How IP Telephony Works
If you regularly make long-distance phone calls, chances are you've already used IP telephony without even knowing it. IP telephony, known in the industry as Voice-over IP (VoIP), is the transmission of telephone calls over a data network like one of the many networks that make up the Internet. While you probably have heard of VoIP, what you may not know is that many traditional telephone companies are already using it in the connections between their regional offices.
This person is using a computer to talk to a friend in another state.
Now, you'll learn about VoIP and the technology that makes it possible. We'll talk about VoIP's major protocols, about the various services provided and the low-cost, often free software that allows you to take advantage of them.
But first, let's discuss the fundamental problem with existing telephone networks -- namely, their reliance on circuit switching.

Circuit Switching

Circuit switching is a very basic concept that has been used by telephone networks for over 100 years. What happens is that when a call is made between two parties, the connection is maintained for the entire duration of the call. Because you are connecting two points in both directions, the connection is called a circuit. This is the foundation of the Public Switched Telephone Network (PSTN).
Here's how a typical telephone call works:
You pick up the receiver and listen for a dial tone. This lets you know that you have a connection to the local office of your telephone carrier. You dial the number of the party you wish to talk to. The call is routed through the switch at your local carrier to the party you are calling. A connection is made between your telephone and the other party's line, opening the circuit. You talk for a period of time and then hang up the receiver. When you hang up, the circuit is closed, freeing your line. Let's say that you talk for 10 minutes. During this time, the circuit is continuously open between the two phones. Telephone conversations over the traditional PSTN are transmitted at a fixed rate of about 64 kilobits per second (Kbps), or 1,024 bits per second (bps), 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 every minute it's open. So in a 10-minute conversation, the total transmission is 9600 KB, which is roughly equal to 9.4 megabytes (MB).
If you look at a typical phone conversation, much of this transmitted data is wasted. While you are talking, the other party is listening, which means that only half of the connection is in use at any given time. Based on that, we can surmise that we could cut the file in half, down to about 4.7 MB. Plus, a significant amount of the time in most conversations is dead air -- for seconds at a time, neither party is talking. If we could remove these silent intervals, the file would be even smaller.
Data networks do not use circuit switching. Your Internet connection would be a lot slower if it maintained a constant connection to the Web page you were looking at. Instead of simply sending and retrieving data as you need it, the two computers involved in the connection would pass data back and forth the whole time, whether the data was useful or not. That's no way to set up an efficient data network. Instead, data networks use a method called packet switching.

Packet Switching

While circuit switching keeps the connection open and constant, packet switching opens the connection just long enough to send a small chunk of data, called a packet, from one system to another. What happens is this: The sending computer chops data into these small packets, with an address on each one telling the network where to send them. When the receiving computer gets the packets, it reassembles them into the original data. Packet switching is very efficient. It minimizes the time that a connection is maintained between two systems, which reduces the load on the network. It also frees up the two computers communicating with each other so that they can accept information from other computers as well. Click "Play" to see how packet switching works.VoIP technology uses this packet-switching method to provide several advantages over circuit switching. For example, packet switching allows several telephone calls to occupy the amount of space occupied by only one in a circuit-switched network. Using PSTN, that 10-minute phone call consumed 10 full minutes of transmission time at a cost of 128 Kbps. With VoIP, that same call may have occupied only 3.5 minutes of transmission time at a cost of 64 Kbps, leaving another 64 Kbps free for that 3.5 minutes, plus an additional 128 Kbps for the remaining 6.5 minutes. Based on this simple estimate, another three or four calls could easily fit into the space used by a single call under the conventional system. And this example doesn't even factor in the use of data compression, which further reduces the size of each call. Let's say that your company had equipment installed and a contract set up so that you can use VoIP. You have installed about a dozen telephones and a digital private branch exchange (PBX) in your office. A PBX is essentially a switch used to connect a number of phones (extensions) to each other and to one or more outside phone lines. In our example, the PBX is also a gateway. Gateways are used to connect devices on two different types of networks so that they can communicate with each other. Our PBX is a gateway because it converts the standard circuit-switched signal from each phone into digital data that can be sent over a packet-switched, IP-based network. IP stands for "Internet protocol," the language used by most data networks. Let's take another look at that typical telephone call, but this time using VoIP over a packet-switched network:

