Data Operations; Downstream Transmissions - Cisco uBR905 Installation Manual

Chapter 1 Product Overview

Data Operations

This section provides an overview of how data is transmitted to and from the Cisco uBR905 router
across the cable system's HFC network. Figure 1-7 illustrates a typical broadband data cable system,
showing the network path between the Cisco uBR905 router and the CMTS headend equipment
(Cisco uBR7200 series universal broadband router or other DOCSIS-compliant CMTS).
Figure 1-7
WAN
Internal backbone
worldwide internet
The Cisco uBR905 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 uBR905 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 percent 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.
OL-0598-01 (9/2000)
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
Tx
Combiner
Fiber
Rx
Cisco uBR900 series
cable access router
HFC
cable
plant
Residence or SOHO
subscriber site:
subscriber RF interface
Ethernet interface
Cisco uBR905 Router Hardware Installation Guide
Functional Overview
1-15