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. 315
would mean that there must be 18 total bytes (k + 2T) required to transmit 6 bytes of data. Given this amount of overhead,
it is normally better to use a low order of modulation such as QPSK and no FEC for IUC 1, assuming the noise on the plant
allows this to work.
Figure 68: Maximum ATDMA Data Rate vs. SNR
The type of traffic that is sent in the upstream direction can affect the optimal FEC values also. For example, assuming
there is a lot of ACKs and small packets (i.e. 64-byte packets), then the FEC codeword length should be set so that there is
no need to pad out the remaining 16 bytes. Assuming a 6-byte MAC header, it would take 70 bytes to send the 64-byte
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