Configuring Cable Modulation Profiles on Cisco CMTSs

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Updated:September 10, 2018
Document ID:23710

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Introduction

Cisco cable modem termination systems (CMTSs) can handle modulation profiles for the RF configuration of a voice and data cable modem plant. Cisco IOS® Software is designed with a default modulation profile that is optimal in most conditions. Therefore, do not change the default configuration. However, if the modulation needs are different for the customer plant, Cisco IOS Software has the ability to customize and to configure the modulation profiles to suit the customer needs.

caution.gif Caution: Only an expert, who understands modulation changes and Data-over-Cable Service Interface Specifications (DOCSIS), should modify these parameters. Otherwise, changes can cause disruption or the degradation of services because the commands affect the physical layer.

This document explains the configuration commands to change the modulation profile. The document also provides the show commands that you use to verify the configured parameters.

The CMTS Cisco IOS Software releases have one preconfigured modulation profile resident in memory, which defines a typical profile for Quadrature Phase-Shift Keying (QPSK) modulation. The Cisco uBR7100, uBR7200, and uBR10000 series CMTSs support up to eight cable modulation profiles. Profile 1 is the default.

Issue the show cable modulation-profile command in order to see the default configured parameters:

ubr7246#show cable modulation-profile

Mod 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  short
1   request qpsk 64     no   0x0    0x10  0x152   0    8     no   yes     952
1   initial qpsk 128    no   0x5    0x22  0x152   0    48    no   yes     896
1   station qpsk 128    no   0x5    0x22  0x152   0    48    no   yes     896
1   short   qpsk 72     no   0x5    0x4B  0x152   6    8     no   yes     944
1   long    qpsk 80     no   0x8    0xDC  0x152   0    8     no   yes     936

Prerequisites

Requirements

There are no specific requirements for this document.

Components Used

This document is not restricted to specific software or hardware versions.

Conventions

Refer to Cisco Technical Tips Conventions for more information on document conventions.

Syntax Description

The cable modulation-profile Command

In order to define a modulation profile, issue the cable modulation-profile global configuration command. In order to remove the specified modulation profile, issue the no cable modulation-profile command.

  • cable modulation-profile profile {iuc | mix | qam-16 | qpsk} fec-tbytes fec-len burst-len guard-t mod scrambler seed diff pre-len last-cw uw-len

  • no cable modulation-profile profile {iuc | mix | qam-16 | qpsk} fec-tbytes fec-len burst-len guard-t mod scrambler seed diff pre-len last-cw uw-len

Note: These commands should each be on one line.

Note: All these options are available in Cisco IOS Software Release 12.1(1)EC and later. In earlier releases, only the initial, long, request, short, or station options were available. There may be later releases of Cisco IOS Software Release 12.0T or 12.0SC that incorporate all the features.

Table 1 – Parameter Descriptions

Parameter Description
profile Modulation profile number—Valid values are from 1 to 8, where 1 is the default modulation profile.

