A-6
Cisco uBR10012 Universal Broadband Router Software Configuration Guide
OL-1520-05
AppendixA DOCSIS and CMTS Architectural Overview
CMTS Traffic Engineering
When configuring your system, configure downstream and upstream parameters based on the fiber nodes
involved, the required services the CM or STB supports, the importance of the data, and desired
performance capabilities.
Your cable plant determines its data performance. Design your network to maximize its performance and
capacity at minimum cost, while meeting subscriber data requirements. Select or customize upstream
profiles for maximum trade-offs between bandwidth efficiency and upstream channel robustness once
you are familiar with the system and have characterized your network. For example, QAM-16 requires
approximately 7 dB higher C/N ratio to achieve the same bit error rate (BER) as QPSK, but it transfers
information at twice the rate of QPSK.
Note Older plants and plants with long amplifier cascades are more susceptible to ingress than
newer plants. These plants produce more noise and signal level variances.
Tips Cisco recommends you keep input to all amplifiers at the same power level in the upstream
direction and keep output of all amplifiers in the downstream direction at the same power level.
This is called unity gain. Tune amplifiers and other equipment properly at desired frequencies. To
characterize and improve your cable plant’s stability, follow procedures in the Cisco uBR10000
Series Universal Broadband Router Hardware Installation Guide on Cisco.com.
A DOCSIS cable plant has the following groups of traffic to size based on current service offerings:
•Basic Internet access data, which is asymmetrical; asymmetrical traffic supports a larger data rate
in one direction—the downstream.
•VoIP traffic, which requires constant bandwidth, has low tolerance to latency and jitter, and is
typically symmetrical—supporting the same data rate in downstream and upstream directions. VoIP
generally requires phase-lock and jitter attenuation.
•VPN traffic, which requires secure transmissions; traffic is typically symmetrical since
telecommuters exchange more data upstream than residential Internet access customers.
•Video, which can include digital video channels based on the services in your network.
•Signaling and maintenance—the DOCSIS MAC layer support includes DOCSIS encapsulation,
initial maintenance, station maintenance, registration, frequency hop, and upstream channel
changes.
You have a wide range of options to engineer your network. Define your network based on your cable
facilities—headend or distribution hub—and your anticipated service offerings, subscription, and
required service levels. Define data requirements relative to the number of subscribers to support and
their usage patterns. Select upstream symbol rate, modulation format, and other parameters based on
data requirements and return path characterizations.
If the service is asymmetrical, determine the ratio of downstream to upstream data rates. For basic
Internet access where the majority of traffic is sent to a subscriber and the subscriber sends only a small
amount of data upstream, use ratios ranging from 5:1 to 10:1.
Determine what data rate the service should support. Define the maximum and minimum data rate,
answering the following questions. Do you want to define the minimum data rate relative to the
maximum? Will the minimum data rate equal the maximum? Will it be a percentage of the maximum?
Will the minimum data rate be zero?