Page 210 - Asterisk™: The Future of Telephony

SONET was created in an effort to standardize optical circuits, but due to its high cost,
coupled with the value offered by many newer schemes, such as Dense Wave Division
Multiplexing (DWDM), there is some controversy surrounding its future.

Digital Signaling Protocols

As with any circuit, it is not enough for the circuits used in the PSTN to just carry (voice)
data between endpoints. Mechanisms must also be provided to pass information about
the state of the channel between each endpoint. (Disconnect and answer supervision
are two examples of basic signaling that might need to take place; Caller ID is an ex-
ample of a more complex form of signaling.)

Channel Associated Signaling (CAS)
Also known as robbed-bit signaling, CAS is what you will use to transmit voice on a
T1 when ISDN is not available. Rather than taking advantage of the power of the digital
circuit, CAS simulates analog channels. CAS works by stealing bits from the audio
stream for signaling purposes. Although the effect on audio quality is not really no-
ticeable, the lack of a powerful signaling channel limits your flexibility.

When configuring a CAS T1, the signaling options at each end must match. E&M (Ear
& Mouth or recEive & transMit) signaling is generally preferred, as it offers the best
supervision. Having said that, in an Asterisk environment the most likely reason for
you to use CAS would be for a channel bank, which means you are most likely going
to have to use FXS signaling.
CAS is very rarely used on PSTN circuits anymore, due to the superiority of ISDN-PRI.
One of the limitations of CAS is that it does not allow the dynamic assignment of
channels to different functions. Also, Caller ID information (which may not even be
supported) has to be sent as part of the audio stream. CAS is commonly used on the
T1 link in channel banks.

ISDN
The Integrated Services Digital Network (ISDN) has been around for more than 20
years. Because it separates the channels that carry the traffic (the bearer channels, or
B-channels) from the channel that carries the signaling information (the D-channel),
ISDN allows for the delivery of a much richer set of features than CAS. In the beginning,
ISDN promised to deliver much the same sort of functionality that the Internet has
given us, including advanced capabilities for voice, video, and data transfer.
Unfortunately, rather than ratifying a standard and sticking to it, the respective tele-
communications manufacturers all decided to add their own tweaks to the protocol,
in the belief that their versions were superior and would eventually dominate the mar-
ket. As a result, getting two ISDN-compliant systems to connect to each other was often
a painful and expensive task. The carriers who had to implement and support this

182 | Chapter 7: Understanding Telephony