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Cisco uBR7225VXR Universal Broadband Router Hardware Installation Guide
OL-17309-02
Chapter 4 Connecting the Cisco uBR7225VXR Router to the Cable Headend
Measuring the Upstream RF Signal
Figure 4-30 Analyzing the Upstream RF Signal—1500-Byte Data Packets
Note Both of the previous examples feature 16-QAM transmission with a channel width of 3.2 MHz, yielding
a 10-Mbit/sec data rate. In addition, these examples have an optimal upstream carrier-to-noise ratio of
approximately 50 dB.
Now it is time to view your upstream RF signal with multiple remote cable modems. Figure 4-31 and
Figure 4-32 both display upstream RF signals encompassing more than one remote cable modem. In
each case, there are two bandwidth requests followed by their respective ping packet returns, both at
slightly different amplitudes. This situation is most commonly caused by a difference in the receive
power from the two cable modems in question. In the example, the remote cable modem with the lesser
amplitude is “cable modem A” and the other is “cable modem B.”
In the following example, cable modem A and cable modem B have been artificially configured to yield
a larger than normal difference in amplitude between their respective upstream RF transmissions. Under
normal conditions, the maximum difference in amplitude between any cable modems will be about
1.5 dB. Differences greater than 1.5 dB indicate a possible cable plant or remote cable modem problem.
Note To further illustrate this point, you can log in to your Cisco uBR7225VXR router using a console
terminal and enter the show cable modem command to obtain a report of the receive power ratings for
each modem. In the example, the receive power ratings for remote cable modems A and B are –2 dBmV
and 0 dBmV, respectively.
The two bandwidth requests and ping packet returns on the upstream RF signal for cable modems A and
B are slightly different in Figure 4-31 and Figure 4-32. Differences in the distance between bandwidth
requests are primarily caused by the contention-based nature of multiple remote cable modems on the
same line. Differences in the distance between ping packet returns are primarily caused by factors such
as packet size and system loading.