connecting and configuring the cable headend - cr72cnrf

7/30/2019 Connecting and Configuring the Cable Headend - Cr72cnrf

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Cisco uBR7200 Series Universal Broadband Router Hardware Installation Guide

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Chapter 4 Connecting the Cisco uBR7200 Series Router to the Cable Headend

Two-Way Data Headend Architecture

Two-Way Data Headend ArchitectureFigure 4-1 shows a typical headend configuration configured for two-way data, including digitized voice

and fax.

Figure 4-1 Typical Cable Headend Configuration for Two-Way Data

Cisco uBR7200series router

DownstreamUpconverter

15429

Upstream0 dBmV

6 dBattenuator

40 dBattenuator

3 dBattenuator

Optical

receiver Fiber-optic

cable Distributionnetwork

+17 dBmV+13 dBmV

Cable

modems

+48 dBmV

+31 dBmV

2-waysplitter

+10 dBmV

6 dBattenuator

40 dB

attenuator

8 dBattenuator

+18 dBmV

+42 dBmV

+58 dBmV

2-way

splitter

+10 dBmV

6 dBattenuator

40 dB

attenuator

21 dBattenuator

+31 dBmV +42 dBmV

+49 dBmV

+50 dBmV

2-way

splitter

+10 dBmV

Lasertransmitter

Broadcast

Narrowcast

0 to 20 dBattenuator

(as required)

8-waytap

3-waysplitter

+17 dBmVvideo carrier

(typical)

+7 dBmVdata carrier

(typical)

Opticalsplitter

Node 1

Node 2

Downstreamforwardtest point

Mainheadend feed

X

X

X

Reverse optical

transmitter

Modem

transmit

levels

Modemtransmit

levels

Modem

transmit

levels

Reverse opticaltransmitter

Reverse optical

transmitter

Optical



+17 dBmV

Opticalreceiver Fiber-optic

cable Distribution

network

+17 dBmV

Downstream(0 to +5 dBmV)

Cablemodem

L H

C

Diplex filter

Downstream

(0 to +5 dBmV)

Cable

modem

L H

C

Diplex filter

Downstream

(0 to +5 dBmV)

Cable

modem

L H

C

Diplex filter


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One-Way Data Headend Architecture

One-Way Data Headend ArchitectureFigure 4-2 shows a typical headend configuration configured for one-way (downstream) data in a telco

return cable system.

Figure 4-2 Typical Cable Headend Configuration for One-Way (Telco Return) Data

RF and Digital Data OverviewThis section describes the interaction of digital and analog RF data as both signals are carried on the

hybrid fiber-coaxial (HFC) network.

Two-way digital data signals are more susceptible than one-way signals to stresses in the condition ofthe HFC network. Degradation in video signal quality might not be noticed, but when two-way digital

signals share the network with video signals, digital signals might be hampered by the following types

of network impairments:

Impulse and electrical noiseImpulse and electrical noise, usually forms of ingress, can enter the

network from sources within a home, such as hair dryers, light switches, and thermostats; or from

high-voltage lines that run near CATV cabling in the network. Areas of signal ingress may be located

and repaired by implementing a comprehensive signal leakage maintenance program.

Amplifier thermal noiseAmplifiers add noise to the HFC network that usually goes unnoticed in

video signals, assuming a properly designed and operated network. Improperly configured

amplifiers will degrade digital data signals. The larger the network, the higher the probability of

amplifier thermal noise affecting the signals.

Ingress noiseIngress noise includes electrical sources (see Impulse and electrical noise above);

amateur radio transmissions; citizens band radios; or high-power shortwave broadcast signals,

which can interfere with frequencies anywhere between 3 and 65 MHz. These often are picked up

by cabling and equipment in the network.

Note Some HFC upstream equipment passes interfering signals below 5-MHz, which may overload the

reverse path.



RADIUS dialsecurity server

IP network access

Cisco network

access server

Telco return

cable modems

Upstream

DHCP

TFTP

TOD

servers

Lasertransmitter

Broadcast

Narrowcast

0 to 20 dBattenuator

(as required)

8-waytap

3-waysplitter


(typical)

+7 dBmVdata carrier

(typical)

Opticalsplitter

Node 1

Node 2


Mainheadend feed

PSTN

27806


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Connecting and Configuring the Downstream

Noise funnelingThe upstream data path to the headend is susceptible to picking up noise and

interference from anywhere in the network, and all upstream noise ultimately ends up at the

headend. This effect is known as noise funneling because of the cumulative nature of the noise from

one or more locations in the network that becomes concentrated at the headend. As a network

serviced by a single upstream receiver increases in size, the probability of noise funneling also

increases.

Variable transmit levelsSignal loss over coaxial cable is affected by temperature. This can cause

variations of 6 to 10 dB per year.

ClippingThe lasers in fiber-optic transmitters can stop transmitting light (clipping) when input

levels are excessive. Excessive input levels may cause bit errors and reduced data throughput in both

the upstream and downstream transmissions. If a laser is overdriven as briefly as a fraction of a

second, clipping can occur.

Connecting and Configuring the DownstreamAfter you install the Cisco uBR7200 series universal broadband router in your headend site, you must

connect the Cisco uBR7200 series to the HFC network and configure the network. The followingsections describe how to connect to and configure the downstream.

Installing and Configuring the Upconverter

If you have not already done so, unpack the IF-to-RF upconverter at your headend site and install it near

your Cisco uBR7200 series universal broadband router.

Note Refer to the user documentation that accompanied your upconverter for safety information and specific

installation instructions.

Caution If you do not properly configure the upconverter, you might see decreased system performance,

increased packet loss, and a reduction in carrier-to-noise ratios.

Note You might need to add attenuation to the downstream path between the Cisco uBR7200 series cable

interface line card and the upconverter. Cisco cable interface cards produce an IF output level of either

+32 dBmV (+/2 dB), +40 dBmV (+/2 dB), or +42 dBmV (+/2 dB) depending upon which version

of cable interface card you have installed. Add enough attenuation to adjust the cable interface line card

IF output level to match the IF input level of your upconverter and to compensate for cable line loss, the

cable interface line cards actual measured output power, and upconverter performance.

For more information refer to the Cisco uBR7200 Series Universal Broadband Router Cable Interface

Line Card Hardware Installation Guide at the following URL:

http://www.cisco.com/en/US/docs/interfaces_modules/cable/line_cards/installation/guide/mcxxfru.htm

l
http://www.cisco.com/en/US/docs/interfaces_modules/cable/line_cards/installation/guide/mcxxfru.htmlhttp://www.cisco.com/en/US/docs/interfaces_modules/cable/line_cards/installation/guide/mcxxfru.htmlhttp://www.cisco.com/en/US/docs/interfaces_modules/cable/line_cards/installation/guide/mcxxfru.htmlhttp://www.cisco.com/en/US/docs/interfaces_modules/cable/line_cards/installation/guide/mcxxfru.html


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Setting the North American Upconverter Input Level

You must set the upconverter IF input level, according to the manufacturers instructions, to match the

downstream output level of the cable interface cards. Earlier Cisco cable interface card output was

+32 dBmV (+/2 dB). More recently, however, Cisco cable interface cards feature an output rating of

+42 dBmV (+/2 dB). Refer to the Measuring the Downstream RF Signal section on page 4-16 and be

sure to check the documentation that accompanied your Cisco cable interface line card or the faceplate

of the cable interface line card itself to determine its downstream output level.

Depending on the upconverter model selected (see Appendix F, Manufacturers for Headend

Provisioning Requirements, for manufacturers and models), you might need to add attenuation between

the cable interface line card IF output and upconverter input to achieve the recommended input level to

the upconverter. For example:

The recommended IF input to the General Instrument C6U is +23 dBmV; this upconverter requires

6 to 9 dB of attenuation on the input cable.

