System information

of suffering later. Echo problems are not pleasant at all, and your users will hate the
system if they experience it.
Several software echo cancellers have recently become available. We have not had a
chance to evaluate any of them, but we know that they employ the same algorithms
the hardware echo cancellers do. If you have a recently purchased Digium analog card,
you can call Digium sales for a keycode to allow its latest software echo canceller to
work with your system.
†
There are other software options available for other types of
cards, but you will have to look into whether you have to purchase a license to use
them.
‡
Keep in mind that there is a performance cost to using software echo cancellers.
They will place a measurable load on the CPU that needs to be taken into account when
you design a system using these technologies.
For more on the topic of echo cancellation, see Appendix B.
Types of Phones
We all know what a telephone is—but will it be the same five years from now? Part of
the revolution that Asterisk is contributing to is the evolution of the telephone, from a
simple audio communications device into a multimedia communications terminal pro-
viding all kinds of yet-to-be-imagined functions.
As an introduction to this exciting concept, we will briefly discuss the various kinds of
devices we currently call “telephones” (any of which can easily be integrated with
Asterisk). We will also discuss some ideas about what these devices may evolve into in
the future (devices that will also easily integrate with Asterisk).
Physical Telephones
Any physical device whose primary purpose is terminating an on-demand audio com-
munications circuit between two points can be classified as a physical telephone. At a
minimum, such a device has a handset and a dial pad; it may also have feature keys, a
display screen, and various audio interfaces.
† This software is not part of a normal Asterisk download because Digium has to pay to license it separately.
Nevertheless, it has grandfathered it into all of its cards, so it is available for free to anyone who has a Digium
analog card that is still under warranty. If you are running a non-Digium analog card, you can purchase a
keycode for this software echo canceller from Digium’s website.
‡ Sangoma also offers free software echo cancellation on its analog cards (up to six channels).
Types of Phones | 657