How it works: VoIP
July 26, 2007
By Grant Buckler, CBC News
The way voice-over-internet protocol, or VoIP, connects two phones is quite different — and more efficient — than the way a traditional phone call works.
VoIP gets its name from the fact that it uses the internet protocol (IP), a standard way of transmitting data over a network.
To transmit voice this way, the first step is to convert it from an analog voice signal into data — digital zeroes and ones.
A coder-decoder, or codec, converts the sound of your voice into a series of bits, which another codec at the other end of the connection will decode to reconstruct the original speech.
Before VoIP, the phone system was circuit-switched. To set up a phone call, the phone system would create a circuit between two phones, and that circuit would stay in place until the call ended. Think of how phone calls worked back in the days of human operators when a caller would pick up the receiver and turn a crank, sending a signal over a line running directly to the operator's office, where it rang a bell.
The operator would plug her phone into the line that was ringing, and ask what number the caller wanted to reach. The caller would give a number, and the operator would plug in a cable leading to that number. Once the call was connected, a set of wires running from one phone to the other was used for nothing but that one phone call until the callers hung up. Automatic switches replaced operators, but the principle is the same.
IP, on the other hand, sends everything — data, voice or even video — in bursts of bits called packets. As the name implies, these are like something you would send at the post office. Inside is some data, and on the outside is an address to indicate where the packet is going. Each data packet is sent over the network, and devices called routers, which are rather like the post office's mail sorters, use the address to send the packet in the right direction until it reaches its destination.
Many packets make up a VoIP phone call. An electronic device called a packet assembler-disassembler organizes data from the codec into packets and passes it to a router that sends it over the network to its destination, where it is taken out of the packets and turned into sound again. Packets may even travel different routes to reach their destination — if your call is from Toronto to Los Angeles, say, one packet might go through Chicago and the next through Vancouver.
This can lead to problems. If packets are delayed or arrive in the wrong order, the phone call doesn't sound right. This is why VoIP systems usually need networks that provide "quality of service" guarantees, meaning that they make sure the packets get through quickly. Today's networks can tell the difference between voice and data packets and give priority to the voice ones, because delays are more noticeable in a voice call than in, say, the transmission of e-mail.
Knowing where to send
How do the network routers know where to send the packets of a voice call?
Internet protocol uses IP addresses, which are like telephone numbers except that their format is different. An IP address is four groups of up to three digits, separated by dots, like 169.254.1.10. Each packet of data has an IP address. Each router that receives that packet uses a lookup table of IP addresses to determine where to send the packet.
But when you make a call using a consumer VoIP service like Vonage, you dial a telephone number, yet your call is routed over your internet connection. How does that work?
When you dial a call, your VoIP router checks the number you dialed against a lookup table. If you're calling another customer of the same VoIP service, it finds an IP address for that customer's phone and sends the call directly over the internet. Otherwise, the router finds the interconnection between the VoIP provider's network and the public telephone network that is nearest to where the call is going. It routes the call over the internet to that point, where it is passed to the phone network and completed.
Before starting to send packets, the router must set up the call — ring the phone at the other end, pass along caller ID information, and so on. Vonage and some other providers use session initiation protocol (SIP) to do this, while some other VoIP systems use other protocols.
Much the same thing happens in the other direction. Say someone in Halifax calls your VoIP phone in Vancouver. The phone system sees the area code 604 and routes the call to Vancouver, where the next three digits send it to the right central office. There the call is passed to a gateway operated by your VoIP service, which uses the lookup table to find the IP address to which your phone number is currently connected.
If your workplace has an IP phone system, it routes internal calls — which can include calls between two offices if your company has its own network connecting them — much the same way as a consumer VoIP service would connect calls between two of its own customers.
Outside calls go to an outside line pretty much the same way as they would with an old-fashioned office phone system.
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