Note: In order to use this 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.
mix Creates a default QPSK/16-QAM1 mix modulation profile where short and long grant bursts are sent with the use of 16-QAM, while request, initial ranging, and station maintenance bursts are sent with the use of QPSK. The burst parameters are set to their default values for each burst type.
qam-16 Creates a default 16-QAM modulation profile, where all bursts are sent with the use of 16-QAM. The burst parameters are set to their default values for each burst type.
qpsk Creates a default QPSK modulation profile, where all bursts are sent with the use of QPSK. The burst parameters are set to their default values for each burst type.
fec-tbytes The number of bytes that can be corrected per the FEC2 code word—Valid values are from 0 to 10, where 0 means no FEC. This is the number of bytes that the FEC decoder can correct within a code word. A code word consists of information bytes (k bytes) and parity bytes for error correction. The number of parity bytes is equal to two times the number of correctable errors (T). The size of T is dictated by channel impairments.
fec-len FEC code word information bytes length—Valid values are from 16 to 253 bytes. This value specifies the number of information bytes (k bytes) per FEC code word.
burst-len Maximum burst length in minislots—Valid values are from 0 to 255, where 0 means no limit. This is used to determine the breakpoint between packets that use the short data grant burst profile and packets that use the long data grant burst profile. If the required upstream time to transmit a packet is greater than this value, the long data grant burst profile is used. If the time is less than or equal to this value, the short data grant burst profile is used.
guard-t Guard time in symbols—The time between successive bursts. Valid values are from 0 to 255 symbols. This is the blank time at the end of a burst transmission that exists to ensure that one burst ends before another burst starts.
mod Modulation—Valid options are 16qam and qpsk. Modulation type is used to select between 4 bits per modulation symbol (16-QAM) or 2 bits per modulation symbol (QPSK). 16-QAM uses both phase and amplitude to carry information. QPSK carries information in the phase of the signal carrier. 16-QAM requires approximately 7 dB3 higher C/N4 in order to achieve the same BER5 as QPSK. But 16-QAM transfers information at two times the rate of QPSK.
scrambler Enable or disable scrambler—Valid options are scrambler and no-scrambler. Scrambler is used to create an almost random sequence of transmission symbols, which ensures an even spectral distribution of energy that is transmitted within the channel. The scrambler seed is an initial value that is used to start the pseudorandomizer to scramble the bits. Because both the transmitter and the receiver know the seed value, the scrambling can be reversed at the receiver to leave only the original data.
seed Scrambler seed, in hexadecimal format—Valid values are from 0x0000 to 0x7FFF.
diff Enable or disable differential encoding—Valid options are diff and no-diff. Differential encoding is a technique wherein the information is transmitted by the phase change between two modulation symbols instead of by the absolute phase of a symbol. This technique makes the absolute phase of the received signal insignificant and effectively doubles the BER for the same C/N.
pre-len Preamble length in bits—Valid values are from 2 to 128. Preamble length (and preamble offset) are used to define a synchronizing string of modulation symbols, which lets the receiver find the phase and timing of the transmitted burst.
last-cw How FEC is handled for last code word—Valid options are fixed for fixed code-word length and shortened for shortened last code word.
uw-len Upstream unique word length—Valid options are uw8 for 8-bit unique words or uw16 for 16-bit unique code words.

1 QAM = quadrature amplitude modulation

2 FEC = forward error correction

3 dB = decibels

4 C/N = carrier-to-noise

5 BER = bit error rate

Defaults

The default is modulation profile 1.

Command Modes

The command mode is global configuration.

Table 2 – Command History

Cisco IOS Software Release Modification
11.3 NA This command was introduced.
12.0(7)XR2 This command was used.
12.0(6)SC and 12.1(3a)EC1 The mix, qpsk and 16qam options were added.

Usage Guidelines

A modulation profile is a collection of six burst profiles that are sent out in an Upstream Channel Descriptor (UCD) message. These profiles configure modem transmit parameters for these upstream message types:

  • Request

  • Initial maintenance

  • Station maintenance

  • Short grant

  • Long grant

You can issue the no cable modulation-profile command in order to remove all modulation profiles except the default 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.

caution.gif Caution: Changes to modulation profiles cause changes to the physical layer. Because changes to physical layer characteristics affect router performance and function, have only an expert user handle this task.

In order 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.

caution.gif Caution: If you turn off the scrambler, you can cause packet loss. Therefore, turn the scrambler off only in lab test environments.

Errors or incompatible configurations in the burst profiles cause modems to either:

  • Drop connectivity

  • Drop short or long data packets

  • Fail to connect to the network

It is possible to build a burst profile set for which no implementation of a DOCSIS receiver can receive the transmissions of the modem.

The data rates of 160 K symbol/second and 2560 K symbol/second 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.

Configuration Samples

Customized Example

The example in this section is a modulation profile for mixed modulation. The initial, request, and station maintenance messages are sent as QPSK, and the short and long data packets are sent as 16-QAM. 16-QAM modulation is more bandwidth-efficient than QPSK, but QPSK is more robust than 16-QAM.

