Table Of Contents

Spectrum Management and Advanced Spectrum Management for the Cisco CMTS

Contents

Prerequisites for Spectrum Management and Advanced Spectrum Management

Restrictions for Spectrum Management

Shared Spectrum Groups

Cisco IOS Releases and Cable Interface Line Card Support

Cisco uBR10012 Router and Cisco IOS Release 12.2(8)BC2 Support

DOCSIS Cable Modem Test Analyzer

Dynamic Upstream Modulation

Fixed-Frequency Spectrum Groups with Advanced Spectrum Management

Limitations on Upstream Modulation Parameters for PacketCable VoIP Calls

HCCP 1+1 and N+1 Redundancy Support

Intelligent and Advanced Spectrum Management Support

Information About Spectrum Management

Spectrum Management Measurements

Signal and Carrier Noise Ratios

Differences Between the SNR and CNR Values

Additional Measurements

Upstream Signal Channel Overview

Upstream Frequency Changes

Upstream Segments and Combiner Groups

Frequency Management Policy

Noise Impairments

Spectrum Groups and Frequency Hopping

Guidelines for Spectrum Management

Guided and Scheduled Spectrum Management

Traffic Shaping

Frequency Hopping Capabilities

Dynamic Upstream Modulation (SNR-based)

Input Power Levels

Intelligent and Advanced Hardware-Based Spectrum Management

Intelligent Spectrum Management Enhancements

Advanced Spectrum Management Support Using the Cisco uBR-MC16S Cable Interface Line Card

Advanced Spectrum Management Suppport Using the Cisco uBR-MC5X20S/U BPE

Benefits

Guided and Scheduled Spectrum Management Benefits

Intelligent and Advanced Spectrum Management Benefits

How to Configure Spectrum Management

Guided and Scheduled Spectrum Management Configuration Tasks

Enabling Upstream Rate Limiting

Enabling Downstream Rate Limiting

Creating and Configuring Spectrum Groups

Assigning a Spectrum Group to One or More Upstream Ports

Configuring Shared Spectrum Groups (Fiber Node Groups) for DOCSIS 3.0

Configuring Dynamic Upstream Modulation (SNR-Based)

Verifying Frequency Hopping

Intelligent and Advanced Spectrum Management Configuration Tasks

Configuring and Assigning Spectrum Groups

Configuring Dynamic Upstream Modulation (CNR-Based)

Configuring Proactive Channel Management

Verifying the Spectrum Management Configuration

Monitoring Spectrum Management

Using CLI Commands

Using SNMP

ccsSNRRequestTable

ccsSpectrumRequestTable

ccsSpectrumDataTable

ccsUpSpecMgmtTable

ccsHoppingNotification

Configuration Examples

Upstream Traffic Shaping and Rate Limiting Examples

Configuring the Low-Peak-Rate Limit Example

Applying the Rate-Limiting Algorithm Without Rate Limiting Example

Enabling Shaping Example

Forcing the Cable Modem to Exceed the Peak Rate Example

Downstream Traffic Shaping and Rate Limiting Examples

Downstream Rate Limiting Example

Verifying Downstream Rate Limiting Example

Spectrum Group and Combiner Group Examples

Verifying Spectrum Group Creation Example

Time-Scheduled Spectrum Group Example

Verifying Spectrum Group Configuration Example

Determining the Upstream Ports Assigned to a Combiner Group Example

Combiner Group Example

Other Spectrum Management Configuration Examples

Dynamic Upstream Modulation Examples

Verifying Your Settings

Modulation Profiles Example

Input Power Level Example

Advanced Spectrum Management Configuration Examples

Advanced Spectrum Management for the Cisco uBR7200 Series Router Example

Advanced Spectrum Management for the Cisco uBR10012 Router Example

Additional References

Related Documents

Standards

MIBs

RFCs

Technical Assistance

Spectrum Management and Advanced Spectrum Management for the Cisco CMTS

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Revised: October 1, 2007, OL-1467-08

This chapter describes the spectrum management features supported by the Cisco Cable Modem Termination System (CMTS) universal broadband routers. Spectrum management support is divided into two main groups:

•Guided and scheduled spectrum management features (supported in software)

•Intelligent and advanced spectrum management features (supported in hardware only on specific cable interfaces)

Cisco IOS software release 12.3(13a)BC introduces advanced spectrum management support (software and hardware) for the Cisco uBR5X20S/U broadband processing engine (BPE) in the Cisco uBR10012 universal broadband router.

Release	Modification
Release 11.3(9)NA, Release 12.0(6)SC, and Release 12.1(2)EC	Guided and scheduled spectrum management was introduced on Cisco uBR7200 series routers.
Release 12.1(2)EC	Support was added for intelligent spectrum management on the Cisco uBR-MC16S cable interface card on the Cisco uBR7200 series router.
Release 12.1(5)EC	Support was added for guided and scheduled spectrum management on Cisco uBR7100 series routers.
Release 12.1(10)EC1, Release 12.2(4)BC1	The SNR algorithm was corrected to display a more accurate value for upstreams.
Release 12.1(7)CX, Release 12.2(4)BC1	Support was added for advanced spectrum management on the Cisco uBR-MC16S cable interface card on Cisco uBR7200 series routers.
Release 12.2(4)BC1	Support was added for guided and scheduled spectrum management on Cisco uBR10012 routers.
Release 12.2(8)BC2	Support was added for intelligent and advanced spectrum management on the Cisco uBR-LCP2-MC16S cable interface card on the Cisco uBR10012 router.
Release 12.2(11)BC3	Support was added for the cable spectrum-group shared command on the Cisco uBR-LCP2-MC16S card on the Cisco uBR10012 router.
Release 12.2(15)BC1	Support was added for guided and scheduled spectrum management on the Cisco uBR-MC5X20S cable interface line card.
Release 12.2(15)BC2	This release added the following support: •Support was added for intelligent and advanced spectrum management on the Cisco uBR-MC16U/X and Cisco uBR-MC28U/X cable interface line cards. •Support was added for guided and scheduled spectrum management on the Cisco uBR-MC5X20U card. •The maximum number of spectrum groups was increased from 32 to 40 groups per router. •The number of predefined modulation profiles was increased.
Release 12.3(9)BC	This release added the following support: •Spectrum management support with the Cisco Broadband Troubleshooter (CBT) 3.2 for the Cisco MC5X20S/U Broadband Processing Engine (BPE).
Release 12.3(13a)BC	This release added the following support: •Advanced spectrum management support for the Cisco MC5X20S/U Broadband Processing Engine (BPE) in the Cisco uBR10012 Universal Broadband router
Release 12.3(21)BC	This release added the following support: •Shared spectrum groups for DOCSIS 3.0, otherwise referred to as Fiber Node Groups, supporting inter-line card or intra-line card group combining. Refer to the "Configuring Shared Spectrum Groups (Fiber Node Groups) for DOCSIS 3.0" section. Note The show controllers cable upstream spectrum command is obsolete.

