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This feature module describes the spectrum management enhancement Dynamic Upstream Modulation feature in Cisco uBR7200 series universal broadband routers. Cisco IOS Release 12.1(3)EC supports the Dynamic Upstream Modulation feature on the Cisco uBR-MC1xC and Cisco uBR-MC16S cable modem cards.
Dynamic Upstream Modulation improves service quality in Data Over Cable Service Interface Specification (DOCSIS) installations offering data, voice, or video services. Cisco uBR-MC16S and Cisco uBR-MC1xC modem cards monitor the signal-to-noise ratio (SNR) and forward error correction (FEC) counters in the active return path of each upstream port. The software with Dynamic Upstream Modulation tracks whether the upstream channel signal quality can support the modulation scheme configured, and adjusts to the most robust modulation scheme when necessary.
This document includes the following sections:
Noise in the upstream line from the consumer to the service provider degrades data transmission from the subscriber home. If the noise impairment is of substantial duration, it may cause the cable modem to temporarily lose communication with the headend facility altogether. As a contingency plan, Multiple Service Operators (MSOs) can reserve multiple upstream frequencies or channels for their subscribers so that if one channel suffers too much interference, the Cable Modem Termination System (CMTS) requests that the cable modems hop to another channel. This method of ingress avoidance is called frequency agility. The Dynamic Upstream Modulation feature in Cisco IOS Release 12.1(3)EC checks that the upstream signal can support the configured modulation scheme, and adjusts to a more robust modulation scheme if necessary. When return path conditions improve, this feature returns the upstream channel to the higher modulation scheme.
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 upstream frequencies, symbol rates and modulation schemes, 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 (16QAM). QPSK carries information in the phase of the signal carrier, but 16QAM 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.
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Tips Customize upstream profiles for maximum trade-offs between bandwidth, efficiency, and upstream channel robustness. For example, 16QAM requires approximately 7dB higher (channel to noise) C/N ratio to achieve the same bit error rate (BER) as QPSK, but transfers information at twice the rate of QPSK. |
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Caution You can assign upstream modulation profiles to specific upstream ports based on the Cisco cable modem card used. But, only those familiar with Data Over Cable Service Interface Specification (DOCSIS) who have received the proper training should create upstream modulation profiles. |
Dynamic Upstream Modulation adjusts the modulation profiles of an upstream channel based on upstream signal quality. A modulation profile is a collection of six burst profiles that are sent out in a UCD message to configure modem transmit parameters for the upstream message types: request, request/data, initial maintenance, station maintenance, short grant, and long grant.
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 frequency hop table associated with a combiner group is called a spectrum group. Dynamic Upstream Modulation can be configured on interfaces with fixed upstream frequencies or on interfaces with assigned spectrum groups.
Dynamic Upstream Modulation can be used along with spectrum groups. If you are using the Cisco uBR-MC16S card, when Dynamic Upstream Modulation and spectrum groups are configured on the same interface, the modulation switchover is chosen as the first corrective action, followed by a frequency hop, and finally a reduction in channel width. For information on spectrum groups and the Cisco uBR-MC16S card, see Cisco uBR7200 Series MC16S Cable Modem Card Spectrum Management.
Based on the SNR estimate of the receiver circuitry and FEC correctable/uncorrectable thresholds of a particular upstream channel on an Cisco uBR-MC16C or Cisco uBR-MC16S cable modem card, the 16QAM channel can change automatically to a QPSK channel. You can enable or disable the SNR thresholds. On a Cisco uBR-MC16S cable modem card, you can enable or disable the automatic switch feature when spectrum management is on.
