Nortel CMTS 1000, Install Manual

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1-2 302693-B Rev 00
CMTS 1000 Installation Manual V1.2
Cable Plant Issues
There is no simple formula to follow when configuring cable plants for
cable modems. However, the goals are as follows:
• Keep the Carrier-to-Noise Ratio (C/N) to 25 dB or better at all points
on the cable. (The allowable number of combined runs depends on this
ratio.)
• Keep the number of translators per modem at a reasonable ratio for
performance and cost.
To make a firm recommendation, it is necessary to take actual measure-
ments on the cable plant. The options that can be taken depend on the
actual system parameters.
In general, every time you double the number of runs combined at the head
end, it is accompanied by a 3 dB loss in C/N. The first 3 dB loss occurs
when 2 runs are combined, the next 3 db loss from 2 to 4, etc. The limit of
combined runs is reached when the loss is great enough so that the C/N
Ratio hits 25 or lower.
In a new installation with only a few cable modems installed, combining
the runs into one CMTS return port is helpful from a cost viewpoint. But as
the concentration of modems increases, runs can be split off. For example,
if two runs have 500 or more cable modems, then these two runs can be
split off into a new CMTS return port, and the remaining runs stay on the
original CMTS return port.
As the density increases even further, it may be necessary to split out the
runs so that each run has its own CMTS return port. Combining and split-
ting runs at the head end is a relatively easy task.
Typically, the fiber optic link is the weakest link; it creates the most noise
in the upstream. This is based on the type of return laser used. In many
cases, Fabler Perot lasers are used in the upstream because of their lower
cost compared to DFB (Distributed Feed Back) lasers. For example, if a
Fabray Perot optical link offers a C/N of 40 dB, then the total system C/N