VoIP uses packet switching rather than circuit switching. Instead of leaving an entire circuit open during the whole conversation, it breaks the conversation into small packets of data. It then transmits that data over the Internet. Instead of monopolizing a circuit, packet switching sends and receives data as needed.
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Let's say you and a friend who uses the same VoIP service are going to talk to each other. You're using analog telephone adapters (ATAs), which let you plug your regular phones into your computers. Here's what would happen:
- You pick up the receiver, which sends a signal to the ATA. The ATA receives the signal and sends a dial tone to let you know that you have a connection to the Internet.
- You dial the phone number. The ATA converts the tones into digital data.
- Your computer sends the data to the VoIP company's call processor.
- The call processor maps the phone number by translating it to an IP address. A specialized mapping program called a soft switch connects your ATA to your friend's. It sends a signal to your friend's ATA, telling it to ring.
- Your friend picks up the phone, establishing a session between your computers. Each system knows to expect packets from the other.
- The system converts your voices into packets of binary data. The normal Internet infrastructure treats the just like it would for an e-mail message or a Web page.
- As you talk, the system transmits packets of data back and forth. The ATAs translate the packets into the analog audio signal that you hear.
- When you finish talking, you hang up the receiver, closing the circuit between your phone and the ATA. The ATA sends a signal to the soft switch to terminate the session.
It's an efficient process -- and it doesn't tie up the line when it doesn't need to. It can also be free. If you're using a free service, like Skype, and calling someone else who also uses the same service, your call costs you nothing. But you might want to make phone calls when you're away from your computer, or you might just want to walk around while you talk.
With 802.11 networking, or WiFi, VoIP can go wireless. A WiFi phone has an antenna that transmits information to a computer, base station or wireless router using radio waves. An antenna at a base station or router picks up the signal and passes it on the Internet. You can read more about this process in How WiFi Works.
Let's look at these phones in more detail.