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Note The automatic switch of a 16QAM to QPSK feature is only available on Cisco uBR-MC16C and Cisco uBR-MC16S cable modem cards. |
For example, if you configure Dynamic Upstream Modulation on the Cisco uBR7200 series using modulation profiles 1 and 2, where:
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Note Cisco recommends that the primary profile uses 16QAM modulation and the secondary uses QPSK, but this is optional. The two modulation profiles can both be QPSK or 16QAM. It is not mandatory that one is 16QAM and the other QPSK. But, modulation profile switchover is tied to 16QAM and QPSK thresholds. |
The criteria for modulation profile 1 to switch to modulation profile 2 (more robust modulation scheme) includes one of the following:
The criteria for modulation profile 2 to switch to modulation profile 1 includes all of the following:
In the previous example, modulation profile 1 switches to modulation profile 2 based on OR conditions. Modulation profile 2 switches to modulation profile 1 based on AND conditions.
Standard
DOCSIS 1.0.
MIBs
For descriptions of supported MIBs and how to use MIBs, see the Cisco MIB web site on CCO at http://www.cisco.com/public/sw-center/netmgmt/cmtk/mibs.shtml.
No new or modified RFCs are supported by this feature.
Configuring Dynamic Upstream Modulation
| Command | Purpose | |||
|---|---|---|---|---|
Step 1 | Router(config)# configure terminal | Enters configuration mode. | ||
Step 2 | Router(config)# cable modulation-profile iuc fec-tbytes fec-len burst-len guard-t mod scrambler seed diff pre-len last-cw uw-len | Creates a modulation profile. A modulation profile is a collection of six burst profiles sent out in a UCD message to configure modem transmit parameters for the upstream message types: request, initial maintenance, station maintenance, short grant, and long grant. profile = Modulation profile number. iuc = Interval usage code or upstream burst type. Valid entries are: initial, long, request, short, or station.
fec-tbytes = The number of bytes that can be corrected per FEC code word. Valid values are from 0 to 10, where 0 means no FEC. fec-len = FEC code word length. Valid values are from 16 to 253. burst-len = Maximum burst length in minislots. Valid values are from 0 to 255, where 0 means no limit. guard-t = Guard time in symbols. The time between successive bursts. mod = Modulation. Valid entries are 16qam and qpsk. scrambler = Enable or disable scrambler. Valid entries are scrambler and no-scrambler. | ||
| Turning the scrambler off can cause packet loss, and is only used in lab testing environments. Errors or incompatible configurations in the burst profile(s) cause modems to either drop connectivity, drop short or long data packets, or fail to connect to the network. It is possible to build a burst profile set for which no implementation of a DOCSIS receiver is capable of receiving the modem's transmission. 160 Ksymbol/sec and 2560 Ksymbol/sec data rates are highly sensitive to unique word length, preamble length, and FEC sizing. Incorrect choices for these values can cause poor or no connectivity at these symbol rates. seed = Scrambler seed in hexidecimal format. Valid values are from 0x0000 to 0x7FFF. diff = Enable or disable differential encoding. Valid entries are diff and no-diff. pre-len = Preamble length in bits. Valid values are from 2 to 128. last-cw = Handling of FEC for last code word. Valid entries are fixed for fixed code word length and shortened for shortened last code word. uw-len = Upstream unique word length. Enter uw8 for 8-bit unique words or uw16 for 16-bit unique code words. | |||
Step 3 | Router(config)# interface type slot/port | Configures the interface where: | ||
Step 4 | Router(config-int)# cable upstream port-number modulation-profile primary-profile-number secondary-profile-number | Adds the modulation profile to the interface. port-number = port number on cable modem slot (port numbers begin with a 0) primary-profile-number = The primary modulation profile added to the interface. secondary-profile-number | ||
Step 5 | Router(config-int)# exit | Enter exit to go back to the configuration mode. |
Router# show running-config
To review changes you make to the configuration, use the show startup-config command to display the information stored in NVRAM.
Step 2 Use the show cable modulation-profile privileged EXEC command to display modulation profile group information.
Router# show cable modulation-profile [profile] [iuc-code]
profile—(Optional) Profile number. Valid values are from 1 to 8.
iuc-code—(Optional) Internal usage code.
