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Circuit Switching and Packet Switching - VoIP and PSTN

Updated on March 22, 2011

A tale of two technologies

The entire IT world is abuzz with the way VoIP is set to become a major competitor to the regular telephone network. Indeed, many businesses have long since made the transition and are using VoIP in one way or the other. So in some sense, the battle is already lost for PSTN. The remaining clean up is only due to the problems (mostly artificial) of inter connectivity between various VoIP providers.

But what is it that really differentiates VoIP from PSTN lines at the core? No - it's not the Internet. The Internet itself is build upon a technology that is a fundamentally different way of communicating compared to the phone network. The real difference lies between what we call circuit switching and packet switching.

One doesn't have to be a techie to understand these two terms and appreciate the difference between them. In this article, I'll attempt to explain in a simple way what the two terms mean and what the implications are for VoIP and PSTN. Each has advantages and disadvantages over the other. However, the landscape of technology has changed dramatically over the last few decades and this has given the edge to one more than it has to the other.

A map of the Internet
A map of the Internet

Circuit Switching and Packet Switching

Circuit switching is the term used when a dedicated channel is created between to elements which want to communicate. You can think of it as a "string" that links the two nodes of a network. While the communication is in progress, no one else can use that particular string which is reserved entirely for the sending and receiving of messages between the participants. Of course, there's no actual "string," but there can be a dedicated path established electronically.

The good thing about circuit switching is that once a connection has been established, you can easily send data over the line without any additional information about who it's supposed to reach etc. The lack of "overhead" information leads to an excellent optimization of the data when the line is in use. In addition, there's no need for the data to try and figure out how to reach. The route is set and there's no more to it.

The problem arises when the line is lying idle as it does for quite a lot of time when two people talk to each other. This leads to a large amount of wastage which led some to wonder if there might not be a better way to go about it. Enter packet switching

In packet switching, each "packet" of information is separate and has to find its own way to the other end. This means that a single network can accommodate many nodes talking over it at the same time since there's no dedicated line for any two of them. With this, we get the optimum utilization of bandwidth. The disadvantage is that each packet has a little extra overhead to identify it. Moreover, packets can arrive at different times and more processing is needed to ensure that they're transmitted sequentially at the other end. Packet switching is far more flexible and this is the strength of the Internet in general.

And there we have it. The core of the difference between PSTN and VoIP. Circuit switching and packet switching in a nutshell.

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