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This chapter describes how to connect the Cisco uBR7200 series to a cable headend. The chapter contains the following sections:
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Note Before installing your Cisco uBR7200 series, analyze the radio frequency (RF) setup at your headend and configure the analog RF signals for interaction with digital data. This chapter guides you through the process of configuring the RF and digital data at the headend for optimal performance. |
Figure 4-1 shows a typical headend configuration configured for two-way data, including digitized voice and fax.
Figure 4-2 shows a typical headend configuration configured for one-way (downstream) data in a telco return cable system.
This section describes the interaction of digital and analog RF data as both signals are carried on the HFC network.
Two-way digital data signals are more susceptible than one-way signals to stresses in the condition of the 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 variations:
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Note Some HFC equipment will pass 3-MHz signals, which can overload the return path. |
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Note Refer to the user documentation that accompanied your upconverter for safety information and specific installation instructions. |
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Caution If you do not properly configure the upconverter, you might see decreased system performance, increased packet loss, and a reduction in signal-to-noise ratios. |
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Note You might need to add attenuation to the downstream path between the Cisco uBR7200 series and the upconverter to configure the Cisco uBR7200 series. Cisco cable modem 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 modem card you have installed. Add enough attenuation to adjust the modem card IF output level to match the IF input level of your upconverter and to compensate for cable line loss, the actual measured output power, and upconverter performance. Refer to the Cisco uBR7200 Series Universal Broadband Router Cable Modem Card Hardware Installation (Cisco document number 78-10494-01) field-replaceable unit document for more information. |
You must set the upconverter IF input level, according to the manufacturer's instructions, to match the downstream output level of the cable modem cards. Earlier Cisco cable modem card output was +32 dBmV (+/-2 dB). More recently, however, Cisco cable modem cards feature an output rating of +42 dBmV (+/-2 dB). Refer to the "Measuring the Downstream RF Signal" section and be sure to check the documentation that accompanied your Cisco cable modem card or the faceplate of the cable modem card, itself, to determine its downstream output level.
Depending on the upconverter model selected (see "Manufacturers for Headend Provisioning Requirements," for manufacturers and models) you might need to add attenuation to the upconverter input to compensate for cable length in the headend, and for the design of the upconverter. For example, the IF input to the General Instrument C6U is +23 dBmV and this upconverter requires 6 to 19 dB of attenuation on the input cable. The IF input to the Wavecom MA4040 is +33 dBmV and 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.
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Tips |
You must set the upconverter IF input level, according to the manufacturer's instructions, to match the downstream output level of your MC16E cable modem card. The MC16E features an output rating of +40 dBmV (+/-2 dB). Refer to the "Measuring the Downstream RF Signal" section and be sure to check the documentation that accompanied your Cisco cable modem card or the faceplate of the cable modem card, itself, to determine its downstream output level.
Depending on the upconverter model selected (see "Manufacturers for Headend Provisioning Requirements," for manufacturers and models) you might need to add attenuation to the upconverter input to compensate for cable length in the headend, and for the design of 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.
You must now set the upconverter output RF level, according to the manufacturer's instructions. DOCSIS specifications permit an RF output level of +50 to +58 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.
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Note You should never set the upconverter output level greater than +60 dBmV. DOCSIS-based cable modems will not be able to communicate with the headend if the upconverter output level is set that high---compression in the upconverter will yield a distorted signal in overdrive state. |
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Tips |
If you are using a GI, Barco, or Scientific Atlanta upconverter, you can measure the 0 dBmV test point at the front of the upconverter. If you are using a Wavecom upconverter, you can measure the 0 dBmV test point at the back of the upconverter. Your spectrum analyzer should display a signal similar to the one shown in Figure 4-5.
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Note The digital signal amplitude should be set between 6 and 10 dB less than the cable network's video carrier amplitude. |
You must now select an output frequency. DOCSIS specifications permit channels from 91 to 857 MHz (center frequency). In the example shown in Figure 4-14, a center frequency of 610 MHz is used.
