CATV Headend Considerations

This chapter describes the cable television (CATV) headend site requirements and considerations for installing the Cisco uBR7246. The chapter contains the following sections:

The Connection to the Internet
RF Configuration
Digital Data Configuration
RF and Digital Data on the Same Network

Before installing your Cisco uBR7246, you should analyze the radio frequency (RF) setup at the CATV headend installation site and configure the analog RF signals for the interaction with digital data. This chapter guides you through the process of configuring the RF and digital data at the headend for optimal performance.

The Connection to the Internet

The Cisco uBR7246 universal broadband router provides a connection to the Internet from your CATV headend through port adapters installed in your Cisco uBR7246. The cabling, or media, used to make this connection depends on which port adapters you choose. The Cisco uBR7246 universal broadband router currently supports high-speed serial interface (HSSI), asynchronous transfer mode (ATM), Fast Ethernet, or Ethernet port adapters.

Your cable customers connect to the headend using the standard hybrid fiber coaxial (HFC) cable network and a cable modem connected to their computer. These connections are enabled by the cable modem cards installed in your Cisco uBR7246.

Figure 4-1 illustrates the connection from the subscriber to the headend, and the headend to the Internet using a Cisco uBR7246 universal broadband router. The digital data is transmitted and received on both sides of this connection using Internet Protocol (IP).

Figure 4-1: The Connection to the Internet

RF Configuration

The configuration of the analog RF signal at the headend is critical to the performance of the Cisco uBR7246 universal broadband router and Cisco cable modems. The following guidelines are provided to assist you in configuring the RF signal to the necessary specifications:

Step 1 Connect a spectrum analyzer to the RF line. Set the resolution bandwidth to 100 kHz and the video bandwidth to 30 kHz. Your analyzer should display a raw RF signal similar to the one shown in Figure 4-2.

Figure 4-2: Raw RF Signal on the Spectrum Analyzer

Step 2 Adjust the spectrum analyzer to set the resolution bandwidth to 1.0 MHz and the video bandwidth to 1 MHz. Your analyzer will now display an intermediate modified RF signal similar to the one shown in Figure 4-3.

Figure 4-3: Intermediate Modified RF Signal on the Spectrum Analyzer

Step 3 Using the video averaging function on the spectrum analyzer, set video averaging to count 100. Add 5 dB to the digital amplitude of the RF signal to adjust for correct channel power.

Note

Figure 4-4: Final Video Averaging RF Signal on the Spectrum Analyzer

Step 4 Verify that your headend RF specifications match the recommended settings listed in Table 4-1. Record your headend settings in the last column in
Table 4-1 while you verify them. This will assist in troubleshooting the Cisco uBR7246 installation later in the process.

Note

Table 4-1

Table 4-1: RF Specifications

Specification	MCNS Specifications¹	Recommended Settings²
System/Channel
Carrier to noise (upstream)	> 25 dB (QPSK³)⁴ > 25 dB (16 QAM⁵)4	> 15 dB (QPSK)4 > 18 dB (16 QAM)4
Carrier to noise (downstream)	> 23.5 dB (64 QAM)4 > 35 dB (256 QAM)4	> 25 dB (64 QAM)4 > 33 dB (256 QAM)4
Carrier to hum	< -26 dBc⁶	< -26 dBc
Carrier to second order	< -50 dBc	< -50 dBc
Amplitude variation	0.5 dB in 6 MHz	0.5 dB in 6 MHz
Group delay	75 ns⁷ in 6 MHz	75 ns in 6 MHz
Digital Signal Levels
From cable modem (upstream)	8 to 55 dBmV	35 to 55 dBmV
Input amplitude to modem card (upstream)	-	-8, 0, or 8 dBmV
From headend (downstream)	-15 to 15 dBmV	-15 to 15 dBmV
Signal as relative to adjacent video signal	-6 dB or -10 dB	-6 dB or -10 dB

