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The following sections describe spectrum management implementation using Cisco uBR7200 series software and the Cisco MC16S cable modem card.
The Cisco uBR7200 series supports the creation of up to 32 cable spectrum groups, allowing multiple upstream ports in a single spectrum group, and specifications of configured channel widths for upstream segments. Upstream frequencies can be assigned as follows:
Note The above frequency agility specifications are based on predetermined sets of frequencies which may or may not have adequate carrier-to-noise (C/N) ratio at any given time.
For more information regarding the MC16S cable modem card and its frequency hopping capabilities, refer to the "MC16S Cable Modem Card" section.
The Cisco uBR7200 series universal broadband routers divide a cable network into downstream and upstream segments. Each upstream segment typically serves one or more fiber nodes. Upstream segments can be defined as:
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 lots of subscribers. Figure 1 illustrates sparse versus dense segments.
As shown in Figure 1, the downstream segment can contain multiple upstream segments. Two fiber nodes can be in one downstream segment, but in different upstream segments.
An upstream frequency has an associated upstream input power level in dBmV. Cable operators must make noise measurements and determine the cable plant's spectrum management policy. Different modulation schemes and symbol rates can be used based on the characteristics of the cable plant and the cable modem card contained in the chassis. Bits are encoded into a two-dimensional mapping called a constellation. Good carrier-to-noise ratio is needed to properly decode symbols into bits.
The upstream channel is characterized by many cable modems transmitting to the CMTS. These signals operate in a burst mode of transmission. Time in the upstream channel is slotted. The CMTS provides time slots and controls the usage for each upstream interval.
The Cisco uBR7200 series equipment periodically broadcasts Upstream Channel Descriptor (UCD) messages to all cable modems. These messages define upstream channel characteristics including the upstream frequencies, symbol rates and modulation schemes, Forward Error Correction (FEC) parameters, and other physical layer values.
Cisco supports all DOCSIS error correction encoding and modulation types and formats. Upstream signals are demodulated using Quadrature Phase Shift Keying (QPSK) or Quadrature Amplitude Modulation (QAM). QPSK carries information in the phase of the signal carrier, whereas QAM uses both phase and amplitude to carry information.
Sending data reliably in the upstream direction is an issue. Because upstream spectrum varies greatly between cable plants, select upstream parameters based on your cable plant's return paths. Select or customize upstream profiles for maximum trade-offs between bandwidth efficiency and upstream channel robustness. For example, 16 QAM requires approximately 7 dB higher C/N ratio to achieve the same bit error rate (BER) as QPSK, but it transfers information at twice the rate of QPSK.
The return path of several fiber nodes can be combined together at a single point to form a single RF frequency domain called a combiner group. The Cisco uBR7200 series universal broadband router software allows a frequency hop table to be associated with a combiner group, called a spectrum group.
Note A combiner group refers to an RF topology point. A spectrum group refers to the frequency hop table associated with a combiner group.
The Cisco uBR7200 series universal broadband routers support up to 32 spectrum groups. Each spectrum group defines the table of frequencies to be used in a specific frequency plan. Cisco cable modem cards interface the downstream and upstream ports to the cable plant.
The Cisco uBR7200 series universal broadband router software includes Cisco IOS commands that can be used to define upstream frequency hop tables. In the global and interface configuration modes, these are grouped under the cable spectrum-group command hierarchy.
The steps to configure a spectrum group are as follows:
Due to the nature of CATV technology, upstream noise management is a significant issue. Cisco recommends that you follow rigorous North American plant maintenance procedures documented in the NCTA Supplement on Upstream Transport Issues to adjust return amplifiers and lasers.
Cisco also recommends that system administrators configure upstream frequency hopping as a countermeasure to long-term, narrowband noise. Cisco IOS Release 12.0(5)T1 or higher CMTS images support blind, time-scheduled, and guided frequency hopping.
With the release of the Cisco MC16S cable modem card, you can perform automatic upstream frequency hopping. You can configure the MC16S to scan the upstream spectrum in your cable network and point to "clean" available upstream channels (of a width you designate) to accept a frequency hop.