The IF input to the Vecima MA4040 is +33 dBmV; this upconverter requires no attenuation on the

input cable. (See Figure 4-3.)

To verify the input level, connect a spectrum analyzer to the test point on the upconverter input. If your

upconverter is equipped with a built-in meter, you do not need to connect a spectrum analyzer. The IF

input level at the test point should be 0 dBmV at 44 MHz.

Figure 4-3 Setting the North American Upconverter Input Level

Tip To adjust the General Instrument (GI) C6U to the correct level at the test point, use the Manual IF

Gain Control (MIGC) on the C6U to adjust the level until the output at the test point reads 0 dBmV

at 44 MHz on a spectrum analyzer, when +23 dBmV is present at the upconverter IF input.

The test point on the GI upconverter has a +/-2 dB accuracy specification.

To adjust the Vecima MA4040 to the correct level at the test point, set the

Automatic Gain Control (AGC) to auto. The Vecima MA4040 has a built-in meter

to verify this setting.

Cisco uBR7200series

Downstream output44 MHz IF

+32 dBmV ( 2 dB)+-

6 dB to 9 dB attenuatorfor GI upconverter

IF input

RF output

Upconverter

Spectrum analyzeror meter at test pointshould equal 0 dBmV 14

041
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Setting the European Upconverter Input Level

You must set the upconverter IF input level, according to the manufacturers instructions, to match the

downstream output level of your Cisco MC16E cable interface card. The Cisco MC16E features an

output rating of +40 dBmV (+/2 dB). Refer to the Measuring the Downstream RF Signal section on

page 4-16 and be sure to check the documentation that accompanied your Cisco cable interface line card

or the faceplate of the cable interface line card itself to determine its downstream output level.

Depending on the upconverter model selected (see Appendix F, Manufacturers for Headend

Provisioning Requirements, for manufacturers and models), you might need to add attenuation between

the cable interface line card IF output and upconverter input to achieve the recommended input level to

the upconverter.

To verify the input level, connect a spectrum analyzer to the test point on the upconverter input. If your

upconverter is equipped with a built-in meter, you do not need to connect a spectrum analyzer. The IF

input level at the test point should be 0 dBmV at 36.125 MHz.

Figure 4-4 Setting the European Upconverter Input Level

Setting the Upconverter Output Level

You must now set the upconverter output RF level according to the manufacturers instructions. DOCSIS

specifications permit an RF output level of +50 to +61 dBmV. Select an output level that falls within this

range that is valid for your upconverter. For example, the configuration in Figure 4-14 shows an output

level of +55 dBmV.

Note You should set the upconverter output to a level no greater than about +58 dBmV. This provides

headroom for test equipment calibration accuracy and measurement uncertainty, and helps to avoid

compression in the upconverter.

Tip The recommended output level of the General Instrument C6U is +53 to +55 dBmV.

The recommended output level of the Vecima MA4040 is +55 to +58 dBmV at 88 to 860 MHz.

Cisco uBR7200series routers

Downstream output36.125 MHz IF

+40 dBmV ( 2 dB)+-

IF input

RF output

Upconverter

Spectrum analyzeror meter at test pointshould equal 0 dBmV 29

428
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Testing the Upconverter Output Level

If you are using a GI, Barco, or Scientific Atlanta upconverter, you can measure the 0 dBmV test point

at thefrontof the upconverter. If you are using a Vecima upconverter, you can measure the 0 dBmV test

point at the backof the upconverter. Your spectrum analyzer should display a signal similar to the one

shown in Figure 4-5.

Note The digitally modulated carriers average power level should be set between 6 and 10 dB less than the

cable networks video carrier amplitude.

Figure 4-5 shows a sample digital channel power measurement made on an Agilent 8591C spectrum

analyzer at a 44 MHz IF test point (North American headend).

Figure 4-5 Sample Digital Channel Power Measurement at 44 MHz IF Test Point

Setting the Upconverter Output Frequency

You must now select an output frequency. DOCSIS specifications permit channels from 91 to 857 MHz

(center frequency of 91 MHz is not required). In the example shown in Figure 4-14, a center frequency

of 610 MHz is used.

The following excerpts from the DOCSIS Radio Frequency Interface Specification define downstream

digitally modulated carrier frequencies.

DOCSIS The downstream frequency plan should comply with Harmonic Related Carrier (HRC),

Incremental Related Carrier (IRC), or Standard (STD) North American frequency plans per [IS-6].

However, operation below a center frequency of 91 MHz is not required.

EuroDOCSIS The downstream frequency plan will include all center frequencies between 112

and 858 MHz on 250 kHz increments. It is up to the operator to decide which frequencies to use to

meet national and network requirements.

Your output frequency should be in your narrowcast band of frequencies, or the narrowcast combiner.

Narrowcast frequencies are defined as frequencies that are transmitted to certain groups of fiber nodes

or regions in your network. (See Figure 4-6.) The same programming content is received within each

group of fiber nodes or regions. Different groups receive different content.


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Tip The Vecima MA4040 displays the channels center frequency.

The GI C6U displays a frequency that is 1.75 MHz below the channels center frequency. This value

is equivalent to the visual carrier frequency for an analog TV channel occupying the same spectrum.

Figure 4-6 Narrowcast Combiner and Output Frequency

Measuring Upconverter Phase Noise Contribution

You may use a spectrum analyzer to measure the relative aggregate phase noise contribution of the

upconverter to the RF output signal originating at your cable headend. By following the steps in this

procedure, you can determine whether or not the amount of phase noise in your RF output signal is

potentially detrimental to downstream data transmission. Perform the following steps to measure the

additive phase noise in your RF output signal.

Note Most spectrum analyzers are not designed to precisely measure phase noise.

Caution Performing the procedures in this section disconnects any active cable modems currently connected to

your Cisco uBR7200 series router.

Viewing the Downstream IF Signal

Note Refer to the user guide that accompanied your spectrum analyzer to determine the exact steps required

to use your analyzer to perform these measurements.

50 - 595 MHz

600 - 700 MHz Narrowcast channels

1

2

3

45

6

3

Each segment isa group of fiber nodes

or a region

14043

Broadcast channels

600 - 700 MHz Narrowcast channels 6

600 - 700 MHz Narrowcast channels


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Step 1 Connect a spectrum analyzer to the downstream connector on a Cisco cable interface card installed in a

Cisco uBR7200 series universal broadband router.

Step 2 Turn the power switch on the spectrum analyzer to the ON position.

Step 3 Set the spectrum analyzer to view the downstream intermediate frequency (IF) signal with a center

frequency of 44 MHz for a North American headend or 36.125 MHz for a European headend.Step 4 Set the resolution bandwidth to 100 kHz, the video bandwidth to 1 MHz, the span to 12 MHz, and the

sweep time to 20 msec. Your analyzer should display a signal similar to the one shown in Figure 4-7.

Figure 4-7 Downstream IF Channel Power

Step 5 Stop downstream digital data transmission from the cable interface card by setting the downstream

symbol rate to 0 on your downstream cable interface. This transmits an unmodulated IF carrier signal

over the downstream. To accomplish this, enter the following Cisco IOS command in interfaceconfiguration mode:

Router(config-if)# cable downstream symbol 0

Your analyzer should display a signal similar to the one shown in Figure 4-8.


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Figure 4-8 Phase Noise Measurement

Note Notice that the total channel power (across 6 MHz) does not change between the modulated carrier and

the unmodulated continuous wave (CW) carrier.

Step 6 Zoom in on the CW signal (the center 6 MHz in Figure 4-8) by reducing the span on your spectrum

analyzer from 12 MHz to 6 MHz.

