As providers prepare for the deployment and maintenance of Internet protocol television (IPTV) service offerings, at the forefront of concern are two issues: service assurance and customer quality of experience (QoE). Success in the IPTV market space will depend on reliable networks and satisfied customers who receive the level of service they expect from a technology which promises revolutionary viewing experiences. The means to address these issues is careful selection and implementation of test and measurement equipment and solutions from a vendor with established digital video, broadcast test and network management solutions expertise.
JDSU brings extensive experience supporting service providers, and the JDSU portfolio of triple-play test solutions leads the industry among both telecommunications and cable operators. With this rich heritage of supplying both the communications and home entertainment industries, JDSU is uniquely positioned to assist providers in successful IPTV service rollout.
ABOUT IPTV
IPTV is not just television over the Internet. IPTV is a means to deliver next-generation, realtime and interactive video service to the home or business via any network capable of delivering IP encapsulated content. At the customer premises, the television is connected to the Internet via either a PC or a set-top box (STB). This connection gives the viewer the capability to experience high-function, two-way communication with the entertainment source. For example, unlike satellite or terrestrial delivery systems where the consumer experience is limited to sending a selection and receiving the content, IPTV enables the consumer to post program content that can be accessed and viewed by others on the network. Figure 1 illustrates the components in a typical IPTV network.
SELECTING IPTV TEST SOLUTIONS
To ensure the end user QoE, service providers deploying IPTV as part of a triple play bundle need to continuously test and monitor their IPTV service offering. IP networks are complicated, with a lengthy IPTV content delivery chain, starting at the broadcaster's head-end, through several network elements, and finally to the end-user's set-top box. The entire service delivery chain must be covered by testing and monitoring at key points across the network.
As with satellite and terrestrial broadcast, testing for IPTV should encompass the full MPEG-2 transport test regimen. Additionally, tests must be performed to ensure the MPEG-2 transport can effectively be implemented at the downstream end of the IP network. Fundamentally, the required IPTV tests can be grouped into three main categories: digital video testing, metro Ethernet/transport testing, and finally access loop testing.
IPTV cannot be tested reliably by traditional transport or access test equipment that may already be in place on network. To ensure customers' QoE, a proactive test and measurement system with enhanced transport and access testing capabilities, as well as a new system that performs digital video testing is required. The digital video test system must perform round-the-clock monitoring of multiple simultaneous signal streams and provide engineers with integrated ability to analyze digital video services remotely. (While it is outside of the scope of this article, please note that additional information is available from JDSU on enhancing metro Ethernet/transport and access loop testing.)
DIGITAL VIDEO TEST CONSIDERATIONS: TIMING
To perform thorough digital video testing, many considerations must be addressed. For IPTV, the most important is ensuring stream composition integrity, synchronization and timing accuracy, TR101 290 compliance and audio/video quality assurance. Also important, at a second tier for IPTV, is testing the metadata contained within the DV stream incoming from content providers.
A good IPTV MPEG-2 test device will make detailed measurements of all the time stamps and timing elements in a given transport stream. This includes, at a minimum, PCR spacing, PCR accuracy, jitter, drift, and PTS and DTS timestamps. Since the TR101 290 specifications defined by the DVB organization provide only guidelines on how to test the timing parameters rather than explicit formulas, it is important to note that one brand of MPEG-2 test instrument may yield different timing measurements than another.
Therefore, it is imperative for the user to perform multi-vendor tests to ensure selection of an instrument that offers the most accurate timing measurements. Another way to identify a digital video protocol analyzer with the most accurate timing analysis is to consult with the various OEMs. They use these types of instruments in their R&D labs and in their field support functions and would provide the name of the vendor they trust most for these testing needs.
TIMING: PROGRAM CLOCK REFERENCE (PCR) ANALYSIS
PCR is the mechanism by which the IPTV MPEG-2 transport stream synchronizes the clock in the receiving device to that of the transmitting/encoding device. Once this synchronization has occurred, further timing data, required for the accurate decoding of the MPEG-2 transport stream, become usable. This synchronization is achieved via a counter, based upon a 27 MHz clock that is sent along with the video data within an MPEG-2 transport stream. As such, the MPEG-2 transport stream test instrument must be able to identify whether inappropriate jitter is present in the PCR timestamps. It should also provide detailed and highly accurate measurements of the actual PCR timestamps, the drift of the 27 MHz PCR clock, the current offset of the PCR clock as well as the drift rate of the PCR clock. For instance, in Figure 2, a PCR jitter graph, the technician should ensure that PCR jitter presents itself in a bell curve with the most number of samples around 0 nsec jitter.
TIMING: SYNCHRONIZATION ANALYSIS
Transportation of IPTV MPEG-2 transport streams requires not only the end-to-end synchronization available through the PCR mechanism, but also synchronization between programs and the synchronization between the audio and video components of each program. The lack of exact synchronization between the audio and video components of a program manifests itself in the form of lip sync errors that are visible to the viewer as the video trailing/leading the audio. An MPEG-2 test and measurement instrument must be able to provide a very accurate and reliable way of identifying such errors in realtime.
PROGRAM SPECIFIC INFORMATION (PSI) TESTING
An IPTV MPEG-2 transport stream is comprised of an endless stream of 188-byte packets, the majority of which typically carry the digital audio and video payloads. There are often multiple programs on each transport stream. In order to sort through the millions of packets and pull out the appropriate audio and video data for presentation, there must be a map of the incoming content of the transport stream for the receiving device.
This map is implemented through the PSI tables, namely the Program Association Table (PAT) and Program Map Tables (PMT). Every element within an MPEG-2 transport stream is assigned a Packet ID (PID). The PAT lists the programs within the stream, and the PID numbers for the PMTs of each program. The PMT, in turn, lists the PID numbers for the audio, video and PCR components of each program.
This would be the minimum required PSI data in an MPEG-2 transport stream. There may also be a Conditional Access Table (CAT), listing subscription and pay-per-view options, as well other ATSC or DVB tables containing data used for Electronic Program Guides (EPG).
TR101 290 TEST CONSIDERATIONS
Along with MPEG-2 standards, there is a set of recommendations called ETR-290 (also called TR 101 290) issued by the Digital Video Broadcasting (DVB) group, which describe how an MPEG-2/DVB stream should operate for a quality broadcast. Highly ranked MPEG-2 analyzers have default measurements that correspond to ETR-290 recommendations, but also allow the user to adjust the error criteria for the system under test so the results are accurate.
ETR-290 test results are categorized into three different levels:
- Priority One indicates errors that directly impact a receiver's ability to make the program presentation.
- Priority Two indicates timing errors likely to have a negative impact on picture quality.
- Priority Three indicates problems with the data being sent for the EPG.
A comprehensive digital video test system should provide one-click access to TR 101 290 results, enabling the technician to view a snapshot of all aspects of the integrity of the transport stream and take action quickly when errors occur.
Bruce Seawood is product marketing manager for the Cable Networks business unit of JDSU's Communications Test and Measurement Group.