Feature History for Spectrum Management for the Cisco CMTS

Finding Support Information for Platforms and Cisco IOS Software Images

Use Cisco Feature Navigator to find information about platform support and Cisco IOS software image support. Access Cisco Feature Navigator at http://www.cisco.com/go/fn. You must have an account on Cisco.com. If you do not have an account or have forgotten your username or password, click Cancel at the login dialog box and follow the instructions that appear.

Contents

•Prerequisites for Spectrum Management and Advanced Spectrum Management

•Restrictions for Spectrum Management

•Information About Spectrum Management

•How to Configure Spectrum Management

•Monitoring Spectrum Management

•Configuration Examples

•Additional References

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Note This chapter provides configuration information but not a complete command reference. For complete information on the commands used in this chapter, see the Cisco Cable Modem Termination System Commands chapter in the Cisco Broadband Cable Command Reference Guide.

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Prerequisites for Spectrum Management and Advanced Spectrum Management

•The appropriate Cisco IOS release for the desired features. For a list of supported Cisco IOS releases by feature, see Table 18-1.

•Guided and scheduled spectrum management features require one of the following Cisco CMTS routers, and one or more of the indicated cable interfaces:

Cisco uBR7100 series (all models)

Cisco uBR7200 series router and one or more of the following cable interfaces:

–Cisco uBR-MC11C

–Cisco uBR-MC12C

–Cisco uBR-MC14C

–Cisco uBR-MC16C/E/S/U/X

–Cisco uBR-MC28C/U/X

Cisco uBR10012 router and one or more of the following cable interfaces:

–Cisco uBR-LCP2-MC16C/E/S

–Cisco uBR-LCP2-MC28C

–Cisco uBR-MC5X20S/U

•Intelligent and advanced spectrum management (hardware-based, CNR frequency hopping) requires the following Cisco CMTS routers and one of more of the indicated cable interfaces:

Cisco uBR7200 series router and one or more of the following cable interfaces:

–Cisco uBR-MC16S/U/X

–Cisco uBR-MC28U/X

Cisco uB10012 router and the following cable interface:

–Cisco uBR-LCP2-MC16S

–Cisco uBR-MC5X20S/U

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Note You must have Cisco IOS Release 12.3(13a)BC installed in your router if you are using the Cisco uBR-MC5X20S/U BPE.

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•Ensure that your network is designed to support reliable broadband data transmission. At minimum, your network must include:

–A Dynamic Host Configuration Protocol (DHCP) server to assign IP addresses to cable modems or set-top boxes on the hybrid fiber-coaxial (HFC) network. This can be a server on the WAN side of the Cisco uBR7200 series router or a Cisco CMTS router that has been configured to act as the DHCP server.

–If you are not using the Cisco uBR7100 series router with integrated upconverter, you must install the appropriate IF-to-RF external upconverter between the Cisco CMTS router and the combiner.

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Note The term "combiner" refers to all cables, amplifiers, and taps at the headend or cable distribution center that connect the Cisco CMTS router to the HFC network.

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–Diplex filters installed in the downstream RF path between the cable modems and the cable interface cards in the router. RG-59 headend coaxial cable with the maximum braid available (60 percent + 40 percent braid), double foil, and the correct connector for this cable.

•Avoid frequencies with known ingress problems such as amateur radio bands or short-wave bands.

•Avoid hostile spectrums below 20 MHz.

•When designing your channel plan, allow extra bands for frequency hopping.

•Place upstream ports in the same combiner group in a shared spectrum group.

•Use the receive power level setting to perform slight equalization adjustments.

•Due to the nature of CATV technology, upstream noise management is a significant issue. We recommend that you follow the rigorous North American plant maintenance procedures that are documented in the NCTA Supplement on Upstream Transport Issues to adjust return amplifiers and lasers.

Restrictions for Spectrum Management

This section describes the restrictions for the following spectrum management features:

•Shared Spectrum Groups

•Cisco uBR10012 Router and Cisco IOS Release 12.2(8)BC2 Support

•Cisco IOS Releases and Cable Interface Line Card Support

•DOCSIS Cable Modem Test Analyzer

•Dynamic Upstream Modulation

•Fixed-Frequency Spectrum Groups with Advanced Spectrum Management

•Limitations on Upstream Modulation Parameters for PacketCable VoIP Calls

•HCCP 1+1 and N+1 Redundancy Support

•Intelligent and Advanced Spectrum Management Support

Shared Spectrum Groups

•Advance spectrum management does not support inter-line-card shared spectrum groups.

•Guided spectrum management does support inter-line-card shared spectrum groups.

Cisco IOS Releases and Cable Interface Line Card Support

The guided and scheduled spectrum management features are available for all currently supported cable interface line cards. These features were released in phases. Table 18-1 summarizes the individual features in this basic spectrum management feature set, and the initial Cisco IOS software releases that introduced them.

Table 18-1 Summary of Guided and Scheduled Spectrum Management Features by Release

Feature	Cisco IOS Release Supported
Traffic Shaping Upstream Traffic Shaping Downstream Traffic Shaping	12.1(2)EC1, 12.2(4)BC1, and later releases
Dynamic Upstream Modulation (SNR-based) Guided Frequency Hopping Time-Scheduled Frequency Hopping	12.1(3a)EC1,12.0(13)SC, 12.2(4)BC1, and later releases
Input Power Levels	12.0(6)SC, 12.1(2)EC1, 12.2(4)BC1, and later releases
Advanced Spectrum Management Suppport Using the Cisco uBR-MC5X20S/U BPE	12.3(13a)BC and later releases

The intelligent and advanced spectrum management features were also released in phases. Table 18-2 shows the minimum software releases that are needed for these features on the cable interface line cards that support them.

Table 18-2 Minimum Cisco IOS Releases for Intelligent and Advanced Spectrum Management Support

Cable Interface Line Card	Minimum Cisco IOS Release
Cisco uBR7200 Series Routers
Cisco uBR-MC16S	12.1(7)EC (intelligent features) 12.2(8)BC2 (advanced features)
Cisco uBR-MC16U/X	12.2(15)CX, 12.2(15)BC2
Cisco uBR-MC28U/X	12.2(15)CX, 12.2(15)BC2
Cisco uBR10012 Router
Cisco uBR-LCP2-MC16S	12.2(8)BC2
Cisco uBR-MC5X20S/U	12.3(13a)BC

Cisco uBR10012 Router and Cisco IOS Release 12.2(8)BC2 Support

The Cisco uBR10012 router using the Cisco uBR-LCP2-MC16S cable interface line card and Cisco IOS Release 12.2(8)BC2 has the following restrictions and limitations:

•Frequency hopping is based on the carrier-to-noise ratio (CNR) and the correctable FEC error and uncorrectable FEC error values. A channel will frequency hop if the CNR falls below the configurable threshold AND either the correctable or uncorrectable FEC error values exceed the configurable threshold values.

