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.
317
after 132 bits. For non-ranging IUCs (any IUC except 3 and 4) for preambles using 16 QAM, equalization begins after 264
bits.
If there is a lot of impulse noise in the upstream channel, then it is probably worth using shorter preambles to lower the
probability that an impulse will actually hit a preamble. Note that if an impulse actually hits a preamble that most likely the
entire frame will be discarded because of a bad preamble. Part of the reason behind this is that the preamble is not
protected with FEC. As such with a lot of impulse noise on the upstream channel, the preamble lengths should be
shortened.
Because of the way that the PHY receiver works in the CMTS, for an SCDMA channel type, there is a slight benefit that can
be gained if the preamble length is a multiple of the SCDMA frame size in symbols. The default value for the SCDMA frame
size is sixteen symbols. Since the preamble for SCDMA is always QPSK, which is two bits per symbol, there is a slight benefit
if the preamble is a multiple of 32.
Differential Encoding
Differential encoding can be enabled since it is a feature in the DOCSIS RFI specifications; however, there is no real gain to
be had in doing this. This mode is rarely used in the field; therefore we recommend that you leave Differential Encoding
disabled. (For more information, see Cable Modem Specifications.)
Scrambler Seeds
This is a value that tends to be tied to the PHY hardware. These values have already been optimized by the PHY
manufacturer. Therefore, do not change the values that come with the default modulation profiles for the scrambler seed.
Maximum Burst Size
This value is contained is something that is displayed in the
show cable modulation-profile
CLI command in the
Max(imum) Bur(st) Size column. This value is in terms of maximum number of minislots that may be used by the associated
IUC. A value of 0 for the maximum burst size means that there is no limitation on the size at least in the modulation profile.
Note: The short data grant (IUC 5) and the advanced PHY short data grant (IUC 9) must have non-zero values.
It is important to understand the type of traffic that is to be sent upstream and the relative priority of that traffic when
adjusting this parameter. For example, if there is VoIP traffic on an upstream that uses a TDMA channel type, then the VoIP
