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. 313
If the Modulation Rate Is…
Then the SNR Threshold Must Be at Least:
FEC
Forward Error Correction (FEC) can correct errors that occur in the upstream channel; however, this comes with a cost of
additional overhead. FEC is typically expressed in terms of two parameters, T and k. T is used to represent the number of
byte errors that can be corrected. The k parameter is used to specify the number of bytes over which the T number of byte
errors can be corrected and is called the codeword length.
The cost of correcting up to T byte errors in k data bytes is that there is an additional 2 * T bytes of overhead. Note, the
values for T are shown in the FEC column in the show cable modulation-profile CLI command. The values for k are shown in
the FEC CW Len column in the show cable modulation-profile CLI command.
show cable modulation-profile 6
Typical output:
Modulation profile 6
Interval Chan Mod Pre Dif FEC FEC Scr Max Guar L Scr ---Atdma--- Prea -----Scdma-----
Usage Type Type Len Enc CW amb Bur Time C amb Int Int mble TCM Int Sp Sub
Code En Len Seed Siz Size S En Depth Block Type En Size En Cod
---------------------------------------------------------------------------------------------
1 request tdma qpsk 56 F 0 16 338 0 8 F T - - - - - - -
3 initial tdma qpsk 640 F 5 34 338 0 48 F T - - - - - - -
4 station tdma qpsk 384 F 5 34 338 0 48 F T - - - - - - -
5 short tdma q16 168 F 6 75 338 7 8 T T - - - - - - -
6 long tdma q16 192 F 10 220 338 0 8 T T - - - - - - -
The figure below illustrates the relation between code words and the packet, and between the T and k parameters. For
more information, see the DOCSIS RFI Specification, version 2.0. (CM-SP-RFIv2.0-C02-090422).
For DOCSIS 3.0, see CM-SP-PHYv3.0-I10-111117.
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