Wednesday, April 05, 2006

VOIP PHONE SERVICE

Circuit switched Vs packet switched networks

As we have discussed in previous post, VoIP technology depends upon Packet switching network. But beforehand we would like to discuss technology behind traditional telephone system. Traditional telephone systems or you can say PSTN (Public switched telephone network) were driven by circuit switching technology. In this a circuit is established in between two nodal points where voice signals are being transmitted and received.

First of all you pick up the receiver of your telephone system. This receiver set has a microphone and a speaker system built in it. Then you wait for the Dial tone. It is a tone employed in a telephone system to indicate to the calling party that the telephone network & equipment is ready to use. It is a continuous sound signal. This dial tone is a signal to make it clear that you are connected to local telephone office. The user than dial the destination number. This call is routed to the called party through a switch at your telephone service provider’s local office. In the process an electrical connection is established between the calling party and the called party. In this process several PSTN Switches interconnected to each other are employed. The telephone rings at the destination point. The called party picks the receiver and the connection opens up. You get a dedicated route line for your call. That is your connection is reliable and of high quality. After talking with each other, you hang up the receiver and the line is free.

The cost for such a call is much higher as compared to VoIP phone calls as you have a dedicated line for yourself for a certain amount of period.

But this technology revolutionized after the invention of Optical Fiber Cables or in short OFC. In this analog sound signal is converted to a digital binary signal and several signals are transmitted simultaneously. Using OFC for telephone routes proved to be much more cost effective. The quality of sound also improved tremendously as sound is transmitted a digital signal. But if you look at much wider aspect of it there is still a lot which can be improved. Generally one person speaks and other person listens. That is half of the bandwidth is wasted and is free. Also there are lots of pauses in a call when neither called party or calling party speaks, here whole of the bandwidth is wasted. This led to the invention of packet switching networking.

Packet switching uses IP Packets for its working. Voice signals are converted to IP packets. These IP Packets flow through a congested network through several routers. In this connection is open for small amount of time to send or receive IP Packet. Each Packet has an address field telling the network devices where to send it. Another field tell about the type of the packet i.e. whether it is from mpg file or .doc file. It also has reassembly information in it i.e. it’s serial number etc. the network devices try to send packet through least congested network line. The two communicating computers are not bound to each other. They can also receive and transmit data packets to other systems too.

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