Use the cable upstream threshold command to change these values. In Cisco IOS Release 12.2(8)BC2, the CNR threshold for the primary modulation profile defaults to 25 dB. The CNR threshold for the secondary modulation profile defaults to 15 dB. The correctable FEC error threshold defaults to 1 percent of total packets received, and the uncorrectable FEC error threshold defaults to 1 percent of total packets received.

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Note In releases previous to Cisco IOS Release 12.2(8)BC2, the channel would hop if either the CNR fell below the configurable threshold OR either the correctable or uncorrectable FEC error values exceeded the configurable threshold values. If you are using only one modulation profile (QPSK), you might need to reduce the CNR threshold value and increase the correctable FEC error value to prevent undesired frequency hopping.

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•Fixed-frequency spectrum groups are not supported for spectrum management in Cisco IOS Release 12.3(9a)BC and earlier releases. Fixed-frequency spectrum groups are supported for advanced spectrum management. Refer to the "Fixed-Frequency Spectrum Groups with Advanced Spectrum Management" section.

•Inter-line-card shared spectrum groups are not supported.

•In Cisco IOS Release 12.2(8)BC2, the short and long grant bursts of the pre-configured mix and qam-16 modulation profiles (see the cable modulation-profile command) default to a unique word offset of 8 (uw8). These values should be changed to uw16 for optimal performance. This can be done with the following commands:

cable modulation-profile n short 6 75 6 8 16qam scrambler 152 no-diff 144 

fixed uw16 

cable modulation-profile n long 8 220 0 8 16qam scrambler 152 no-diff 160 

fixed uw16 

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Note This default was tracked by caveat CSCdx35070 and was fixed in Cisco IOS Release 12.1(19)EC1 and Cisco IOS Release 12.2(11)BC3.

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•With Cisco IOS Release 12.2(8)BC2, in rare situations, using CLI commands to configure the Cisco uBR-LCP2-MC16 card could result in the error message "Resources are busy with other msg requests. Try later." The workaround is to repeat the same configuration command again until the error disappears.

DOCSIS Cable Modem Test Analyzer

Cisco IOS Release 12.2(8)BC2 through Cisco IOS Release 12.2(11)BC3 support the DOCSIS Cable Modem Test Analyzer (DCMTA) software from Acterna Corp. This spectrum management tool is designed for troubleshooting ingress and other problems on the return path in real time, not for ongoing monitoring of the upstream spectrum.

In Cisco IOS Release 12.2(15)BC1 and later releases, the Acterna DCMTA tool is no longer supported. The Cisco Broadband Troubleshooter (CBT), release 3.0 or later, replaces the DCMTA tool. For more information, see the Cisco Broadband Troubleshooter documentation, at the following URL:

http://www.cisco.com/univercd/cc/td/doc/product/cable/trblshtr/index.htm

Dynamic Upstream Modulation

•The Cisco router has one preconfigured (primary) modulation profile that defines a typical profile for QPSK modulation. To use the Dynamic Upstream Modulation feature, you must create a second modulation profile that has a higher modulation scheme than the preconfigured profile.

•Upstream modulation profiles are assigned to upstream ports and affect all cable modems on those upstream ports.

•Modulation profiles affect the physical layer of the cable network, so only trained technicians who are familiar with the DOCSIS specifications should create modulation profiles.

•When using the Dynamic Upstream Modulation feature with Voice over IP (VoIP) services, frequent changes to the upstream modulation or channel width could briefly impact the quality of voice calls

•The Dynamic Upstream Modulation feature operates differently on the Cisco uBR-MC16S cable interface line card than on other supported cable interface line cards. See the description of this feature in the "Guided and Scheduled Spectrum Management" section and the "Intelligent and Advanced Hardware-Based Spectrum Management" section for more information.

Fixed-Frequency Spectrum Groups with Advanced Spectrum Management

When using cable interface line cards that support advanced spectrum management (such as Cisco uBR-16S/U/X, Cisco uBR-MC28U/X, and the Cisco uBR-MC5X20S/U), do not configure fixed-frequency spectrum groups by specifying a frequency using the cable spectrum-group frequency command (for example, cable spectrum-group 3 frequency 76000000). If fixed-frequency spectrum groups are desired, configure a band with a starting and ending range, which, along with the desired channel width, specifies the desired center frequency. In this situation, you must also configure a static channel width so that the Dynamic Upstream Modulation feature does not attempt to hop to a different frequency using a smaller channel width.

For example, to specify a center frequency of 7.6 MHz with a 3.2 MHz channel width, specify a starting frequency of 6.0 MHz (7.6 MHz - 1.6 MHz) and an ending frequency of 9.2 MHz (7.6 MHz + 1.6 MHz):

CMTS(config)# cable spectrum-group 15 band 6000000 9200000 

CMTS(config)# interface cable 6/0 

CMTS(config-if)# cable upstream 0 channel-width 3200000 3200000 

CMTS(config-if)# cable upstream 0 spectrum-group 15 

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Note Cisco IOS Release 12.2(8)BC2 does not support spectrum groups with fixed frequencies on the Cisco uBR10012 router.

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Limitations on Upstream Modulation Parameters for PacketCable VoIP Calls

When PacketCable support is enabled on the Cisco CMTS to provide Voice over IP (VoIP) support, the following combinations of upstream modulation parameters should not be used, because the channel width is too small to allow the upstream MAC scheduler to provide sufficient grants for reliable VoIP communications.

Table 18-3 lists the modulation, channel width, and minislot size parameters that should not be used when an upstream is providing support for PacketCable VoIP calls:

Table 18-3 Unsupported Upstream Parameter Combinations for VoIP Calls 

Modulation1	Channel Width	Minislot Size
QPSK	200 KHz	32, 64, 128
QPSK	400 KHz	16, 32, 64
16-QAM	400 KHz	32, 64, 128
16-QAM	400 KHz	16, 32, 64

1 The above combinations assume that you are using one of the predefined QPSK and 16-QAM upstream modulations (see the cable modulation-profile command). Although it is possible to fine-tune the modulations for VoIP support by manually specifying each of the burst parameters, this should be done only by engineers who are very knowledgeable about RF issues.

