Cisco MC16S Uživatelský manuál stáhnout pdf (Strana 2)

Spectrum Management Overview

12.0(7)XR2

Two-Way Cable

The Cisco uBR7200 series universal broadband routers divide a cable network into downstream and

upstream segments. Each upstream segment typically serves one or more ﬁber nodes. Upstream

segments can be deﬁned as:

• 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 modem card supports either sparse or dense segments—not 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 lots of subscribers. Figure 1 illustrates sparse versus dense

segments.

Figure 1 Sparse Versus Dense Segment

As shown in Figure 1, the downstream segment can contain multiple upstream segments. Two ﬁber

nodes can be in one downstream segment, but in different upstream segments.

An upstream frequency has an associated upstream input power level in dBmV. Cable operators must

make noise measurements and determine the cable plant’s spectrum management policy. Different

modulation schemes and symbol rates can be used based on the characteristics of the cable plant and

the cable modem card contained in the chassis. Bits are encoded into a two-dimensional mapping

called a constellation. Good carrier-to-noise ratio is needed to properly decode symbols into bits.

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Dense

Segment

Sparse

US0

US1

US2

US3

1x4 CM card

US0

US1

US2

US3

1x4 CM card

attenuator

Cable

modems

Fiber-optic

cable

Optical

receiver

Reverse optical

transmitter

Distribution

network

Segment

attenuator

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