Step 7 Change the resolution bandwidth from 100 kHz to 1 kHz so that the sweep time slows to around

18 seconds. Be sure, however, that you retain the 1-kHz video bandwidth and the same center frequency.


Figure 4-9 6-MHz Channel Band

Note Your signal might show additional signals or spurs similar to the ones displayed in Figure 4-9. These

are characteristics of the modulator in this mode and do not affect downstream data transmission.


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Step 8 Narrow your span even further from 6 MHz to 100 kHz to zoom in on the center frequency of the channel

carrier and reduce your resolution bandwidth and video bandwidth to absolute minimum values for your

particular spectrum analyzer. Your analyzer should display a signal similar to the one shown in

Figure 4-10.

Figure 4-10 100-kHz Zoom View of Carrier Center Frequency

Step 9 Save this plot on your spectrum analyzer display for comparison with the RF output signal of the

upconverter.

Viewing the RF Output Signal

After you have isolated, displayed, and saved the downstream IF signal on your spectrum analyzer, you

must go through the same sort of procedure for the RF output signal from your upconverter.

Step 1 Disconnect the spectrum analyzer from the downstream IF interface on the cable interface card and

reconnect the downstream IF interface to the IF input on your upconverter.

Note Be sure that you have added the appropriate attenuat ion to your downstream path, if necessary. For more

information, refer to the Setting the North American Upconverter Input Level section on page 4-5.

Step 2 Connect the spectrum analyzer to the RF output of the upconverter.

Step 3 Set the center frequency on the spectrum analyzer so that it matches the RF output frequency of yourupconverter. For this example, 555 MHz is the RF output frequency.

Step 4 Allow the spectrum analyzer to complete at least one complete sweep of the RF output.

Note Be sure to adjust the reference level of the current measurement so that its peak value is equal to that of

the saved signal.


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A second signal appears on the spectrum analyzers screen, yielding a display similar to that in

Figure 4-11.

Figure 4-11 Comparison of IF Input Sweep and RF Output Sweep

Step 5 Qualify the 100-kHz phase noise measurement of your upconverter by comparing the shapes of the two

signals. If the signal is substantially higher (with the peak amplitude measurements lined up), your

upconverter might have high-frequency phase noise, and might be incompatible with digitally modulated

signals. Upconverters suffering from excessive gain, power supply difficulties, or design deficiencies can

introduce unacceptable levels of phase noise into your cable headend. If your comparison resembles

Figure 4-11, your upconverter is operating properly.

Note If you suspect that your upconverter is drifting over time, place two additional traces on your screen,min-hold and max-hold, while comparing your two signals. Should either trace change shape, your

upconverter has failed. This method can help you discover mechanical tapping or vibration-related

problems, which can have an adverse effect on upconverter phase noise.

To be certain of your findings, we recommend that you check your upconverter with a QAM analyzer

capable of displaying constellation phase noise impairments.

Step 6 Attach the spectrum analyzer to the output of your cable interface card and clear the saved trace.

Step 7 Reduce the span on the spectrum analyzer from 100 kHz to 10 kHz and allow the spectrum analyzer to

complete at least one full sweep of the RF output.

Step 8 Save this new trace and return to the RF output of the upconverter (reconnecting the upconverter in the

process). Your analyzer should display a signal similar to the one shown in Figure 4-12.


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Figure 4-12 Increased Frequency Resolution on IF Input Sweep and RF Output Sweep

Note This step displays an even greater resolution for both the IF input and RF output signals at your

upconverter.

Fine-Tuning the Center Frequencies

The signals in Figure 4-12 feature a very slight frequency error. Continue with the final steps in this

procedure to resolve this frequency error and display the low-frequency additive phase noise at and

below 10 kHz.

Step 1 Eliminate the offset between the IF and RF output traces by fine-tuning the RF output center frequency

on the spectrum analyzer. The difference between the center frequencies in these two signals is referred

to as the frequency error of the upconverter.

Note The FCC specifies that the frequency error of an upconverter cannot exceed 5 kHz in the aviation bands

Step 2 Repeat Step 3 and Step 4 under the Viewing the RF Output Signal section on page 4-11, substituting

the exact center frequency for 555 MHz. In the example, the exact center frequency is 544.99970 MHz.


Note Your spectrum analyzer must feature frequency counting capability in order for you to be able to work

out the new, adjusted center frequency.


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Figure 4-13 Frequencies Lined Up to Illustrate Additive Phase Noise in RF Output

Step 3 Measure the increase in phase noise (the difference in amplitude between the IF and RF output traces)

at 5 kHz from the center frequency to establish a reliable reading. This value is the phase noise

contribution from your upconverter.

Step 4 Compare the value derived in Step 3 with the minimum specifications for your cable network headend

to get an indication of how well your upconverter is operating.

The spectrum analyzer signals displayed in this procedure are of a high-quality DOCSIS-based

upconverter operating in an ideal headend environment. If your particular results reveal significantly

greater levels of phase noise (that is, the skirts of the RF signal are significantly higher than the IF signal

skirts when the peak amplitudes have been lined up), your upconverter might not be suitable for digital

modulation formats.

If possible, double-check your upconverter performance with a specialized analyzer such as a QAM

analyzer or other type of dedicated phase-noise measurement equipment.

Note If you are concerned about possible upconverter drift or intermittency, try viewing three curves on your

spectrum analyzer: IF plot or real-time RF plot; max-hold RF plot; and min-hold RF plot. If the

IF or real-time RF plot differs in shape from the max-hold or min-hold RF plots, your upconverter

has most likely suffered an intermittency error.

Step 5 Re-establish 64-QAM downstream digital data transmission from the cable interface card by setting the

downstream symbol rate to 5056941 symbols/second on your downstream cable interface. To

accomplish this, enter the following Cisco IOS command in interface configuration mode:

Router(config-if)# cable downstream symbol 5056941

Note The value 5056941 symbols/second is specified by DOCSIS for 64-QAM transmissions in a 6-MHz

downstream channel plan. For 256-QAM transmission, the value is 5360537 symbols/second.

Step 6 After you determine that your upconverter is suitable for reliable downstream data transmission, proceed

to the following section, Completing the Downstream Configuration.


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Completing the Downstream Configuration

To complete the downstream configuration, you must combine the upconverter output with the main

headend broadcast feed into the laser transmitter in the headend. In the example shown in Figure 4-14,

the laser transmitter has two inputs. These inputs are designed for +17 dBmV video carriers. The

narrowcast feed, which includes cable modem service and digital video and local access channels, isconnected to the laser transmitter input using an 8-way tap and a 3-way splitter. (See Figure 4-14.)

Note In the example, there is an optical splitter on the output of the laser transmitter that allows you to transmit

to two fiber nodes.

The 8-way tap has an insertion loss of 11 dB and the 3-way splitter has an insertion loss of 7 dB; the

combined loss is 18 dB. With this combined insertion loss, you will overdrive the input on the transmitter

and it will not work properly. To compensate for this insertion loss, you must add attenuation to the

digital carrier laser input. The input level for the data carrier is +7 dBmV, or 10 dB below the video

carriers. In this example, start with a 20-dB attenuator to adjust for the insertion loss and passive loss in

the headend cables. (See Figure 4-14.)

Figure 4-14 Complete Downstream Configuration

Tip If you have a very large, complex headend system with many outputs, you might notice a very large

passive loss in your headend combining network. For example, in a headend with 100 feet of 59-series

headend coaxial cable (RG-59), you can see losses of 6 to 8 dB. To compensate for this loss, you can

install the upconverter closer to the laser transmitters.


