PDF of this tutorialThe existing analog transport network can also play a role in carrying interactive traffic. For video-on-demand, for example, a typical hub may need to receive several hundred digital streams to meet the traffic requirements for 20-percent or 30-percent penetration.
To deliver this amount of bandwidth, up to eight 1550 transmitters, each carrying 200 MHz of QAM loading, can be multiplexed onto a single fiber and combined at the hub with the broadcast signal (see Figure 12). Depending on architectures chosen, either externally modulated or directly modulated transmitters may be used. This technology is also being used in the upstream direction to carry reverse path signals from hubs to head ends.
Figure 12. Using the Analog Network for Interactive Traffic
In the longer term, systems will need to carry, over the same fiber, not only multiple wavelengths of baseband digital, but also multiple QAM channels and an analog tier as well. In this way, backbone metropolitan-area networks (MANs) can be constructed to meet the transport goal of efficient connectivity of multimedia services in multiple formats, avoiding unnecessary conversion and processing costs whenever possible.
