Specifications
Configure T1 CAS Voice Ports
Voice over IP for the Cisco AS5300 21
Configure T1 CAS Voice Ports
CAS is the transmission of signalling information within the voice channel. Various types of CAS
signalling are available in the T1 world. The most common forms of CAS signalling are loop-start,
ground-start, and E&M. The main disadvantage of CAS signalling is its use of user bandwidth to
perform signalling functions. CAS signalling is often referred to as robbed-bit signalling because
user bandwidth is being “robbed” by the network for other purposes. In addition to receiving and
placing calls, CAS signalling processes the receipt of DNIS and ANI information, which is used to
support authentication and other functions.
T1 CAS capabilities have been implemented on the Cisco AS5300 VFC to enhance and integrate T1
CAS capabilities on common central office (CO) and PBX configurations for voice calls. The
service provider application for T1 CAS includes connectivity to the public network using T1 CAS
from the Cisco AS5300 to the end office switch. In this configuration, the Cisco AS5300 captures
the dialed-number or called-party number information and passes it along to the upper level
applications for interactive voice response (IVR) script selection, modem pooling, and other
applications. Service providers also require access to calling party number, ANI, for user
identification, for billing account number, and in the future, for more complicated call routing.
Service providers who implement VoIP include traditional voice carriers, new voice and data
carriers, and existing Internet service providers. Some of these service providers might use
subscriber side lines for their VoIP connectivity to the PSTN; others might use tandem-type service
provider connections.
T1 CAS Signalling Systems
Voice over IP for the AS5300 supports the following T1 CAS signalling systems:
• E&M—E&M signalling is typically used for trunks. It is normally the only way that a CO switch
can provide two-way dialing with direct inward dialing. In all the E&M protocols, off-hook is
indicated by A = B = 1, and on-hook is indicated by A = B = 0. If dial pulse dialing is used, the
A and B bits are pulsed to indicate the addressing digits. There are several further important
subclasses of E&M robbed-bit signalling:
— E&M Wink Start—Feature Group B
In the original Wink Start protocol, the terminating side responds to an off-hook from the
originating side with a short wink (transition from on-hook to off-hook and back again). This
wink tells the originating side that the terminating side is ready to receive addressing digits.
After receiving addressing digits, the terminating side then goes off-hook for the duration of
the call. The originating endpoint maintains off-hook for the duration of the call.
— E&M Wink Start—Feature Group D
In Feature Group D Wink Start with Wink Acknowledge protocol, the terminating side
responds to an off-hook from the originating side with a short wink (transition from on-hook
to off-hook and back again) just as in the original Wink Start. This wink tells the originating
side that the terminating side is ready to receive addressing digits. After receiving addressing
digits, the terminating side then provides another wink (called an Acknowledgment Wink)
that tells the originating side that the terminating side has received the dialed digits. The
terminating side then goes off-hook to indicate connection when the ultimate called endpoint
has answered. The originating endpoint maintains off-hook for the duration of the call.