Stall The Ball

 

 

Introduction

As IP based networks gain popularity the logical decision to combine voice and data traffic is becoming more and more a no brainer. Hopefully we will explain here why VoIP is becoming the defacto standard and how it works. Where possible the article will use layman's terms but it is inevitable that some technical terminology is used.

VoIP what is it and how does it work

Voice traffic and data traffic are fundamentally different animals. Voice is an analogue signal such as a piano string vibrating whereas data is a digital technology similar to Morse code. However for many years now voice signals have been readily converted to digital, the ubiquitous CD is proof of that, so the real issue is more about how to transmit that information in real time. A 10 second delay in downloading a file isn't a real problem but a 10 second delay in a speech is annoying.

Voice telephony utilises technology called a “circuit-switched infrastructure”. When a call is placed you are given a dedicated line to the other number in much the same fashion that operators used to physically connect wires from one line to another line in old telephone exchanges. However in the digital era this happens automatically and the entire process converts your voice to a digital signal and effectively gives you your own connection direct to the other caller.

When you place a call, you are given 64kbs of bandwidth for the duration of the call, on a fixed line called a “channel”. Voice calls do not use the full bandwidth for several reasons. Firstly the line is a Full Duplex line. This means that it is capable of using 64kbs in both directions at once. However it is rare for two people to be speaking to each other at the same time typically one person speaks, the other listens and visa versa, unless you are married in which case the rules go out the window. Secondly there are times when no one is speaking, albeit for small durations i.e. between words, as one person stops and the other starts etc. All of these delays and quiet periods are still using the whole line. But the important point is that your speech has absolute priority over everything else and as soon as you speak the information is being sent in real time to the destination.

With VoIP networks, the conversion of voice also occurs in real-time; a process called packetisation. But there the similarities end. IP based data traffic is designed to allow packets of data to take separate routes take different lengths of time and resend lost packets.
To understand how this works imagine trying to post by ordinary mail a 26 volume encyclopaedia. You could put it all in one large box, a packet, but it would be very unwieldy and expensive. However a better way would be to send each volume in its own packet, address it, mark on it volume 1 or 2 etc. and post it.

The person receiving it will not necessarily receive the packets in the same order they were posted, packet 10 might arrive before packet 1 because the postman forgot one packet one day or another packet got routed through a different postal office etc. However it is quiet easy for the person receiving the encyclopaedia to wait until all the packets arrive and reassemble them using the numbers we put on the packets in the beginning. A no harm no foul situation. Computers do this all the time with data, collect packets, request missing packets to be resent and reassemble the packets into the correct order and present the completed product. With voice this is not a good thing. If VoIP has to wait several seconds for a missing packet before converting it to speech then you notice a delay a stutter. The busier the networks the worse the stuttering until you end up with unintelligible speech. Thankfully VoIP can use a few tricks to minimise this problem.

Firstly it can send several packets sequentially far faster than you can talk. In effect it can send redundant packets. Secondly it can utilise QoS (Quality of Service) technology to ensure that its packets get at least a minimum amount of bandwidth to ensure packets are received in a timely manner Think of paying extra for a priority service by using first class stamps.

So how does VoIP actually do all this stuff.

First it has to create a packet.
1 Voice signal is converted to a digital signal known as a stream.

2 The line echo is removed from the stream, silences removed, low level noise removed and any dialling tones analysed.

3 The resulting samples are converted to frames (packets without all the addressing details etc) and are compressed much like using WinZip. Typically a 10ms long frame contains 10Bytes of speech. It compresses the 128kbps (64kps each way) to 8kbps because we are not worried about absolute fidelity.

4 The voice frames are then transformed into voice packets by adding some useful information to the beginning and end of the frame such as how big the packet is, what is in the packet, the source address and the destination address.

5 The packet is sent through the Internet where routers and switches examine the destination address, and route and deliver the packet appropriately to the destination equivalent to the postman and post offices in ordinary mail terms.

6 When the destination receives the packet, the packet goes through the reverse process for playback.

The IP packets are numbered as they are created and sent to the destination address. The receiving end must reassemble the packets in their correct order (when they arrive out of order) to create the voice signal.

Sounds complicated and messy why would I use this

In a nutshell cost.
Once the hard work is done of converting the voice to data the difference between sending the data locally or to the far side of the world are insignificant. Telco's like it because it offers more efficient use of their network 8kbps rather than 64kbps. It runs on the same network as data so no need for two separate networks voice and data. You can setup your VoIP telephone number to route to anywhere in the world, collect it on a web server have voice messages emailed to you etc.

Usage in the real world.

Several months ago I signed up with blueface.ie for VoIP. For the extortionate amount of 9.99 EURO, yes that’s right nine euro ninety nine cents, I was given a VoIP number, offered to have my existing telephone number pointed at VoIP, 300 minutes of free calls, free calls to any other VoIP user, and rates like 1 cent per minute to Canada, 1.6 cent per minute to USA.

The important bit in there is FREE calls to other VoIP users. Free as in costs nothing. And this is where broadband truly comes to the fore. If you use a dial-up account, you are paying roughly 4 cent per minute in peak times plus say 1 cent per minute to Canada so the call costs 5 cents per minute. However with broadband you pay a fixed price per month for broadband and your calls are 100% free to other VoIP users and 1 cent a minute to Canada.

But there’s more. If you use a wireless broadband connection from the likes of IBB or Last Mile then there is no real reason to have a telephone line. Ring up eircom or BT Ireland to cancel your line and save yourself the cost of line rent every month. Suddenly VoIP starts to make a lot of financial sense.

And if you don’t want to use your computer to make phone calls, no problem. Buy a box called a VoIP router typically 90 euro new or 50 euro on eBay. It connects to you broadband connection and plug an ordinary telephone handset into the box. It does all the complicated stuff all you have to do is pick up the handset and dial as normal. Did I mention that calls to other VoIP users are free.