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Two-Way Cable

Cisco uBR7200 Series MC16S Cable Modem Card Spectrum Management 3

Upstream Signals

The upstream channel is characterized by many cable modems transmitting to the CMTS. These

signals operate in a burst mode of transmission. Time in the upstream channel is slotted. The CMTS

provides time slots and controls the usage for each upstream interval.

The Cisco uBR7200 series equipment periodically broadcasts Upstream Channel Descriptor (UCD)

messages to all cable modems. These messages deﬁne upstream channel characteristics including

the upstream frequencies, symbol rates and modulation schemes, Forward Error Correction (FEC)

parameters, and other physical layer values.

Cisco supports all DOCSIS error correction encoding and modulation types and formats. 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.

Sending data reliably in the upstream direction is an issue. Because upstream spectrum varies greatly

between cable plants, select upstream parameters based on your cable plant’s return paths. Select or

customize upstream proﬁles for maximum trade-offs between bandwidth efﬁciency and upstream

channel robustness. For example, 16 QAM 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.

Combiner Groups

The return path of several ﬁber nodes can be combined together at a single point to form a single RF

frequency domain called a combiner group. The Cisco uBR7200 series universal broadband router

software allows a frequency hop table to be associated with a combiner group, called a spectrum

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.

The Cisco uBR7200 series universal broadband routers support up to 32 spectrum groups. Each

spectrum group deﬁnes the table of frequencies to be used in a speciﬁc frequency plan. Cisco

cable modem cards interface the downstream and upstream ports to the cable plant.

Frequency Allocation Table

The Cisco uBR7200 series universal broadband router software includes Cisco IOS commands that

can be used to deﬁne upstream frequency hop tables. In the global and interface conﬁguration

modes, these are grouped under the cable spectrum-group command hierarchy.

The steps to conﬁgure a spectrum group are as follows:

• Determine the upstream ports assigned to a combiner group.

• Create the frequency hop table under Cisco IOS global conﬁguration.

• Assign the upstream port to the frequency hop table under Cisco IOS interface conﬁguration.

Note For more information, refer to “Beneﬁts” section on page 13.

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Cisco MC16S Bedienungsanleitung Seite 3