Downstream Transmissions - Cisco uBR924 Installation Manual

Chapter 1 Product Overview
Figure 1-4
WAN
Internal backbone
worldwide internet
The Cisco uBR924 router provides the connection between the PC and the cable system, modulating
the data transmitted to and from the PC so that it can be carried over the coaxial cable installed by the
service provider. To avoid interfering with the cable video signals that are also transmitted over this
same coaxial cable, the DOCSIS specification allows only certain frequencies to be used for data
transmissions. Separate frequencies are used for the data sent from the CMTS to the cable modem (the
downstream direction) and for the data sent from the cable modem to the CMTS (the upstream
direction).
The CMTS divides the cable plant into downstream channels and upstream segments or clusters of
nodes. Each Cisco uBR924 router on the network is configured to receive data on a particular
downstream channel. A downstream channel contains one or more upstream segments; partitioning the
upstream plant into smaller segments significantly reduces the number of potential ingress sources and
failure points.

Downstream Transmissions

Because 90% of the data transmitted on the Internet is, on average, sent from the network to the user,
the cable system allocates the majority of bandwidth for downstream data (data sent from the CMTS to
the router). Downstream transmissions use a 6 MHz data channel in the 88 to 860 MHz range, providing
an approximate maximum bandwidth of 27 or 26 Mbps. This bandwidth is shared among all subscribers
who have been assigned to this particular downstream channel.
The CMTS receives the downstream data from its Internet or other WAN connections. It addresses the
data to the appropriate Cisco uBR924 router and modulates it for transmission on the cable network.
When the data arrives at the subscriber's site, the router modulates it for transmission over the Ethernet
connection to the appropriate CPE device.
OL-0338-01 (3/2000)
Cisco Broadband Data Cable System
CMTS
Switch/router
rack equipment
Servers
and
Cable System Headend
Upstream and downstream data interfaces
Operation support system interface
Downstream RF interface
Upstream RF interface
Cisco uBR924 Cable Access Router Description
Combiner Tx
Fiber
Rx
Cisco uBR900 series
cable access router
HFC
cable
plant
Residence or SOHO
subscriber site:
subscriber RF interface
Ethernet interface
Cisco uBR924 Router Hardware Installation Guide
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