September 30, 2011 - 12:00am

In the last post we reviewed some terminology and configuration parameters that are common when setting up an MPEG/IP delivery network. In that post, we mentioned that IP was originally developed for delivering sporadic burst of data. We also mentioned that in order to deliver video data it must arrive on time and in sequence at the intended destination.  Because timing is everything for optimal video delivery, the ability to accurately monitor points throughout the network helps to ensure the best possible video experience for the viewer.

Here are some more terms and their definitions that are commonly used to describe network quality and some metrics that should be monitored throughout the network to be able to isolate any issues and quickly determine the source of the problem.

Terms

PCR (Program Clock Reference) – is a value in the transport stream that provides a means for the decoder to lock its video output clock to the video input clock to the encoder that is providing the transport stream.

Jitter – is an error in a digital signal caused by timing variations.

Network Jitter – is the deviation between the time when a packet is expected and when the packet actually arrives. A packet can either arrive earlier or later relative to other packets causing a variable arrival of data packets.

PCR Jitter – is inaccuracy in the PCR as received by the decoder. PCR Jitter can be attributed to two sources, PCR accuracy error caused by the encoder and jitter in the network.

Metrics

There are a variety of test devices available on the market for IP monitoring, but the most fundamental job of any IP monitoring equipment is to keep track of the packet arrival times with a high degree of accuracy so that it can provide an indication of the quality of the network. The following metrics should be provided by a network monitoring device:

PCR_OJ (PCR overall jitter) – is a metric defined by TR 101 290. This metric includes both encoder caused PCR inaccuracies as well as jitter induced by variability in the network.

IAT (Interpacket Arrival Time) – is a metric that indicates the variability of the arrival of IP packets on the network. If the IAT has too much variability, the decoder will have difficulty recovering the PCR. And in a really bad case, the decoder buffers could underflow.

Packet Loss – is simply the failure of one or more of the transmitted packets to arrive at the desired destination. If the stream is sent using RTP, a monitoring device can use the RTP sequence number to determine if any packets have been lost.

Monitoring multiple points throughout the network provides valuable insight to potential issues and helps insure signal quality to the viewer. By monitoring multiple points, when a timing problem is identified, it can be narrowed down to the equipment between two monitoring points.

Sencore is happy to discuss possible monitoring solutions that will best fit your system needs and requirements. Contact us today.W.jones@sencore.com

In the last post we reviewed some terminology and configuration parameters that are common when setting up an MPEG/IP delivery network. In that post, we mentioned that IP was originally developed for delivering sporadic burst of data. We also mentioned that in order to deliver video data it must arrive on time and in sequence at the intended destination.  Because timing is everything for optimal video delivery, the ability to accurately monitor points throughout the network helps to ensure the best possible video experience for the viewer.

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