Why would another pick Divicom for ATM expertise? Only HP know for sure......................................
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Validating ATM-based digital video networks
MPEG-2 and ATM compliance
Detecting subtle errors that cause failures
Network impairment emulator verifies jitter removal
Programs for compliance testing
MPEG offers many test challenges
DiviCom, a Milpitas, California, manufacturer of MPEG-2 encoding systems for broadcast television, runs an extensive program for integrating and testing ATM-based digital video networking systems. Dan Maltbie, Vice-President of the company's system integration division, shares his insights into digital video network testing using the HP Broadband Series Test System.
“A ‘typical' digital video broadcast system can deliver over 100 channels of programming, including audio services and near video on demand, using real-time encoding,” explains Maltbie. “For flexibility, the system may have a distributed architecture with different encoding centers and use ATM networks to distribute the MPEG-encoded signals to transmission sites. Today we are building MMDS transmission sites, but other types of transmission networks--such as switched digital video--are possible. A distributed architecture can include different network delivery systems, all based on MPEG carried over an ATM backbone distribution system.”
The digital video system must be tested to ensure that everything is interoperating and performing to specification. DiviCom's extensive test bed incorporates a complement of all the equipment they will deploy--the ATM networks, the encoding systems, the modulation systems, the conditional access systems, the automation systems, and the program-guide systems.
In the test setup for an MMDS delivery system with an ATM distribution network, coding systems and video servers are attached to the ATM network, as are the transmission systems. Within this setup, DiviCom uses HP's Broadband Series Test System (BSTS). Maltbie explains, “First we use the BSTS on the ATM network to validate the sources--are they delivering valid MPEG over ATM? Are they meeting the interface specification? Then we use it to introduce impairments into the ATM network in order to verify that our transmission systems can remove jitter and correctly handle errors coming in from the ATM network.”
MPEG-2 and ATM compliance
An important part of testing is the verification of interfaces to different vendors' equipment. “Everything comes together over ATM,” states Maltbie, “so ATM is the point at which we monitor the various MPEG sources, analyzing them for compliance to ATM, to MPEG, and to the interface specifications that we've established for a particular customer. We use the test system in essence as a referee to verify that network equipment is meeting specs.
“We have to do more than just analyze ATM formats and look at ATM headers. Because we are carrying MPEG information, it requires us to set up filters that are “MPEG-aware” and capture information that is relevant either as a transport header or the packetized elementary stream header to allow us to analyze the MPEG information over ATM on the fly.
“HP's test system with the MPEG-2 Protocol Viewer software gives us the ability to filter on some fairly complex MPEG information. For example, we can capture only packets that have PCRs (program clock references) in them, or only packets with starter sequence headers. This filtering allows us to maximize the capture buffer by storing only essential information.”
Detecting subtle errors that cause failures
Testing is also used to determine various failure conditions. “Because MPEG is generating multimegabit data streams with lots of information flying by,” explains Maltbie, “it's hard to get a handle on what that information means. And yet some very subtle problem could have a catastrophic effect--dropping a cell now and then could cause the video to break up. From the viewer's standpoint, there is something wrong with the picture, yet the error is hard to find because of the volume and complexity of information flying by. To help define what is missing, we use HP's test system, which can detect different kinds of errors in the ATM network that affect MPEG performance, such as dropped cells or cell delay variation.”
Network impairment emulator verifies jitter removal
Another important area of digital video network testing is timing and jitter analysis. Says Maltbie, “MPEG is very specific about the types of jitter it allows; only hundreds of nanoseconds. The transport of ATM introduces large amounts of jitter; milliseconds. So the ability to measure that jitter and understand its effect on the MPEG stream is essential to making these ATM networks work in terms of carrying MPEG. We have to be able to isolate components or switching systems that may be introducing too much jitter.
“For example, we developed equipment to remove jitter from a set top so that the set top can operate in a standard MPEG mode. In order to verify that our equipment removes jitter, we must first introduce it, which we do with the HP test system's network impairment emulator.
“We also use the impairment emulator to verify that our system's error handling and detection are operational--first by causing errors in the data stream at various bit error rates, and then by analyzing the effects on the transmission system and on the set top itself.”
Programs for compliance testing
“We use the programming capabilities of the HP test system to help us develop compliance tests,” adds Maltbie. “For example, we have written filtering scripts that run allow us to demonstrate to our customer that the encoding systems in the NVOD servers are indeed performing as specified. In this case, the HP BSTS is not so much a tester as a device that we connect to the ATM network to provide a higher level of programming and analysis capability for verifying the compliance of MPEG and of our system.”
MPEG offers many test challenges
Suggests Maltbie, “One of the challenges of developing test equipment is dealing with all the network interfaces--QAM, QPSK, and all the flavors of ATM--OC3, DS3, and so on. Test equipment has to be very flexible in terms of how it interfaces to the network.
“MPEG itself has some very difficult test challenges. There is a lot of complex and high speed data that requires a sophisticated processing platform for data capture and processing. It's important to capture long streams of data over time in order to analyze the behavior of MPEG over time, and that requires test equipment with large capture buffers or sophisticated filtering capabilities--or real-time testing.
“The ultimate challenge for designers of MPEG test equipment will be to quantify the quality of the video and audio. It is difficult to do this in a lossy system that uses data compression. We can't use comparative methods and say that the bits that went into the system are the same ones that came out, because the compression algorithm is throwing out as many bits as it can. The challenge is to define a test set that can state that an MPEG signal is ‘good'--because that is what people look for in a test environment.”
DiviCom's experience creating MPEG-based systems for broadcast television and near video on demand is helping HP define new capabilities for its test equipment, including the real-time HP E6271A MPEGscope ATM for the HP BSTS. (See article, Growing family of products for MPEG-2 testing.)
Visit DiviCom on the World Wide Web at divi.com
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