Triple-Play Service Deployment

Chapter 6: Troubleshooting Video in the Headend
This problem is increasingly more likely with the proliferation of bandwidth
maximizing technologies. Rate shaping, transrating, and stat-muxing are
processes designed to maximize the use of bandwidth. As the video element is
always the largest bandwidth consumer in the MPEG stream, it often is the focus
of bandwidth optimization. In fact, in a Constant Bit Rate (CBR) MPEG transport
stream, most of the stream consists of video PIDs, and the balance is filled with
stuffing of null PIDs to accomplish the constant output bit rate. If a stat mux
experiences an interruption in its process, rather than drop the video PID
altogether, it will likely continue to push it out with some nominal rate, and the
Null PID rate will jump to cover the gap. The resulting program presentation will
either have degraded video or none at all. TR 101-290 has no provision for
alarming on his scenario, but if the technician can monitor the PID flows it is
possible to detect when the video PID rate and null PID rates change.
In an IPTV system, bit rate is at such a premium that Variable Bit Rate (VBR)
transport streams may be used in lieu of CBR streams.The VBR stream has no Null
PID, but the issue with Video bit rate being constrained too heavily persists. An
analyzer that can track the rate of individual PIDs or Programs can provide
valuable visibility to an IPTV headend engineer. The better analyzers will extend
an ability to set rate thresholds, allowing for active rate monitoring and alarming
when the rate falls too low.
PCR Analysis
As the primary synchronization device for the set top box, the PCR is critical to a
quality program presentation. It essentially consists of a time code that has been
stamped into the outgoing transport stream at an interval of 27 MHz. In the MPEG
environment, the PCR must be accurate to within 500 nanoseconds. That
threshold applies to the accuracy of the time code within each successive PCR, as
well as the jitter on arrival of each PCR at the receiver.Thus, there is an element of
accuracy and network jitter. There are multiple threats to the integrity of the PCR
integrity as it travels through the MPEG and IP system. Sophisticated MPEG
multiplexers such as stat muxes, transcoders and rate shapers make making
significant changes to the MPEG transport streams.While the video programming
is intended to look identical going in as coming out of these devices, the MPEG
transport streams often look completely different. Yet, the PCR integrity must be
preserved. Often these devices will author the PCR values in concert with respacing
them at the output, and that presents significant risk to the PCR integrity.
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