DownstreamIF input0 dBmV

RF output

Upconverter

14042

610 MHz center frequency+55 dBmV output level

UpstreamLaser

transmitter

Broadcast

Narrowcast

-3 dBmV video carrier

Mainheadend

feed

0-20 dBattenuator

(as required)8-way

tap

3-waysplitter


(typical)

+7 dBmVdata carrier

(typical)

Opticalsplitter

Node 1

Node 2


40 dBattenuator

IF

RF

Cable

modem

L H

C

Diplex filter


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Measuring the Downstream RF Signal

Testing the Downstream Configuration

To test the downstream configuration, you can connect a cable modem to the downstream forward test

point of the laser transmitter. Use a diplex filter and an attenuator, as needed, to connect the cable modem

to an upstream port on the Cisco uBR7200 series universal broadband router cable interface card.

The nominal input level for a Cisco uBR7200 series router upstream port is 0 dBmV, but it can also beadjusted as low as 10 dBmV or as high as +25 dBmV using Cisco uBR7200 series software. The router

instructs the modem to adjust its output level to provide the correct cable interface line card input level.

Your test cable modem will require a minimum of 8 to 10 dB of attenuation between the upstream of the

diplex filter and the upstream port on the router. In the example in Figure 4-14, a 40-dB attenuator pad

is used.

If this configuration is working properly, you have a very good chance of getting the rest of the network

up. If this configuration generates a low carrier-to-noise (C/N) ratio estimate in the cable interface, you

need to make further adjustments.

Note You can measure the preliminary C/N ratio estimate at the headend downstream laser test point. This

measurement may be used to verify the performance of the upconverter, headend combiner, and forwarddistribution system before cable modems are installed on the HFC network. Typically, this is the only

place to test for downstream interference from the forward path. Ensure that your sweep equipment is

not programmed to transmit on the digital carrier. Otherwise, this can cause bit errors and packet loss,

resulting in unreliable cable modem operation.

You can verify the C/N ratio estimate on a Cisco uBR900 series cable access router by following the

directions described in the About Verifying the Downstream Signal section on page 6-10.

Tip The C/N estimate for a Cisco uBR900 series access router installed at the headend should be between

35 and 39 dB.

A maximized C/N ratio estimate optimizes cable modem reliability and service quality.

Measuring the Downstream RF SignalThe configuration of the downstream digitally modulated carrier at the headend is critical to the

performance of the Cisco uBR7200 series universal broadband router and cable modems. The following

guidelines are provided to assist you in configuring the RF signal to the necessary specifications. There

are two options you can use to measure the RF signal with a spectrum analyzer. These options are

described in the following sections:

Measuring the Downstream RF Signal Using the Channel Power Option on a Spectrum Analyzer

section on page 4-17

Measuring the Downstream RF Signal Using CATV Mode on a Spectrum Analyzer section on

page 4-23 (equipped with digital channel power mode)

These two sections describe the procedures necessary to use a spectrum analyzer. You may also use a

signal level meter that has the ability to measure the average power level of digitally modulated carriers,

as well as a QAM analyzer. Some instruments to perform these measurements include:

Acterna SDA-5000 w/Option 4 (http://www.jdsu.com)
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Agilent 8591C, N1776A, 2010 or 3010

(http://www.home.agilent.com/agilent/home.jspx?cc=US&lc=eng)

Sunrise Telecom AT-2000RQ, CM1000 or CR1200R

(http://www.sunrisetelecom.com/products/cable.php)

Telsey DMA-120, DMA-121 or DMA122 (http://www.telsey.it)

Trilithic 860DSP w/Option QA1 (http://www.trilithic.com)

If you complete these measurements using one of the previously mentioned options, your downstream

signal can be verified as correctly configured and it can assist you with troubleshooting your network

later on.

If you want to measure the downstream RF signal using the spectrum analyzer channel power option,

proceed to the following section, Measuring the Downstream RF Signal Using the Channel Power

Option on a Spectrum Analyzer. If you want to measure the downstream RF signal using CATV mode,

proceed to the Measuring the Downstream RF Signal Using CATV Mode on a Spectrum Analyzer

section on page 4-23.

Note An analog TV channel modulator with external IF loops is not suitable for use as a QAM digitally

modulated carrier upconverter. These units typically do not have the phase noise and linearity

performance levels required for 64- and 256-QAM digital signals, and they might cause degraded

performance and possible system failure.

Measuring the Downstream RF Signal Using the Channel Power Option on aSpectrum Analyzer

The following sections describe how to measure the downstream RF signal using the channel power

option on a spectrum analyzer:

Measuring the Downstream IF Signal at the Cisco uBR7200 Series Router section on page 4-17

Measuring the Downstream RF Signal at the Upconverter Output section on page 4-19

Measuring the Downstream IF Signal at the Cisco uBR7200 Series Router




Cisco uBR7200 series router.


Step 3 Set the spectrum analyzer to view the downstream intermediate frequency (IF) signal with a center

frequency of 44 MHz for a North American headend or 36.125 MHz for a European headend.

Step 4 Set the span to 10 MHz. Your analyzer should display a signal similar to the one shown in Figure 4-15
http://www.home.agilent.com/agilent/home.jspx?cc=US&lc=enghttp://www.sunrisetelecom.com/products/cable.phphttp://www.telsey.it/homepage.asphttp://www.trilithic.com/http://www.telsey.it/homepage.asphttp://www.trilithic.com/http://www.sunrisetelecom.com/products/cable.phphttp://www.home.agilent.com/agilent/home.jspx?cc=US&lc=eng


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Figure 4-17 Measuring the IF Channel Power Using Video Averaging

Note The approximate in-channel peak-to-valley flatness may be verified using the spectrum analyzers video

averaging feature. Be aware, however, that amplitude values registered while in the video averaging

mode are typically around 2.5 dB below the actual channel power.

Measuring the Downstream RF Signal at the Upconverter Output

Step 1 Disconnect the spectrum analyzer from the cable interface card downstream connector.

Step 2 Connect the downstream output of the cable interface card to the upconverter input connector.

Step 3 Connect the spectrum analyzer to the RF output of the upconverter. If your spectrum analyzer input is

overloaded, you might see artifacts that are internally generated by the spectrum analyzer. The artifacts

are circled on the analyzer trace shown in Figure 4-18. Add attenuation as necessary to correct the

overload condition.


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Figure 4-18 Overloaded Spectrum Analyzer Input

Step 4 Set the input of the upconverter to a digital QAM signal and the output level to the manufacturersrecommended settings. Typical output amplitudes range from +50 to +58 dBmV, although DOCSIS

specifies +61 dBmV.

Step 5 Set the spectrum analyzer to view the RF signal at the center frequency you selected for your headend.

In this example, the RF center frequency is 699 MHz. Set your span to 20 MHz. Finally, set your channel

spacing and your channel bandwidth to 6 MHz.

If the RF signal is causing an overload condition on the spectrum analyzer input, your analyzer might

display a signal similar to the one shown in Figure 4-19. The sloping of the lines at the sides of the signal

indicates a false reading.

Figure 4-19 Measuring the RF Signal at the Upconverter OutputOverload Condition

Step 6 If you add attenuation to the input to the spectrum analyzer you can correct the overload condition as



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Figure 4-20 Measuring the RF Signal at the Upconverter OutputOverload Condition Corrected

with Attenuation

Step 7 Change the spectrum analyzer settings to view the digital channel power. This setting enables you to see

if there is too much power on the upconverter output. In Figure 4-21, the upconverter output is reading

+64.31 dBmV, which is beyond the DOCSIS-specified range of +50 to +61 dBmV.

Tip A spectrum analyzer might become overloaded and produce false readings (such as internally generated

spurs) when measuring a signal at this amplitude.

