Riverstone Networks WICT1-12 - Manual (Page 119)

Riverstone Networks RS Switch Router User Guide Release 8.0 7-7
CMTS Configuration Guide Connecting the Upstream to the Laser Receiver
7.4.5 Completing the Downstream Configuration
To complete the downstream configuration, you must combine the upconverter output with the main headend
broadcast feed into the laser transmitter in the headend. The narrowcast feed, which includes cable modem service and
digital video and local access channels, is connected to the laser transmitter input using an 8-way tap and a 3-way
splitter.
The 8-way tap has an insertion loss of 11 dB and the 3-way splitter has an insertion loss of 7 dB. The combined loss
is 18 dB. With this combined insertion loss, you will overdrive the input on the transmitter and it will not work
properly. In order to compensate for this insertion loss, you must add attenuation to the digital carrier laser input. The
input level for the data carrier is +7 dBmV, or 10 dB below the analog carrier. In this example, start with a 20-dB
attenuator to adjust for the insertion loss and passive loss in the headend cables.
If you have a very large, complex headend system with many outputs, you might notice a very large passive loss in
your headend combining network. For example, in a very large headend with 100 feet of RG-59 or 59 series headend
coaxial cable, you might see losses of 6 to 8 dB. To compensate for this loss, you can install the upconverter closer to
the laser transmitters.
7.4.6 Testing the Downstream Configuration
To test the downstream configuration, you can connect a cable modem to the downstream forward test point of the
laser transmitter. Use a diplex filter and an attenuator, as needed, to connect the cable modem to an upstream port on
the RS 8000/8600 CMTS module.
The nominal input level for a RS 8000/8600 CMTS module upstream port is 0 dBmV, but it can also be adjusted as
low as -16 dBmV or as high as +26 dBmV using RS 8000/8600 CMTS module software.
The RS 8000/8600 CMTS module will instruct the modem to adjust its output level to match the current input level.
Your test cable modem will require a minimum of 8 to 10 dB of attenuation between the upstream of the cable modem
and the upstream port on the RS 8000/8600 CMTS module.
If this configuration is working properly, you have a very good chance of getting the rest of the network up. If this
configuration generates a low carrier-to-noise ratio (C/N) estimate in the cable modem, you need to make further
adjustments.
You can measure the preliminary C/N ratio estimate at the headend downstream laser test point. This measurement can
be used to verify the performance of the upconverter, headend combiner, and forward distribution system before cable
modems are installed on the HFC network.
7.5 CONNECTING THE UPSTREAM TO THE LASER
RECEIVER
To connect the upstream to the laser receiver, use a 2-way splitter as a combiner to leave the RS 8000/8600 CMTS
module cable access router connected at the headend, and connect the upstream headend cable to the laser receiver.
You must adjust the upstream input level to the RS RS 8000/8600 CMTS line card so the output of the laser receiver
is the same as the input to your upstream port or your RS 8000/8600 CMTS module.