MPEG2 TS tests and measurements have already been discussed. This section will cover testing strategies for core and wired access networks.

Core ATM Network Testing
SONET/SDH layer testing will not be covered here since SONET/SDH networks have been operational for a number of years, and there is a well-established set of tests. ATM parameters which may affect the quality of service of digital video have been specified by the ATM Forum and include (See Table 5):

Quality of Service Parameter	Effect on Digital Video
cell errors	degrades picture quality
cell delay	picture motion becomes jerky
cell delay variation (cell jitter)	random jerkiness
cell loss	lost frames, picture jumps

Table 5. Quality of Service and Its Effect on Digital Video

While cell errors and loss are of concern no matter what type of payload is being carried in ATM cells, cell delay and cell-delay variation are of particular concern in digital video since timing is so crucial.

Access Network Testing
Here, we are concerned with the impairments introduced in wired residential access networks. HFC and SDV networks use similar RF–modulation techniques, and, therefore, many of the necessary measurements are the same. The classes or layers of measurements may be divided into the following categories:

• RF
• modulation quality
• data quality

RF Measurements
Signal- and power-level measurements have been a staple of analog TV systems and will continue to be important in digital-video systems. In HFC systems, in particular, level measurements are important because, with so many channels carried simultaneously in one cable, adjacent channel interference can degrade the signal quality. At the head end or other RF distribution point, accurate level measurements are required, while near the customers, a go/no-go test is adequate. Compared to analog video, measuring the average power of a digital-video signal is more difficult because of the wide-band, noise-like nature of the digital RF spectrum. The peak power of a digital signal is typically 6 to 10 dB higher than the average power. This will require cable operators to back off the level of digital channels approximately 10 dB below analog video carrier levels to avoid adjacent channel interference. The RF tests required for digital video services include (see Table 6):

Measurement	Why Important?
average channel power (or level)	prevent adjacent channel interference peak-to-average
channel power or (level) ratio	determine back-off level for digital channels
channel carrier-to-noise ratio	determine adjacent channel effects, internal and external noise
adjacent channel power (or level) and spillover	determine adjacent channel effects

Table 6. Measurements and Their Functions

Modulation-Quality Measurements
To achieve spectral efficiency in digital-video transmission, high-level digital modulation techniques, such as QAM and its closely-related derivative, CAP, are employed. QAM attains its spectral efficiency by assigning a group of bits to a symbol. There are a discrete number of symbols, each of which is translated to a particular RF amplitude and phase. Figure 7 shows the symbols and resulting signal constellation of a 16–QAM signal.

Figure 7. 16–QAM Signal Symbols

Each point on the constellation diagram is the ideal location, in amplitude and phase, of a symbol. The number of bits per symbol, and their amplitude/phase translation, varies with specific modulation scheme. Spectral efficiency increases as the number of bits per symbol increases. The increase in efficiency comes with a trade off—decrease in signal-to-noise margin. As the number of points in the constellation increases, each point is closer to its neighboring points (to maintain constant spacing would require an increase in power, which is not feasible). This makes it possible for noise and other impairments to cause a demodulation error.

This effect can be observed by viewing a real-time constellation display. Impairments cause the actual symbols to deviate from the ideal target location. The degraded signal will sometimes be misinterpreted as a different symbol, resulting in bit errors. Viewing the shape of the cloud around each constellation is indicative of the type of impairment causing the degradation.

QAM has been in use for a number of years in non-video applications, and a body of knowledge exists pertaining to the identification of impairments by viewing constellation displays. Numerical measures of the constellation have also been devised to quantifying the amount of degradation in a digital signal. These measurements, error vector magnitude (EVM) and modulation error ratio (MER), are explained in Figure 8.

Figure 8. Measurements of Viewing Display Impairments

The DVB has singled out MER as a single figure of merit test for modulation quality. Its units of measurement, in dB, are analogous to traditional analog TV signal-to-noise ratio (SNR) and carrier-to-noise ratio (C/N) measurements.

Data-Quality Measurements
A decisive proof of digital transmission quality is the BER. An important reason for measuring BER is the so-called cliff effect in digital transmission. In analog-transmission systems, as impairment levels continuously increase, the degradation in quality continuously increases. In analog TV, this results in snow, and other visual and audio impairments which gradually worsen as the SNR degrades. In digital transmission, increasing impairment levels will have no visible effect on quality until a threshold—or cliff—is reached, at which point the entire picture will be severely degraded or lost without warning. The cliff effect in digital video is due to the following factors:

• compression—MPEG2 compression removes most of the redundancy in the original audio and video signals. Loss or corruption of the TS can have a major impact on the ability to decode and reconstruct audio and video signals.
• forward error correction (FEC)—In lossy networks, FEC is used to automatically correct bit errors. FEC, however, will only be effective up to a certain BER level beyond which the FEC will not function.