¹ MCNS specifications are baseline settings for an MCNS-compliant, two-way data-over-cable system.
² Recommended settings are slightly higher than the MCNS settings to account for cable system variations over time and temperature. Using these settings should increase the reliability of MCNS-compliant, two-way data-over-cable systems.
³ QPSK = Quadrature Phase-Shift Keying: a method of modulating digital signals onto a radio-frequency carrier signal using four phase states to code two digital bits.
⁴ These settings are measured relative to the digital carrier. Add 6 or 10 dB, as determined by your company's policy and derived from the initial cable network setup, relative to the analog video signal.
⁵ QAM = Quadrature Amplitude Modulation: a method of modulating digital signals onto a radio-frequency carrier signal involving both amplitude and phase coding.
⁶ dBc = decibels relative to carrier.
⁷ ns = nanoseconds.

Once you have analyzed and adjusted the RF signal according to the steps outlined on the preceding pages, proceed to the next section "Digital Data Configuration."

Digital Data Configuration

Once you have configured the RF signal, you must configure the digital data signal that will be carried between the Cisco uBR7246 universal broadband router and cable modems.

You should install a Cisco cable modem at the headend to verify the digital data configuration. For instructions on how to install a Cisco cable modem, refer to the Cisco Cable Modem Installation and Configuration Guide (Document Number 78-4769-xx). This document accompanies every Cisco cable modem that is shipped from the factory.

The output of the Cisco uBR7246 universal broadband router is in standard intermediate frequency (IF) signals. IF signals are converted to RF signals through an upconverter. Upconverter output levels should be set to carry the digital signal data at 6 dB or 10 dB below the adjacent analog video signal. The value chosen is at each cable operator's discretion.

Note The value chosen for the digital data in relation to the adjacent video signal must be made available to field technicians installing Cisco cable modems.

At a cable modem connection, this value can be measured to verify the correct operation 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 QAM 64-based digital network:

Society of Cable Television Engineers (SCTE) standard recommended procedures
FCC regulations of 1993
Standard engineering practices

For example, if your headend overdrives the fibre optic lasers, in either the upstream or downstream path, clipping (the self-protecting, automatic shutoff feature in fiber-optic lasers) can occur. Fiber optic clipping leads to damaged signal integrity. This signal damage is not visible on an analog video signal, but it can completely disrupt the digital transmission path. Digital signals with forward error correction (FEC) are 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 spectrum analyzers, measure to an accuracy of +/- 1.0 dB. However, field meters measure to an accuracy of +/- 3dB; therefore, maintaining 6 dB margins above the minimum levels can provide reliable long term service.

RF and Digital Data on the Same Network

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 video signals to stresses in the overall 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:

Ingress signals--Noise from sources such as amateur radio transmissions; citizen band radios; or high-frequency, high-power broadcast signals that are often picked up by cabling and equipment on the network.
Electrical signal ingress--Noise can enter the network from electrical 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 can be located and repaired by implementing a cumulative leakage index (CLI) maintenance program.
Amplifier noise--Amplifiers add noise to the HFC network that go unnoticed in video signals. Improperly configured amplifiers will degrade digital data signals. The larger the network, the higher the probability of amplifier noise affecting the signals.
Noise funneling--The upstream data path to the headend is susceptible to picking up noise and interference from throughout the network and all upstream noise ultimately ends up at the headend. This effect is know as noise funneling because of the cumulative nature of the noise from anywhere on the network that becomes concentrated at the headend. As a network serviced by a single RF receiver increases in size, the probability of noise funneling also increases.
Variable transmit levels--Signal loss over coaxial cable is affected by temperature and can vary by up to 6 to 10 dB per year.
Clipping--The lasers in fiber-optic transmitters can shut themselves off (clipping) if unusual current voltage peaks are detected. This protects the laser from damage but introduces bit errors in both the upstream and downstream transmissions. If a laser is overdriven as briefly as 1 microsecond, clipping can occur.

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