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Chapter 7 Basic M-CMTS Data Specific Operation
Generally, a service group will first be implemented using a single existing
downstream primary from the CMTS RF linecard (i.e. the 5x20 linecard). When
cable modems come online using the local 5x20 primary downstream, they will
exhibit a timing offset value that is indicative of processing delays in the 5x20
and other characteristics of the physical plant (i.e. length of cables, temperatures,
etc.). Incidentally, the processing delays in the 5x20 linecard are fixed - the user
cannot add offsets to them. Therefore, the timing offset from the local 5x20
primary becomes the benchmark timing offset for the service group.
Timing offsets are critical because they are the mechanism by which the CMTS
communicates the system time to cable modems. Cable modems need to know
the system time so that they will only transmit in the upstream during specified
timeslots (so that collisions are avoided on the shared upstream medium).
As a service group grows, operators may choose to add primary downstream
capacity to improve the service level of the mac-domain (thus creating MxN
domains). Primary downstream capacity can be added using additional 5x20
carriers, or, can be added using the M-CMTS architecture with one or more
eQAM channels.
If the operator chooses to add primary downstreams from an eQAM, he or she
will discover that the eQAM's own internal processing delays contribute to result
in timing offsets (for cable modems that register online on eQAM primaries) that
are different than the same cable modems that previously registered online on
the local 5x20 primary. Therefore, eQAMs must implement a feature for
offsetting the value of the DTI timestamp to calibrate eQAM primaries to match
local 5x20 primaries in the service group. All eQAM primaries must be
manually calibrated.
Generically, the calibration process to implement load-balancing in a mac-
domain occurs as follows:
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Click Apply.
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Click Save.
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Allow a test group of cable modems to register online on a benchmark
local 5x20 primary downstream channel.
Using the "show cable modem" command at the CMTS command line,
record the average Timing Offset for all modems in the group.
Add a primary downstream channel to an eQAM carrier in the service
group. Allow the same group of cable modems to register online on it, so
that the operator can record the difference in the average Timing Offset
for the group.
Manually calibrate out the timing offset difference using the DTI Offset
feature of the eQAM.
Finally, load-balancing can be configured and enabled for the service
group.
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