1. You pick up the receiver, which sends a signal to the PBX.

2. The PBX receives the signal and sends a dial tone. This lets you know that you have a connection to the PBX.

3. You dial the number of the party you wish to talk to. This number is then temporarily stored by the PBX.

4. Once you have entered the number, the PBX checks it to ensure that it is in a valid format. 5. The PBX determines whom to map the number to. In mapping, the number is attached to the IP address of another device called the IP host. The IP host is typically another digital PBX that is connected directly to the phone system of the number you dialed. In some cases, particularly if the party you are calling is using a computer-based VoIP client, the IP host is the system you wish to connect with.

6. A session is established between your company's PBX and the other party's IP host. This means that each system knows to expect packets of data from the other system. Each system must use the same protocol to communicate. The systems will implement two channels, one for each direction, as part of the session.

7. You talk for a period of time. During the conversation, your company's PBX and the other party's IP host transmit packets back and forth when there is data to be sent. The PBX at your end keeps the circuit open between itself and your phone extension while it forwards packets to and from the IP host at the other end.

8. You finish talking and hang up the receiver.

9. When you hang up, the circuit is closed between your phone and the PBX, freeing your line. 10. The PBX sends a signal to the IP host of the party you called that it is terminating the session. The IP host terminates the session at its end, too.

11. Once the session is terminated, the PBX removes the number-to-IP-host mapping from memory. Probably one of the most compelling advantages of packet switching is that data networks already understand the technology. By migrating to this technology, telephone networks immediately gain the ability to communicate the way computers do. Of course, having the ability to communicate and understanding the methods of communication are two very different things. For telephones to communicate with each other and with other devices, such as computers, over a data network, they need to speak a common language called a protocol.

Protocol

There are two major protocols being used for VoIP. Both protocols define ways for devices to connect to each other using VoIP. Also, they include specifications for audio codecs. A codec, which stands for coder-decoder, converts an audio signal into a compressed digital form for transmission and back into an uncompressed audio signal for replay. The first protocol is H.323, a standard created by the International Telecommunications Union (ITU). H.323 is a comprehensive and very complex protocol. It provides specifications for real-time, interactive videoconferencing, data sharing and audio applications such as IP telephony. Actually a suite of protocols, H.323 incorporates many individual protocols that have been developed for specific applications.H.323 Protocol SuiteVideo Audio Data TransportH.261H.263 G.711G.722G.723.1G.728G.729 T.122T.124T.125T.126T.127 H.225H.235H.245H.450.1H.450.2 H.450.3RTPX.224.0As you can see, full implementation of H.323 requires a lot of overhead. This page provides detailed information about the entire H.323 suite of protocols and how they relate to the OSI Reference Model. An alternative to H.323 emerged with the development of Session Initiation Protocol (SIP) under the auspices of the Internet Engineering Task Force (IETF). SIP is a much more streamlined protocol, developed specifically for IP telephony. Smaller and more efficient than H.323, SIP takes advantage of existing protocols to handle certain parts of the process. For example, Media Gateway Control Protocol (MGCP) is used by SIP to establish a gateway connecting to the PSTN system. You can learn more about the architecture of SIP on this page.