We recommend configuring upstreams that are being used for PacketCable operations and VoIP calls for a channel width that is larger than 400 KHz. (These channel widths and upstream parameter combinations can still be used, however, for best-effort data communications.)

HCCP 1+1 and N+1 Redundancy Support

HCCP redundancy requires that the Working and Protect cable interface line cards be identical. This ensures that the Protect interface supports the same exact configuration as the Working interface. When protecting cards that support intelligent and advanced spectrum management (Cisco uBR-MC16S/U/X, Cisco uBR-MC28U/X, and Cisco uBR-MC5X20S/U), a switchover preserves the spectrum management configuration, and the Protect interface initially uses the same upstream frequency as the Working interface. However, the Protect interface does not begin using the advanced spectrum management features until the system stabilizes, so as to avoid any unnecessary frequency hops or channel width changes.

In addition, the only exception to the rule that like cards must protect like cards is that the Cisco uBR-MC16C and Cisco uBR-MC16S cards can be used to protect one another. This configuration, however, has the limitations on the use of intelligent and advanced spectrum management that are listed in Table 18-4:

Table 18-4 Switchover Operation for Advanced Spectrum Management Configurations 

Working Cable Interface	Protect Cable Interface	Operation After Switchover
Cisco uBR-MC16C	Cisco uBR-MC16S	The protect card uses the same upstream frequency as the working card, but after the system stabilizes, the protect card begins using the advanced spectrum management features that are configured on the protect CMTS.
Cisco uBR-MC16S	Cisco uBR-MC16C	The protect card uses the same upstream frequency as the working card. If the upstream becomes unstable, the protect card performs only guided frequency hopping.

For example, if a Cisco uBR-MC16S is configured as the Working interface and a Cisco uBR-MC16C is configured as the Protect interface, the Cisco uBR-MC16S can be configured for advanced spectrum management features. If a switchover occurs, the Cisco uBR-MC16C comes online using the same upstream frequency configuration, but the Cisco uBR-MC16C can use only guided frequency hopping to correct any future upstream problems. If another switchover occurs, and the Cisco uBR-MC16S comes back online, it again uses the advanced spectrum management features that have been configured.

Intelligent and Advanced Spectrum Management Support

•Intelligent and advanced spectrum management is supported on the Cisco uBR-MC5X20S/U BPE in Cisco IOS Release 12.3(13a)BC.

•Cable interfaces use standard DOCSIS, EuroDOCSIS, and the extended Japanese frequency ranges (5 to 55 MHz for upstream interfaces) to support the intelligent and advanced spectrum management features.

•Intelligent and advanced spectrum management features are supported only in the DOCSIS 1.0 and DOCSIS 1.1 Time Division Multiple Access (TDMA) mode of operation. These features cannot be used when a cable interface is operating in the DOCSIS 2.0 mixed and Advanced TDMA (A-TDMA) modes of operation.

•Upstream channels must meet the CNR and carrier-to-ingress power ratio values given in the DOCSIS specifications. The minimum value for both parameters is 25 dB in the 5-65 MHz frequency range.

•The intelligent and advanced Spectrum Management features do not support inter-line-card shared spectrum groups. Spectrum Management features require that upstream ports on different line cards (Cisco uBR-MC16U/X, Cisco uBR-MC28U/X, and Cisco uBR-MC5X20S/U) have their own RF domain (a unique set of nonoverlapping frequencies).

•HCCP 1+1 redundancy is not supported on any cable interface line card that has defined spectrum groups, which typically is the normal configuration for advanced spectrum management.

•If you are using only one modulation profile and are using a software release prior to Cisco IOS Release 12.2(8)BC2, you need to change the CNR and FEC threshold parameters from their default values to prevent undesired frequency hopping. This is because in these releases, a frequency hop would occur if just one of the measured values (CNR value, correctable FEC counter, or uncorrectable FEC counter) crosses the configured threshold value. Reducing the CNR threshold or increasing one of the FEC threshold values would limit the number of frequency hops.

This situation no longer occurs in Cisco IOS Release 12.2(8)BC2 and later releases, because a frequency hop can occur only when both the CNR value and one of the FEC counters falls below its threshold value. See the "Advanced Spectrum Management Support Using the Cisco uBR-MC16S Cable Interface Line Card" section for more information.

Information About Spectrum Management

Spectrum management allows a Cisco Cable Modem Termination System (CMTS) to sense both downstream and upstream plant impairments, report them to a management entity, and automatically correct them where possible. The spectrum management feature performs these functions without reducing throughput or latency and without creating additional packet overhead on the radio frequency (RF) plant.

In particular, because the cable interfaces on the Cisco CMTS router receives upstream packets, it can directly detect upstream transmission errors. The router can also indirectly monitor the condition of the plant by keeping a record of modem state changes, such as the number and frequency of cable modems that are "flapping" (modems that either miss a station maintenance message or that go offline and then come back online).

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Note For more information about the cable modem flapping and how to monitor the cable modem flap list, see the chapter "Flap List Troubleshooting for the Cisco CMTS" in this guide.

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Spectrum management can prevent long-term service interruptions caused by upstream noise events in the cable plant. It is also used for fault management and troubleshooting the cable network. When cable modems are detected to go online and offline by flap detectors, the cable operators can look at the flap list and spectrum tables to determine the possible causes.

Because to the nature of cable television (CATV) technology, upstream noise management is a significant issue. Frequency bands must have a sufficient carrier-to-noise ratio (CNR) and carrier-to-ingress power ratio to support the transmission of quadrature phase-shift keying (QPSK) and quadrature amplitude modulation (QAM) data. The Data-over-Cable Service Interface Specifications (DOCSIS) set the minimum value for both of these ratios to 25 dB in the 5 to 65-MHz frequency range. If the CNR drops below 25 dB on a particular channel due to noise, the cable modem on that channel degrades and can drop off the hybrid fiber-coaxial (HFC) network.

This overview contains the following subsections:

•Spectrum Management Measurements—Provides an overview of fundamental concepts and terms that are used in spectrum management.

•Upstream Signal Channel Overview—Describes how signals are sent and how changes occur in upstream channels.

•Upstream Segments and Combiner Groups—Describes sparse and dense segments and combiner groups.

•Frequency Management Policy—Describes the types of noise impairments and how to counteract ingress noise with spectrum groups and frequency hopping.

•Guided and Scheduled Spectrum Management—Describes the following Guided and Scheduled spectrum management features: traffic shaping, frequency hopping capabilities, dynamic upstream modulation (SNR-based), and input power levels.

