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There are typically three different types of VoIP systems in place now.
Analog Telephone Adaptor (ATA)
The most common type is to send the voice signals through a device called analog telephone adaptor (ATA). This device allows connection of a regular phone to a PC for use as VoIP. The ATA is simply an analog-to-digital converter, similar in operation to a modem. It takes the analog signal from your traditional phone and converts it into digital data for transmission over the Internet.
Then there are IP Phones which look like ordinary phones with a handset, cradle and buttons but have RJ-45 connectors instead of the RJ-11 connectors we see on phones. The IP phones connect directly to the router and have in built programs and hardware to administer the VoIP calls.
Lastly, there is computer-to-computer system which is of course the easiest way to carry out a VoIP call. All you need for this type of connection is microphone, speakers, a sound card and a fast Internet connection. There is no charge for computer-to-computer calls; you only pay your ISP fees.
When you place a VoIP all, your voice signals are converted to digital signals that traverse the internet to reach the other party at the end. If you place the call using a phone with an adapter, you will observe that the phone has a dialling tone just like an ordinary phone. As soon as the other party picks the call, a connection is established between the two of you. Then the packet exchange takes place. The voice signals, converted to packets, are delivered to the receiver and the receiverís gets to the senderís. In between the two is the internet infrastructure like routers, etc which handle all the tasks relating to the call. Actually the internet handles all the communication like an email or a music file. It hardly recognises that you are communicating by voice as it is only aware of the packets as signals.
During this time, the ATAs at each end reassemble packets of information as they are received and convert them to the analog audio signal that you hear. The ATA is also responsible for maintaining the connection open as you talk so the system does not hang up. When you hang up, the circuit is closed between your phone and the ATA.
Packet Switching VS Circuit Switching
Packet switching has several advantages over circuit switching. If a packet is sent to a recipient and it bounces back, it will be sent again on several tries until it goes trough. So if you make the call and the there is a problem at the receivers end, the system will keep on trying like it does with an ordinary internet connection till the message goes through. Circuit switching is not like that. At first try, there should be 99.9% connectivity or acceptance to get the call through otherwise the call will bounce back as engaged.
Again, data networks already understand packet switching so passing voice signals over them is not a thing new. This way, they can handle the new service using their existing protocol without any limitations.
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