PDF of this tutorialAs new services are added to the broadcast video suite, the first network goal is to increase the reusable bandwidth per user. This can be accomplished in two ways: by making the pipe bigger and, as the interactive services are shared, by decreasing the number of users on a given node (see Figure 2). Bigger pipes, in the form of 862-MHz systems, currently are being deployed in larger metropolitan areas. Fiber-deeper architectures are beginning to be deployed as well, taking node size down to 50 homes, which yields up to 10 times the interactive bandwidth per user.
Figure 2. Increasing Reusable Bandwidth
As the volume of interactive traffic grows, the transport network must be enhanced to provide flexible, efficient connections to servers. These servers may be located anywhere in the network, but they typically start out in a centralized manner at the primary head end, putting a significant strain on today's transport structures. A key consideration for network design is to be able, as much as possible, to match the equipment deployment expense with the expected revenue from the service.
Network evolution goals thus are twofold: an access goal of maximizing reusable bandwidth per user and a transport goal of efficient flexible connection to servers anywhere in the network.
Four key technologies are needed to address these goals:
- High-power 1550-nm optics can be used both in the transport area to carry multiple quadrature amplitude modulation (QAM) bundles for interactive traffics and in the access area to lower the network costs to facilitate fiber-deeper architectures.
- Digital transmission, using video-optimized SONET multiplexers, is critical to building high-speed multimedia backbones.
- Wave division multiplexing (WDM) is used not only for increased bandwidth, but also for optical routing and reduction in access cost.
- Passive optical technology becomes critical to both cost and performance as the amount of fiber in the network grows.
The first consideration for determining the optimal access architecture is the amount of necessary bandwidth, either broadcast and interactive or narrowcast. An HFC network has four dimensions involved with delivering interactive bandwidth: frequency, spatial multiplexing, spectral efficiency, and wavelength.
Frequency gives the ability both to decide the size of the pipe (750 MHz, 862 MHz, or 1 GHz) and to determine what type of signal a given subcarrier offers. Each frequency can be reused over time as the service set changes, providing a unique flexibility in comparison to other architectures. Spatial multiplexing determines how many fibers to run in the backbone and to each node and how to load them. Spectral efficiency allows migrating over time to modulation techniques such as 256 versus 64 QAM, which effectively increases bandwidth. Finally, multiple wavelengths, whether DWDM or 1310/1550 combinations, can be used within a given fiber to increase capacity.
