How Does VoIP Allow Calls Between IP Networks and PSTN?

How These Two Technologies Make Calls Happen

Businessman dialing telephone in office
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With VoIP, you use an IP network such as the Internet, through an ADSL or other Internet connection, to make/receive phone calls between VoIP service but also to/from PSTN landline networks. For instance, you can use your VoIP service to make calls to landline and mobile numbers that are out of the fold of IP networks. An example is using Skype to call a fixed line. The Internet and the PSTN line works in very different ways. One is analog and one is digital. Another big difference is the way data is transferred. VoIP on the Internet uses packet switching while the PSTN uses circuit switching. Here is how the communication between these two different systems works in very different ways. One is analog and one is digital. Another big difference is the way data is transferred. VoIP on the Internet uses packet switching while the PSTN uses circuit switching. Here is how the communication between these two different systems works. 

Address Translation

The answer lies in one term: address translation. It is a mapping done between different types of addressing. On the one hand, there is the VoIP service using the Internet on which each device is identified by an IP address. On the other hand, each phone on the PSTN number is identified by a phone number. The handshaking takes place between these two addressing elements. 

In VoIP, every phone number has an IP address to which it maps. Each time a device (PC, IP phoneATA etc.) engages in a VoIP call, its IP address is translated into the phone number, which is then handed over to the PSTN network. This is analogous to the way web addresses (domain names) and email addresses are mapped to IP addresses.

In fact, when you register for a service that offers the type of service (VoIP to PSTN or mobile), you are given a phone number. This number is your handle to and from the system. You can even choose a number in a given location so as to cut down the cost. For example, if your pool of correspondence is found in New York, you will want to have a number in that region. You can also port your existing number to your VoIP service, such that people who know you can still all you through the number they know without you having to notify everyone of a change in contact details. 

The Cost

The cost of a call between VoIP and the PSTN is in two parts. There is the VoIP-VoIP part, which takes place on the Internet. This part is generally free and does not depend on the duration of the call. The actual cost for this part is in the investment on technologies, space, server functionalities etc., which is shared over time and by users and is therefore negligible for the user. 

The second part is the part where the call continues as soon as it leaves the IP network and transits to the plain old telephone line. Circuit switching takes place here, and the circuit is dedicated throughout the duration of the call. This is the part you pay for, hence the per-minute rates. It is much cheaper than traditional telephony since a lot of it takes place on the Internet. Some destinations remain expensive due to factors like poor network dealings, poor underlying hardware, and technology, remoteness etc.