Installation guide

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Cisco uBR7100 Series Universal Broadband Router Software Configuration Guide

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Chapter 3 Configuring the Cisco Cable Interface

Configuring Cable Modulation Profiles

To configure cable profiles (deviating from defaults), perform the following as necessary:

• “Configuring Cable Modulation Profiles” section on page 3-41

• “Configuring QoS Profiles” section on page 3-43

• “Setting QoS Permission” section on page 3-45

• “Enforcing a QoS Profile Assignment” section on page 3-45 (Optional)

• “Monitoring and Maintaining QoS for Higher Priority Traffic” section on page 3-47.

Configuring Cable Modulation Profiles

In this step, you are defining the elements used in a cable modulation profile. The Cisco uBR7100 series

CMTS supports as many as 8 cable modulation profiles. Profile 1 is the default.

Caution If you modify a cable modulation profile from default parameters, you are changing the physical layer.

Changing physical layer characteristics affects router performance and function; therefore, only an

expert should perform this task.

The following modulation profile values are configurable:

• Interval usage code (iuc)—Valid values are initial, long, reqdata, request, short, and station.

• Fec-tbytes—The number of bytes that can be corrected per FEC codeword. Valid range is

0 to 10 bytes, where 0 means no FEC. This is the number of bytes that the FEC decoder can correct

within a codeword. A codeword consists of information bytes, called k-bytes and parity bytes for

error correction. The number of parity bytes is equal to two times the number of correctable error

(T). The size of T is dictated by channel impairments.

• Fec-len—FEC codeword length. Valid range is 16 to 254 bits. This value enables an efficiency mode

wherein all codewords are fixed in size. In a fixed operation, all codewords are the same size with

the last codeword padded with nulls if there are not enough information bytes to fill it entirely. The

efficiency is gained by not having to transmit the nulls that pad the last codeword.

• Burst-len—Maximum burst length in bytes. 0 means no limit on burst length. 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. This is the time between successive bursts. It 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 values are “16qam” and “qpsk”. Modulation type is used to select between

four bits per modulation symbol (QAM-16) or two bits per modulation symbol (QPSK). QAM-16

uses both phase and amplitude to carry information. QPSK carries information in the phase of the

signal carrier. QAM-16 requires approximately 7 dB higher C/N to achieve the same BER as QPSK,

but it transfers information at two times the rate of QPSK.