Figure 4-21 Measuring the RF Signal at the Upconverter OutputUpconverter Output Level Too

High

Step 8 Adjust the power on the upconverter output to ensure that it is between +50 and +61 dBmV. In

Figure 4-22, the upconverter output is reading +57.06 dBmV, which is within the correct range.


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Figure 4-22 Measuring the RF Signal at the Upconverter OutputOutput Adjusted to Correct

Range

Step 9 Select the video averaging feature on the spectrum analyzer. The signal becomes smoother and

frequency response problems might become visible. Your analyzer now displays an RF signal similar to

the one shown in Figure 4-23.

Figure 4-23 Measuring the RF Signal at the Upconverter Output Using Video Averaging

Tip The approximate in-channel peak-to-valley flatness may be verified using the spectrum analyzers video

averaging feature. Be aware, however, that values registered while in the video averaging mode are

typically around 2.5 dB below the actual channel power.


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Note Any in-channel frequency response problems at the headend can impair network performance or prevent

a cable modem on the HFC network from operating. The specified maximum peak-to-valley

measurement from a Cisco cable interface card is +/1.5 dB across 5.6 MHz. At the output of the

upconverter, the maximum tilt should not exceed +/1.5 dB across 5.6 MHz. If the tilt is greater than

+/1.5 dB across 5.6 MHz when measured, the upconverter might not be compatible with digital QAMsignals, or the upconverter might be defective. Remember, however, that when using your spectrum

analyzer in video averaging mode, amplitude accuracy adjustments must also be taken into

consideration.

Step 10 Verify that your headend RF measurements meet the recommended DOCSIS parameters listed in the

tables in Appendix B, RF Specifications. Record your headend settings and measurements in your

headend site log (Appendix G, Site Log). This will assist in troubleshooting the

Cisco uBR7200 series router installation later in the process.

This completes the procedure to measure the downstream RF signal using the channel power option.

Proceed to the Measuring the RF Signal at the Forward Test Point on a Laser Transmitter section onpage 4-49.

Measuring the Downstream RF Signal Using CATV Mode on aSpectrum Analyzer

The following two sections describe the methods you may use to measure the downstream RF signal

using CATV mode (digital channel power option) on a spectrum analyzer:

Measuring the Downstream IF Signal at the Cisco uBR7200 Series Router Using CATV Mode

section on page 4-23

Measuring the Downstream RF Signal at the Upconverter Output Using CATV Mode section onpage 4-26

Note We recommend using as recent a model of spectrum analyzer as possible to perform the two analyses

described here. You can use spectrum analyzers, such as the Agilent 8591C

http://www.home.agilent.com or the Tektronix 2715 (http://www.tek.com) to help you perform the tasks

outlined in this section.

Measuring the Downstream IF Signal at the Cisco uBR7200 Series Router Using CATV Mode




Cisco uBR7200 series router.

http://ub72rf.pdf/http://ub72stlg.pdf/http://www.home.agilent.com/agilent/home.jspxhttp://www.tek.com/http://ub72rf.pdf/http://ub72stlg.pdf/http://www.tek.com/http://www.home.agilent.com/agilent/home.jspx


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Step 3 Set the spectrum analyzer to CATV mode (CATV analyzer option) and select the channel measurement

option to view the downstream intermediate frequency (IF) signal. Your analyzer should display a signal

similar to the one shown in Figure 4-24.

Note Figure 4-24 shows the first of three screens that will be displayed by an Agilent 8591C when you use the

analyzer in this mode. Figure 4-25 is the last of the three screens displayed.

Figure 4-24 Viewing the Downstream IF Signal on a Spectrum Analyzer in CATV ModeInitial

Screen

Step 4 Advance to the last of the three screens in this display. Your analyzer should display a signal similar to


Figure 4-25 Viewing the IF Signal on a Spectrum Analyzer in CATV ModePreliminary Digital

Channel Power Screen


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Step 5 Enter a digital channel to measure and select digital channel power. Your spectrum analyzer will display

a signal similar to the one shown in Figure 4-26.

Figure 4-26 Measuring the IF Signal on a Spectrum Analyzer in CATV ModeDigital Channel

Power Screen

Step 6 Using the spectrum analyzers reference level control, adjust the amplitude of the displayed signal until

the shape of the signal is clearly distinguishable as a digitally modulated carrier, as shown in

Figure 4-27.

Figure 4-27 Measuring the IF Signal on a Spectrum Analyzer in CATV ModeAdjusted Digital


Note The IF channel power in Figure 4-27 is +33 dBmV, as displayed on the spectrum analyzer.

Step 7 Select the video averaging feature. Your spectrum analyzer should display a signal similar to the one



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Figure 4-28 Measuring the IF Signal on a Spectrum Analyzer in CATV ModeDigital Channel

Power Screen (Video Averaging)

Note The approximate in-channel peak-to-valley flatness can be verified using the spectrum analyzers video



Proceed to the next section, Measuring the Downstream RF Signal at the Upconverter Output Using

CATV Mode.

Measuring the Downstream RF Signal at the Upconverter Output Using CATV Mode

Step 1 Disconnect the spectrum analyzer from the cable interface card downstream connector.

Step 2 Connect the downstream output of the cable interface card to the upconverter input connector.

Step 3 Connect the spectrum analyzer to the RF output of the upconverter.

Step 4 Set the upconverter output level to the manufacturers recommended settings. Typical output amplitudes

range from +50 to +58 dBmV, although DOCSIS specifies levels as high as +61 dBmV.

Step 5 Set the spectrum analyzer to view the RF signal at the center frequency you selected for your headend.

In this example, the RF center frequency is 705 MHz.

Step 6 Set the spectrum analyzer to CATV mode (CATV analyzer option) and select the channel measurement

option to view the downstream RF signal. Your spectrum analyzer should display a signal similar to theone shown in Figure 4-24.

Note Figure 4-29 shows the first of three screens that will be displayed by an Agilent 8591C when you use the

analyzer in this mode. Figure 4-30 is the last of the three screens displayed.


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Figure 4-29 Viewing the Downstream RF Signal at the Upconverter Output in CATV ModeInitial

Screen

Step 7 Advance to the last of the three screens in this display. Your analyzer should display a signal similar to


Figure 4-30 Viewing the RF Signal on a Spectrum Analyzer in CATV ModePreliminary Digital


Step 8 Enter a digital channel to measure and select digital channel power. Your spectrum analyzer will display

a signal similar to the one shown in Figure 4-31.


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Figure 4-31 Measuring the RF Signal at the Upconverter Output in CATV ModeDigital Channel

Power Screen

Step 9 Using the spectrum analyzers reference level control, adjust the amplitude of the displayed signal until

the signal peak is within the top graticule of the analyzers display grid as shown in Figure 4-32.

Figure 4-32 Measuring the RF Signal at the Upconverter Output in CATV ModeAdjusted Digital


Step 10 Select the video averaging feature. Your spectrum5 analyzer should display a signal similar to the oneshown in Figure 4-33.


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Figure 4-33 Measuring the RF Signal at the Upconverter Output in CATV ModeDigital Channel

Power Screen Using Video Averaging

Note The approximate in-channel peak-to-valley flatness can be verified using the spectrum analyzers video



Note Any in-channel frequency response problems at the headend can impair network performance or prevent

a cable interface on the HFC network from operating. The specified maximum peak-to-valley

measurement from a Cisco cable interface card is +/1.5 dB across 5.6 MHz. At the output of the

upconverter, the maximum tilt should not exceed +/1.5 dB across 5.6 MHz. If the tilt is greater than

+/1.5 dB across 5.6 MHz when measured, the upconverter might not be compatible with digital QAM

signals, or the upconverter might be defective.


tables in Appendix B, RF Specifications.

