System information

eBay. Look for units from Adtran and Carrier Access Corp. (Rhino makes great channel
banks, and they are very competitively priced, but they may be hard to find used.) Don’t
forget that you will need a T1 card in order to connect a channel bank to Asterisk.
Other types of PSTN interfaces
Many VoIP gateways exist that can be configured to provide access to PSTN circuits.
Generally speaking, these will be of most use in a smaller system (one or two lines).
They can also be very complicated to configure, as grasping the interaction between
the various networks and devices requires a solid understanding of both telephony and
VoIP fundamentals. For that reason, we will not discuss these devices in detail in this
book. They are worth looking into, however; popular units are made by Sipura, Grand-
stream, Digium, and many other companies.
Another way to connect to the PSTN is through the use of Basic Rate Interface (BRI)
ISDN circuits. BRI is a digital telecom standard that specifies a two-channel circuit that
can carry up to 144 Kbps of traffic.
*
Due to the variety of ways this technology has been
implemented, and a lack of testing equipment, we will not be discussing BRI in very
much detail in this book.
Connecting Exclusively to a Packet-Based Telephone Network
If you do not need to connect to the PSTN, Asterisk requires no hardware other than
a server with a Network Interface Card. However, you still may need to install the
DAHDI kernel modules, as DAHDI is required for using the MeetMe() application for
conferencing.
Echo Cancellation
One of the issues that can arise if you use analog interfaces on a VoIP system is echo.
Echo is simply what you say being reflected back to you a short time later. The echo is
caused by the far end, but you are the one that hears it. It is a little-known fact that
echo would be a massive problem in the PSTN were it not for the fact that the carriers
employ complex (and expensive) strategies to eliminate it. We suggest that you consider
adding echo-cancellation hardware to any card you purchase for use as a PSTN inter-
face. 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
* BRI is very rarely used in North America but is very popular in Europe, and Digium has produced the B410P
card to address this need.
656 | Appendix C: Preparing a System for Asterisk