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In order to understand how VoIP systems really work and how it's an improvement over the traditional analog phone system, it helps to first understand how that traditional phone system works.
The Standard Phone System -vs- The VoIP System
Circuit Switching
Our existing phone systems are driven by a very reliable but inefficient method for connecting calls. This system is called circuit switching.
Circuit switching has been used by telephone networks for over a century. When you call someone, the connection is maintained for the entire duration of the call. Because you are connecting two phones in both directions, the connection is called a circuit. This is the foundation of the Public Switched Telephone Network (PSTN).
How a typical circuit switched telephone call works:
1. Pick up the receiver and listen for a dial tone. The tone signifies that you have a connection to the local office of your telephone carrier.
2. You dial the number you want to call.
3. The call is routed through the switch at the local center to the number you are calling.
4. A connection is made between your telephone and the other line, typically using several interconnected switches along the way.
5. The phone at the other end rings, and someone answers your call.
6. The connection automatically "opens" the circuit.
7. You talk for a while and then hang up.
8. When you hang up, the circuit is closed. This frees up your line and all the lines in between.
Let's say that you talk for 10 minutes. During this entire time, the circuit is continuously open between both phones. Up until the 60's, every call you made had to have a dedicated wire stretching from one phone to the other for the entire duration of the call.
So if you were in Seattle and you called Miami, the switches between would physically connect pieces of copper wire all the way across the US! You would use all those pieces of wire just for your call - for the full 10 minutes. You paid a lot for the call, because you actually "owned" that 3,500-mile-long copper wire for 10 minutes.
Since the 60's, infrastructure upgrades have made the phone network a little more efficient and such long-distance calls cost a lot less. Typically, your voice is now digitized, and your call (along with thousands of others) can be combined onto a single fiber optic cable for much of that 3,500 miles.
The average call on fiber optic cable is transmitted at a fixed rate of 64 kilobits per second (Kbps), in each direction, for a total transmission rate of 128 Kbps. Since there are 8 kilobits (Kb) in a kilobyte (KB), this translates to a transmission of 16 KB each second the circuit is open and 960 KB for every minute it's open.
So in a 10-minute phone call, the total transmission is 9,600 KB, which is roughly equal to 10 megabytes*. If you look at a typical phone conversation, much of this transmitted data is wasted.
Wasted? Basically, yes. Because while you are talking, the other guy is listening. This means that only half of the connection is actually being used at any given time. Plus, a significant amount of the time, often for seconds at a time, nobody is talking and the connection is not being used - but it's still connected.
Packet Switching
Packet switching uses existing interconnected data networks (aka: the internet) and data networks do not use circuit switching at all. Instead, data networks simply send and retrieve data as needed.
You are using packet switching on a data network right now - your internet connection. Web surfing would be a lot slower if your web browser had to maintain a constant connection to the web at all times.
More significantly, instead of routing the data you request over a dedicated line, tying that line up for just one conversation, data network packet switching opens a brief connection to the internet that only stays open long enough to send a small chunk of data (the packet) from one system to another.
How a typical Packet Switch telephone call works:
* The sending computer chops data into small packets, with an address on each that tells the network devices where to send them.
* Inside of each packet is a "payload". For web browsing, the payload could be a piece of an e-mail, a music file or whatever... for a phone call, it's a sound bite.
* The sending computer sends the packet to a nearby router, which sends the packet to another router that is closer to the recipient computer. That router sends the packet along to another, even closer router, and so on.
* When the receiving computer finally gets the packets (which often have traveled along different routes), it uses instructions contained within the packets to reassemble the data into its original state.
* This all happens at light speed, which explains how your voice conversation can seem to be one continuous event, even though each word or phrase has been broken up into little packets and sent over assorted routes.
At first glance packet switching may seem chaotic, but in practice it is very efficient. It lets the network route the packets along the least congested (and cheapest) lines. It also frees up the computers communicating with each other so that they can accept information from other computers as well.
It's so efficient, in fact, that you can surf your favorite websites or search for something while you are making a long-distance phone call.
A packet-switched phone network is the logical next step in business and personal communication, the obvious replacement to our century-old system of circuit switching. In packet switching, instead of sending a continuous stream of data (much of which is wasted, remember), it sends just the packets of data as they are created. By removing silent intervals and using both sides of the connection (incoming and outgoing) simultaneously, the data transmission rates are smaller and allow more efficient use of the same infrastructure.
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