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 will receive different content.
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Tips |
You can 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.
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Note Most spectrum analyzers are not designed to precisely measure phase noise. |
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Note Performing the procedures in this section will disconnect any active cable modems currently connected to your Cisco uBR7200 series. |
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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. |
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 of 20 msec. Your analyzer should display a signal similar to the one shown in Figure 4-7.
Step 5 Stop downstream digital data transmission from the cable modem card by setting the downstream symbol rate to 0 on your downstream cable interface. This isolates the unmodulated IF carrier wave (CW) signal for transmission over the downstream. To accomplish this, enter the following Cisco IOS command in interface configuration mode:
Router(config-if)# cable downstream symbol 0
Step 6 Reestablish 64 QAM downstream digital data transmission from the cable modem 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
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Note The value 5056941 symbols/second is specified by DOCSIS for 64-QAM transmissions in a 6-MHz downstream channel plan. A different value exists for 256-QAM transmission. |
Your analyzer should display a signal similar to the one shown in Figure 4-8.
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Note Notice that the total channel power (across 6 MHz) does not change between the modulated carrier and the unmodulated sine wave. |
Step 7 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 8 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-MHz video bandwidth and the same center frequency. Your analyzer should display a signal similar to the one shown in Figure 4-9.
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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. |
Step 9 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.
Step 10 Save this plot on your spectrum analyzer display for comparison with the RF output signal of the upconverter.
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.
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Note Be sure that you have added the appropriate attenuation to your downstream path, if necessary. For more information, refer to the "Setting the North American Upconverter Input Level" section. |
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 your upconverter. 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.
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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. |
A second signal appears on the spectrum analyzer's screen yielding a display similar to that in Figure 4-11.
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.
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Note If you suspect that your upconverter is "drifting over time," place two additional traces on your screen, "min-hold" & "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, which can have an adverse effect on upconverter phase noise. To be certain of your findings, Cisco recommends that you check your upconverter with a QAM analyzer capable of accurate phase noise measurements. |
Step 6 Attach the spectrum analyzer to the output of your cable modem 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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Note This step displays an even greater resolution for both the IF input and RF output signals at your upconverter. |
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.
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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 "Viewing the RF Output Signal," substituting the exact center frequency for 555 MHz. In our example, the exact center frequency is 544.99970 MHz. Your analyzer should display a signal similar to the one shown in Figure 4-13.
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Note Your spectrum analyzer must feature frequency counting capability in order for you to be able to work out the new, adjusted center frequency. |
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 in your cable network headend.
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.
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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" and/or "min-hold" RF plots, your upconverter has most likely suffered an intermittency error. |
After you determine that your upconverter is suitable for reliable downstream data transmission, proceed to the following section, "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, is connected to the laser transmitter input using an 8-way tap and a 3-way splitter. (See Figure 4-14.)
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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. In order 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.)
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Tips 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 RG-59 or 59 series headend coaxial cable, you can see losses of 6 to 8 dB. To compensate for this loss, you can install the upconverter closer to the laser transmitters. |
The nominal input level for a Cisco uBR7200 series upstream port is 0 dBmV, but it can also be adjusted as low as -10 dBmV or as high as +25 dBmV using Cisco uBR7200 series software. The Cisco uBR7200 series will instruct the modem to adjust its output level to match the current input level. Your test cable modem will require a minimum of 8 to 10 dB of attenuation between the upstream of the cable modem and the upstream port on the Cisco uBR7200 series. 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 ratio (C/N) estimate in the cable modem, you need to make further adjustments.
