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cancellation hardware to any card you purchase for use as a PSTN interface. While
Asterisk can do some work with echo in software, it does not provide nearly enough
power to deal with the problem. Also, echo cancellation in software imposes a load on
the processor; hardware echo cancellers built into the PSTN card take this burden away
from the CPU.
Hardware echo cancellation can add several hundred dollars to your equipment cost,
but if you are serious about having a quality system, invest the extra money now instead
of suffering later. Echo problems are not pleasant at all, and your users will hate the
system if they experience it.
As of this writing, several software echo cancellers have become available. We have not
had a chance to evaluate any of them, but we know that they employ the same algory-
thems 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 measureable load on the CPU that needs to be taken into account
when you design a system using these technologies.
Types of Phones
Since the title of this book is Asterisk: The Future of Telephony, we would be remiss if
we didn’t discuss the devices that all of this technology ultimately has to interconnect:
telephones!
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 As-
terisk). We will also discuss some ideas about what these devices may evolve into in
the future (devices that will also easily integrate with Asterisk).
† 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 web site.
‡ Sangoma also offers free software echo cancellation on their analog cards (up to six channels).
Types of Phones | 29