•Intelligent and Advanced Hardware-Based Spectrum Management—Describes spectrum management features that are supported by a number of cable interface line cards that have onboard spectrum management hardware. These features include a real-time spectrum analyzer, CNR-based, proactive frequency hopping, and a more robust dynamic upstream modulation.

Spectrum Management Measurements

Measuring the signal-to-noise ratio (SNR) and carrier-to-noise ratio (CNR) are the major ways of determining the quality of a downstream or upstream signal. The following sections provide an overview of these two ratios, as well as explaining the differences between them, and some additional values that might be useful:

•Signal and Carrier Noise Ratios

•Differences Between the SNR and CNR Values

•Additional Measurements

Signal and Carrier Noise Ratios

Measuring the SNR and CNR of a downstream or upstream is the first step in determining the quality of the signal, and whether spectrum management needs to be performed to correct any errors. The following are brief descriptions of these two values:

•Signal-to-Noise Ratio (SNR)—An estimate of signal strength that is done on the upstream after ingress noise cancellation is performed. This means the SNR takes into account a variety of modulation impairments, including frequency response distortions (such as in-channel amplitude tilt and ripple), group delay, microreflections, and phase noise. The SNR is a good gauge of the overall end-to-end quality of the cable network, because it includes the impact that the transmitter circuitry, receiver circuitry, and transmission media have on the upstream signal.

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Note The SNR value was incorrectly calculated in early Cisco IOS software images, reporting a value that was 4 dB larger than expected. This defect (reported as caveat CSCdv78225) was corrected in Cisco IOS Release 12.1(10)EC1 and Release 12.2(4)BC1, and later releases. For more information, see Field Notice 44400, at the following URL:

http://www.cisco.com/en/US/products/sw/iosswrel/ps1835/products_field_notice09186a00801adb75.shtml

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•Carrier-to-Noise Ratio (CNR)—A ratio of the measured modulated power, in dB, on the upstream (before ingress noise cancellation is done) that compares the channel power to the noise power. This measurement is usually provided only by an external spectrum analyzer, but the cable interface line cards that support intelligent and advanced hardware spectrum management features can provide two types of CNR measurement:

–CNR measured for a particular upstream—An overall CNR for all of the cable modems on an upstream, as determined by measuring the RF power at the cable interface's upstream receiver. This value is always just a snapshot in time for a particular upstream. The cable interface measures the RF power at a time when no bursts are expected from the cable modems, but it can be skewed by a small number of cable modems that are experiencing or creating signal problems.

–Per-modem CNR—A CNR for a particular cable modem, as measured by the signal strength of the modem's burst transmissions at the cable interface's upstream receiver. The per-modem CNR measurement is a very accurate measure of a particular cable modem's signal, but you should not use a single modem's CNR to make assumptions about other cable modems on that upstream or about the upstream itself. However, you can get a good picture of the upstream's signal quality by polling the CNR for a number of cable modems over a representative time period.

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Tip Changing the channel width has a direct impact on CNR. Doubling the channel width (for example, from 400 KHz to 800 KHz) decreases the CNR for an upstream by approximately 3 dB. Cutting the channel width in half (for example, from 3.2 MHz to 1.6 MHz) increases the CNR for an upstream by approximately 3 dB.

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Differences Between the SNR and CNR Values

In a perfect network, such as a test lab where the only impairment is additive white Gaussian noise (AWGN), you can expect the CNR and SNR values to be comparable throughout all of the allowable power levels and frequency ranges. In a live network, however, it is expected that the SNR value should be a few dB lower than the CNR value, given that the SNR value takes into account noise impairments and distortions that are not accounted for by the CNR power measurements.

In general, when the CNR value is in the 15 to 25 dB range, you can expect the SNR value to have a comparable value. The difference between the SNR and CNR values is expected to be larger when the CNR value falls outside of the 15 to 25 dB range.

Table 18-5 provides a comparison for the SNR and CNR values, listing the major reasons for why the SNR and CNR values might diverge on an active network that is passing live traffic:

Table 18-5 Comparison of SNR and CNR in a DOCSIS Cable Network 

Signal-to-Noise (SNR)	Carrier-to-Noise (CNR)
Post-detection measurement of the RF signal.	Pre-detection measurement of the RF signal.
Measurement of the RF frequency domain	Measurement of the baseband domain.
Includes the effect of signal distortions and impairments on the signal. These include: •Group delay in the channel such as occurs during operation near the diplexer band edge. •Channel amplitude variation and echoes. •Data collisions. •Microreflections. •Narrow band ingress in the channel. •Non-linearities in the cable plant. •Phase noise. •Poor selection of the preamble. •Poor symbol fidelity in a cable modem's transmissions, despite a good SNR value. •Unrecoverable carrier offsets. •Unrecoverable symbol timing offsets.	Measures only the RF modulated carrier power versus noise power.
Provides an indication of overall, end-to-end network quality (what the transmitter, receiver, and transmission media are doing to the signal).	Provides an indication of network performance (what the transmission media or network is doing the signal).
Average over time with current data traffic patterns, useful for tracking long-term trends in signal quality.	Real-time spectrum analysis.
Reflects the CNR value as part of its value.	Does not reflect the SNR value as part of its value.
Averaged over 10,000 symbols, and an accurate reading requires that short and long grants are being transferred.	Unaffected by the type of traffic being transmitted.
Does not use packets with uncorrectable FEC errors to determine its value. Bursts of uncorrectable errors, therefore, could result in a deceptively high SNR value.	Unaffected by uncorrectable FEC packet bursts.
DOCSIS specifications do not define any required SNR values for upstreams and downstreams.	Minimum downstream CNR of 35 dB in a 6 MHz band (44 dB in DOCSIS 2.0 for 8 MHz band) Minimum upstream CNR of 25 dB (22 dB in DOCSIS 2.0)

Additional Measurements

In addition to SNR and CNR values, you should be aware of and monitor the following indicators of signal quality:

•Modulation Error Ratio (MER)—A measure of RF signal strength, in dB, which is similar to the SNR value for an upstream, in that it includes distortions and signal impairments such as phase noise and group delay. However, the MER is preferred for data networks, because it also includes additional factors that affect the signal, such as analog-to-digital and digital-to-analog conversions, rounding errors, and phase jitter. For this reason, the DOCSIS 2.0 RF specification adds a requirement for the minimum MER value for a signal, supplementing the existing CNR minimum requirements.

. A simple formula for calculating the MER value for an upstream is:

MER = 20 x log (RMS error magnitude / average symbol magnitude) 

You can also calculate the Error Vector Modulation (EVM) to find the equivalent value expressed as a percentage of noise on an upstream:

EVM = Average error magnitude / Max symbol magnitude * 100 

See the DOCSIS 2.0 specification for more complete information on calculating and using the MER value.