Step 12 Record your headend settings and measurements in Appendix G, Site Log, as you verify them. This

will assist in troubleshooting the Cisco uBR7200 series router installation later in the process.

Step 13 After you have analyzed and adjusted the RF signal according to the steps outlined on the preceding

pages, proceed to Connecting and Configuring the Upstream section on page 4-30.
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Connecting and Configuring the Upstream

Connecting and Configuring the UpstreamThe following sections describe how to connect and configure the upstream for digital data.

Connecting the Upstream to the Optical ReceiverTo connect the upstream to the optical receiver, use a 2-way splitter as a combiner to leave the

Cisco uBR900 series access router connected at the headend, and connect the upstream headend cable

to the optical receiver. (See Figure 4-34.)

The default upstream input level to the Cisco uBR7200 series cable interface line card is 0 dBmV. You

may adjust the upstream input level to other values using the Cisco IOS software running on your router.

The Cisco uBR7200 series router uses automatic power control when transmitting to remote

cable modems. Accurately setting the power level helps to ensure reliable cable modem operation.

Table 4-1 provides upstream input power ranges for the various cable interface cards available for the

Cisco uBR7200 series router, depending on the channel bandwidth you are using.

Note If you have a Cisco MC16x cable interface card (six upstream ports and one downstream port) installed

in your Cisco uBR7200 series router, the 2-way splitter described above would be replaced by six 2-way

splitters (one splitter per upstream port). This would enable you to connect to all of the available

upstream ports on the Cisco MC16x.

Table 4-1 Upstream Input Power Ranges According to Cable Interface Line Card Type

Channel Bandwidth Cisco MC11 FPGA

Cisco MC16B, MC1xC1,

MC16E, and MC16S

1. The designation MC1xC includes the MC11C, MC12C, MC14C, and MC16C cable interface cards.

DOCSIS Specification

200 KHz n/a 10 to +25 dBmV 16 to +14 dBmV



1.6 MHz 10 to +10 dBmV 10 to +25 dBmV 7 to +23 dBmV

3.2 MHz n/a 10 to +25 dBmV 4 to +26 dBmV


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Figure 4-34 Connecting and Configuring the Upstream

Testing the Upstream Configuration

To test the upstream configuration, insert a test signal of known amplitude (+17 dBmV is shown in this

example) into the fiber node and measure the amplitude output level at the output of the headends

optical receiver. This measurement depends on return laser performance and optical distance. This

procedure is known as establishing the X-level test point. (See Figure 4-35.)

Figure 4-35 The X-level Test Point


14044

Upstream

Opticalreceiver

To downstream forwardtest point on laser transmitter

40 dBattenuator

2-waysplitter

(combiner)

10 dBattenuator

Optional attenuatorto adjust X-level point

measurement

X-level test point signal levelmust be the same for all

optical receivers (+/- 0.5 dB),See Fig. 4-36 and Fig 4-37

X

Cablemodem

L H

C

Diplex filter

14045

Spectrum analyzerOpticalreceiver

Fiber node0.5 milliwatt

Fiber node1.0 milliwatt

Spectrum analyzerOpticalreceiver

1dB attenuator

+11 dBmV

X-level test point = +10 dBmV(in this example)

30 km

10 km+17 dBmVInsert +17 dBmV signal

X

Measure +10 dBmV at this point

Insert +17 dBmV signal


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This X-level test point measurement will be different for every fiber node in the HFC network until

you adjust the attenuation on the upstream. You must adjust the attenuation so that this measurement is

the same on every fiber node. If you change a receiver or a transmitter at the fiber node, or if you unplug

a connector and plug it back in, you must recheck this amplitude measurement. Figure 4-36 shows how

three distribution network X-level test points connected to the same upstream port are all calibrated to

+10 dBmV using different attenuators.

Figure 4-36 Calibrating Multiple X-level Test Points Connected to One Upstream Port

Figure 4-37 shows how three distribution network X-level test points connected to the three different

upstream ports are all calibrated to +10 dBmV using different attenuators.

Cisco uBR7200series


1404

6

Upstream0 dBmV

3 dB

attenuator

3 dBattenuator

3 dBattenuator

40 dBattenuator

3 dB

attenuator

8 dBattenuator

21 dBattenuator

Cablemodems

+48 dBmV

+31 dBmV

+42 dBmV

+50 dBmV

+46 dBmV

+49 dBmV

+58 dBmV

4-waysplitter

X

X

X

+10 dBmV

+10 dBmV

+10 dBmV

To downstreamforward test pointon laser transmitter

+13 dBmV

+18 dBmV

+31 dBmV


Opticalreceiver

Fiber-opticcable Distribution

network

+17 dBmV


Opticalreceiver


network

+17 dBmV


Opticalreceiver


network

+17 dBmV

Modem transmit levels



Cablemodem

L H

C

Diplex filter


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Figure 4-37 Calibrating Multiple X-level Test Points Connected to Multiple Upstream Ports



15428

Upstream

0 dBmV

Downstream

(0 to +5 dBmV)

Cable

modem

L H

C

6 dBattenuator

40 dB

attenuator

3 dBattenuator

+13 dBmV

Cable

modems

+48 dBmV

+31 dBmV

2-way

splitter

+10 dBmV

Diplex filter

Downstream

(0 to +5 dBmV)

Cable

modem

L H

C

3 dB

attenuator

Diplex filter

Downstream

(0 to +5 dBmV)

Cable

modem

L H

C

Diplex filter

6 dB

attenuator

40 dB

attenuator

+18 dBmV

+42 dBmV

+58 dBmV

2-way

splitter

+10 dBmV

6 dB

attenuator

40 dBattenuator

21 dB

attenuator

+31 dBmV +42 dBmV

+49 dBmV

+50 dBmV

2-way

splitter

+10 dBmV

X

X

X

Reverse optical

transmitterOptical

receiver Fiber-opticcable Distribution

network

+17 dBmV

Reverse optical

transmitterOptical



+17 dBmV

Reverse optical

transmitterOptical



+17 dBmV

Modem

transmit

levels

Modemtransmit

levels

Modemtransmit

levels


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Measuring the Upstream RF Signal

Measuring the Upstream RF SignalYou can use a spectrum analyzer to measure the upstream signal from one or more remote cable modems

in a two-way data cable network. Performing this procedure can help alert you to potential problems in

your cable networks upstream configuration before a problem occurs. This helps to avoid trying to solve

a problem after a remote cable modem has experienced a failure in service. This procedure is referred toas the zero-span method.

Measuring the Upstream RF Signal Using a Spectrum Analyzer

This procedure is designed to help you accurately measure an upstream RF signal where no adjacent

channels are in use. To measure an upstream RF signal with active adjacent channels, refer to the Using

the Zero-Span Method with Adjacent Upstream Channels section on page 4-40.



Step 1 Connect the spectrum analyzer to the upstream signal from your cable network.


Step 3 Set the spectrum analyzer to view the upstream RF signal with a center frequency matching the actual

upstream center frequency defined in your Cisco uBR7200 series configuration file.

Step 4 Set the spectrum analyzers span to 0 MHz.

Note You can view the configuration file for your Cisco uBR7200 series router by using the

show controller cableslot/upstream-port| include frequency command, available in Cisco IOS

Release 11.3(6)NA or later releases and Cisco IOS Release 12.0(5)T1 or later releases. For example, if

you wanted to view the center frequency of port 0 on a cable interface card in slot 3, you would enter the

show controller cable 3/0 | include frequency command.

If you have assigned spectrum groups in your configuration file, use the show cable hop command to

display the current upstream center frequency for each cable interface.

