Cisco Model D-PCG1000 PowerKEY CAS Gateway Installation Guide
Chapter 1 Introducing the DAVIC QPSK Demodulator
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System Overview
Introduction
The QPSK demodulator is an integral component of the Cisco DBDS. The QPSK
demodulator works with QPSK modulators and Digital Home Communication
Terminals (DHCTs) to provide a forward signaling and reverse communications
path for interactive two-way video and data services.
The Modulating/Demodulating Process
The QPSK modulator initiates and controls configuration and setup through the
QPSK forward path. The QPSK modulator splits messages into Asynchronous
Transfer Mode (ATM) cells, formats the messages in DAVIC-compliant frames, adds
QPSK modulation, and then transmits the messages to the DHCT at a rate of 1.544
Mbps. After the DHCTs are configured, all control and status information travels
through the QPSK forward path, while all video and audio sources are carried by
high-bandwidth Quadrature Amplitude Modulation (QAM) channels to the DHCT.
The QPSK demodulator receives the messages that originate from a DHCT, such as a
The QPSK demodulator receives the messages that originate from a DHCT, such as a
request for a service, on a 1.544 Mbps reverse-path channel. The QPSK demodulator
demodulates the incoming QPSK signal, performs error correction on the detected
data, and transmits the message as an ATM packet to the QPSK modulator through
an ATM-25 interface that operates at 18.5 Mbps and uses RJ-45 connectors.
You can connect up to eight QPSK demodulators to one QPSK modulator, so the
You can connect up to eight QPSK demodulators to one QPSK modulator, so the
maximum sustained input rate to a modulator will be eight times the 1.544 Mbps
rate, or approximately 12.4 Mbps to the DAVIC router function of the QPSK
modulator.
The QPSK modulator receives the ATM cells and uses the slot number information
The QPSK modulator receives the ATM cells and uses the slot number information
inserted by the QPSK demodulator in the ATM cells, along with the demodulator
port number (for example, reverse channel number) to create a “success feedback”
word to acknowledge or confirm receipt to the DHCT. These words generate the
“acknowledge bits.” The DHCT needs these bits to determine whether its cell was
received successfully. ATM cells from DHCTs are routed to the main memory of the
modulator, where complete messages are reassembled. The modulator processes
these reassembled messages as a part of its Media Access Control (MAC) functions.
The QPSK modulator serves as a DAVIC Router by implementing the DAVIC MAC
functions, and by communicating signaling and status information back to the
DNCS through an Ethernet/IP connection.