Completing The Downstream Configuration - Cisco 7246 - uBR Router Hardware Installation Manual

Chapter 4 Connecting the Cisco uBR7200 Series Router to the Cable Headend

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. In the example shown in
the laser transmitter has two inputs. These inputs are designed for +17 dBmV video carriers. 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. (See
In the example, there is an optical splitter on the output of the laser transmitter that allows you to transmit
Note
to two fiber nodes.
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. 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 video
carriers. In this example, start with a 20-dB attenuator to adjust for the insertion loss and passive loss in
the headend cables. (See
Figure 4-14 Complete Downstream Configuration
Upstream Downstream
Diplex filter
40 dB
L H
attenuator
C
Cable
modem
If you have a very large, complex headend system with many outputs, you might notice a very large
Tip
passive loss in your headend combining network. For example, in a headend with 100 feet of 59-series
headend coaxial cable (RG-59), you can see losses of 6 to 8 dB. To compensate for this loss, you can
install the upconverter closer to the laser transmitters.
OL-5421-03
Figure
Cisco uBR7200
series router
Upconverter
IF
IF input
0 dBmV
RF output
RF
0-20 dB
attenuator
(as required)
-3 dBmV video carrier
Cisco uBR7200 Series Universal Broadband Router Hardware Installation Guide
4-14.)
610 MHz center frequency
+55 dBmV output level
Laser
Main
transmitter
headend
feed
+17 dBmV
Broadcast
video carrier
(typical)
Narrowcast
+7 dBmV
data carrier
3-way
(typical)
splitter
8-way
tap
Connecting and Configuring the Downstream
Figure
Node 1
Optical
splitter
Node 2
Downstream
forward
test point
Figure 4-14,
4-14.)
4-15