In this example, the request burst has these values by definition:

  • fec-tbytes: 0

  • fec-len: 16 KB

  • burst-len: 1

  • guard-t: 8

  • mod: qpsk

  • Scrambler enabled

  • seed: 152

  • Differential encoding disabled

  • pre-len: 64 bits

  • last-cw: fixed

  • uw-len: uw8

The remaining initial, station, short, and long bursts are defined in similar fashion for profile 2.

ubr7246#configure terminal

Enter configuration commands, one per line.  End with CNTL/Z.

ubr7246(config)#cable modulation-profile 2 request 0 16 1 8 qpsk
                scrambler 152 no-diff 64 fixed uw8
ubr7246(config)#cable modulation-profile 2 initial 5 34 0 48 qpsk
                scrambler 152 no-diff 128 fixed uw16
ubr7246(config)#cable modulation-profile 2 station 5 34 0 48 qpsk
                scrambler 152 no-diff 128 fixed uw16
ubr7246(config)#cable modulation-profile 2 short 6 75 6 8 16qam
               scrambler 152 no-diff 144 fixed uw8
ubr7246(config)#cable modulation-profile 2 long 8 220 0 8 16qam
                scrambler 152 no-diff 160 fixed uw8
ubr7246(config)#^Z

ubr7246#

Note: The commands in this output should each be on one line.

Note: Use the modulation profile command in order to create all the bursts (request, initial, station, short, and long) for this modulation profile. If you fail to do so, you can cause performance problems or service interruption.

This example uses this equipment:

Table 2 – Configuration Example Equipment

Name Model Usage Software Version Image Name
Cable Head End Station uBR7246 CMTS Cisco IOS Software Release 12.1(1a) ubr7200-ik1st-mz.121-1a.T1
Home Office uBR924 Cable Modem Cisco IOS Software Release 12.2(1) ubr920-k8o3v6y5-mz.122-1.bin
  PC DHCP Server WinNT1 Server 4.0 Cisco Network Registrar 3.5
TFTP Server WinNT Server 4.0 Cisco TFTP
ToD2 Server WinNT Server 4.0 Any NTP3 or ToD server

1 WinNT = Microsoft Windows NT

2 ToD = Time of Day

3 NTP = Network Time Protocol

This example configuration shows the entire configuration of the uBR7246. The commands that appear in boldface are the commands that are relevant to the configuration of modulation profiles:

uBR7246 Configuration 
ubr7246#show run

Building configuration...
Current configuration:
!
version 12.1
no service pad
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
service compress-config
!
hostname ubr7246
!
enable password ww
!
cable modulation-profile 2 request 0 16 1 8 qpsk scrambler 
152 no-diff 64 fixed uw8
cable modulation-profile 2 initial 5 34 0 48 qpsk scrambler 
152 no-diff 128 fixed uw16
cable modulation-profile 2 station 5 34 0 48 qpsk scrambler 
152 no-diff 128 fixed uw16
cable modulation-profile 2 short 6 75 6 8 16qam scrambler 
152 no-diff 144 fixed uw8
cable modulation-profile 2 long 8 220 0 8 16qam scrambler 
152 no-diff 160 fixed uw8

!--- Note: These commands should each be on one line.

no cable qos permission create
no cable qos permission update
cable qos permission modems
cable time-server
!
!
!
!
ip subnet-zero
no ip domain-lookup
!
interface Ethernet2/0
 ip address 172.16.30.20 255.255.255.192
!
interface Cable3/0
 ip address 192.168.5.1 255.255.255.0 secondary
 ip address 10.2.3.1 255.255.255.0
 no keepalive
 cable downstream rate-limit token-bucket shaping
 cable downstream annex B
 cable downstream modulation 256qam
 cable downstream interleave-depth 32
 cable upstream 0 frequency 20000000
 cable upstream 0 power-level 0
 cable upstream 0 modulation-profile 2
 no cable upstream 0 shutdown
 cable upstream 1 shutdown
 cable upstream 2 shutdown
 cable upstream 3 shutdown
 cable upstream 4 shutdown
 no cable upstream 5 shutdown
 cable source-verify dhcp
 cable dhcp-giaddr policy
 cable helper-address 172.16.30.3
!
ip classless
ip route 0.0.0.0 0.0.0.0 172.16.30.1
no ip http server
!
!
line con 0
 transport input none
line aux 0
line vty 0 4
 password ww
 login tacacs
!
end

