Cisco Headend System Release 2.5 用户指南
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Overlay Technology
4004831 Rev B
Differences Between a DBDS and an Overlay Environment
Overview
You can offer subscribers the services and capabilities provided by a Cisco DBDS by
introducing Cisco components into an existing non-Cisco network.
The key innovation that Overlay technology provides to cable service providers is
The key innovation that Overlay technology provides to cable service providers is
the ability for multiple conditional access (CA) systems to coexist on the same
network with only a minimal increase in required plant bandwidth.
To deploy set-tops in a system that utilizes multiple CA and set-top vendors without
To deploy set-tops in a system that utilizes multiple CA and set-top vendors without
Overlay technology, the digital content typically must be encrypted by each CA
vendor’s equipment and dual-carried on the cable plant. Most cable systems simply
cannot support a 100% increase in bandwidth for their encrypted digital services.
Overlay technology partially encrypts the transport stream video to reduce the
Overlay technology partially encrypts the transport stream video to reduce the
overhead requirement from 100% to a theoretical minimum of 2%. Instead of 100%
of the MPEG content being dual-carried, only “critical packets” from within each
MPEG stream are dual-carried and the remainder are left unencrypted and carried
once. A “critical packet” is loosely defined as a packet which, once removed from the
MPEG stream, will prevent reconstruction of the video or audio signal by the
receiving device.
Overlay Environment
While a typical Cisco DBDS will support any Cisco set-top, an Overlay environment
only supports Cisco set-tops that have versions of the Cisco Resident Application
(SARA) written especially for an Overlay environment.
When introducing Cisco set-tops into a non-Cisco network, hardware components
When introducing Cisco set-tops into a non-Cisco network, hardware components
must be added to create the digital streams that the Cisco set-tops require. The
components that control the set-tops (BFS channel, QPSKs, etc.) are the same in an
Overlay environment and a Cisco DBDS. The difference is how the MPEG streams
for the digital programs are added into the existing system. These streams are
unique to an Overlay environment.