Calling

There are four ways that you might talk to someone using VoIP. If you've got a computer or a telephone, you can use at least one of these methods without buying any new equipment:
Computer-to-computer - This is certainly the easiest way to use VoIP. You don't even have to pay for long-distance calls. There are several companies offering free or very low-cost software that you can use for this type of VoIP. All you need is the software, a microphone, speakers, a sound card and an Internet connection, preferably a fast one like you would get through a cable or DSL modem. Except for your normal monthly ISP fee, there is usually no charge for computer-to-computer calls, no matter the distance. A good example of this software is MSN Explorer.
The Net2Phone software client is easy to set up and use.
Computer-to-telephone - This method allows you to call anyone (who has a phone) from your computer. Like computer-to-computer calling, it requires a software client. The software is typically free, but the calls may have a small per-minute charge. For example, Net2Phone offers free calls to anywhere in the United States for the first five minutes. If the call is over five minutes, a rate of 3.9 cents per minute kicks in. Net2Phone's international rates vary widely, ranging from 3.9 cents to $7.52 per minute, depending on where you call.
Telephone-to-computer - A few companies are providing special numbers or calling cards that allow a standard telephone user to initiate a call to a computer user. The caveat is that the computer user must have the vendor's software installed and running on his or her computer. The good news is that the cost of the call is normally much cheaper than a traditional long-distance call.
Telephone-to-telephone - Through the use of gateways, you can connect directly with any other standard telephone in the world. To use the discounted services offered by several companies, you must call in to one of their gateways. Then, you enter the number you wish to call, and they connect you through their IP-based network. The downside is that you have to call a special number first. The upside is that the rates are typically much lower than standard long distance. Although it will take some time to happen, you can be sure that, eventually, all of the circuit-switched networks will be replaced with packet-switching technology. IP telephony just makes sense, in terms of both economics and infrastructure requirements. More and more businesses are installing VoIP systems, and the technology will continue to grow in popularity as it makes its way into our homes

BLUETOOTH TECHNOLOGY TO HARNESS THE SPEED OF 802.11

Wireless transfer of large format entertainment data – music, video, and photos – between devices at short range is imminent. The Bluetooth Special Interest Group (SIG) today announced a new way it will provide for consumers’ growing need for speed.
The Bluetooth SIG is developing an innovative method of radio substitution. It will allow the well known Bluetooth protocols, profiles, security and pairing to be used in consumer devices while achieving faster throughput with momentary use of a secondary radio already present in the device. This architecture, called ‘Alternate MAC/PHY’ by Bluetooth SIG members working on the specification, is taking on a two-phased approach as SIG member companies drive the specification forward.
“This is the wireless technology equivalent of ‘low hanging fruit,’” said Michael Foley, Ph.D., executive director, the Bluetooth SIG. “What we’re doing is taking classic Bluetooth connections – using Bluetooth protocols, profiles, security and other architectural elements – and allowing it to jump on top of the already present 802.11 radio, when necessary, to send bulky entertainment data, faster. When the speed of 802.11 is overkill, the connection returns to normal operation on a Bluetooth radio for optimal power management and performance.”
In 2006, the Bluetooth SIG announced the selection of the WiMedia Alliance brand of ultra wideband technology as a high speed channel for Bluetooth technology. This development work continues between the two organizations in advance of widespread ultra wideband technology adoption – expected to be co-located in many Bluetooth devices. In the meantime, however, the SIG will make use of IEEE 802.11, a technology already present in many of the devices demanding greater speeds.
This two-phased roadmap for higher speeds will allow for a steady evolution in Bluetooth devices utilizing the presence of 802.11 today while continuing preparations for the presence of ultra wideband in the near future. “We’re committed to speedy wireless personal area network connections and we’ll always be looking for the best near term and long term way to accomplish that,” adds Foley. “The greatness of a generic alternate radio architecture being developed is that it’s adaptable.”
With the availability of high speed Bluetooth technology, device users can expect to move their entertainment data between their own devices and the trusted devices of friends, without the need for cables and wires. Some applications consumers will experience include:
Wirelessly bulk synchronize music libraries between PC and MP3 player Bulk download photos to a printer or PC Send video files from camera or phone to computer or televisionMeanwhile, Bluetooth devices will continue to offer the well known, low power and secure connections that make up the nearly 2 billion products already on the market. The core specification enabling the Alternate MAC/PHY is expected to be published to members in mid-2009 with work already well underway.
“The Bluetooth SIG is taking a logical step by applying Bluetooth protocols over an existing 802.11 radio to achieve efficient transfers of high data throughput applications," said Flint Pulskamp, wireless and mobile analyst at IDC. "Since Bluetooth and 802.11 already have significant traction in mobile devices, this coupled solution could prove to be an efficient interim solution, as the Bluetooth SIG continues to develop UWB for the future."

Bluetooth Basics

Bluetooth is a standard developed by a group of electronics manufacturers that allows any sort of electronic equipment -- from computers and cell phones to keyboards and headphones -- to make its own connections, without wires, cables or any direct action from a user. Bluetooth is intended to be a standard that works at two levels:

• It provides agreement at the physical level -- Bluetooth is a radio-frequency standard.