In order to be complete, this document also shows the configuration of one of the uBR924 cable modems. This is the basic bridge configuration that the cable modems receive after they come on line and obtain an IP address from the DHCP server:

uBR924 Configuration 
uBR924-445b#show run

Building configuration...
version 12.2
no service single-slot-reload-enable
no service pad
service timestamps debug uptime
service timestamps log uptime
no service password-encryption
!
hostname uBR924-445b
!
logging rate-limit console 10 except errors
!
clock timezone - -8
ip subnet-zero
no ip routing
no ip finger
!
ip audit notify log
ip audit PO max-events 100
no ip dhcp-client network-discovery
call RSVP-sync
!
!
!
interface Ethernet0
 ip address 10.2.3.3 255.255.255.0
 no ip route-cache
 bridge-group 59
 bridge-group 59 spanning-disabled
!
interface cable-modem0
 no ip route-cache
 bridge-group 59
 bridge-group 59 spanning-disabled
 h323-gateway voip interface
 h323-gateway voip id 3620-gk ipaddr 172.16.30.5 1718
 h323-gateway voip h323-id test2
!
ip classless
ip http server
no ip http cable-monitor
!
snmp-server packetsize 4096
snmp-server manager
!
voice-port 0
 input gain -2
 output attenuation 0
!
voice-port 1
 input gain -2
 output attenuation 0
!
dial-peer voice 1 pots
 destination-pattern 3333
 port 0
!
dial-peer voice 2 pots
 destination-pattern 4444
 port 1
!
dial-peer voice 10 voip
 destination-pattern 1111
 session target ras
!
gateway
!
!
line con 0
 transport input none
line vty 0 4
!
end

Modulation Profile Built-In Templates

In order to simplify the definition of new modulation profiles, you can use the cable modulation-profile command. This command quickly creates a complete modulation profile on the basis of one of three predefined templates. These are the three templates:

  • qpsk—This template is a modulation profile that is optimized for all interval usage codes (IUCs) to use QPSK.

  • qam-16—This template is a modulation profile that is optimized for all IUCs to use 16-QAM.

    Note: Cisco does not recommend the use of this modulation profile.

  • mix—This template is a modulation profile that is optimized to use 16-QAM for data traffic and QPSK for maintenance and request traffic.

Use mix in order to gain the robustness of QPSK for maintenance traffic and the speed of 16-QAM for data traffic. In order to create a new modulation profile that uses one of these templates, issue the cable modulation-profile command. These sections of this document show the use of the cable modulation-profile command:

QPSK Modulation Profile Example

In order to configure a CMTS with the QPSK template, issue the cable modulation-profile profile qpsk command in global configuration. This output shows the use of the command: 

ubr7246#configure terminal

Enter configuration commands, one per line.  End with CNTL/Z.

ubr7246(config)#cable modulation-profile 2 qpsk

ubr7246(config)#end

After you issue this command, the CMTS loads all the preconfigured settings for this profile. These settings appear in the configuration:

ubr7246#show run | include modulation-profile 2

cable modulation-profile 2 request 0 16 0 8 qpsk scrambler 152 no-diff 64 fixed uw8
cable modulation-profile 2 initial 5 34 0 48 qpsk scrambler 152 no-diff 128 fixed uw16
cable modulation-profile 2 station 5 34 0 48 qpsk scrambler 152 no-diff 128 fixed uw16
cable modulation-profile 2 short 5 75 6 8 qpsk scrambler 152 no-diff 72 fixed uw8
cable modulation-profile 2 long 8 220 0 8 qpsk scrambler 152 no-diff 80 fixed uw8

Note: Notice that all the IUCs were built automatically with QPSK modulation.

Note: After you issue the cable modulation-profile profile {mix | qam-16 | qpsk} command, you can manually alter some of the parameters.

