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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Scrambler Enable
The scrambler enable is shown in the
show cable modulation-profile
CLI command in the Scramb En column. The
scrambler might be better called a randomizer. Having this field set to true enables hardware that randomizes the bit
stream to avoid a long pattern of either zeroes or ones. This helps in the overall transmission efficiency of the entire
system. In general this should always be set to true (T).
ATDMA Interleaver Depth
With an ATDMA upstream channel, there is the capability of using an interleaver that is not available in the DOCSIS 1.X
version of TDMA. This interleaver works on a byte basis. When enabled, the interleaver will change the order in which
bytes are transmitted. This has a side effect of causing additional latency in the upstream direction. The benefit is the
additional protection against impulse noise. In general an impulse will corrupt some number of bytes that are transmitted
consecutively with time. If the bytes are all from the same FEC codeword, and if FEC is not able to correct for this problem,
then the data is lost. However, by ordering the transmission of bytes such that bytes from multiple FEC codewords are
intermixed, the same impulse will hit fewer bytes from the same FEC codeword giving a better chance that FEC will be able
to recover the corrupted data.
The ATDMA interleaver depth is shown in the show cable modulation-profile CLI command in the Atdma Int Depth column.
This value controls how this interleaver works. A value of 0, puts the interleaver into a dynamic mode such that the
interleaver adjusts the way that it interleaves the data based upon the size of the data to transmit. A value of 1, turns off
this interleaver. Any other value directly controls how many FEC codewords are interleaved together. When directly
controlling how many FEC codewords are interleaved together, the value has a range from 2 to the floor (2048 / (k + 2T))
where k and T are the FEC parameters described in the FEC section of this document.
The default modulation profiles use the dynamic mode of operation in order to get as much protection from impulse noise
as possible. If a system has extremely tight restrictions in terms of upstream latency, then the amount of interleaving may
be changed to either be off or of a lesser amount. This comes at the cost of reduced impulse noise immunity.
ATDMA Interleaver Block Size
This is another control for the ATDMA byte interleaver and is shown in the show cable modulation-profile CLI command in
the Atdma Int Block field. According to the DOCSIS 2.0 RFI specification, both the CMTS and a cable modem must contain
2048 bytes of memory to perform the ATDMA byte interleaving. This parameter controls how much of that memory is