Note The choice of spectrum is usually limited to those bands having sufficient C/N ratio and carrier-to-ingress power ratio to support transmission of QPSK and QAM data. The MC16S card operates with default FEC values at approximately 15 dB SNR for QPSK, and 25 dB SNR for 16QAM. Bands with known ingress should be avoided.
Higher frequency bands are also preferred because they are least likely to suffer noise interference. Measurement of noise power levels with a spectrum analyzer is a mandatory part of the setup procedure to characterize your cable plant.
Upstream port frequency should be set to a fixed value during system installation and testing. This is chosen from the allocation plan for the RF domain for the RF plant segment connected to the upstream.
Note Having fixed frequency settings is recommended during early deployment at least until amplifier cascade adjustments or plant repair becomes infrequent for the nodes connected to the upstream port. When multiple upstream ports are combined together to provide increased bandwidth, care should be taken to prevent overlapping frequency bands.
When the system has reached sufficient stability, the RF domain topology can be entered into the Cisco IOS configuration file to enable RF spectrum management. This management feature—called spectrum groups described earlier—applies a common frequency management policy to a set of upstream ports.
Note When each upstream port has its own RF domain, the group is called a non-shared spectrum group. When multiple upstream ports share the same RF domain, the group is called a shared spectrum group.
This section presents tips, capabilities, and guidelines to keep in mind when configuring the Cisco uBR7200 series for spectrum management.
In general, when defining your spectrum, be sure to take the following into consideration:
Using the Cisco uBR7200 series cable modem cards and Cisco IOS Release 12.0(7)XR2 CMTS images, you can configure the router to:
Note Use the show cable spectrum-group command to display the current allocation table and frequency assignments.
Following is an example topology for a Cisco uBR7223 with combiner groups designated A through J. Combiner groups C and E have multiple upstream ports. These should be configured in a shared spectrum group. The other upstreams should be configured in a non-shared spectrum group.
In this example, ten combiner groups are served with frequency hop tables from three spectrum groups:
The laser group term refers to the set of fiber nodes that share the same downstream signal. An optical splitter is often used to create individual feeds-per-node.
In the downstream direction, two 6 MHz channel slots are assigned. All fiber nodes in combiner groups A-E should have a channel slot containing the downstream signal from Cable3/0. Combiner groups A-E are said to belong to "laser group 1."
All fiber nodes in combiner groups E-J should have a channel slot containing the downstream signal from Cable4/0. Combiner groups E-J are said to belong to "laser group 2."
Because combiner group E belongs to two laser groups, there should be two different downstream channel slots for Cable3/0 and Cable4/0.
Assuming that all combiner groups use the frequency band from 20 to 26 MHz, the following configuration enables spectrum management for all upstream ports:
A description of the spectrum groups 1 through 3 follows:
For the 20 to 26 MHz band of each RF domain, the spectrum is channelized according to the channel width settings of each member port. For example, if the ports U2 and U3 of Cable3/0 are set to 3.2 MHz and 1.6 MHz channel widths, respectively, then spectrum group 2 uses the following channelization:
Because the group is shared, ports U2 and U3 will be assigned channels 1 and 4, respectively, to prevent overlap.
Note There are no alternate frequency assignments for either port and bandwidth is wasted from 24.8-to-26.0 MHz. To create alternate channels, increase the upper boundary from 26.0-to-28.0 MHz.
Care should be taken to reduce the spectrum allocation when 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, then there will be 40 possible slots for that channel width. Blind frequency hopping may require quite a long time to find the clean slot since it will try each available slot, one at a time for several seconds each try.
In addition to frequency, the nominal input power level of the upstream receivers can be set at the time of frequency hop. This allows the cable operator to perform minor equalization as a function of frequency.
In this example, the upstream port nominal receive power at 21.6 MHz is 0 dBmV, at 24.8 MHz is 1 dBmV, and at 28.0 MHz is 2 dBmV. At any time, the power level set in the interface configuration overrides the spectrum group power level.
You can add spectrum on a weekly schedule by including an optional weekday and time:
Deletion is performed using the "delete" keyword:
If your cable plant has an upstream noise characteristic on a weekly cycle, use time-scheduled spectrum allocation.