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Note You can measure the preliminary C/N ratio estimate at the headend downstream laser test point. This measurement can be used to verify the performance of the upconverter, headend combiner, and forward distribution 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 equipment is not programed to transmit on the digital carrier. Otherwise, this can cause bit errors and packet loss, resulting in unreliable remote cable modem operation. You can verify the C/N ratio estimate on a Cisco uBR904 or Cisco uBR924 cable access router by following the directions described in the "Using a Cable Modem at the Cable Headend to Verify Downstream Signals" section. |
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Note These two sections describe the procedures necessary to use a spectrum analyzer. You can also use a digital signal level meter designed specifically for this purpose. Some models include the Agilent 2010 or 3010 (http://www.tm.agilent.com), Hukk Engineering CR1200 (http://www.hukk.com), the Tektronix DMA120 (http://www.tek.com), or the Sencore DSL757 (http://www.sencore.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 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.
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Note An analog channel modulator with external IF loops is not suitable for use as a digital Quadrature Amplitude Modulation (QAM) upconverter. These units typically do not have the phase noise performance levels required for 64- and 256-QAM digital signals, and they might cause degraded performance and possible system failure. |
The following sections describe how to measure the downstream RF signal using the channel power option on a spectrum analyzer:
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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. |
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 span to 10 MHz. Your analyzer should display a signal similar to the one shown in Figure 4-15.
Step 5 Measure the IF signal using the channel power option on your spectrum analyzer. Set your channel spacing and your channel bandwidth to 6 MHz. Your analyzer should display a signal similar to the one shown in Figure 4-16.
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Note The IF channel power in Figure 4-16 is +34.23 dBmV, as displayed on the spectrum analyzer. |
Step 6 Select the video averaging feature. Your spectrum analyzer should display a signal similar to the one shown in Figure 4-17.
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Note The peak-to-valley flatness can be verified using the spectrum analyzer's 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. |
Step 2 Connect the downstream output of the cable modem 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.
Step 4 Set the input of the upconverter to a digital QAM signal and the output level to the manufacturer's recommended settings. Typical output amplitudes range from +50 to +58 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.
Step 6 If you add attenuation to the input to the spectrum analyzer you can correct the overload condition as shown in Figure 4-20.
Step 7 Change the spectrum analyzer settings to view the digital channel power. This setting will enable 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 typical range of +50 to +58 dBmV.
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Note A spectrum analyzer might become overloaded and produce false readings (such as internally generated spurs) when measuring a signal at this amplitude. |
Step 8 Adjust the power on the upconverter output to ensure that it is between +50 and +58 dBmV. In Figure 4-22, the upconverter output is reading +57.06 dBmV, which is within the correct range.
Step 9 Select the video averaging feature on the spectrum analyzer. The signal will become smoother and frequency response problems might become visible. Your analyzer will now display an RF signal similar to the one shown in Figure 4-23.
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Note The peak-to-valley flatness can be verified using the spectrum analyzer's 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 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 modem 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. 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 match the recommended settings listed in Table B-4 of "RF Specifications." Record your headend settings in the last column in Table B-4 as you verify them. This will assist in troubleshooting the Cisco uBR7200 series universal broadband 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.
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Note Cisco recommends 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 Hewlett-Packard HP 8591C (http://www.tm.agilent.com) or the Tektronix 2715 (http://www.tek.com) to help you perform the tasks outlined above. |
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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. |
Step 2 Turn the power switch on the spectrum analyzer to the ON position.
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.
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Note Figure 4-24 shows the first of three screens that will be displayed by a Hewlett Packard HP 8591C when you use the analyzer in this mode. Figure 4-25 is the last of the three screens displayed. |
Step 4 Advance to the last of the three screens in this display. Your analyzer should display a signal similar to the one shown in Figure 4-25.
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.
Step 6 Using the display on the analyzer screen, adjust the amplitude of the signal until the shape of the signal is clearly distinguishable as a digitally modulated carrier, as shown in Figure 4-27.
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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 shown in Figure 4-28.
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Note The peak-to-valley flatness can be verified using the spectrum analyzer's 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. |
Proceed to the next section, "Measuring the Downstream RF Signal at the Upconverter Output Using CATV Mode."
Step 2 Connect the downstream output of the cable modem card to the upconverter input connector.
Step 3 Connect the spectrum analyzer to the RF output of the upconverter.
