Data Operations; Downstream Transmissions - Cisco CVA122 Hardware Installation Manual

Chapter 1 Product Overview

Data Operations

This section provides an overview of how data is transmitted to and from the Cisco CVA122/CVA122E
Cable Voice Adapter across the cable system's HFC network. Figure 1-3 illustrates a typical broadband
data cable system, showing the network path between the Cisco CVA122/CVA122E Cable Voice
Adapter and the CMTS headend equipment (Cisco uBR7200 series universal broadband router or other
DOCSIS-compliant CMTS).
Figure 1-3
WAN
Internal backbone
worldwide internet
The Cisco CVA122/CVA122E Cable Voice Adapter 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 CVA122/CVA122E Cable Voice Adapter 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 cable voice adapter). 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-0799-02 (05/2001)
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 CVA122/CVA122E Cable Voice Adapter Description
Tx
Combiner
Fiber
Rx
Cisco CVA120 Series
cable voice adapter
HFC
cable
plant
Cisco CVA122/CVA122E Hardware Installation Guide
Residence
subscriber site:
cable RF interface
Ethernet interface
USB interface
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