Per DOCSIS RFI specification section 6.3.2.2, RF channel migration occurs by broadcasting a change in the UCD message to all cable modems. The UCD message contains the upstream frequency and transmission parameters associated with an upstream channel.
The speed of channel migration via the UCD message is typically less than 20 ms. During that period of time, upstream transmission is interrupted until the cable modem transmitter adjusts to its new frequency. Data is stored in the cable modem's buffers for this brief period of time and sent when the frequency hop is complete.
Also, per the DOCSIS RFI, station maintenance intervals are used to perform per-modem keepalive polling. The Cisco uBR7200 series routers poll each cable modem at least once every 30 seconds, with the default polling count of once every 25 seconds. When there are fewer than 25 modems per downstream, the default polling rate is every modem per second. The system takes the total number of modems to be polled every 25 seconds, and evenly distributes the number of modems polled per second.
When ingress noise or interference causes loss of keepalive messages from a configurable percentage of all cable modems (resulting in missed polls) connected to an MC16S with Cisco IOS Release 12.0(7)XR2, a new frequency is selected from the allocation table and an UCD update is performed. But, the system must wait until a hop threshold time interval has elapsed before it can act on that hop.
The migration time is 2 msec for any upstream UCD update. Once the UCD is updated, the hop has occurred. The system must wait the hop period until it changes the UCD a second time.
The system will not hop endlessly, because one cable modem is generating 90% of the errors and 90% of the traffic.
If the CMTS does not receive two polls in a row, it will signal a frequency hop. A single modem missing two polls in a row counts as 10% missed polls. The CMTS polls all modems sequentially.
It waits for 20 seconds. If it misses two polls in a row again, it frequency hops again. This pattern is repeated.
Note Regardless of the number of missed polls during the 20 second hop threshold time, it will not hop again thus preventing endless frequency hopping.
The default threshold of 100% can be modified using the following IOS global configuration command:
The percentage threshold method prevents a single failing cable modem from affecting service to other good cable modems. To prevent excessive frequency hop, the minimum period between hops can be changed from the default value of 300 seconds using the following Cisco IOS global configuration command.
If the destination channel is expected to be impaired, then the frequency hop period threshold can be reduced to a small value such as 10 seconds. This allows the frequency hop to continue more rapidly until a clear channel is found.
The current implementation is known as blind frequency hop because there is no "look-ahead" mechanism. In this case, the search time is 0 seconds and the switching time is 20 ms.
The upstream port becomes "up" when it is assigned an upstream frequency and it is configured to be administratively up. Configuring a spectrum group enables frequency agility and disables the fixed upstream frequency setting.
Note Frequency assignment is independent of the configured shutdown state. A configured no cable upstream n shutdown places the port in the "adminup" state. If cable upstream n shutdown is configured and no frequency assigned, then the port transitions to the "down" state.
A maximum of 32 spectrum groups can be configured in the system. Configuring a spectrum group allows the upstream frequency and input power level to change whenever noise impairs upstream data traffic.
Specifies the spectrum group number. (The valid range is 1 to 32.)
If the time parameter is present, then the frequency setting or band is made available or is deleted at the specified time.
Specifies that the frequency setting or band should be removed from use at the specified time.
Specifies that a center frequency setting should be a unit of allocated spectrum in this spectrum group.
Note A channel width of 3.2 MHz is assumed for allocation purposes. For example, if the center frequency is configured as 20.0 MHz, then the band 18.4 to 21 MHz; 6 MHz will be added to the allocated spectrum. This applies even if the configured channel width is smaller.
Upstream center frequency. Valid range is 5,000,000 to 42,000,000 Hz (5 to 42 MHz) depending on the cable modem card used. Half of the upstream carrier energy is distributed below and the other half above this point.
Specifies that a continuous band setting should be a unit of allocated spectrum in this spectrum group.
Specifies the lower boundary of a frequency band.
Specifies the upper boundary of a frequency band.
Specifies the nominal input power level. (The valid range is -10 to +25 dBmV.)
Note Some cable plants may wish to change only the input power level and not frequency on a daily time schedule.
Specifies that a given spectrum group should be deleted.
