Traditional Phone Systems

To truly understand how VoIP works, one must first understand the basics of how traditional phone service works. Traditional phone systems connect calls by utilizing a system known as circuit switching. Essentially, when a call is made, a connection between the two parties is established and that connection is maintained for the entire duration of the phone conversation. Thus, you have a circuit, or a connection which connects two points in both directions. (1) In very basic terms, two copper wires are connected to form the circuit.

Perhaps the easiest way to understand the traditional phone system, is to look back to the days of the manual switchboard. In those days, each time you picked up the receiver at your house or apartment, a circuit was established between you and the switchboard operator. The circuit manifested itself as a light on the switchboard. Essentially, when you picked up the receiver, you completed the circuit which in turn caused a light bulb on the switchboard to light. The operator would then plug his or her headset into the jack under the lighted bulb and a connection was made. After asking you who you desired to speak with, the operator would ring the phone at their house or office. If they picked up, the operator established a circuit between your house and theirs, and your conversation could take place. (2) One of the key things to understand about this system is that, depending upon the distance of the call, any number of switches could be necessary. For instance, a call from New York to Los Angeles would connect pieces of copper wire all the way across the country and maintain those connections for the duration of the call. (3) Obviously, that was expensive.

The process has obviously gotten simpler and more efficient. Today's traditional phone systems still utilize a copper wire to connect to your house, but the phone system now digitizes your voice and transmits it over a fiber optic cable. That process is handled by something known as a digital concentrator. The digital concentrator digitizes your voice, combines it with dozens of other voices and sends them all down a single wire, to the phone company's office. Your voice travels over the fiber optic line to the office of the receiving party.

VoIP Phone Systems

In the broadest terms, VoIP allows your voice to be converted into a digital signal and thus transmitted over the Internet. According to to the Federal Communications Commission,

"VoIP services convert your voice into a digital signal that travels over the Internet. If you are calling a regular phone number, the signal is converted to a regular telephone signal before it reaches the destination. VoIP can allow you to make a call directly from a computer, a special VoIP phone, or a traditional phone connected to a special adapter." (4)

VoIP is made possible by a technology called packet switching. We mentioned this briefly when we discussed the ARPANET project. Packet switching is essentially the way in which the Internet operates. As opposed to circuit switching, which continuously maintains an open connection, packet switching, "opens a brief connection, just long enough to send a small piece of data, called a packet, from one system to another." (5) Essentially, the sending computer breaks the data up into small packets complete with instructions and sends them off to a nearby router. Each packet is then moved towards its destination by a series of routers, each successively closer to the destination computer. Once at the destination computer, the packets are decoded and the transmitted data is reassembled. (6) Packet switching is important to VoIP because it allows multiple phone calls to occupy the same space that a single phone call occupies in a traditional phone system.

Placing a call using VoIP essentially begins in the same manner as a traditional call. That is, with a dial tone and a phone number. However, in this case the dial tone is an Internet connection and as such, the phone number must be transposed from a number to an IP address. This transition of number to IP address is accomplished by a central call processor, which is nothing more than a piece of hardware using a specialized database/mapping program called a soft switch. The soft switch is responsible for identifying where a device is on the network, identifying what phone number is associated with that device and identifying what IP address is currently assigned to that device.(7) Then the soft switch connects the two devices. For instance, if you were to dial (123) 456-7890, the soft switch would identify where you are on the network and the IP address associated with your location. Then it would identify the location and IP address of the device at the number you dialed. "If the soft switch does not have the information it needs, it passes the request to other soft switches until it finds one to answer the request." (8) Once all devices and IP addresses are identified, a connection is made and, assuming the party you are calling answers the phone, your conversation takes place. If necessary, a coder-decoder (codec) is used to convert the audio signals of the conversation into a compressed digital signal. These digital signals are then transmitted as packets and decoded by the other system.

