Cisco Model D-PCG1000 PowerKEY CAS Gateway Installation Guide
Theory of Operation
4012215 Rev B
13
Overlay Headroom Requirements
In order to avoid a QAM over-provision condition, the data rate of each MPTS must
be less than or equal to the QAM data rate. The NOBE, however, adds overlay data
to each MPTS, and so each incoming MPTS rate must be reduced in order to allow
for this additional overhead. This reduction, called overlay headroom, must be
maintained. When the overlay headroom falls below this threshold, the encryption
percentage is temporarily reduced in order to avoid dropping packets. This
encryption percentage dialback will occur to the point where all packets are sent in
the clear. It is therefore very important to provide adequate headroom for the
overlay operation.
be less than or equal to the QAM data rate. The NOBE, however, adds overlay data
to each MPTS, and so each incoming MPTS rate must be reduced in order to allow
for this additional overhead. This reduction, called overlay headroom, must be
maintained. When the overlay headroom falls below this threshold, the encryption
percentage is temporarily reduced in order to avoid dropping packets. This
encryption percentage dialback will occur to the point where all packets are sent in
the clear. It is therefore very important to provide adequate headroom for the
overlay operation.
If, for example, an MPTS that is destined for a 256 QAM modulator contains 12
programs with one 3 Mbps video and one 200 kbps audio channel, the required
headroom would be determined as follows:
programs with one 3 Mbps video and one 200 kbps audio channel, the required
headroom would be determined as follows:
The overlay data that is added to the MPTS must be estimated; this data includes
PowerKEY encrypted video and encrypted audio packets as well as Cisco ECM
and PMT PIDs.
PowerKEY encrypted video and encrypted audio packets as well as Cisco ECM
and PMT PIDs.
Since video is unviewable when 4 percent of data is encrypted, we can estimate
the Overlay headroom needed for PowerKEY encrypted video as 0.12 Mbps (3.0
x 0.04 = 0.12 Mbps).
the Overlay headroom needed for PowerKEY encrypted video as 0.12 Mbps (3.0
x 0.04 = 0.12 Mbps).
When only the audio start packets are encrypted (with the remaining audio
packets in the clear), the audio is unintelligible. These packets occur in the audio
stream at a rate of 47 kbps, regardless of the audio data rate. Therefore, Overlay
headroom required for each audio PID is 47 kbps, and we can estimate Overlay
headroom for encrypted audio as 0.047.
packets in the clear), the audio is unintelligible. These packets occur in the audio
stream at a rate of 47 kbps, regardless of the audio data rate. Therefore, Overlay
headroom required for each audio PID is 47 kbps, and we can estimate Overlay
headroom for encrypted audio as 0.047.
The ECM and PMT PIDs are minor and can be estimated to be 0.01 Mbps each.
Using these figures, we can determine the amount of Overlay headroom as
indicated here:
indicated here:
-
The MPTS Overlay Headroom = 12 programs (0.12 + 0.047 + 0.01 + 0.01) =
2.25 Mbps
2.25 Mbps
-
Allow 38.8 Mbps for the full 256 QAM data rate
-
Therefore the MPTS feeding the NOBE would need to be groomed to 36.5
Mbps (38.8 – 2.25 = 36.5 Mbps).
Mbps (38.8 – 2.25 = 36.5 Mbps).
Note: The audio encryption percentage on the overlay session setup screen should
always be configured to 98%. This value signals the NOBE to only encrypt the start
packets. Use of any other encryption percentage value could result in audio being
intelligible.
always be configured to 98%. This value signals the NOBE to only encrypt the start
packets. Use of any other encryption percentage value could result in audio being
intelligible.