The Cisco uBR7200 series IOS Release 12.1(3)EC has one preconfigured modulation profile resident in memory, which defines a typical profile for QPSK modulation. In order to use the Dynamic Upstream Modulation feature, a second profile must be created that is unique from the first profile, and typically provides a higher modulation scheme.
The following example is a modulation profile for 16QAM, in which the initial, request, and station maintenance messages are sent as QPSK, and the short and long data packets are sent as 16QAM. 16QAM modulation is more bandwidth-efficient than QPSK, but QPSK is more robust than 16QAM.
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Note The upstream request and station maintenance messages use less time on the cable network when configured in QPSK for symbol rates of 640, 1280 and 2560 Ksymbols/sec, Thus, these messages are actually more efficient when used in QPSK mode and they ensure a more reliable modem connection. The upstream initial maintenance message takes exactly the same amount of time on the cable network no matter how it is configured. Modems will connect more quickly and experience fewer cycles of power-adjustment during initial maintenance if the system is set for QPSK. |
Router# configure terminal Router(config)# cable modulation-profile 2 request 0 16 1 8 qpsk scrambler 152 no-diff 64 fixed uw8 Router(config)# cable modulation-profile 2 initial 5 34 0 48 qpsk scrambler 152 no-diff 128 fixed uw16 Router(config)# cable modulation-profile 2 station 5 34 0 48 qpsk scrambler 152 no-diff 128 fixed uw16 Router(config)# cable modulation-profile 2 short 6 75 6 8 16qam scrambler 152 no-diff 72 fixed uw8 Router(config)# cable modulation-profile 2 long 8 220 0 8 16qam scrambler 152 no-diff 160 fixed uw8
In the following example, all message types are carried with 16QAM modulation. Although 16QAM modulation offers a consistent modulation scheme for all five types of messages, the added length of the 16QAM preamble offsets the increased bandwidth efficiency of the MAC data message for the station maintenance messages and bandwidth request messages.
Router# configure terminal Router(config)# cable modulation-profile 2 request 0 16 1 8 16qam scrambler 152 no-diff 128 fixed uw16 Router(config)# cable modulation-profile 2 initial 5 34 0 48 16qam scrambler 152 no-diff 256 fixed uw16 Router(config)# cable modulation-profile 2 station 5 34 0 48 16qam scrambler 152 no-diff 256 fixed uw16 Router(config)# cable modulation-profile 2 short 5 75 6 8 16qam scrambler 152 no-diff 144 fixed uw8 Router(config)# cable modulation-profile 2 long 8 220 0 8 16qam scrambler 152 no-diff 160 fixed uw8
Add the cable upstream port-number modulation-profile primary profile-number secondary profile-number command to the appropriate interface(s). In this example, modulation-profile 2 is for 16QAM modulation and profile 1 is for QPSK modulation.
Router# configure terminal Router(config)# interface Cable6/0 Router(config-if)# cable upstream 0 modulation-profile 2 1
To define the modulation profile, use the cable modulation-profile global configuration command. To remove the specified modulation profile, use the no form of this command.
cable modulation-profile profile iuc fec-tbytes fec-len burst-len guard-t mod scrambler seed diff pre-len last-cw uw-len
Syntax Description
profile Modulation profile number. iuc Interval usage code. Valid entries are: initial, long, request, short, or station. fec-tbytes The number of bytes that can be corrected per FEC code word. Valid values are from 0 to 10, where 0 means no FEC. fec-len FEC code word length. Valid values are from 16 to 253. burst-len Maximum burst length in minislots. Valid values are from 0 to 255, where 0 means no limit. guard-t Guard time in symbols. The time between successive bursts. mod Modulation. Valid entries are 16qam and qpsk. scrambler Enable or disable scrambler. Valid entries are scrambler and no-scrambler. seed Scrambler seed in hexidecimal format. Valid values are from 0x0000 to 0x7FFF. diff Enable or disable differential encoding. Valid entries are diff and no-diff. pre-len Preamble length in bits. Valid values are from 2 to 128. last-cw Handling of FEC for last code word. Valid entries are fixed for fixed code word length and shortened for shortened last code word. uw-len Upstream unique word length. Enter uw8 for 8-bit unique words or uw16 for 16-bit unique code words.