Making a Call

There are currently three ways to make a call using VoIP: using an Analog Telephone Adapter (ATA), using an IP Phone or computer-to-computer. Regardless of the method, an Internet connection is obviously needed (in most cases a broadband connection).

ATA - An ATA is probably the most common of the equipment types. In fact, in many cases companies like Vonage and AT&T are bundling ATA's with new service. An ATA is an analog to digital converter (a codec) which means that it takes the analog signals from your traditional phone and converts them to digital data. (9) As previously discussed, this data is then able to be transmitted over the Internet. ATA's simply connect to your existing phone which instead of being plugged into the wall jack, is now plugged into the ATA.




IP Phones - IP Phones work exactly like a regular phone with the exception of the connection. Instead of using a traditional phone connection and connecting to a telephone jack in your house, IP Phones use an Ethernet connector and are connected directly to your router. The hardware and software required to make Internet calls is normally pre-installed in the phone. (10)



Computer-to-Computer - Making computer-to-computer VoIP calls requires slightly more work than making VoIP calls with either an ATA or an IP Phone. To make a computer-to-computer call, you must install the appropriate software as well as a microphone, speakers ans sound card onto your computer (if they do not already have these components installed).

Protocols

The final piece of the VoIP technological mix is protocols. Protocols are complicated and difficult to understand, thus we will not linger with them long. However, it is important to understand that there are several competing protocols in the VoIP world and, as yet, no standard has been established. The most significant of the protocols are H.323, Session Initiation Protocol (SIP) and Media Gateway Control Protocol (MGCP). Protocols make communication between all of the different pieces of hardware and software possible. However, because there is not yet an established standard, it is important that you realize that the different protocosl do not always communicate with each other and thus, at times, VoIP calls bog down when conflicting protocols meet.

Key Terms

Circuit Switching - A telecommunications network that establishes a constant and dedicated connection between two users. This is the basis of the traditional phone system.

Digital Concentrator - A device, about the size of a refrigerator, which digitizes your voice so that it can be sent over a fiber optic line.

Packet Switching - Is the basis for the Internet. Instead of a constant connection, packet switching networks open a connection between two endpoints just long enough to send or receive data. Thus the connection is not constantly established.

Central Call Processor - A piece of hardware running a specialized database/mapping program called a soft switch. This system is responsible for mapping a specific phone number to an IP address.

Soft Switch - A database/mapping program run by a central processor which allows the processor to map a standard phone number to an IP address.

Coder-Decoder (Codec) - Converts an audio signal into a compressed digital form for transmission and then back into an uncompressed audio signal for replay.

Analog Telephone Adapter (ATA) - A device that connects to your standard phone and allows you to make VoIP calls by converting your analog voice signals into digital data.



IP Phone - A special phone which connects, via an Ethernet connection, directly to your router and allows you to make VoIP calls.

CitationsSources

(1) Valdes, Robert. "How VoIP Works" Available at www.howstuffworks.com

(2) Bain, Marshal. "How Telephones Work." Available at www.howstuffworks.com

(3) Valdes, Robert. How VoIP Works. Available at www.howstuffworks.com

(4) Voice Over Internet Protocol. Available at www.fcc.gov

(5) Valdes, Robert. "How VoIP Works." Available at www.howstuffworks.com

(6) Valdes, Robert. "How VoIP Works." Available at www.howstuffworks.com

(7) Valdes, Robert. "How VoIP Works." Available at www.howstuffworks.com

(8) Valdes, Robert. "How VoIP Works." Available at www.howstuffworks.com

(9) Valdes, Robert. "How VoIP Works." Available at www.howstuffworks.com

(10) Valdes, Robert. "How VoIP Works." Available at www.howstuffworks.com

Images

ATA and IP Phone Connection Diagrams
Kuhn, Dr. Robert M. "Understanding Voice Over IP." Compass Consulting International, Inc. Available at www.compassconsulting.com

Images of ATA and IP Phones
Cisco Systems, Inc. Available at www.cisco.com