Specifications

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Cisco Cable Modem Termination System Feature Guide
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Chapter 3 Spectrum Management for the Cisco Cable Modem Termination System
Feature Overview
Upstream Segments and Combiner Groups
The Cisco routers divide a cable plant into downstream channels. Downstream channels contain
upstream segments. Each upstream segment typically serves more than one fiber node. Upstream
segments can be defined as one of the following:
• Sparse segment—Containing one upstream channel per upstream segment.
• Dense segment—Containing multiple upstream channels per upstream segment; frequencies must
be different.
Note A cable interface line card can support sparse or dense segments, or both.
Defining sparse segments allows the cable operator to share upstream bandwidth among fiber nodes with
fewer subscribers. Defining dense segments allows the cable operator to provide larger upstream
bandwidth to fiber nodes with many subscribers.
Figure 3-1 illustrates sparse versus dense segments.
Figure 3-1 Sparse Versus Dense Segment Illustrations
As shown in Figure 3-1, the downstream segment can contain multiple upstream segments. Two fiber
nodes can be in one downstream segment but in different upstream segments.
The return path of several fiber nodes can be combined at a single point to form a single RF frequency
domain called a combiner group. The CMTS software allows a frequency hop table called a spectrum
group to be associated with a combiner group.
Note A combiner group refers to an RF topology point. A spectrum group refers to the frequency hop table
associated with a combiner group.
27979
Dense
Segment
1
Sparse
DS
US0
US1
US2
US3
US4
US5
1x4 CM card
DS
US0
US1
US2
US3
US4
US5
1x4 CM card
attenuator
Cable
modems
Fiber-optic
cable
Optical
receiver
Reverse optical
transmitter
Distribution
network
Segment
2
Segment
x
=
Segment
3
Segment
4
Segment
5
Segment
6
x
attenuator