•Forward Error Correction (FEC) Counters—Counters that keep track of how many correctable and uncorrectable FEC errors occur on the upstream. The FEC error counters are useful for tracking fast transient errors such as impulse noise that are not usually reflected in SNR or CNR values.

A correctable error count of more than 1 percent can be used a warning sign of possible physical plant or cable modem problems that might be developed. An uncorrectable error count of more than 1 percent can indicate an existing problem that is blocking traffic on the upstream. Cable interface line cards that support the intelligent and advanced spectrum management features can use the FEC counters as one of the indicators to be monitored to determine whether an upstream must change frequencies so as to correct noise problems.

•Microreflections—Additional copies of a signal that arrive at the receiver, usually at different times and attenuated by different amounts, causing the receiver to misidentify the incoming signal's true phase and amplitude. Microreflections typically are caused by impedance mismatches in the physical cable plant, and can indicate either equipment that has been degraded by weather or other causes, or equipment that has not been installed correctly.

Upstream Signal Channel Overview

The upstream channel is characterized by many cable modems transmitting to the CMTS. These signals operate in a burst mode of transmission. Time in the upstream channel is slotted. The CMTS provides time slots and controls the usage for each upstream interval. The CMTS periodically broadcasts Upstream Channel Descriptor (UCD) messages to all cable modems. The UCD message contains the upstream frequency and transmission parameters associated with an upstream channel. These messages define upstream channel characteristics including the upstream frequencies, symbol rates and modulation schemes, forward error correction (FEC) parameters, and other physical layer values.

Cisco supports all DOCSIS error-correction encoding and modulation types and formats. Upstream signals are demodulated using QPSK or QAM. QPSK carries information in the phase of the signal carrier, whereas QAM uses both phase and amplitude to carry information.

Sending data reliably in the upstream direction is an issue. Because upstream spectrum varies greatly between cable plants, select upstream parameters based on your cable plant's return paths. Select or customize upstream profiles for maximum trade-off between bandwidth efficiency and upstream channel robustness. For example, QAM-16 requires approximately 7 dB higher CNR to achieve the same bit error rate as QPSK, but it transfers information at twice the rate of QPSK.

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Note The above specifications are based on predetermined sets of frequencies that may or may not have an adequate CNR at any given time.

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Upstream frequencies can be assigned as follows:

•Fixed—Configuring a spectrum group disables the fixed upstream frequency setting.

•Single subband—The CMTS administrator can define a center frequency and symbol rate such that the boundaries of the upstream carrier stay within the subband. The frequency and symbol rate can change within the boundary in response to noisy line conditions, based on the defined upstream parameters.

•Multiple subbands—The data carrier can remain in a particular subband for a duration of time and then hop to another subband based on the defined upstream parameters.

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Tip Measurement of noise power levels with a spectrum analyzer should be part of the procedure in initially selecting and setting up frequency allocations. Cisco recommends having fixed frequency settings during early deployment, at least until amplifier cascade adjustments or plant repair have become infrequent enough that they no longer significantly affect the nodes connected to the upstream port.

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Upstream Frequency Changes

As stated in the DOCSIS radio frequency interface (RFI) specification, RF channel migration or upstream frequency change occurs when a change in the UCD message is broadcast to all cable interfaces.

The speed of channel migration via the UCD message is typically less than 20 milliseconds (ms). During this time, upstream transmission is interrupted until the cable interface transmitter adjusts to its new frequency. Data is stored in the cable interface's buffers during this time and is sent when the frequency hop is complete.

Station maintenance intervals are used to perform per-modem keepalive polling. The CMTS polls each cable modem at least once every 30 seconds, with the default being once every 25 seconds. When ingress noise causes loss of keepalive messages from a configurable percentage of all cable interfaces, resulting in missed polls, a new frequency is selected from the allocation table and a UCD update is performed. The migration time is 2 msec for any upstream UCD update. After the UCD is updated, the hop occurs. The system must wait until a hop-threshold time interval has elapsed before it can change the UCD a second time.

Upstream Segments and Combiner Groups

The Cisco routers divide a cable plant into downstream channels. Downstream channels contain upstream segments. Each upstream segment typically serves more than one fiber node. Upstream segments can be defined as one of the following:

•Sparse segment—Containing one upstream channel per upstream segment.

•Dense segment—Containing multiple upstream channels per upstream segment; frequencies must be different.

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Note A cable interface line card can support sparse or dense segments, or both.

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Defining sparse segments allows the cable operator to share upstream bandwidth among fiber nodes with fewer subscribers. Defining dense segments allows the cable operator to provide larger upstream bandwidth to fiber nodes with many subscribers.

Figure 18-1 illustrates sparse versus dense segments.

Figure 18-1 Sparse Versus Dense Segment Illustrations

As shown in Figure 18-1, the downstream segment can contain multiple upstream segments. Two fiber nodes can be in one downstream segment but in different upstream segments.

The return path of several fiber nodes can be combined at a single point to form a single RF frequency domain called a combiner group. The CMTS software allows a frequency hop table called a spectrum group to be associated with a combiner group.

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Note A combiner group refers to an RF topology point. A spectrum group refers to the frequency hop table associated with a combiner group.

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Frequency Management Policy

Spectrum management applies a common frequency-management policy to a set of upstream ports to ensure that data is delivered reliably over the cable plant. Cable plant operators must make noise measurements and determine the cable plant's spectrum management policy. Different modulation schemes, upstream frequency techniques, and symbol rates are used based on the cable plant characteristics and the cable interface line card in the chassis.

See the following sections for more information about these topics:

•Noise Impairments

•Spectrum Groups and Frequency Hopping

•Guidelines for Spectrum Management

Noise Impairments

Upstream noise impairments such as signal degradation on cable networks can negatively affect service to subscribers. 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 noticeable in one-way cable TV service, but when two-way digital signals share the network with video signals, digital signals can be hampered by:

•Impulse and electrical signal ingress—Noise can enter the network from electrical sources within a residence or from high-voltage lines that run near CATV cabling. Two types of ingress noise include broadband and narrowband. Broadband noise is generally of lower frequency (below 10 MHz) and results in harmonic rolloff. Narrowband noise is a more significant interference source. Cable equipment and infrastructure often pick up noise from amateur radio transmissions, citizen band radios, or high-power shortwave broadcast signals. Implement a signal leakage maintenance program to locate and repair areas of signal ingress.

•Amplifier noise—Amplifiers add noise to the HFC network that typically goes unnoticed in video signals, but degrades digital data signals if amplifiers are improperly configured. The larger the network, the higher the probability of amplifier noise affecting signals.