• It also provides agreement at the next level up, where products have to agree on when bits are sent, how many will be sent at a time and how the parties in a conversation can be sure that the message received is the same as the message sent. The companies belonging to the Bluetooth Special Interest Group, and there are more than 1,000 of them, want to let Bluetooth's radio communications take the place of wires for connecting peripherals, telephones and computers. There are already a couple of ways to get around using wires. One is to carry information between components via beams of light in the infrared spectrum. Infrared refers to light waves of a lower frequency than human eyes can receive and interpret. Infrared is used in most television remote control systems, and with a standard called IrDA (Infrared Data Association) it's used to connect some computers with peripheral devices. For most of these computer and entertainment purposes, infrared is used in a digital mode -- the signal is pulsed on and off very quickly to send data from one point to another. Infrared communications are fairly reliable and don't cost very much to build into a device, but there are a couple of drawbacks. First, infrared is a "line of sight" technology. For example, you have to point the remote control at the television or DVD player to make things happen. The second drawback is that infrared is almost always a "one to one" technology. You can send data between your desktop computer and your laptop computer, but not your laptop computer and your PDA at the same time. These two qualities of infrared are actually advantageous in some regards. Because infrared transmitters and receivers have to be lined up with each other, interference between devices is uncommon. The one-to-one nature of infrared communications is useful in that you can make sure a message goes only to the intended recipient, even in a room full of infrared receivers. The second alternative to wires, cable synchronizing, is a little more troublesome than infrared. If you have a Palm Pilot, a Windows CE device or a Pocket PC, you know about synchronizing data. In synchronizing, you attach the PDA to your computer (usually with a cable), press a button and make sure that the data on the PDA and the data on the computer match. It's a technique that makes the PDA a valuable tool for many people, but synchronizing the PDA with the computer and making sure you have the correct cable or cradle to connect the two can be a real hassle.

Bluetooth

Bluetooth is intended to get around the problems that come with both infrared and cable synchronizing systems. The hardware vendors, which include Siemens, Intel, Toshiba, Motorola and Ericsson, have developed a specification for a very small radio module to be built into computer, telephone and entertainment equipment. From the user's point of view, there are three important features to Bluetooth:

• It's wireless. When you travel, you don't have to worry about keeping track of a briefcase full of cables to attach all of your components, and you can design your office without wondering where all the wires will go.

• It's inexpensive.

• You don't have to think about it. Bluetooth doesn't require you to do anything special to make it work. The devices find one another and strike up a conversation without any user input at all. Bluetooth communicates on a frequency of 2.45 gigahertz, which has been set aside by international agreement for the use of industrial, scientific and medical devices (ISM). A number of devices that you may already use take advantage of this same radio-frequency band. Baby monitors, garage-door openers and the newest generation of cordless phones all make use of frequencies in the ISM band. Making sure that Bluetooth and these other devices don't interfere with one another has been a crucial part of the design process.

Avoiding

InterferenceOne of the ways Bluetooth devices avoid interfering with other systems is by sending out very weak signals of 1 milliwatt. By comparison, the most powerful cell phones can transmit a signal of 3 watts. The low power limits the range of a Bluetooth device to about 10 meters, cutting the chances of interference between your computer system and your portable telephone or television. Even with the low power, the walls in your house won't stop a Bluetooth signal, making the standard useful for controlling several devices in different rooms. With many different Bluetooth devices in a room, you might think they'd interfere with one another, but it's unlikely that several devices will be on the same frequency at the same time, because Bluetooth uses a technique called spread-spectrum frequency hopping. In this technique, a device will use 79 individual, randomly chosen frequencies within a designated range, changing from one to another on a regular basis. In the case of Bluetooth, the transmitters change frequencies 1,600 times every second, meaning that more devices can make full use of a limited slice of the radio spectrum. Since every Bluetooth transmitter uses spread-spectrum transmitting automatically, it’s unlikely that two transmitters will be on the same frequency at the same time. This same technique minimizes the risk that portable phones or baby monitors will disrupt Bluetooth devices, since any interference on a particular frequency will last only a tiny fraction of a second. When Bluetooth-capable devices come within range of one another, an electronic conversation takes place to determine whether they have data to share or whether one needs to control the other. The user doesn't have to press a button or give a command -- the electronic conversation happens automatically. Once the conversation has occurred, the devices -- whether they're part of a computer system or a stereo -- form a network. Bluetooth systems create a personal-area network (PAN), or piconet, that may fill a room or may encompass no more distance than that between the cell phone on a belt-clip and the headset on your head. Once a piconet is established, the members randomly hop frequencies in unison so they stay in touch with one another and avoid other piconets that may be operating in the same room.