16-QAM Modulation Profile Example

In order to configure a CMTS with the 16-QAM template, issue the cable modulation-profile profile qam-16 command in global configuration. This output shows the use of the command: 

ubr7246#configure terminal

Enter configuration commands, one per line.  End with CNTL/Z.

ubr7246(config)#cable modulation-profile 3 qam-16

ubr7246(config)#end

After you issue this command, the CMTS loads all the preconfigured settings for this profile. These settings appear in the configuration:

ubr7246#show run | include modulation-profile 3

cable modulation-profile 3 request 0 16 0 8 16qam scrambler 152 no-diff 128 fixed uw16
cable modulation-profile 3 initial 5 34 0 48 16qam scrambler 152 no-diff 256 fixed uw16
cable modulation-profile 3 station 5 34 0 48 16qam scrambler 152 no-diff 256 fixed uw16
cable modulation-profile 3 short 6 75 6 8 16qam scrambler 152 no-diff 144 fixed uw8
cable modulation-profile 3 long 8 220 0 8 16qam scrambler 152 no-diff 160 fixed uw8

Mixed Modulation Profile Example

There is a more efficient template, which combines QPSK and 16-QAM modulation profiles and is more robust at a higher speed.

In order to configure a CMTS with the mixed template, issue the cable modulation-profile profile mix command in global configuration. This output shows the use of the command: 

ubr7246#configure terminal

Enter configuration commands, one per line.  End with CNTL/Z.

ubr7246(config)#cable modulation-profile 4 mix

ubr7246(config)#end

After you issue this command, the CMTS loads all the preconfigured settings for this profile. These settings appear in the configuration:

ubr7246#show run | include modulation-profile 4

cable modulation-profile 4 request 0 16 0 8 qpsk scrambler 152 no-diff 64 fixed uw16
cable modulation-profile 4 initial 5 34 0 48 qpsk scrambler 152 no-diff 128 fixed uw16
cable modulation-profile 4 station 5 34 0 48 qpsk scrambler 152 no-diff 128 fixed uw16
cable modulation-profile 4 short 6 75 6 8 16qam scrambler 152 no-diff 144 fixed uw8
cable modulation-profile 4 long 8 220 0 8 16qam scrambler 152 no-diff 160 fixed uw8

Note: Notice that the short and long IUCs use 16-QAM. The request, initial, and station use QPSK.

Note: The default unique word setting for long and short IUCs in the mixed profile is unique word 8 (UW8). Cisco recommends the use of UW16. The use of a longer UW helps in noisy situations and reduces uncorrectable FEC error generation.

Issue the cable modulation-profile profile command in order to change the unique word from UW8 to UW16. This is the output of show run | include modulation-profile 4 command after you change the unique word from UW8 to UW16: 

ubr7246#show run | include modulation-profile 4

cable modulation-profile 4 request 0 16 0 8 qpsk scrambler 152 no-diff 64 fixed uw16
cable modulation-profile 4 initial 5 34 0 48 qpsk scrambler 152 no-diff 128 fixed uw16
cable modulation-profile 4 station 5 34 0 48 qpsk scrambler 152 no-diff 128 fixed uw16
cable modulation-profile 4 short 6 75 6 8 16qam scrambler 152 no-diff 144 fixed uw16
cable modulation-profile 4 long 8 220 0 8 16qam scrambler 152 no-diff 160 fixed uw16

The section The show cable modulation-profile Command details the use of the show cable modulation-profile command. Issue this command after you configure modulation profiles in order to ensure that the CMTS correctly understood all the parameters.

Related Commands

The cable upstream modulation-profile Command

In order to assign a modulation profile to an interface, issue the cable upstream n modulation-profile interface configuration command. In order to assign the default primary modulation profile (profile 1) to the interface, issue the no cable upstream n modulation-profile interface command.

  • cable upstream n modulation-profile primary-profile-number [secondary-profile-number]

  • no cable upstream n modulation-profile primary-profile-number [secondary-profile-number]

Table 4 – Parameter Descriptions

Parameter Description
n Port number on the cable modem slot Port numbers begin with a 0.
primary-profile number Default modulation profile that is added to the interface
secondary-profile number Additional modulation profile that is added to the interface

Defaults

The default is the primary modulation profile (profile 1).

Command Modes

The command mode is interface configuration.

