Chapter 9: Upstream Cable Access Modules (UCAMs)
STANDARD Revision 1.0 C4® CMTS Release 8.3 User Guide
© 2016 ARRIS Enterprises LLC. All Rights Reserved. 320
used. The parameter can vary from 2 * (k + 2T) up to 2048 where k and T are the FEC parameters described in the FEC
section.
When the ATDMA byte interleaver is in the dynamic mode of operation, this parameter should really be left at the 2048
value; otherwise, the byte interleaver will perform sub-optimally. Note, the dynamic mode of operation and a block size of
2048 are used in the default modulation profiles.
Preamble Type
An upstream channel using either ATDMA or SCDMA, has the capability to change the power level at which the preamble is
transmitted. This is displayed in the show cable modulation-profile CLI command in the Preamble Type column. There are
two different values that are possible for the preamble power levels. The first value is QPSK0 which corresponds to the
transmit levels that are used by a DOCSIS 1.x upstream channel. The second value is QPSK1, which uses a higher power
level. For actual differences in the power levels, see the RFI specification for DOCSIS 2.0 or later.
By transmitting at a higher power level, there are times when a preamble of type QPSK1 will be heard when the QPSK0
preamble type would not be heard. Therefore QPSK1 is used for the default modulation profiles.
SCDMA TCM Enable
SCDMA upstream channels can use Trellis Coded Modulation (TCM). The Scdma Tcm En column of the show cable
modulation-profile command shows whether it is enabled. Theoretically there is a slight gain to be had by using TCM.
However, in practice there are sometimes problems with TCM. There are cases where TCM will actually compound
problems and cause more errors than it solves. Consequently, these issues tend to offset the possible gain. TCM is not
enabled in the default modulation profiles.
SCDMA Interleaver Step Size
This parameter controls how the interleaver actually interleaves symbols in an SCDMA frame. It appears in the show cable
modulation-profile command in the Scdma Int Size column. According to the DOCSIS RFI specification, this parameter
has a value from 1 to 31. The PHY chip that is used places an additional restriction in the value must be at least 1 and has a
maximum of SCDMA frame size minus 1. The SCDMA frame size is also called the SCDMA spreading interval.
In general increasing this number will helps reduce impulse noise. The cost associated with increasing this value is
additional latency. The default modulation profiles have a value of 5 for this parameter.
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