Page 59 - Asterisk™: The Future of Telephony

phone is different is that inside the telephone, the analog signal is sampled and con-
verted into a digital signal—that is, a numerical representation of the analog waveform.
We’ll leave a detailed discussion of digital signals until Chapter 7; for now, suffice it to
say that the primary advantage of a digital signal is that it can be transmitted over
limitless distances with no loss of signal quality.
The chances of anyone ever making a proprietary digital phone directly compatible
§
with Asterisk are slim, but companies such as Citel (http://www.citel.com) have created
gateways that convert the proprietary signals to Session Initiation Protocol (SIP). ‖

ISDN telephones
Prior to VoIP, the closest thing to a standards-based digital telephone was an ISDN-
BRI terminal. Developed in the early 1980s, ISDN was expected to revolutionize the
telecommunications industry in exactly the same way that VoIP promises to finally
achieve today.

There are two types of ISDN: Primary Rate Interface (PRI) and
Basic Rate Interface (BRI). PRI is commonly used to provide trunking
facilities between PBXes and the PSTN, and is widely deployed all over
the world. BRI is not at all popular in North America, but is common
in Europe.

While ISDN was widely deployed by the telephone companies, many consider the
standard to have been a flop, as it generally failed to live up to its promises. The high
costs of implementation, recurring charges, and lack of cooperation among the major
industry players contributed to an environment that caused more problems than it
solved.
BRI was intended to service terminal devices and smaller sites (a BRI loop provides two
digital circuits). A wealth of BRI devices have been developed, but BRI has largely been
deprecated in favor of faster, less expensive technologies such as ADSL, cable modems,
and VoIP.
BRI is still very popular for use in video-conferencing equipment, as it provides a fixed
bandwidth link. Also, BRI does not have the type of quality of service issues a VoIP
connection might, as it is circuit-switched.

§ Citel has produced a fantastic product that is limited by the fact that it is too expensive. If you have old
proprietary PBX telephones, and you want to use them with your Asterisk system, Citel’s technology can do
the job, but make sure you understand how the per-port cost of these units stacks up against replacing the
old sets with pure VoIP telephones.
‖ The SIP is currently the most well-known and popular protocol for VoIP. We will discuss it further in
Chapter 8.

Types of Phones | 31