Table 5 – Command History

Cisco IOS Software Release Modification
11.3 NA This command was first introduced.
12.0(7)XR2 and 12.1(1a)T1 This command was introduced into the Cisco IOS Software Release 12.x trains.
12.1(3a)EC1 This command was modified to add the primary-profile-number and secondary-profile-number parameters in order to enable the Dynamic Upstream Modulation feature.
12.1(5)EC This command was introduced for the Cisco uBR7100 series universal broadband routers.
12.1(7)CX This command was enhanced for the Cisco uBR-MC16S line card.

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 in order to track upstream signal quality. The feature 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, the feature returns the upstream channel to the higher modulation scheme.

When you configure Dynamic Upstream Modulation and spectrum groups on the same interface, Cisco uBR-MC1xC and Cisco uBR-MC16S cable modem cards attempt corrective actions in this order:

  1. Modulation switchover

  2. Frequency hopping

  3. Channel width reduction

Example

This 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

The show cable modulation-profile Command

The syntax for this command is show cable modulation-profile [profile] [iuc-code] .

Table 6 – Parameter Descriptions

Parameter Description
profile (Optional) Profile number—Valid values are from 1 to 8.
iuc-code (Optional) Internal usage code—Valid options are:
  • request—Request burst (IUC 1)
  • reqdata—Request/data burst (IUC 2)
  • initial— Initial ranging burst (IUC 3)
  • station— Station ranging burst (IUC 4)
  • short—Short grant burst (IUC 5)
  • long— Long grant burst (IUC 6)

Defaults

There is no default behavior nor default values.

Command Modes

The command mode is privileged EXEC.

Table 7 – Command History

Cisco IOS Software Release Modification
11.3 XA This command was first introduced.
12.(0)7XR2 This command was used.
12.1(3a)EC1 This command was supported and the reqdata type was added.

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. These profiles configure modem transmit parameters for these upstream message types:

  • Request

  • Reqdata

  • Initial maintenance

  • Station maintenance

  • Short grant

  • Long grant

This is sample output from the show cable modulation-profile command:

ubr7246#show cable modulation-profile 1

Mod 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  short
1   request qpsk 64     no   0x0    0x10  0x152   1    8     no   yes     952
1   initial qpsk 128    no   0x5    0x22  0x152   0    48    no   yes     896
1   station qpsk 128    no   0x5    0x22  0x152   0    48    no   yes     896
1   short   qpsk 72     no   0x5    0x4B  0x152   6    8     no   yes     944
1   long    qpsk 80     no   0x8    0xDC  0x152   0    8     no   yes     936

This table describes the fields that display in the show cable modulation-profile command output:

Table 8 – Field Descriptions

Field Description
Mod 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 that is called the IUC. Bandwidth maps messages by IUC codes that are used to allocate upstream time slots. These types are currently defined:
  • request—Request burst (IUC 1)
  • reqdata—Request/data burst (IUC 2)
  • initial—Initial ranging burst (IUC 3)
  • station—Station ranging burst (IUC 4)
  • short—Short grant burst (IUC 5)
  • long—Long grant burst (IUC 6)
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 k bytes The number of information bytes within an FEC code word
FEC CW size Size, in bytes, of the post-FEC code word This is typically k + 2T bytes, or the number of FEC Information Bytes (k) plus two times the number of bytes that can be corrected within each FEC code word (T).
Scrambl seed Scrambler seed value, in hexadecimal format
Max B size Maximum burst size
Guard time size Time, in symbols, between successive bursts
Last CW short Handling of FEC for shortened last code word
Scrambl Scrambler enabled (yes) or not enabled (no)
Preamb offset The preamble value bits

Example

In order to verify that the modulation profile that you configured has the correct parameters, issue the show cable modulation-profile command for profile 2. 

ubr7246#show cable modulation-profile 2

Mod 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  short
2   request qpsk 64     no   0x0    0x10  0x152   1    8     no   yes     440
2   initial qpsk 128    no   0x5    0x22  0x152   0    48    no   yes     384
2   station qpsk 128    no   0x5    0x22  0x152   0    48    no   yes     384
2   short   qam  144    no   0x6    0x4B  0x152   6    8     no   yes     864
2   long    qam  160    no   0x8    0xDC  0x152   0    8     no   yes     848

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