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Cisco uBR10012 Universal Broadband Router Software Configuration Guide
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AppendixA DOCSIS and CMTS Architectural Overview CMTS Traffic Engineering
Note The minimum data rate has the greatest impact on the network. The network must be sized
to accommodate this level of traffic to fulfill the defined service data requirements. The
amount of bandwidth available to a group of subscribers establishes where, within the
defined maximum and minimum data rates, a subscriber within a group is able to operate.
For video traffic planning purposes, use a typical bit rate to calculate densities of video streams within
a channel. For QoS calculations, limit the number of video streams per channel to prevent packet drops.
The key traffic parameter is how many IP video streams will fit into the RF channel.
Ideally, the network is sized so that it supports all subscribers being active at the same time at the
maximum data rate. This results in an expensive network, however, where full capacity, particularly for
residential subscribers, is rarely used. Cisco recommends designing your network to support a given
level of over-subscription.
Note Configure your network to support a percentage of all subscribers at a given data rate. At
this level, the network supports the bandwidth needs of all active users. Provided the
over-subscription rate is low enough, such that service definitions are met, all subscribers
receive the service to which they subscribed.
Caution With over-subscription, the network is unable to support all subscribers being active at the
maximum data rate. If the over-subscription is severe enough, subscribers may be denied
service.
Parameters to determine the over-subscription level include:
•Peak percentage of simultaneous users—Not all subscribers access the network at the same time.
Subscribers have different access patterns that vary based on profiles; working hours; family
demographics; type of user—telecommuter or residential Internet access customer. Only a portion
of subscribers are active at a given time. This number serves as the “peak percentage of
simultaneous users parameter”— busy hour number of subscribers.
•Average data rate per subscriber—Not only are all subscribers not active at the same time, but they
do not continuously operate at peak rate. Using basic Internet access as an application, data that
subscribers request and send downstream and upstream is subject to bursts. A group of subscribers,
therefore, has an average data rate less than the maximum rate defined by the service.
Note For some services, the average value might be the maximum rate. VoIP is such an
application.
How bandwidth contention is handled depends on the mix of services defined and individual service
definitions.
Percentage of homes passed subscribing to the service is another factor to consider. If this parameter is set too
conservatively, the network is under-engineered and requires modification to grow the service. If set too
aggressively, the network is over-engineered and costs for services are higher than they should be.
Full implementation of service levels requires additional higher layer items including scheduling,
queuing priorities, bandwidth allocation. These items are addressed in DOCSIS 1.0 extensions. Refer to
the “Overview of CiscouBR10012 Universal Broadband Router Software” section on page1-1 and to
additional chapters of this guide for additional information.
For detailed engineering calculations, refer to the Cisco Multimedia Traffic Engineering for HFC
Networks publication (PDF format) on CIsco.com.