Step 4 Set the output of the upconverter to a digital QAM signal and the output level to the manufacturer's recommended settings. Typical output amplitudes range from +50 to +58 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 analyzer should display a signal similar to the one shown in Figure 4-24.
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Note Figure 4-29 shows the first of three screens that will be displayed by a Hewlett Packard HP 8591C when you use the analyzer in this mode. Figure 4-30 is the last of the three screens displayed. |
Step 7 Advance to the last of the three screens in this display. Your analyzer should display a signal similar to the one shown in Figure 4-30.
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.
Step 9 Using the display on the analyzer screen, adjust the amplitude of the signal until the signal peak is within the top graticule of the analyzer's display grid.
Step 10 Select the video averaging feature. Your spectrum analyzer should display a signal similar to the one shown in Figure 4-33.
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Note The peak-to-valley flatness can be verified using the spectrum analyzer's 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 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 modem 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. |
Step 11 Verify that your headend RF measurements match the recommended settings listed in Table B-4 in "RF Specifications." Record your headend settings in the last column in Table B-4 as you verify them. This will assist in troubleshooting the
Cisco uBR7200 series universal broadband router installation later in the process.
After you have analyzed and adjusted the RF signal according to the steps outlined on the preceding pages, proceed to the next section, "Measuring the Upstream RF Signal."
The following sections describe how to connect and configure the upstream for digital data.
To connect the upstream to the laser receiver, use a 2-way splitter as a combiner to leave the
Cisco uBR904 or Cisco uBR924 cable access router connected at the headend, and connect the upstream headend cable to the laser receiver. (See Figure 4-34.)
You must adjust the upstream input level to the Cisco uBR7200 series using the Cisco IOS software running on your router so the output of the laser receiver is the same as the input to your upstream port or your Cisco uBR7200 series. The Cisco uBR7200 series uses automatic power control when transmitting to remote cable modems. Accurately setting the power level will help to ensure reliable cable modem operation.
Table 4-1 provides upstream input power ranges for the various cable modem cards available for the Cisco uBR7200 series, depending on the channel bandwidth you are using.
| Channel Bandwidth | MC11 FPGA | MC16B, MC1xC1, MC16E, and MC16S | DOCSIS Specification |
|---|---|---|---|
200 KHz | n/a | -10 to +25 dBmV | -16 to +14 dBmV |
400 KHz | n/a | -10 to +25 dBmV | -13 to +17 dBmV |
800 KHz | n/a | -10 to +25 dBmV | -10 to +20 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 |
| 1The designation "MC1xC" includes the MC11C, MC12C, MC14C, and MC16C cable modem cards. |
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Note If you have an MC16 cable modem card (six upstream ports and one downstream port) installed in your Cisco uBR7200 series universal broadband router, the 2-way splitter described above would be replaced by six 2-way splitters. This would enable you to connect to all of the available upstream ports on the MC16. |
To test the upstream configuration, insert a test signal of known amplitude (such as +17 dBmV) into the fiber node and measure the amplitude output level at the output of the headend's optical receiver. This measurement will depend on return laser performance and optical distance. This measurement is known as the "X" point. (See Figure 4-35.)
This "X" 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" points connected to the same upstream port are all calibrated to +10 dBmV using different attenuators.
Figure 4-37 shows how three distribution network "X" points connected to the three different upstream ports are all calibrated to +10 dBmV using different attenuators.
You 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 network's 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 to as the "zero-span" method.
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.
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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. |
Step 2 Turn the power switch on the spectrum analyzer to the ON position.
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 and set the span to 0 MHz.
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Note You can view the configuration file for your Cisco uBR7200 series by using the show controller cable slot/upstream-port | include frequency command, available in Cisco IOS Release 11.3(6)NA or later and Cisco IOS Release 12.0(5)T1 or later. For example, if you wanted to view the center frequency of port 0 on a cable modem 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 4 Set both the resolution bandwidth and the video bandwidth on the spectrum analyzer to 3 MHz. 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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Note The horizontal line passing through the center of the spectrum analyzer display in Figure 4-38 is the trigger line. |
Step 5 Set the sweep value to 80 µsec. Your spectrum analyzer should display a signal similar to the one shown in Figure 4-39.
