Information Technology Reference
In-Depth Information
Delay dial:
In this technique, the originating side goes off-hook, waits for about 200
ms, and then checks whether the far end is on-hook. If the far end is on-hook, it out-
puts dial digits. If the far end is off-hook, it waits until it goes on-hook and then out-
puts dial digits.
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CAS and CCS Signaling
Digital signaling has two major forms: Channel Associated Signaling (CAS) and Common
Channel Signaling (CCS). The major difference is that with CAS the signaling is included in
the same channel as the voice call. With CCS, the signaling is provided in a separate chan-
nel. Table 14-2 shows the common types of CAS and CCS. They are covered in the follow-
ing sections.
Key
To p i c
Ta b l e 1 4 - 2
Common CAS and CCS Signaling Types
Signaling
Signaling/Circuit Type
CAS T1 or E1 signaling
CCS ISDN PRI or BRI
QSIG
SS7
T1/E1 CAS
Digital T1 CAS uses selected bits within each channel to transmit signaling
information. CAS is also called robbed-bit signaling or in-band signaling in the T1
implementation. Robbed-bit CAS works with digital voice because losing an occasional
voice sample does not affect the voice quality. The disadvantage of robbed-bit CAS is that
it cannot be used on channels that might carry voice or data without reducing the data
rate to 56 Kbps to ensure that signaling changes do not damage the data stream. Because
of the implementation of signaling via a limited number of bits, CAS signaling is limited in
signaling feat ure s uppor t.
E1 CAS uses a separate channel in the shared medium for CAS, so it does not have this
disadvantage. The E1 signaling bits are channel associated, but they are not in-band.
CCS
CCS circuits set aside one channel specifically for carrying signaling information
for all the other channels. This architec t ure allow s for CC S c irc uits to prov ide a more
robust feature set between switches. ISDN PRI uses CCS signaling and is the preferred
connection type for PSTN-to-PBX or PBX-to-PBX connections.
ISDN PRI/BRI
ISDN T1 PRI provides 23 64-kbps B (bearer) channels for voice, with a
separate 64-kbps D (data signaling) channel for signaling. The ISDN E1 PRI provides 30 B
channels. The use of messages in a separate channel, rather than preassigned bits, is also
called Common Channel Signaling. ISDN provides the advantage of not changing bits in
the channels and thus is useful for data traffic in addition to voice traffic.
The ISDN BRI interface includes two 64-kbps B channels for voice or data and a separate
16-kbps D channel that provides signaling for the interface.
Q.SIG
Q.SIG is the preferred sig naling protocol used bet ween PBX sw itches. It is a
