3.1
Introduction
A common joke among IP telephony engineers is to say that if they had proposed to carry voice over IP a couple years ago, they would have been fired. This remains a private joke until you make your first IP phone call to someone on an old PC without a headset, to find out that the only person you heard was yourself (this is no longer true today … even PCs have software echo cancelers). Another way to find out why there really is a problem with IP telephony is to try a simple game: “collaborative counting”.
Collaborative counting has a simple rule: if you hear the person you talk to say V, you immediately say ‘n + 1′. In order to compare ‘classic’ telephony with IP telephony, you first make a regular phone call to someone you know and say ’1′, he goes ’2′, etc. Keep an eye on your watch and measure how long it takes to count to 25.
Then you make an IP phone call and play the same game. In all cases, it will take much longer .. .
The problems we have just emphasized, echo and delay, have been well known to telephone network planners since the early days of telephony, and today s telephone networks have been designed to keep these impairments imperceptible to most customers.
When carrying voice over IP, it becomes much more difficult to control echo, delay, and other degradations that may occur on a telephone line. As we will see, it will require state-of-the-art technology and optimization of all components to make the service acceptable to all customers.
However, once echo and delay are maintained within acceptable limits by proper network engineering, VoIP can use voice coders, such as G.722 (‘wide-band’ coder), which provide an absolute voice quality beyond that of current PSTN networks. It is frequently heard that VoIP can reach ‘toll quality’; in fact, in the future VoIP will provide a voice quality that exceeds ‘toll quality’ through rigorous planning and design.
