Cisco Headend System Release 2.5 User Guide
4004831 Rev B
Overlay Technology
1-3
Differences Between a DBDS and an Overlay Environment,
Continued
What Additional Hardware is Required in the Overlay Environment?
At the heart of the Cisco Overlay system is the Cisco Gigabit Overlay QAM
modulator (GoQAM). The GoQAM provides many new digital broadcast features
and innovations for your system and is an integral component of the Overlay
environment. The GoQAM allows set-tops from different vendors, running unique
conditional access protocols, to operate on the same system while optimizing
bandwidth usage. Depending on the system architecture, the GoQAM can be used in
either headends or hubs.
Each GoQAM channel accepts two inputs: an unencrypted MPEG transport stream,
Each GoQAM channel accepts two inputs: an unencrypted MPEG transport stream,
and the same MPEG transport stream after encryption by a third-party CA vendor.
When supplied with these clear and encrypted inputs, the GoQAM will construct the
partially encrypted Overlay MPEG stream. Specifically, the GoQAM determines the
critical packets, encrypts the clear version of these critical packets using Cisco’s
PowerKEY encryption, selects the corresponding critical packets from the third-
party encrypted input, selects the remaining clear packets, and multiplexes the
selected packets together to create the partially encrypted Overlay transport stream.
This stream is then modulated and upconverted to either an IF or RF frequency for
output to the HFC plant.
Note: Additional hardware may be required to integrate the GoQAM into the
overall Overlay environment, as illustrated on the following pages.
Modulator
PowerKEY
Encryption
Overlay
Multiplexor
Encrypted Feed
Clear Feed
GoQAM
RF or IF Ou
t