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Note Be sure your particular spectrum analyzer is capable of supporting sweep times as little as 80 µsec. |
Step 6 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 Please refer to the documentation that accompanied your particular spectrum analyzer for detailed instructions on activating and positioning the trigger line. A known workaround exists for the HP 8591C spectrum analyzer. After activating and positioning the trigger line in video mode, you must press the "video" button on the spectrum analyzer once more to enable proper functionality. |
Step 7 Adjust the amplitude on your spectrum analyzer so that the uppermost portion of the upstream RF signal is in the top graticule of the analyzer's 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.
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Note Cisco does not recommend using the spectrum analyzer's "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. |
Step 8 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.
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Note To verify this reading, you can also measure the power rating with an HP 89441A vector signal analyzer (http://www.tm.agilent.com). |
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 for instructions on how to overcome this phenomenon.
Step 9 Verify that your headend RF measurements match the recommended settings listed in Table B-3 in "RF Specifications." Record your headend settings in the last column in Table B-3 as you verify them. This will assist in troubleshooting the Cisco uBR7200 series universal broadband router installation later in the process.
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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 measurement(s) would register lower than the actual transmission level(s). |
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 2 Adjust the sweep time on your spectrum analyzer to 20 msec.
Step 3 Ping the remote cable modem 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 modem. 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 larger spikes are the actual 64- or 1500-byte ping packet returns.
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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. We will label the remote cable modem with the lesser amplitude "cable modem A" and the other "cable modem B."
In the following example, cable modem A and cable modem B have been artificially configured to yield a 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.
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Note To further illustrate this point, you can log into your Cisco uBR7200 series universal broadband router via a console terminal and enter the show cable modem command to obtain a report of the receive power ratings for each modem. In our 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 due to the contention-based nature of multiple remote cable modems on the same line. Differences in the distance between ping packet returns are primarily due to factors such as packet size and system loading.
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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% occupied during this time. |
Figure 4-45 shows 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, you will notice that the carrier-to-noise ratio measurement between the two diamond-shaped markers is only about 12 dB. (A few other noise peaks are even worse.)
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.
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Note This illustration depicts an upstream RF signal whose carrier-to-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. |
When measuring upstream signals using the zero-span method, a very wide resolution and video bandwidth gives very accurate readings, but renders 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.)
| Center Frequency | Channel Width | Symbol Rate | 1/2 Symbol Rate | Center Frequency +/- 1/2 Symbol Rate | Minimum Resolution Bandwidth |
|---|---|---|---|---|---|
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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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 like the HP 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.
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.
c. Switch your vector signal analyzer over to Digital Demodulation Mode. Your vector signal analyzer will display a set of screens similar to those in Figure 4-49.
d. Switch your vector signal analyzer over to QPSK Demodulation Mode. Your vector signal analyzer will display a set of screens similar to those in Figure 4-50.
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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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 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 our 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 5 Change the center frequency on the spectrum analyzer to this value (33.248 MHz in our 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, your spectrum analyzer should display a signal similar to the one in Figure 4-52.
To get an even better look at the patterns and dramatic shifts in amplitude within the preamble, itself, you can accelerate the sweep time for your zero-span signal processing.
Step 6 Return the center frequency value to 32.608 MHz, reset both the resolution and video bandwidth of the signal back to 3 MHz, but reduce the sweep time from 200 µsec to 60 µsec. 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.
Step 7 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 µsec. 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% of any upstream DOCSIS channel, and the peak signal energy is spread across the entire upstream channel width. |
Step 8 Narrow the resolution bandwidth from 1 MHz to 100 kHz and increase the video bandwidth to 3 MHz, still retaining the 60 µsec sweep time. Your spectrum analyzer should display a signal similar to Figure 4-55.
