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. 316
packet. Also assuming BPI is enabled then there is an additional 5-byte extended header such that it would take 75 bytes
to send a 64-byte packet. Note, that the preamble is not included in FEC and should not be included in these calculations.
In general, the predominant packet size plus the associated overhead determines that the k value should not be a value
between 1 and 15; otherwise there is additional overhead in padding of the codeword.
In the case of a DOCSIS 2.0 upstream, the modulation profile will include an IUC 11. The IUC 11 is used for UGS data flows.
The thing that most commonly uses UGS data flows is VoIP. There are several different codecs that are used in the
industry; however, typically there is only one codec with one sampling period (5, 10 or 20 milliseconds) on a given cable
plant. This tends to cause all of the upstream UGS data packets to be of the same size. Knowing the size of these UGS data
packets, the value of k for the FEC should be such that no additional padding is required for FEC.
Preamble Lengths
The preamble length is something that is displayed in the show cable modulation-profile CLI command in the Pre Len
column. This value represents the number of bits that are in the preamble.
In general the preamble is transmitted as a QPSK symbol no matter what the modulation type for the IUC actually is. One
exception to this is that a 16 QAM TDMA upstream channel will use 16 QAM symbols or 4 bits per symbol.
The preamble length is used by the upstream receiver to decode the upstream burst even if the upstream burst is not
perfectly aligned with the proper spot in the upstream spectrum. The preamble helps to recognize where an upstream
burst actually begins, and is also used to perform equalization on the upstream burst, provided that the preamble is long
enough.
In general a longer preamble is desired for IUC 3, Initial Ranging, since this is the first time that the CMTS has heard from
this modem. Additionally, the extra equalization also helps in IUC3. The amount of gain for equalization does not normally
warrant the additional overhead in terms of length of the preamble especially in IUCs other than the ranging IUCs (IUCs 3
and 4).
The point where the PHY chip switches from simply recognizing the start of the preamble and making sure that it has the
correct pattern to recognizing when extra symbols are used for equalization depends upon several parameters. Some of
these parameters are not currently configurable and require very detailed knowledge of how the PHY chip operates. As
such, this paper will not explain all of the details behind the following numbers. The point where equalization begins for
preambles using QPSK on IUCs 3 and 4 is currently after 136 bits. For IUCs 3 and 4 when 16 QAM is used, equalization
begins after 272 bits. For non-ranging IUCs (any IUC except IUCs 3 and 4) for preambles using QPSK, equalization begins
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