Note Member upstream ports may transition to the "down" state since there is no spectrum to assign. Reconfigure more bands or move the upstream ports to another spectrum group to bring them back up.
"Shared" indicates that upstream ports must share the same spectrum, and, therefore, must never be set for overlapping carriers.
Note By default, upstream may overlap for all ports in the same spectrum group if this option is not configured.
Indicates that a frequency hop parameter is to be configured for a period.
Valid range is 1 to 3600 seconds. This parameter limits the frequency hop rate to a configured period value in seconds. The configured value may be returned to its default of 300 seconds using the no keyword.
Valid range is 0 to 100 percent.
Specify upstream channel width in Hz. Valid values are 200000, 400000, 800000, 1600000, and 3200000 based on the cable modem card used. The corresponding symbol rate is 160000, 320000, 640000, 1280000, and 2560000 symbols/sec. A default value of 1600000 Hz (1280000 symbols/sec) is configured by the no keyword.
The channel width set by this command determines the amount of spectrum needed during frequency assignment.
Disables the upstream port. In this mode, downstream UCD and MAP messages for the upstream channel are disabled which inhibits the upstream transmitters on the cable modems.
Enables the upstream port. The operator must configure this option to enable data traffic.
Enable frequency agility and spectrum management of the upstream frequency and power. The global spectrum group specified by groupnum defines a frequency table; otherwise, the upstream port will assume the "down" state.
Upstream parameters must be configured manually. After the setup facility is run, upstream ports have a default state of "shutdown." To configure upstream channel frequencies, you have two choices:
The cable modem card receiver accepts time-division multiplexed burst transmissions from cable modems which are DOCSIS compliant. The upstream port becomes "up" when it is assigned an upstream frequency and is configured to be administratively up.
Upstream burst parameters can be configured by defining individual modulation profiles. A modulation profile consists of a table of physical layer characteristics for the different types of upstream bursts, for example, initial maintenance, long grant, request/data, request, short grant, and station maintenance.
To activate upstream interfaces on the MC16S, follow these steps:
For detailed information regarding the Setup Facility on the Cisco uBR7200 series universal broadband router, refer to the "Configuring the Cisco uBR7200 Series" chapter of the Cisco uBR7200 Series Universal Broadband Router Hardware Installation Guide.
Step 2 Enter the configure terminal command to get into global configuration mode.
Step 3 In global configuration mode, configure modulation profiles and/or spectrum groups for your Cisco uBR7200 series using the cable modulation-profile and cable spectrum-group commands.
For detailed information regarding parameters and allowable values for the cable modulation-profile and cable spectrum-group commands, refer to the "cable upstream hopping blind" section and the "Debug Commands" section.
Step 4 To configure individual upstream interfaces on the MC16S, enter the cable upstream interface number command to get into cable interface configuration mode.
Step 5 While in cable interface configuration mode, you can configure various characteristics for the interface in question using the cable upstream commands defined in this feature module in the "Command Reference" section and the Cisco uBR7200 Series Universal Broadband Router Cable Modem Card Installation and Configuration document.
You can check your settings and review any changes to your configuration using various software commands.
To check the value of the settings you have entered, enter the show running-config command at the Router# prompt:
To review changes you make to the configuration, use the EXEC show startup-config command to display the information stored in NVRAM.
You can use a variety of show commands to view information specific to the hardware and cable interface configuration on your Cisco uBR7200 series universal
broadband router. Please refer to the configuration notes that accompanied the various components in your Cisco uBR7200 series for detailed information.
The MC16S consists of a motherboard (based on the MC16C cable modem card) and an additional "piggy-back" spectrum management card. The MC16S supports 256 QAM at 40 Mbps downstream and 16 QAM at 5 Mbps upstream default data rates. The MC16S supports all DOCSIS upstream channel widths—3.2 MHz, 1.6 MHz, 800 kHz, 400 kHz, and 200 kHz—and the card output is +42 dBmV +/- 2 dB. In addition, the MC16S features advanced spectrum management capabilities made possible by hardware and software enhancements.