Step 5 Set both the resolution bandwidth and the video bandwidth on the spectrum analyzer to 3 MHz and the

sweep rate to 20 ms. Provided there is a large amount of activity on your upstream channel, the spectrum

analyzer should display a signal similar to the one shown in Figure 4-38.


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Figure 4-38 Measuring the Upstream RF SignalSetting the Resolution and Video Channel

Bandwidth

Tip The horizontal line passing through the center of the spectrum analyzer display in Figure 4-38 is the

trigger line.

Step 6 Set the sweep value to 80 microseconds. Your spectrum analyzer should display a signal similar to the

one shown in Figure 4-39.

Note Be sure that your particular spectrum analyzer is capable of supporting sweep times as short as 80

microseconds.

Figure 4-39 Measuring the Upstream RF SignalSetting the Sweep Time Period

Step 7 Position the trigger line on the spectrum analyzer so that it is roughly in the middle (approximately

halfway between the highest and lowest portions) of the upstream RF signal.


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Note Refer to the documentation that accompanied your particular spectrum analyzer for detai led instructions

on activating and positioning the trigger line.

A known workaround exists for the Agilent 8591C spectrum analyzer. After activating and positioning

the trigger line in video mode, you mustpress the video button on the spectrum analyzer once more toenable proper functionality.

Step 8 Adjust the amplitude on your spectrum analyzer so that the uppermost portion of the upstream RF signal

is in the top graticule of the analyzers display grid and adjust the trigger line accordingly. Your spectrum

analyzer will then display an upstream RF signal similar to the one shown in Figure 4-40.

Note We do not recommend using the spectrum analyzers max-hold feature while analyzing upstream

signals in the frequency domain. Max-hold readings in the frequency domain can be inaccurate

because the analyzer focuses on the peak power of the strongest ranging modem rather than the power

levels of cable modems that are operating in a more ideal range.

Figure 4-40 Measuring the Upstream RF SignalAccurately Measured Amplitude on Spectrum

Analyzer

Step 9 Position a marker about 7/8 of the way into the preamble of the signal, as illustrated in Figure 4-40. (The

preamble is the regular pattern displayed at the front of the signal and the length of the preamble is a

function of the channel width/data rate, modulation format, and DOCSIS burst-profile configurations.)

The peak amplitude of the marker, which registers +31.07 dBmV in this case, will be within 1 dB of the

true burst power.

Note To verify this reading, you can also measure the power rating with an Agilent 89441A vector signal

analyzer (http://www.home.agilent.com/agilent/home.jspx?cc=US&lc=eng) .

If the preamble of your upstream signal is displayed with a significantly lower amplitude than the rest

of the RF signal, refer to the Using the Zero-Span Method with Adjacent Upstream Channels section

on page 4-40 for instructions on how to overcome this phenomenon.
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tables in Appendix B, RF Specifications.

Step 11 Record your headend settings in Appendix G, Site Log. This will assist in troubleshooting the Cisco

uBR7200 series universal broadband router installation later in the process.

Note Be sure not to narrow the focus of your analysis any further than approximately 3-MHz channel width.

Doing so can yield incorrect readings. For example, if you were to view an upstream RF signal with a

resolution bandwidth of only 300 kHz and a video channel bandwidth of only 100 kHz, your

measurements would register lower than the actual transmission levels.

Analyzing the Upstream RF Signal

When you have set up your spectrum analyzer to accurately read the upstream RF signal, you can verify

that a remote cable modem is operating as it should by pinging the modem via a console terminal.

Step 1 Log in to your Cisco uBR7200 series universal broadband router with a console terminal.

Step 2 Adjust the sweep time on your spectrum analyzer to 20 msec.

Step 3 Ping the remote cable interface card using first a 64-byte, then a 1500-byte ping packet request and take

note of the upstream RF signal in each case. Several hundred or thousand ping packets might be required

for a usable pattern to emerge.

Figure 4-41 and Figure 4-42 provide two examples of an ideal upstream RF signal based on a simple 64-

or 1500-byte ping of a single remote cable interface. The more slender of the data spikes in the RF signal

(the first and third spikes in Figure 4-41) are bandwidth request packet transmissions, while the largerspikes are the actual 64- or 1500-byte ping packet returns.

Figure 4-41 Analyzing the Upstream RF Signal64-Byte Data Packets
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Figure 4-42 Analyzing the Upstream RF Signal1500 Byte Data Packets

Note Both of the previous examples feature 16-QAM transmission with a channel width of 3.2 MHz, yielding

a 10-Mbit/sec data rate. In addition, these examples have an optimal upstream carrier-to-noise ratio of

approximately 50 dB.

Now it is time to view your upstream RF signal with multiple remote cable modems. Figure 4-43 and

Figure 4-44 both display upstream RF signals encompassing more than one remote cable modem. In

each case, there are two bandwidth requests followed by their respective ping packet returns, both at

slightly different amplitudes. This situation is most commonly caused by a difference in the receive

power from the two cable modems in question. In the examples the remote cable modem with the lesser

amplitude is cable modem A and the other is cable modem B.

In the following example, cable modem A and cable modem B have been artificially configured to yielda larger than normal difference in amplitude between their respective upstream RF transmissions. Under

normal conditions, the maximum difference in amplitude between any cable modems will be about

1.5 dB. Differences greater than 1.5 dB indicate a possible cable plant or remote cable modem problem.

Note To further illustrate this point, you can log in to your Cisco uBR7200 series router using a console

terminal and by entering the show cable modem command to obtain a report of the receive power ratings

for each modem. In the example, the receive power ratings for remote cable modems A and B are 2

dBmV and 0 dBmV, respectively.

The two bandwidth requests and ping packet returns on the upstream RF signal for cable modems A and

B are slightly different in Figure 4-43 and Figure 4-44. Differences in the distance between bandwidth

requests are primarily caused by the contention-based nature of multiple remote cable modems on thesame line. Differences in the distance between ping packet returns are primarily caused by factors such

as packet size and system loading.


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Figure 4-43 Analyzing the Upstream RF SignalMultiple Active Remote Cable Modems (A)

Figure 4-44 Analyzing the Upstream RF SignalMultiple Active Remote Cable Modems (B)

Note When viewing the upstream RF signal on your spectrum analyzer, two ping packet returns (for example,

from remote cable modems A and B) can be so close together that they appear to be one rather large

packet with a slight jump or decline in amplitude halfway through the measurement. This is an indication

that the upstream is 100 percent occupied during this time.

Figure 4-45 shows an upstream RF signal from a remote cable modem in a real-life scenario including

outside plant noise. Notice the relatively tall spike at the very left edge of the ping packet return. This

spike is mainly additive noise associated with an upstream RF signal mired by excessive amounts of

severe outside plant noise (as in this example). In addition, notice that the carrier-to-impulse noise ratio

measurement between the two diamond-shaped markers is only about 12 dB. (A few other noise peaks

are even worse.)


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The importance of this example is to bring to your attention the need for minimal outside plant noise.

Time-varying, fast noise can cause bit errors in packet transmissions, rendering your communication link

unreliable, if not unusable.

Figure 4-45 Analyzing the Upstream RF SignalOutside Plant Noise Included

Note This illustration depicts an upstream RF signal whose carrier-to-impulse noise ratio does not meet

DOCSIS 1.0 specifications. The data packet in Figure 4-45 was dropped due to severe noise

interference with a more narrow resolution bandwidth.

Using the Zero-Span Method with Adjacent Upstream Channels

When measuring upstream signals using the zero-span method, a very wide resolution and video

bandwidth give very accurate readings, but render your readings susceptible to energy in adjacent

channels. As the number of upstream services increases, so does the likelihood of interference from

adjacent channels. This section describes using the zero-span power measurement method, with a more

narrow resolution bandwidth.

