A-5
Cisco uBR10012 Universal Broadband Router Software Configuration Guide
OL-1520-05
AppendixA DOCSIS and CMTS Architectural Overview CMTS Traffic Engineering
Upstream signals are demodulated using Quadrature Phase Shift Keying (QPSK) or quadrature
amplitude modulation (QAM). QPSK carries information in the phase of the signal carrier, whereas
QAM uses both phase and amplitude to carry information.
Tip If your cable plant is susceptible to ingress or noise, Cisco recommends QPSK, based on the
importance of the data. Frequencies below 20MHz are more susceptible to noise and might require
lower symbol rates. Higher frequencies might be able to support higher rates and use QAM
modulation instead.

Overview of DOCSIS Two-Way Server Requirements

A TFTP server, DHCP server, and ToD server are required to support DOCSIS1.0-based CMs on the
network. A DOCSIS 1.0-based CM does not boot if these servers are not available.
Log server and security servers are not required to configure and operate a CM. If the log server or
security servers are not present, a CM generates warning messages, but continues to boot and function
properly.
ToD and TFTP servers are standard Internet implementations of the RFC 868 and RFC 1350
specifications. Most computers running a UNIX-based operating system, supply ToD and TFTP servers
as a standard software feature. Typically, the ToD server is embedded in the UNIX inetd and requires
no additional configuration. The TFTP server is usually disabled in the standard software, but can be
enabled by modifying the inetd.conf file. Microsoft NT server software includes a TFTP server that can
be enabled with the services control panel. Microsoft NT does not include a ToD server. A public domain
version of the ToD server for Microsoft NT can be downloaded from several sites. For configuration
information, refer to Chapter3, “Configuring Cable Interface Features for the CiscouBR10012Router.”
CMTS Traffic Engineering
Sending data reliably upstream is a critical issue. Designing a robust upstream architecture requires
balancing system parameters, establishing subscriber data requirements, and configuring the network to
support those requirements.
Upstream spectrum varies greatly between cable plants. Maintaining stable return paths also differs
based on varying patterns and levels of ingress noise and interference. Common problems in cable plants
include:
•Electrical and magnetic interference (EMI)
•Thermal noise
•Carrier to noise (C/N) imbalances
•Interference of leaking signals
•Ingress due to other channels appearing at the desired channel frequency
•Distortion due to non-linearities of cable equipment
•Cross modulation—carrier to frequency distortion
•Hum and low frequency distortion
•Improper RF amplifier tuning
•Non-unity gains due to incorrect usage of attenuators
•Low-quality subscriber equipment
•Out of range signal power from the CMTS to the CM