Information Technology Reference
In-Depth Information
Note: Although the values in each time slot are shown in decimal in Figure 1-10, they are
actually transmitted and interpreted in binary.
Corporations use digital voice connections to the PSTN as T1 circuits in the United
States, Canada, and Japan. A T1 circuit is built from 24 separate 64-kbps channels known
as a digital signal 0 (DS0). Each one of these channels is able to support a single voice call.
Corporations in areas outside the United States, Canada, and Japan use E1 circuits, which
allow you to use up to 30 DS0s for voice calls.
Although digital technology solves the problems of signal degradation and the inability to
send multiple calls over a single line that occur in analog technology, it creates a new is-
sue: signaling. With analog circuits, supervisory signals were passed by connecting the tip
and ring wires together. The phone company generated informational and address signals
through specific frequencies of electricity. By solving the problems associated with analog
signaling, digital signaling also removed the typical signaling capabilities. To solve this,
two primary styles of signaling were created for digital circuits:
Channel associated signaling (CAS): Signaling information is transmitted using
the same bandwidth as the voice.
Key
To p i c
Common channel signaling (CCS): Signaling information is transmitted using a
separate, dedicated signaling channel.
The following sections discuss these two styles of signaling.
Channel Associated Signaling
T1 digital connections that use CAS actually “steal” binary bits that would typically have
been used to communicate voice information and use them for signaling. Initially, this may
seem crazy; if you take the binary bits that are used to resynthesize the voice, won't the
voice quality drop significantly? Although the voice quality does drop some, the number
of binary bits stolen for signaling information is small enough that the change in voice
quality is not noticeable.
Note: Because T1 CAS steals bits from the voice channel to transfer signaling informa-
tion, it is often called robbed bit signaling (RBS).
The voice device running the T1 line uses the eighth bit on every sixth sample in each T1
channel (DS0). Figure 1-8 illustrates this concept.
As you can see from Figure 1-8, the 24 channels of the digital T1 circuit carry only voice
data for the first five frames that they send. On the sixth frame (marked with an S in
Figure 1-11), the eighth bit (also called the least significant bit) is stolen for the voice de-
vices to transmit signaling information. This process occurs for every sixth frame after
this (12 th , 18 th , 24 th , and so on). This stolen bit relays the signaling information for each re-
spective DS0 channel. For example, the bits stolen from the third DS0 channel relay the
signaling information only for that channel.
 
 
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