•Noise funneling—The upstream data path to the headend is susceptible to interference from the entire network. All upstream noise ultimately ends up at the headend because the cumulative nature of noise 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—Temperature affects signal loss over coaxial cable. This can cause variations of 6 to 10 dB per year.

•Clipping—The lasers in fiber-optic transmitters can stop transmitting light when input levels are excessive. Excessive input levels introduce bit errors in both the upstream and downstream transmissions. If a laser is overdriven as briefly as a fraction of a second, clipping can occur.

To adjust your return amplifiers and lasers, follow rigorous plant maintenance procedures documented in the NTSC Supplement on Upstream Transport Issues or appropriate cable plant standard. Also refer to the hardware installation guide that ships with your CMTS.

Spectrum Groups and Frequency Hopping

We recommend that CMTS administrators configure upstream frequency hopping to counteract long-term, narrowband noise. Cisco CMTS routers support a combination of guided frequency hopping and time-scheduled frequency hopping.

The frequency hop to proactively avoid noise ingress is sometimes called frequency agility. Frequency agility is configured and activated using spectrum groups. Spectrum management supports the creation of a number of cable spectrum groups, allowing multiple upstream ports in a single spectrum group. Each spectrum group defines the table of frequencies to be used in a specific frequency plan. Upstream frequencies can be a fixed single frequency, a single continuous range of frequencies (band), or multiple ranges (or bands) of frequencies.

The cable interface does not operate until you assign a frequency to the upstream, which can be done either by configuring and assigning a spectrum group or assigning a fixed frequency. The spectrum group takes precedence, so if you configure both a spectrum group and a fixed frequency on an upstream, the spectrum group overrides the fixed upstream frequency setting.

From the interface point of view, a spectrum group also represents the set of upstreams connected to the same group of fiber nodes. The spectrum manager software in Cisco routers examines all the RF parameters that have been configured on an upstream to determine whether the upstream frequencies need to be managed together. For example, if you configure a spectrum group with several fixed frequencies, but those frequencies are all within the configured channel width, the spectrum manager software combines the frequencies into a single band.

The upstream ports use the spectrum group to determine which frequencies are available if frequency hopping is needed to deal with noise or other path impairments. The types of frequency hopping techniques are guided, time-scheduled, and a combined guided and time-scheduled. See the "Frequency Hopping Capabilities" section for more information on the types of frequency hopping techniques.

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Note When each upstream port has its own RF domain, the group is called a nonshared spectrum group. When multiple upstream ports share the same RF domain, the group is called a shared spectrum group.

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Guidelines for Spectrum Management

In general, when defining your spectrum, use the following guidelines:

•Avoid frequencies with known ingress problems, such as amateur radio bands or short-wave bands.

•Avoid hostile spectrum below 20 MHz.

•Allow extra bands for frequency hopping.

•Take the possible channel widths into account when creating frequency bands. The range of frequencies being used must be able to hop between at least two different frequencies when using the channel width that is configured on the upstream.

•Place upstream ports in the same combiner group in a shared spectrum group.

•Use the receive power level setting to perform slight equalization adjustments.

•If you combine multiple upstream ports to provide increased bandwidth, you must avoid overlapping frequency bands. Each port should be using a discrete band of frequencies that does not overlap the bands being used by other ports in the group. We recommend adding at least 20 KHz between the ending frequency of one band and the starting frequency of the next band, to ensure that the bands do not overlap.

Guided and Scheduled Spectrum Management

Guided and Scheduled spectrum management constitutes a set of basic features for all currently supported cable interface line cards. These features are considered basic because they are available for all cable interfaces, and constitute the elementary, cornerstone features upon which the Intelligent and Advanced spectrum management features are built.

See the following sections for more information about each feature:

•Traffic Shaping

•Frequency Hopping Capabilities

•Dynamic Upstream Modulation (SNR-based)

•Input Power Levels

Traffic Shaping

Traffic shaping basically uses queues to limit data surges that can congest a network. The data is buffered and then sent into the network in regulated amounts to ensure that the traffic fits within the expected traffic envelope for the particular connection.

Traffic shaping reduces the chance that information must be retransmitted to hosts on the cable plant. When cable modems (CMs) have rate limits established, the CMTS typically drops data packets to enforce the rate limit. Dropping packets from the requesting CM causes the host sending the information to retransmit its information, which wastes bandwidth on the network. If both hosts sending and requesting information are on the cable plant, the upstream bandwidth is wasted as well.

Traffic shaping allows the CMTS to perform upstream and downstream rate limiting on the DOCSIS upstream and downstream channels. Rate limiting restricts the data rate to and from a CM; the MAC scheduler supports traffic-shaping capabilities for downstream and upstream traffic. Rate limiting ensures that no single CM consumes all of the channel bandwidth and allows a CMTS administrator to configure different maximum data rates for different subscribers. Subscribers requiring higher peak rates and willing to pay for higher rates can be configured with higher peak rate limits in their CM DOCSIS configuration file over regular subscribers, who pay less and get lower rate limits.

Each time a packet belonging to a flow is transmitted on an output channel, the token-bucket policer function checks the rate limit status of the flow, passing the following parameters:

•Token bucket peak rate in bits/msec.

•Token bucket depth (maximum transmit burst) in bits.

•Length of current packet to be sent in bits.

•Pointer to the flow's token bucket.

•Pointer to the flow's token bucket last update time stamp.

•Variable to return the msec buffering delay in case the packet needs to be shaped.

•Maximum buffering delay that the subsequent traffic shaper can handle in msecs.

Every flow has its own shaping buffer where rate-exceeded packets are typically held back in first-in/first-out (FIFO) order for later transmission.

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Tip Token bucket policing with shaping is the new per-upstream default rate limiting setting at the CMTS. Shaping can be enabled or disabled for the token-bucket algorithm.

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Upstream Traffic Shaping

Upstream traffic shaping allows the CMTS to perform rate limiting on a DOCSIS upstream channel. The upstream traffic shaping feature delays the scheduling of the upstream packet, which in turn, causes the packet to be buffered on the cable modem device, instead of being dropped. This allows the user TCP/IP stack to pace the application traffic appropriately and approach throughput commensurate with the subscriber's defined quality of service (QoS) levels. Upstream traffic shaping enables the CMTS to enforce the peak upstream rate for each CM without degrading overall TCP performance for the subscriber CMs.

When you do not enable the shaping option for upstream rate limiting, the CMTS upstream-rate-policing code drops bandwidth requests from cable modems that are found to have exceeded their configured-peak-upstream rate (using different local drop policies). The effect of bandwidth requests (eventually upstream packets) being dropped causes degraded throughput performance of window-based protocols (like TCP) for these rate-exceeded modems because of the timeouts and retransmits that follow.