An Example

Let’s take a look at how the Bluetooth frequency hopping and personal-area network keep systems from becoming confused. Let’s say you’ve got a typical modern living room with the typical modern stuff inside. There’s an entertainment system with a stereo, a DVD player, a satellite TV receiver and a television; there's a cordless telephone and a personal computer. Each of these systems uses Bluetooth, and each forms its own piconet to talk between main unit and peripheral. The cordless telephone has one Bluetooth transmitter in the base and another in the handset. The manufacturer has programmed each unit with an address that falls into a range of addresses it has established for a particular type of device. When the base is first turned on, it sends radio signals asking for a response from any units with an address in a particular range. Since the handset has an address in the range, it responds, and a tiny network is formed. Now, even if one of these devices should receive a signal from another system, it will ignore it since it’s not from within the network. The computer and entertainment system go through similar routines, establishing networks among addresses in ranges established by manufacturers. Once the networks are established, the systems begin talking among themselves. Each piconet hops randomly through the available frequencies, so all of the piconets are completely separated from one another. Now the living room has three separate networks established, each one made up of devices that know the address of transmitters it should listen to and the address of receivers it should talk to. Since each network is changing the frequency of its operation thousands of times a second, it’s unlikely that any two networks will be on the same frequency at the same time. If it turns out that they are, then the resulting confusion will only cover a tiny fraction of a second, and software designed to correct for such errors weeds out the confusing information and gets on with the network’s business. Most of the time, a network or communications method either works in one direction at a time, called half-duplex communication, or in both directions simultaneously, called full-duplex communication. A speakerphone that lets you either listen or talk, but not both, is an example of half-duplex communication, while a regular telephone handset is a full-duplex device. Because Bluetooth is designed to work in a number of different circumstances, it can be either half-duplex or full-duplex. The cordless telephone is an example of a use that will call for a full-duplex (two-way) link, and Bluetooth can send data at more than 64,000 bits per second in a full-duplex link -- a rate high enough to support several human voice conversations. If a particular use calls for a half-duplex link -- connecting to a computer printer, for example -- Bluetooth can transmit up to 721 kilobits per second (Kbps) in one direction, with 57.6 Kbps in the other. If the use calls for the same speed in both directions, a link with 432.6-Kbps capacity in each direction can be made.

Specs

Here are some specification details from the Bluetooth Web site

• The devices in a piconet share a common communication data channel. The channel has a total capacity of 1 megabit per second (Mbps). Headers and handshaking information consume about 20 percent of this capacity.

In the United States and Europe, the frequency range is 2,400 to 2,483.5 MHz, with 79 1-MHz radio frequency (RF) channels. In practice, the range is 2,402 MHz to 2,480 MHz. In Japan, the frequency range is 2,472 to 2,497 MHz with 23 1-MHz RF channels.

• A data channel hops randomly 1,600 times per second between the 79 (or 23) RF channels. • Each channel is divided into time slots 625 microseconds long.

• A piconet has a master and up to seven slaves. The master transmits in even time slots, slaves in odd time slots.

• Packets can be up to five time slots wide.

• Data in a packet can be up to 2,745 bits in length.

• There are currently two types of data transfer between devices: SCO (synchronous connection oriented) and ACL (asynchronous connectionless).

• In a piconet, there can be up to three SCO links of 64,000 bits per second each. To avoid timing and collision problems, the SCO links use reserved slots set up by the master.

• Masters can support up to three SCO links with one, two or three slaves.

• Slots not reserved for SCO links can be used for ACL links.

• One master and slave can have a single ACL link.

• ACL is either point-to-point (master to one slave) or broadcast to all the slaves.

• ACL slaves can only transmit when requested by the master