Voice over IP

How does a VoIP call work?

So how does VoIP work? VoIP is largely dependent on a protocol called SIP (Session Initiation Protocol). The idea behind SIP is to provide a simple, lightweight means for creating and ending connections for real-time interactive communications over IP networks -- mainly for voice, but also for videoconferencing, chat, gaming or even application sharing. In other words, the protocol initiates call setup, routing, authentication and other communication features to endpoints within an IP domain.

VoIP is quite similar to e-mail. That is, you have the Internet, a server and a client. If users want to check their e-mail they would have to register with their e-mail server and download mail from that server. If they send an e-mail that also goes via that mail server. Nor do users have to be tied to one location -- they can send and receive mail while travelling. As long as they can connect to the Internet, they can use the service.

VoIP is exactly like that. There is a server, in this case a SIP proxy server or a softswitch. These are both the same -- software applications running on general purpose computing. In the first instance, the client (ATA, VoIP phone or soft phone) will register with a user's SIP server. From then on, when a caller picks up the phone to make a phone call, because the ATA has a current registration, there will be dial tone.

This works because the first thing an ATA does when it boots up is get a DHCP address so it has an address on the Internet, and then the DHCP server points to a DNS server on the Internet and it will use that to resolve this unique server and it will register with the service provider to say "hey, I am alive".

As a plug-and-play experience, this whole DHCP event is important. For most ATAs on the market, users do not need a computer to make it work. All that is required is for the ATA to be powered up from the wall and plugged into a phone.

At the boot-up process, the ATA sends a SIP message (an invite) to the SIP register (softswitch) which is maintained by a user's VoIP service provider. The softswitch says "OK". From then on, there is a two-way communication which happens every 60 seconds, in much the same way as a heartbeat. The softswitch does a lot. It is constantly listening to thousands of people, even if they are not making calls.

VoIP service providers usually charge by the minute. Ensuring a call is routed efficiently and cheaply also lies with the softswitch.

Once the softswitch determines where the subscriber is, it does a thing called Least Cost Routing (LCR). It works out the least-cost path to get to its endpoint by using algorithms inside the softswitch and by provisioning. Provisioning is the process of entering subscriber information into the softswitch. This is used by the LCR algorithms, so really it's the softswitch's knowledge of the network that allows it to do LCR. For example, if someone dials Europe from a Sydney suburb, it will state the best path to that final destination. All of this is transparent to the end user.

Once the softswitch finds the least cost and sends it on to the gateway (also owned by the VoIP service provider), that gateway will do an IP to TDM (Time Division Multiplexing is the language of the POTS) conversion. It will then send it on to the PSTN via a protocol called SS7, which is the signalling system for the PSTN. In other words, SS7 is the language which telcos use to talk to each other. Once in the PSTN "cloud" it will eventually be routed to a phone number and that phone will then ring.

Join the Good Gear Guide newsletter!

Error: Please check your email address.

Our Back to Business guide highlights the best products for you to boost your productivity at home, on the road, at the office, or in the classroom.

Keep up with the latest tech news, reviews and previews by subscribing to the Good Gear Guide newsletter.

PC World Staff

PC World
Show Comments

Cool Tech

Crucial Ballistix Elite 32GB Kit (4 x 8GB) DDR4-3000 UDIMM

Learn more >

Gadgets & Things

Lexar® Professional 1000x microSDHC™/microSDXC™ UHS-II cards

Learn more >

Family Friendly

Lexar® JumpDrive® S57 USB 3.0 flash drive 

Learn more >

Stocking Stuffer

Plox Star Wars Death Star Levitating Bluetooth Speaker

Learn more >

Christmas Gift Guide

Click for more ›

Most Popular Reviews

Latest News Articles


GGG Evaluation Team

Kathy Cassidy


First impression on unpacking the Q702 test unit was the solid feel and clean, minimalist styling.

Anthony Grifoni


For work use, Microsoft Word and Excel programs pre-installed on the device are adequate for preparing short documents.

Steph Mundell


The Fujitsu LifeBook UH574 allowed for great mobility without being obnoxiously heavy or clunky. Its twelve hours of battery life did not disappoint.

Andrew Mitsi


The screen was particularly good. It is bright and visible from most angles, however heat is an issue, particularly around the Windows button on the front, and on the back where the battery housing is located.

Simon Harriott


My first impression after unboxing the Q702 is that it is a nice looking unit. Styling is somewhat minimalist but very effective. The tablet part, once detached, has a nice weight, and no buttons or switches are located in awkward or intrusive positions.

Featured Content

Latest Jobs

Don’t have an account? Sign up here

Don't have an account? Sign up now

Forgot password?