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Note The slight "ramp-up" at the beginning of the preamble when viewed in this mode is attributed to the time required to charge the spectrum analyzer's detector circuit. |
Figure 4-55 shows a smooth and easily measured signal amplitude, providing accurate measurement of a very fast burst upstream carrier. You can compare the measurements obtained using a spectrum analyzer with those of specialized test equipment. In general, the readings from the spectrum analyzer will be within 1 to 2 dB of the (more expensive) specialized equipment. Because 1 to 2 dB is well within the calibration accuracy of spectrum analyzers, you can reliably use these procedures in the cable headend environment.
This section describes RF signal measurements that should be taken with a spectrum analyzer at the downstream forward test point on the fiber-optic laser transmitter. (See Figure 4-14 for the location of the downstream forward test point.)
Use the following steps to measure the downstream forward test point on the fiber-optic laser transmitter:
Step 2 Using the spectrum analyzer zoom feature, zoom the display in on the first individual video channel. In the example in Figure 4-57, the first video channel is channel 48.
Step 3 Select the next channel option. Figure 4-58 shows the detailed display of the analog carrier level and frequency screen for the channel 48 (in this example).
Step 4 Return to the main menu on your spectrum analyzer.
Step 5 Select a digital channel to measure. In the example in Figure 4-59, the digital channel shown is channel 50.
Step 6 Go to the main menu on the spectrum analyzer and advance the screen displays (next screen) until the digital channel power display is shown. (See Figure 4-60.)
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Note The +3.6 dBmV digital power rating is nearly the same as the previously measured video carrier level (+4.3 dBmV). This value is too high to provide reliable digital data transmission. |
Step 7 Select the video averaging feature to verify flatness through the headend combiner. After 10 averages, the power rating will decrease by approximately 2.5 dB from actual digital channel power. While video averaging is in progress, your spectrum analyzer should display a signal similar to the one shown in Figure 4-61.
Cisco recommends installing a Cisco uBR904 or Cisco uBR924 cable access router at the headend to verify the digital data configuration. For instructions on how to install a Cisco uBR904 or
Cisco uBR924 cable access router, refer to the Cisco uBR904 Cable Modem Installation and Configuration Guide or the Cisco uBR924 Cable Access Router Installation and Configuration Guide.
The output of the Cisco uBR7200 series is a standard 44 MHz center frequency IF signal. IF signals are converted to RF signals through an upconverter. Upconverter output levels should be set to carry the digital signal data at 6 to 10 dB below the adjacent analog video signal. The value chosen is at the discretion of each cable operator.
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Note The value chosen for the digital data in relation to the adjacent video signal must be made available to field technicians installing Cisco uBR904 or Cisco uBR924 cable access routers. At a cable modem connection, this value can be measured to verify the correct operation of the cable modem. |
Careful system design and operation can prevent potentially serious intermittent performance problems across your cable modem network. Each cable operator should make use of the following guidelines and practices to ensure reliable operation of any 64-QAM based digital network:
For example, if your headend overdrives the fiber-optic lasers, in either the upstream or downstream path, clipping can occur. Fiber-optic clipping leads to damaged signal integrity. In minor doses, this signal damage is not immediately visible on an analog video signal, but it can completely disrupt the digital transmission path. (That is, digital signals are more sensitive to clipping than analog signals and will more readily display the negative effects of laser clipping.)
If a digital signal employing forward error correction (FEC) is near its impairment limit, it is very susceptible to changes in signal level---on the order of 0.1 dB. If there is no amplitude margin available in the transmission path between the headend and any one cable modem, the typical signal level variations of a properly functioning cable system (3 to 6 dB) can create intermittent service outages that are difficult to isolate.
Typical CATV measurement equipment, such as digital signal level meters, measure to an accuracy of +/-1 dB. However, some older analog meters only measure to an accuracy of +/-3 dB; therefore, maintaining 6 dB margins above the minimum levels can provide reliable long-term service.
Posted: Tue Mar 28 08:48:27 PST 2000
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