The Hybrid Fiber Coaxial HFC Network

The first networks to include a fiber node-type technology were built beginning in 1990. CATV networks today are built exclusively using HFC design and older networks are being retrofitted to receive the benefits of HFC. As a result, most CATV networks today are based on fiber node-based architecture, but they also include elements of the older coaxial networks. The HFC network design helps reduce many of the amplification and attenuation issues and other issues associated with all coaxial cable plants. HFC cable networks can be extended significantly further than coaxial networks without the need for amplification. Because the signal travels over optical fiber, the physical distances between headend and subscriber can extend to more than 100 kilometers—much further than in a coaxial-based plant.

With the advent of HFC topologies, the old trunk cable was renamed express cable. Figure 13-2 shows a typical HFC network design with a remote fiber node attached to express cable, distribution cables, and feeder cables. Express cable typically runs between amplifier cascades and does not have subscriber households directly attached.

Figure 13-2 Example HFC Network

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Figure 13-2 Example HFC Network

Feeder Cable

Hfc Node Fiber Tail

HFC is a hierarchical network design. Each headend includes fiber receiver/transmitter pairs that are connected through long strands of fiber to a remote fiber node. A remote fiber node converts received downstream optical signals to electrical signals and puts them onto the attached coaxial cable. Likewise, when the remote fiber node receives upstream electrical signals on the coaxial cable, it converts them to optical signals and places them on the attached optical fiber. The remote fiber node is connected to the headend transmit and receive lasers through attached fiber and to the subscriber premises through the attached coaxial cable. Fiber nodes also perform important tasks such as filtering frequencies in the upstream outside of a set range (typically frequencies between 5 and 45 MHz are not filtered, although this varies based on the type of cable network).

A fiber node serves a set number of homes or potential subscribers. Because the number of potential subscribers in a specific geographic location does not change frequently, fiber node deployments can usually be designed with a specific number of potential subscribers (known as households passed [HHP]) in mind. In 1990, during initial HFC rollouts, each fiber node was designed to serve 10,000 HHP. By 1995, this number had decreased to 2000 HHP. As optical equipment prices continue to decrease and demands for plant segmentation increase, node size decreases. In 2001, nodes with as few as 250 HHP became common. The number of homes served by a fiber node has significant impact on the deployment of two-way IP cable technologies on that physical section of cable plant.

Cable operators are gradually upgrading their cable plants and reducing the number of subscribers per fiber node in a process called plant segmentation. Because the cable plant is fundamentally a shared transmission medium, plant segmentation allows the cable operator to more closely target subscribers with specific services. Each fiber node can be considered its own small cable plant because upstream and downstream signals are isolated on a per-node basis. Specifically, this implies that the cable operator can make different television channels or other separate services available to separate nodes. Plant segmentation is one of the most important techniques used to control the number of cable modems that are transmitting on a single shared section of the cable plant. The cable operator can also use additional frequencies, called frequency-division multiplexing (FDM), to segment traffic. An operator weighs the costs of additional frequency spectrums versus the capital costs of further segmenting the network, and typically chooses the lower cost alternative.

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