Essay Preview: Voip
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CONTENTS
Introduction: What is VoIP?
Circuit Switching
Packet Switching
Packet Switching in VoIP
Protocols
Setting up VoIP
Calling
Why was Internet telephony illegal?
VoIP limitations and solutions
VoIP- The Future
1. Introduction: What is VoIP?
VoIP stands for Voice over Internet Protocol, popularly known as IP telephony. As the term says VoIP tries to let go voice (mainly human) through IP packets and, in definitive through Internet. Thus 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.
Let us learn about VoIP and the technology that makes it possible. Well talk about VoIPs major protocols, about the various services provided and the low-cost, often free software that allows you to take advantage of them.
But first, lets discuss the fundamental problem with existing telephone networks — namely, their reliance on circuit switching.
2. 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).
Heres 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 partys 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.
Lets 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), 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 its 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. Thats no way to set up an efficient data network. Instead, data networks use a method called packet switching.
3. 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.
4. PACKET SWITCHING IN VOIP
VoIP technology uses this packet-switching method to provide several advantages over circuit switching. In VoIP, analog voice signal is digitized using PCM. These digital voice samples are then buffered on an IP gateway. This device converts the PCM data stream into a compressed IP packet stream using
DSPs (Digital Signal Processors). DSPs are responsible for converting from analog to digital as well as compression. The set of PCM samples are analyzed as a discrete set of binary data. It checks the speech for all the moments of silence, which are a lot. Even when we speak, there are pauses in between that go unnoticed to the human ear, but are quite discernible to the sampling device. The length