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.
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traffic should be given a higher priority in terms of FEC protection. Assuming that there is more FEC associated with a short
data grant (IUC 5) than with a long data grant (IUC 6), then the maximum burst size should be set such that the higher
priority traffic, in this case VoIP, uses the IUC with the higher level of FEC protection.
When the modulation profile is for an ATDMA or an SCDMA channel, the modulation profile will contain the UGS IUC 11.
The VoIP traffic will tend to use this IUC on a DOCSIS 2.0 upstream channel. In this case make sure you do not include the
UGS data packets into the process of determining the best value for the maximum burst size.
Guard Time Size
The guard time size is given in the Guard Time Size column in the
show cable modulation-profile
CLI command. The
guard time size is related to processing delays with a non-SCDMA upstream channel and is defined to be zero for an
SCDMA upstream channel. The times for non-SCDMA channels are already optimized based upon the hardware to be as
small as possible without losing data. If the numbers are increased, then upstream bandwidth is lost without any additional
gains. Changing the default is not recommended.
Shortened Last Codeword
A codeword is specified by the k parameter for FEC. Assuming the data packet is not an even multiple of k, then the last
codeword used to transmit a data packet will have less than k bytes to send. This parameter controls the format of that
last codeword. The last codeword must always contain at least 16 bytes whether shortened or not. This parameter control
whether or not the last codeword must contain k bytes or if it can contain between 16 and k bytes. If the last codeword
can contain between 16 and k bytes, the last codeword is allowed to be shortened and this parameter has a values of true
(T). If the last codeword must be padded out to contain k bytes, then the shortened last codeword value is set to false (F).
The shortened last codeword is displayed in the LCS (Last Codeword Shortened) field in the
s
how cable modulation-
profile
CLI command.
In general having the shortened last codeword set to true will improve upstream efficiency in that there is less overhead
associated with the additional padding in the last codeword. The default modulation profiles in general for IUCs 1, 3 and 4
do not have this set to true, simply because the messages sent on IUCs 1, 3 and 4 are of a fixed length and the values for k
are already optimized for this length. As such there is no benefit to turning on shortened last codeword with the default
modulation profiles unless the value of k is changed for these modulation profiles to a non-optimal value.
