PDF of this tutorialFor voice and data, the obvious choice, at least in the near term, is SONET technology. However, SONET traditionally does not do a good job at video transport. Video coders/decoders (CODECs), which compress one or more video signals to digital service, level 3 (DS–3) rates, are expensive and yield relatively poor performance specs. Furthermore, being an outboard device, the SONET network management sees just the DS–3 circuits; it cannot monitor the video performance. Consequently, many broadband operators install two networks: a SONET system for voice and data, and an analog or proprietary digital system for video. From both an architectural and operational perspective, this is not an optimal solution.
To address this essential transport issue, a video-optimized OC–48 SONET multiplexer has been developed (see Figure 11). Video-optimized input/output (I/O) devices take analog video channels in various formats and map them directly in STS–3 slots in the SONET payload. This lets the operator carry analog video in an uncompressed, high-performance manner while maintaining all the benefits of SONET for duplex services such as voice and data. To encode the video, the operator can use 10-bit samples and map both baseband and IF video signals, scrambled or clear channel. This technology allows building a true multimedia backbone in which a single pair of fibers carries a full range of video, voice, and data services. Eliminating service-specific architectures creates the flexibility needed to cope with the uncertainty inherent in new service rollouts.
Figure 11. A Video-Optimized OC–48 SONET Multiplexer
One drawback of carrying uncompressed analog video signals digitally is the large amount of bandwidth needed—16 analog channels occupy an entire OC–48. To address this, DWDM is used to combine up to eight systems on a single fiber. This combination enables the transport of 80 analog video channels and several hundred digital video streams, while maintaining 5 gigabits for voice and data services.