Simply narrowing the resolution bandwidth will not yield accurate readings. See Table 4-2.

Table 4-2 Sample Channel Width and Symbol Rate Combinations with Their Respective Minimum Resolution

Bandwidth Measurements

Center Frequency Channel Width Symbol Rate1/2Symbol Rate

Center Frequency+/1/2 Symbol Rate

Minimum ResolutionBandwidth

20.000 200 kHz 160 80 20.080 and 19.020 MHz 10 kHz

30.000 400 kHz 320 160 30.160 and 29.840 MHz 30 kHz

40.000 800 kHz 640 320 40.320 and 39.680 MHz 100 kHz

25.000 1.6 MHz 1280 640 25.640 and 24.360 MHz 100 kHz

28.000 3.2 MHz 2560 1280 29.280 and 27.720 MHz 300 kHz


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Step 1 Display a signal complete with preamble and upstream data transmission information similar to the

resulting signal from Step 3 through Step 10 under the Measuring the Upstream RF Signal Using a

Spectrum Analyzer section on page 4-34. Your spectrum analyzer should display a signal similar to the

one in Figure 4-46.

Figure 4-46 Preamble Amplitude Before Resolution and Video Bandwidth Reduction

Note Figure 4-46 is a display from a standard spectrum analyzer. The following figures, Figure 4-47 through

Figure 4-50, are taken from a vector signal analyzer. If you do not have access to a vector signal analyzer,

or wish to skip the following section describing its use when viewing your upstream signal, proceed to

Step 3.

Step 2 (Optional) View your upstream signal using a vector signal analyzer such as the Agilent 89441A.

The advantage of displaying these signals with the vector signal analyzer is that you can view them over

the time domain for a specified time interval. In addition, the vector signal analyzer enables you to

measure the digital channel power of a very short duration data transmission, like the preamble of a

digital signal.

a. Set up your vector signal analyzer to view both the frequency domain and time domain of your

upstream signal. Your vector signal analyzer should display a pair of signals similar to those in

Figure 4-47.


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Figure 4-47 Vector Signal Analyzer Plot of Upstream Data Burst

The upper graph in Figure 4-47 represents the frequency domain and the lower graph represents the

time domain.

In the time domain, the channel power of the preamble of a digital upstream signal is not spreadacross the entire channel. However, the channel power of the remainder of the digital transmission

is spread across the entire channel. Even though it may not seem so, the total channel power across

both the preamble and the subsequent data segment remains constant.

b. Narrow the view on your vector signal analyzer to display only the preamble of the digital data

signal in both the frequency domain and time domain.

The upper display in Figure 4-48 is a plot of only the preamble portion of the digital signal in

Figure 4-47. Notice how the amplitude of the signal experiences many peaks and valleys. When

you are measuring the preamble power using the zero-span method, be sure that you measure the

actual signal energy (a peak), rather than accidentally measuring the power level of a valley in the

preamble.


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Figure 4-48 Vector Signal Analyzer Plot of Upstream Data Burst (Preamble Only)

Figure 4-47 and Figure 4-48 illustrate the benefit of properly adjusting the center frequency of your

spectrum analyzer to enable the power measurement of the preamble to match the power

measurement of the rest of the digital transmission.

Figure 4-47 and Figure 4-48 show how adjusting the spectrum analyzer in the time domain revealsthis frequency domain phenomenon. The spectrum analyzer is unable to capture the data as shown

in the vector signal analyzer plots.

The power level in the upstream channel fluctuates by approximately 1 dB between Figure 4-47 and

Figure 4-48. This difference is within both the measurement tolerance of the vector signal analyzer

and the accuracy requirement for any DOCSIS-based cable modem.

c. Switch your vector signal analyzer over to Digital Demodulation Mode. Your vector signal analyzer

displays a set of screens similar to those in Figure 4-49.

Using this mode to view your upstream signal allows you to view the same time and frequency

domain information found in Figure 4-48, as well as the upstream signals phase characteristics,

shown in the bottom-right portion of the vector signal analyzer display screen.


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Figure 4-49 Vector Signal Analyzer Plot of Upstream Data Burst (Preamble Only)QPSK

Demodulation Mode

d. Switch your vector signal analyzer over to quaternary phase shift keying (QPSK) demodulation

mode. Your vector signal analyzer will display a set of screens similar to those in Figure 4-50.

Figure 4-50 displays the QPSK demodulation information for the same upstream signal as in

Figure 4-49. However, there are some notable differences in the information presented. For example,

notice that the constellation and transition graphs (top and bottom left) both indicate only two of thefour QPSK data points handling any bits. Because this graph is covering only the preamble of the

data transmission, you get to see only a portion of the whole signal performance. (If you were to

view the entire signal transmission in this mode, all four QPSK data points would display bits.)


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Figure 4-50 Vector Signal Analyzer Plot of Upstream Data Burst (Preamble Only)Digital

Demodulation Mode

Note Before moving on to Step 3, be sure to hook your spectrum analyzer back up to the upstream signal

source.

Step 3 On your spectrum analyzer, narrow both the resolution and video bandwidth to 1 MHz. You will notice

that the preamble of the signal has dropped in amplitude, yielding a spectrum analyzer display similar

to the one in Figure 4-51.


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Figure 4-51 Preamble Amplitude Before Center Frequency Adjustment

Note The slight amplitude variations shown in these figures are normal signal level variations between bursts

in the upstream channel. Expect modems to vary upstream transmit power by nearly 1 dB between

bursts. This is well within the requirements for DOCSIS compliance. The default variation between

modems is up to 1.5 dB for most DOCSIS CMTS equipment.

Step 4 Using the examples in Table 4-2 on page 4-40 as a basis for the formula, calculate the correct center

frequency offset necessary to measure the preamble peak power when viewed in a narrow bandwidth.

In the example, the channel width is 1.6 MHz, which has a symbol rate of 1280 ksym/sec; therefore, the

appropriate offset value is 640 kHz.

Step 5Change the center frequency on the spectrum analyzer by this offset value (33.248 MHz in the example)and check to see that the preamble has regained its lost amplitude by comparing it to the amplitude of

the rest of the signal. If so, the spectrum analyzer should display a signal similar to the one in

Figure 4-52.


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Figure 4-52 Preamble Amplitude Recovery After Center Frequency Adjustment

To get an even better look at the patterns and dramatic shifts in amplitude within the preamble i tself, you

can accelerate the sweep time for your zero-span signal processing.

Step 6 Tune the spectrum analyzer to the original center frequency (32.608 MHz in this example).

Step 7 Reset both the resolution and video bandwidth of the signal back to 3 MHz, but reduce the sweep time

from 200 microseconds to 60 microseconds. The resulting display, similar to Figure 4-53, clearly shows

the tight pattern of the preamble stretched across three-quarters of the spectrum analyzer display.

Figure 4-53 Original Preamble Viewed with Accelerated Sweep Time

Step 8 Change the center frequency back to 33.248 MHz and both the resolution and video bandwidth values

to 1 MHz, retaining the new sweep time of 60 microseconds. The peak amplitude is clearly displayed

with approximately 4.25 dB difference between the preamble and the rest of the upstream data

transmission. (See Figure 4-54.)


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Note The 4.25 dB decrease in amplitude is due to a combination of half of the channel bandwidth (3 dB) and

an additional 1.25 dB decrease attributed to the digital channel filter mask, known as the alpha. The

value of alpha is 25 percent of an upstream DOCSIS channels width, and the peak signal energy spread

across the entire upstream channel width.

Figure 4-54 Preamble with Decreased Amplitude and Condensed Sweep Time

Step 9 Narrow the resolution bandwidth from

connecting and configuring the cable headend - cr72cnrf

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