Defaults
Modulation profile #1
Command Modes
Global configuration
Command History
11.3 NA This command was introduced. 12.0(7)XR2 This command was used 12.1(3)EC This command was used.
Release
Modification
Usage Guidelines
A modulation profile is a collection of six burst profiles that are sent out in an Upstream Channel Descriptor (UCD) message to configure modem transmit parameters for the upstream message types: request, initial maintenance, station maintenance, short grant, and long grant.
You can use the no cable modulation-profile command to remove all modulation profiles except modulation profile 1. In the case of modulation profile 1, the no cable modulation-profile command sets all the parameters in a burst to default values.
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Caution Changes to modulation profiles causes changes to the physical layer. Because changing physical layer characteristics affects router performance and function, this task should be reserved for expert users. |
To use the cable modulation-profile command correctly, enter a line with all parameters for each upstream burst type. An incomplete burst profile causes unreliable operation, or loss of modem connectivity.
Turning the scrambler off can cause packet loss, and is only used in lab testing environments.
Errors or incompatible configurations in the burst profile(s) cause modems to either drop connectivity, drop short or long data packets, or fail to connect to the network. It is possible to build a burst profile set for which no implementation of a DOCSIS receiver is capable of receiving the modem's transmission.
The data rates 160 Ksymbol/sec and 2560 Ksymbol/sec are highly sensitive to unique word length, preamble length, and FEC sizing. Incorrect choices for these values can cause poor or no connectivity at these symbol rates.
Examples
The following example defines the burst parameters for profile 2:
The request burst is defined to have:
The remaining initial, station, short, and long bursts are defined in similar fashion for profile 2.
Router(config)# cable modulation-profile 2 request 0 16 1 8 qpsk scrambler 152 no-diff 64 fixed uw8 Router(config)# cable modulation-profile 2 initial 5 34 0 48 qpsk scrambler 152 no-diff 128 fixed uw16 Router(config)# cable modulation-profile 2 station 5 34 0 48 qpsk scrambler 152 no-diff 128 fixed uw16 Router(config)# cable modulation-profile 2 short 6 75 6 8 16qam scrambler 152 no-diff 144 fixed uw8 Router(config)# cable modulation-profile 2 long 8 220 0 8 16qam scrambler 152 no-diff 160 fixed uw8
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Note You must create all of the bursts (request, initial, station, short and long) for this modulation profile using the modulation profile command. |
See the show cable modulation-profile command for a description of the output display fields.
Related Commands
Assigns a modulation profile to an interface. Displays modulation profile group information.
Command
Description
To assign a modulation profile to an interface, use the cable upstream modulation-profile interface configuration command. To assign the default primary modulation profile (profile #1) to the interface, use the no form of this command.
cable upstream n modulation-profile primary-profile-number secondary-profile-number
Syntax Description
n Port number on the cable modem slot (port numbers begin with a 0). primary-profile number Default modulation profile added to the interface. secondary-profile number Additional modulation profile added to the interface.
Defaults
Primary modulation profile (profile #1)
Command Modes
Interface configuration
Command History
11.3 NA This command was first introduced. 12.0(7)XR2 This command was used. 12.1(3)EC This command was modified to add the primary-profile-number and secondary-profile-number.
Release
Modification
Usage Guidelines
You can configure modulation profiles with fixed upstream frequencies or on interfaces with assigned spectrum groups. The Dynamic Upstream Modulation feature uses modulation profiles to track upstream signal quality. It checks that the upstream signal can support the configured modulation scheme, and adjusts to a more robust modulation scheme if necessary. When return path conditions improve, it returns the upstream channel to the higher modulation scheme.
