Voice over IP (VoIP)--The basics

You might already be using VoIP and not even know or realize it. Many telephone service providers are starting to use some form of VoIP (transparently to you) inside their networks because of the cost efficiencies it can afford them. Many online voice chat services, such as Xbox Live voice chat, Skype, and so on, rely on VoIP. You might find it worthwhile to spend a few minutes to understand how VoIP works. Don't worry, you don't need to know how it works to use it, but it might help to understand the advantages and limitations we discuss later. Circuit Switching and Packet Switching

The main difference between traditional phone systems and VoIP systems is circuit switching versus packet switching. The public switched telephone network (PSTN) uses circuit switching to carry your voice from your phone to the person you are calling (See Figure 2-1). This means that while you are on the phone, a connection is made end-to-end through the phone system. This requires resources (in this case, a series of wires, switches, and connections) in the phone network that are dedicated for the duration of your call. While you are using them, no one else can use them. The end-to-end circuit is reserved for your conversation.

This approach works well, but imagine the resources that are required to carry millions of calls each day coast to coast. At first, each call required a separate set of copper wires. Technology got better, and now millions of calls can be carried over fiber-optic cables (and still circuits get overloaded on Mother's Day). But even though density improved, the basic principles of circuit switching still apply today—each call consumes a channel on the wire end to end for the duration of the call.

Transoceanic fiber cables can carry more than 100 million phone calls each. Even the more ordinary fiber cables have thousands of strands but can carry 1 million+ calls.

Packet switching works differently (See Figure 2-1). Instead of having a dedicated connection end-to-end, packet switching breaks the voice conversation into pieces, transmits the pieces, and then reassembles the pieces at the other side back into the voice conversation. You might be asking yourself: How does that save anything? Well, if you remember in circuit switching, you are consuming a dedicated resource end-to-end. But in packet switching, many people can share that same resource at the same time.

In the example shown in Figure 2-1, the word Hello spoken by the caller is broken into five packets, one per letter sound, and transmitted across the network with millions of other packets from other phone conversations. The receiving switch or phone knows how to reassemble these five packets into the sounds spoken by the caller, and the word Hello is played out the handset speaker.

Note Alas, we are intentionally oversimplifying again. In reality, it takes about 50 packets to transmit each second of speech. But we had a hard time finding a word in the dictionary with 50 letters that could be spoken within one second for use in our example.

The important difference to understand is that during a traditional phone call, you are using a dedicated circuit for the duration of your call. Transmission is constant. In packet switching, the pieces of the conversation find their own way through the network and are re-assembled on the other end, which allows many more conversation to take place than in curcuit switching. So, lots of other folks can use the same circuit at the same time you are.

How VoIP Works
Now that you understand a fundamental difference in the way VoIP compares to the PSTN, we look in more detail at how VoIP works. Any phone service has the following four primary components:

• Signaling--Refers to the communications between your handset and the phone service, for example, how the system recognizes you want to make a call, how it receives the number you want to call, and so on.
• Conversation--Sometimes referred to as the "bearer" component. This is the actual voice conversation being transmitted and received across the network.
• Features--Phone services offer many features including call waiting, call forwarding, voice mail, and so on.
• Power--How the handset in your home receives electric power for it to operate.