the exchange.
Circuit Switched Network
The POTS uses circuit switching. It can be both low capacity as well as a high capacity network. The resources are static and are dedicated to the circuit (copper lines) during the entire message transfer. The network use is initiated by a connecting phase which sets up a dedicated circuit connection between the source and destination. This is terminated by a disconnect phase.

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The telephone company reserves a specific physical path to the number you are calling. This can be routed through your local exchange and out station exchange depending on the number you are calling. During the duration of your call no one else can use the physical lines involved. The network achieves the routing using a telephone number. Only after establishing a circuit, the information is transferred over the circuit. Delays are minimized using faster switching mechanism. Initially used only for voice it soon came to be used for data transfer when internet started becoming popular.

PSTN-(Public Switched Telephone Network) was initially analog. Later digital techniques started to be used. Yet the connection between home and central office on a PSTN is analog. ISDN-Integrated Services Digital Network has a digital dialup service for voice and data. Analog telephones and fax machines are used over ISDN lines.

The ISDN terminal adapter converts the analog signals into digital. For the basic rate interface it has two B channels of 64Kbps bandwidth and one D channel of 16Kbps bandwidth (2B + 1D) . Primary rate interface has 23B + 1D. The ISDN has made use of larger bandwidth and multiplexing techniques to provide voice, video and data transfer on the PSTN/POTS.
Packet Switched Network
Voice, video or data is converted into digital format, which is split into packets and put onto a network. These packets take different routes to the destination were they are compiled to form the original message. Breaking of data into small packets of data is advantageous as the communication path is shared among many users as compared to circuit switched network where it is dedicated to a single user. The destination address is contained in each packet. There is no need to establish a circuit and therefore is called a connectionless circuit. Internet is the most well known model of packet switching which exists as a layer-3 protocol. Ethernet, X.25 and Frame relays are other examples but are layer-2 protocols.
Future of POTS
The traditional telephone system has served its purpose and will continue to serve its purpose for years to come. Migration to the Internet telephony though being touted as the replacement for PSTN may take a long time. There are many issues that remain to be resolved like standards. Voice over Internet may raise cost for internet connections which are cheap till now. Service taxes on internet voice services may soon make the cost differences meager.

Internet Telephony voip is here to stay and may take a long time to establish itself as the means of voice communication.

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