Upstream grant shaping is on a per-CM (SID) basis. The grant shaping feature is a configurable option for the current upstream token-bucket rate-limiting algorithm.

A traffic shaping feature is restricted QoS class assignment, which allows a CMTS administrator to override the class of service provisioned for a CM. When this feature is enabled, the user-defined QoS profile is enforced on the CM attempting to register with the CMTS, regardless of the CM's provisioned class of service. Use the cable qos profile command to configure a QoS profile.

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Note The restricted QoS class assignment feature is added to address instances where a cable operator implemented rate limiting incorrectly. The feature allows an administrator to override the statically provisioned QoS parameters of the CM and force the CM to use a specific QoS profile defined at the CMTS.

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For configuration task information on upstream traffic shaping, refer to the "Enabling Upstream Rate Limiting" section.

Downstream Traffic Shaping

The CMTS supports basic downstream traffic shaping by effecting data rate limiting on a per-modem basis. A downstream traffic shaping feature called downstream rate limiting with type-of-service (ToS) bits extends that capability by allowing the CMTS administrator to configure the ToS byte to calculate the data rate for a specified flow.

Downstream rate limiting with ToS bits enables you to partition downstream traffic for a CM into multiple classes of service and multiple data rates by using the three precedence bits in the ToS byte in the IP header to specify a class of service assignment for each packet. Those packets with the precedence bit set in the ToS field are given higher priority. Using the ToS byte, you can calculate the data rate for a specified flow, in addition to the data rate configured on a per-CM basis. By specifying a maximum data rate for a particular ToS, you can override the common maximum downstream data rate.

The administrator can override the maximum common downstream data rate limits by configuring the ToS byte.

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Note Packets that contain ToS bytes that have not been configured for downstream data rates continue to use the common data rate limits.

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Frequency Hopping Capabilities

Noise in the upstream transmission line, that is from the consumer to the service provider, can degrade data transmission from the subscriber's home. If the noise impairment is of substantial duration, it may cause the cable modem to temporarily lose communication with the headend facility. As a contingency plan, the multiple service operators (MSOs) can reserve multiple channels or upstream frequencies for their subscribers. If one channel suffers too much interference, the CMTS requests that the cable modems "hop" to another channel.

To provide frequency hopping capability, Cisco CMTS routers contain a spectrum manager that continuously monitors the noise in unused upstream channels. If the CNR reaches an unacceptable level on a particular channel, the spectrum manager automatically assigns a new upstream channel to the cable modem using that channel.

Cisco CMTS routers support the following techniques for upstream frequency hopping when the frequency band in use is not clean:

•Guided frequency hopping—In guided frequency hopping (also known as blind hopping), the spectrum manager automatically assigns a new upstream channel frequency when a configurable threshold of station maintenance (keepalive) messages fails. Failed station maintenance messages represent an impairment of the upstream channel due to noise, plant, or equipment failure. Explicit frequency subbands and associated input power levels are assigned in a spectrum group in guided frequency hopping.

•Time scheduled frequency hopping—Frequency reassignment is scheduled by the time of day or by a specific day of the week.

•Combined guided and time-scheduled frequency hopping.

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Note Frequency hopping is not effective against broadband noise phenomena such as impulse noise.

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Time-scheduled and guided hopping techniques are independent concepts:

•The spectrum is controlled by a script, not a frequency table.

•The available spectrum is time-scheduled as an option.

•A guided hopping frequency is selected from the available spectrum at the current time.

You can configure and activate frequency hopping by using spectrum groups. You can create up to 40 cable spectrum groups, each containing multiple upstream ports. The configured channel width is used for each upstream frequency.

After you have created one or more spectrum groups for your cable network, you can add characteristics to them, providing you with more definitive control over frequency usage and frequency hopping.

You can configure hopping thresholds. For example, the frequency hop threshold percentage method prevents a single failing cable modem from affecting service to other working cable modems. As long as a high enough threshold is configured, the system does not hop endlessly due to a single cable modem failing to respond to 90 percent of its station maintenance (keepalive) messages.

You can also configure the minimum period between frequency hops, with a default setting of 300 seconds. If the destination channel is expected to be impaired, you can reduce the minimum period between frequency hops to a small value, such as 10 seconds. This allows the frequency hop to continue more rapidly until a clear channel is found. If excessive frequency hop is an issue, you can increase the minimum period between hops.

To configure different techniques of frequency hopping, see the "Creating and Configuring Spectrum Groups" section.

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Note Spectrum management is not supported for one-way (telco return) cable modems, because spectrum management capabilities focus on the upstream path over an HFC network.

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Guided Frequency Hopping

Guided frequency hopping is called "guided" because the frequency hopping uses the frequencies that are specified in the spectrum group, which can be either a set of discrete frequencies or a band. The cable interface cards that support guided frequency hopping do not have a "look-ahead" mechanism that would allow them to determine the quality of the new frequency or band ahead of time, which is why previous documents referred to this as blind hopping. Because of this, though, the cable interface does not need to perform any search on the new potential frequencies, so the switching time between frequencies is only approximately 20 ms.

You can specify some rules the system uses when hopping to another frequency when the frequency band in use is not clean. You can assign explicit frequency subbands and associated input power levels in a spectrum group. All cable modems then on the upstream port migrate to the next frequency with an assigned input power level. The number of lost station management messages exceeding a configured threshold can initiate an upstream channel frequency reassignment. For example, you can specify a frequency hop based on lost station management messages that exceed a threshold. The default threshold may be 10 to 20 percent depending on the Cisco IOS release. The frequency change occurs rapidly without data loss and with minimal latency.

Take care to reduce the spectrum allocation when it is used with small channel widths. Otherwise, there will be a large number of upstream channel slots. For example, if the allocation is from 20.0-to-28.0 MHz and an upstream port has its channel width set to 0.2 MHz, there are 40 possible slots for that channel width. Guided frequency hopping can require a long time to find the clean slot, because it tries each available slot, one at a time, for several seconds during each try.

Time-Scheduled Frequency Hopping

You can specify upstream channel frequency reassignment based on a configured time of every day or of a specific day of the week. If your cable plant has an upstream noise characteristic on a weekly cycle, use time-scheduled spectrum allocation. With a time-scheduled policy, a single frequency becomes valid at any given time.

Dynamic Upstream Modulation (SNR-based)

The basic Dynamic Upstream Modulation feature is supported on all Cisco cable interface line cards beginning with Cisco IOS Release 12.1(3a)EC1, Cisco IOS Release 12.2(4)BC1b, and later releases. This section describes the operation of this feature, which is based on evaluating the signal-to-noise ratio (SNR) of an upstream.

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Note