The MC16S spectrum management daughter card is able to sample the 5-to-42 MHz upstream spectrum and identify clean channels acceptable for a dynamic frequency hop. When a user-defined threshold percentage of "offline" cable modems has been reached, the spectrum management card takes a snapshot of the available upstream spectrum and then passes this information to the Cisco IOS software, where it is analyzed for indications of significant ingress and/or impulse noise. From this analysis, the Cisco IOS software draws informed conclusions regarding the "clean" portion(s) of the upstream frequency spectrum and automatically initiates a frequency hop to a clean upstream channel. The user-defined threshold value is specified by a command in the configuration file of the Cisco uBR7200 series router.
The advanced spectrum management capabilities of the MC16S include the ability to hierarchically scan portions of the upstream spectrum for clean channels of varying widths. For more specific information on MC16S capabilities and programming, refer to the "Command Reference" section.
Table 1 lists the Cisco IOS universal broadband router features related to the scope of this feature module.
Table 1 Cisco uBR7200 Series Universal Broadband Router Features
The Cisco uBR7200 series universal broadband router is described in Cisco IOS Release 12.1 and in the following online feature modules:
No new or modified standards are supported by this feature.
Table 2 lists the MIBs necessary to support spectrum management functionality on Cisco uBR7200 series universal broadband routers.
| MIB Filename | Description | Release |
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CISCO-CABLE-SPECTRUM-MIB.my |
This module describes the spectrum management flap list attributes. |
Note In Cisco IOS Release 12.0(7)XR2, a new table entitled "cdxIfUpstreamChannelExtTable" was added to CISCO-DOCS-EXT-MIB supporting the use of a second channel width designation for hierarchical spectrum analysis, as in the cable upstream interface number channel-width width1 width2 command.
No new or modified RFCs are supported by this feature.
This section documents commands necessary to configure upstream cable interfaces for spectrum management. Configuring your Cisco uBR7200 series universal broadband router for spectrum management involves setting up the basic (global) attributes of your upstream interfaces as well as specifically configuring spectrum management capability for those upstream interfaces.
To configure spectrum management capabilities for the MC16S cable modem card, you must configure your upstream interface(s) using the following commands:
Note An upstream cable interface will not operate until you either set a fixed upstream frequency or you create and configure a spectrum group (see the "Debug Commands" section 32).
Define the elements that are used in a cable modulation profile. You can have up to 8 cable modulation profiles. Profile 1 is the default.
The following example to specifies a FEC codeword length of 200 to modulation profile 2:
The following example removes modulation profile 2 from your router configuration:
Frequency agility is configured and activated using spectrum groups that are controlled by the spectrum manager. You can create from 1 to 32 spectrum groups for each cable modem card upstream port.
To create spectrum groups, specify a list of upstream frequencies and nominal power levels that each spectrum group can use when an upstream frequency change is necessary. Each spectrum group should have its own list of upstream frequencies. At 1.6 MHz, the valid range is -10 dBmV to 25 dBmV. The power level value should only be changed if you want to change only the power level as part of spectrum management. The standard power level is 0 dBmV.
Note You must repeat this command for each frequency or power level that you want to add to a spectrum group's list of valid values.
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.
Note The cable interface will not operate until you either create and configure a spectrum group or set a fixed upstream frequency. See the "cable upstream hopping blind" section on page 27.
Specifies the spectrum group for which you are specifying a parameter value or specifies the number of the spectrum group you wish to remove from your router configuration. Valid range is from 1 to 32. |
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The spectrum group values that can be changed or added are:
Note The time command can be combined with the frequency and band commands, as shown in the "Example" section.
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The corresponding parameter value for the parameter you are defining for a given spectrum group. |
| Release | Modification |
|---|---|
The band parameter for this command was added to enable frequency range scanning capabilities in the MC16S cable modem card. |
See the following example to configure spectrum group 1 with an upstream frequency of 6,500,000 Hz and a default power level of 0 dBmV:
See the following example to add the upstream frequency 7,000,000 Hz to the list of valid frequencies with a default power level of 0 dBmV for spectrum group 1:
See the following example to configure spectrum group 2 with an upstream frequency 7,500,000 Hz and change the power level to 5 dBmV:
See the following example to configure spectrum group 3 with an upstream band of 12,000,000 to 18,000,000 Hz and default power level of 0 dBmV:
