Information Technology Reference
In-Depth Information
Frame 1:
…
F
1
st
DS0
2
nd
DS0
3
rd
DS0
4
th
DS0
24
th
DS0
Frame 2:
…
F
1
st
DS0
2
nd
DS0
3
rd
DS0
4
th
DS0
24
th
DS0
Frame 3:
1
st
DS0
2
nd
DS0
3
rd
DS0
4
th
DS0
…
24
th
DS0
F
Frame 4:
1
st
DS0
2
nd
DS0
3
rd
DS0
4
th
DS0
…
24
th
DS0
F
Frame 5:
…
F
1
st
DS0
2
nd
DS0
3
rd
DS0
4
th
DS0
24
th
DS0
Frame 6:
1
st
DS0
S
2
nd
DS0
S
3
rd
DS0
S
4
th
DS0
S
…
24
th
DS0
S
F
0101101 0
1010111 0
0011010 0
0110110 0
1101010 0
Signaling Bit
T1 Framing Bit
Figure 1-8
CAS T1 Signaling Bits
Common Channel Signaling
CCS dedicates one of the DS0 channels from a T1 or E1 link for signaling information.
This is often called out-of-band signaling because the signaling traffic is sent completely
separate from the voice traffic. As a result, a T1 connection using CCS has only 23 usable
DS0s for voice. Because CCS dedicates a full channel of the circuit for signaling, the
“stolen bit” method of signaling using ABCD bits is no longer necessary. Rather, a full sig-
naling protocol sends the necessary information for all voice channels. The most popular
signaling protocol used is Q.931, which is the signaling protocol used for ISDN circuits.
CCS is the most popular connection used between voice systems worldwide because it
offers more flexibility with signaling messages, more bandwidth for the voice bearer chan-
nels, and higher security (because the signaling is not embedded in the voice channel).
CCS also allows PBX vendors to communicate proprietary messages (and features) be-
tween their PBX systems using ISDN signaling, whereas CAS does not offer any of these
capabilities.
Tip:
When using CCS configurations with T1 lines, the 24
th
time slot is always the signal-
ing channel. When using CCS configurations with E1 lines, the 17th time slot is always the
signaling channel.
Key
To p i c
Note:
Although ISDN is the most popular protocol used with CCS configurations, CCS
can use other protocols. For example, telephone companies use the Signaling System 7
(SS7) protocol (described later) with CCS configurations to communicate between COs.
