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.
304
Parameter
Description
fec-tbytes Forward Error Correction (T) The number of bytes with errors that can be corrected with FEC in the size specified in
the codeword information byte length. A value of zero indicates that FEC is disabled. For each byte that can be
corrected there are two additional FEC parity bytes that are added to the FEC codeword. The FEC codeword contains
both the FEC information bytes and the FEC parity bytes. The number of codeword parity bytes is 2 x T, where T =
0-10 for a DOCSIS 1.x upstream channel and 0-16 for a DOCSIS 2.0 upstream channel.
fec-len
Forward Error Correction, Length of Codeword (K) The number of bytes in the information bytes of the FEC
codeword. Assuming that FEC is enabled, the FEC codeword can contain from 16 to 253 information bytes. The FEC
codeword contains both the FEC information bytes and the FEC parity bytes and can be between 18 and 255 bytes. A
shorter codeword will increase the amount of overhead but allow for more errors to be corrected in the total data
frame.
pre-len
Preamble Length The preamble serves to put the FEC and randomizer (also called the scrambler) into known states
before the data is transmitted. The preamble also helps the receiver to receive an upstream burst accurately. DOCSIS
1.X (type 4 TLV in the UCD) requires the preamble length to be between 0 and 1024 bits. DOCSIS 2.0 (type 5 TLV in
the UCD) requires the preamble length to be between 0 and 1536 bits.
12U or 24U CAM
• For a TDMA or tdma-atdma upstream channel, IUCs of 3 and 4 with 16 QAM must have a preamble length in the
range of 208 to 768.
• All other cases for IUCs of 3 and 4 must have a preamble length in the range of 104 to 768.
• For a TDMA or tdma-atdma upstream channel, IUCs of 1, 5 and 6 with 16 QAM must have a preamble length in the
range of 72 to 256.
• All other cases for IUCs of 1, 5 and 6 must have a preamble length in the range of 36 to 256.
• For IUCs 9, 10 and 11, the preamble length must be in the range of 36 to 512.
Note: Even with the above guidelines, it is possible to choose parameters such that the Preamble Superstring which
contains the preamble strings for all the different IUCs does not fit within either the 1024 or the 1536 bit limits.
Contact technical support for further assistance if experiencing these problems.
seed
The 15-bit seed value for the scrambler polynomial. The pseudo-random generator (randomizer) is used so that the
data stream will not produce a long string of either 1's or 0's. Changing the seed for the pseudo-random generator
will cause the generator to produce a different pattern of ones and zeroes if the same input data is sent to the
pseudo-random generator. Some seeds will work better than others for producing a good distribution of 1's and 0's
without a contiguous string of either 1's or 0's. The range of possible values is from 0-32767.