In Cisco uBR-MC1xC and Cisco uBR-MC16S cable modem cards, when Dynamic Upstream Modulation and spectrum groups are configured on the same interface, the first corrective action is modulation switchover, then frequency hopping, and finally reduction in channel width.
Examples
The following example assigns the primary modulation profile 2 and the secondary modulation profile 1 to port (interface) 0:
Router(config-if)# cable upstream 0 modulation-profile 2 1
Syntax Description
profile (Optional) Profile number. Valid values are from 1 to 8. iuc-code (Optional) Internal usage code. Valid options are: initial— Initial ranging burst
long— Long grant burst
request—Request burst
short—Short grant burst
station— Station ranging burst
Defaults
No default behavior or values.
Command Modes
Privileged EXEC
Command History
11.3 XA This command was first introduced. 12.(0)7XR2 This command was used. 12.1(3)EC This command was used.
Release
Modification
Usage Guidelines
The show cable modulation-profile command displays modulation profile group information. A modulation profile is a collection of six burst profiles that are sent out in a UCD message to configure modem transmit parameters for the following upstream message types: request, initial maintenance, station maintenance, short grant, and long grant.
The following is sample output from the show cable modulation-profile command:
router# show cable modulation-profile 1Mo IUC Type Preamb Diff FEC FEC Scrambl Max Guard Last Scrambl Preamb
length enco T CW seed B time CW offset
bytes size size size short1 request qpsk 64 no 0x0 0x10 0x152 1 8 no yes 56
1 initial qpsk 128 no 0x5 0x22 0x152 0 48 no yes 0
1 station qpsk 128 no 0x5 0x22 0x152 0 48 no yes 0
1 short qpsk 72 no 0x5 0x4B 0x152 0 8 no yes 48
The following table describes the fields shown in the show cable modulation-profile display.
| Field | Description |
|---|---|
Mo | Modulation profile group number. A modulation profile group is the set of burst profiles that define upstream transmit characteristics for the various types of upstream transmission classes. |
IUC | Internal usage code. Each upstream transmit burst belongs to a class which is given a number called the IUC. Bandwidth maps messages by IUC codes used to allocate upstream time slots. The following types are currently defined: Request—bandwidth request slot Initial maintenance—initial link registration contention slot Station maintenance—link keep-alive slot Short data grant—short data burst slot Long data grant—long data burst slot |
Type | Modulation type. |
Preamb length | Preamble length. |
Diff enco | Differential encoding enabled (yes) or not enabled (no). |
FEC T bytes | Number of bytes that can be corrected for each FEC code word. |
FEC CW size | Size, in bytes, of the FEC codeword. |
Scrambl seed | Scrambler seed value in hex format. |
Max B size | Maximum burst size. |
Guard time size | Time between successive bursts measured in symbols. |
Last CW short | Handling of FEC for shortened last code word. |
Scrambl | Scrambler enabled (yes) or not enabled (no). |
Preamb offset | The bits to be used for the preamble value. |
Related Commands
Configures a spectrum group to use a specified frequency.
Command
Description
To display debug messages for frequency hopping, use the debug cable freqhop EXEC command. To disable debugging output, use the no form of this command.
debug cable freqhopSyntax Description
This command has no arguments or keywords.
Defaults
Debugging for frequency hopping is not enabled.
Command Modes
EXEC mode.
Command History
12.0(4)XI This command was introduced. 12.(0)5T This command was used. 12.1(3)EC This command was used.
Release
Modification
Examples
Router# debug cable freqhop CMTS freqhop debugging is on
Related Commands
debug cable hw-spectrum Displays messages for frequency hopping.
Command
Description
Posted: Thu Sep 28 12:20:09 PDT 2000
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