Fig. 1: Comparison of useful test points for three video delivery architectures.
ORANGE, CONN.—Over-the-air DTV transmitters are relatively easy to test, at least with respect to the integrity of the outgoing video stream. Once the RF signal radiates from the antenna, impairments to the digital stream come from physical-layer phenomena, such as loss, noise, and reflections. The same is true for satellite broadcasts, and an also applies to some extent for CATV systems.
In contrast, an IPTV delivery system consists of a number of active devices (routers, switches, etc.) between the signal origin and the viewer’s receiver. Any of these devices along the way can cause impairments to the IP video stream without leaving any trace of an issue at the physical layer when it reaches the consumer. Fig. 1 compares the number of test points (shown as red circles) required for three different video distribution architectures, showing how IPTV systems may require monitoring at a number of intermediate points along the path to the viewer.
Another complication is the fact that IPTV streams are usually single program transport streams that are sent individually through a network whereas other systems such as CATV rely heavily on multiprogram transport streams. This fragmentation creates more streams that need to be tested, and reduces the value of comparing different streams, further intensifying the testing problem.
TYPES OF TESTING
Physical layer testing looks at the cables, fibers, or other transport medium and the electrical or optical signals that ride on them. These tests typically measure parameters such as BER (Bit Error Rate/Ratio), jitter and latency. Physical layer tests can determine if a particular connection between two devices is faulty or suffering from interference, or if the devices on either end are causing errors.
Network layer testing deals with the Ethernet data frames that are travelling between devices on the network. Monitoring can determine if frames are being corrupted and need to be discarded, and if traffic levels are within the limits of the physical connection. Tests can also be done to determine if multicast packets are being correctly forwarded by switches using IGMP (Internet Group Management Protocol) snooping.
Karl Kuhn IP layer monitoring looks at IP packets, both collectively and within individual streams. End-to-end addressing can be validated alongside the ability of the network to correctly handle multicast join and leave requests. Packet loss, network latency, and packet jitter are also important parameters to monitor.
Video and audio testing can be performed at the application layer, both in the compressed domain and after the streams have been decoded. This type of testing can help determine if the video data is complete and in the proper syntax, whether audio loudness limits are being followed, and whether the all-important MPEG PCR (Program Clock Reference) is being properly carried along the path.
“Monitoring PCR clock behavior in an MPEG stream as it passes through various portions of a network acts as a fingerprint that can be used to perform forensic analysis of performance issues,” said Karl Kuhn, senior video applications engineer for Tektronix, in Beaverton, Ore. “If the PCR is smooth and well-behaved, then the network is doing a good job in transporting the MPEG stream. If the PCR is varying in a periodic, sinusoidal manner, then the prime suspect is in the RF transmission domain. If the PCR is experiencing random, sharp discontinuities, then IP network packet loss and excessive packet jitter are the likely culprits.”
IPTV systems can create difficulties for performance monitoring and troubleshooting because of the large number of routers and switches which must be configured correctly and operate properly. Such devices can create subtle downstream faults that can be hard to localize because they may only occur when certain traffic patterns are present.
“Modern service providers need to monitor transport system behavior at multiple locations throughout the network and look at both data delivery statistics as well as video and audio signal quality,” said Joel Daly, director of product management for IneoQuest Technologies, a Mansfield, Mass.- based provider of video quality and service assurance technology. “This requires a system that can gather large amounts of data from multiple sources, including test probes installed at critical locations in the network, reports from system operators who are monitoring signals, and inquiries from users who are experiencing troubles.
“Large multichannel video programming distributors are currently busy building large data analysis systems that can look at current network status and historical trends to more accurately spot problems as they arise, and to anticipate where failures could happen in the future before they occur,” he added.
INSIDE THE STUDIO
As broadcast studios move towards greater use of IP/Ethernet infrastructure, testing live, in-service video streams will become increasingly important. Broadcasters must ensure that link capacities are not exceeded and that packet buffers limits are not violated. IP routers and switches must provide detailed information about their current configuration and performance, and will need a mechanism to reserve bandwidth for specific flows. In addition, network probes will likely be needed to monitor system traffic and provide independent data to sectionalize faults, along with a management system to analyze data from multiple sources.
IP networks offer many benefits for broadcasters, including speed, cost reduction, and flexibility. Harnessing this power requires the ability to analyze system performance across several network layers and to gather data at multiple points along each network path. Fortunately, new technologies are being developed today to meet these challenges.