Cable Access Technologies - Evolution from One-Way Broadcast to Two-Way Hybrid Fiber Coax

Evolution from One-Way Broadcast to Two-Way Hybrid Fiber Coax

A CATV network consists of a head-end location where all incoming signals are received and, regardless of their source, frequency-division multiplexing (FDM) is applied, amplified, and transmitted downstream for distribution to the complete cable plant.

Original CATV networks, as shown in Figure 22-1, were exclusively one-way, comprised of diverse amplifiers in cascade to compensate for the intrinsic signal loss of the coaxial cable in series with taps to couple video signal from the main trunks to subscriber homes via drop cables.

Figure 22-1 A Simple, One-Way Video Broadcast Topology Using Coaxial Cabling Exclusively

Besides being unidirectional, the long amplifier cascades resulted in a system with high noise that was inherently unreliable and failure-prone, in addition to being susceptible to lightning strikes and ingress noise from foreign radio frequency (RF) signals.

The first significant improvement to the CATV plant was the introduction of fiber-optic technology and the advent of the HFC plant (see Figure 22-2).

Figure 22-2 Simple HFC Distribution Network

Portions of the coaxial cable and supporting amplification elements are replaced with multifiber optic cable from a head end or hub location. The aggregated video signal is used to modulate a downstream laser, which transmits the optical signal to an optical node, which in turn converts the signal from an optical to an electrical signal that can then be propagated downstream to the entire customer serving area.

It can be readily seen that the introduction of the fiber can significantly reduce the number of cascaded amplifiers consequently improving system reliability, the signal-to-noise ratio (SNR) of the downstream video signal, and potential system bandwidth. In addition, this makes the system ready for the next step to two-way operation. As an added benefit, HFC reduces operational and maintenance costs, and improves the immunity of the system to ingress noises.

Two-way operation is achieved by the addition of requisite upstream amplifiers in the amplifier housings, the addition of a narrow-band upstream laser in the optical node, a dedicated upstream fiber to the head end, and a compatible optical receiver to convert any upstream information to an electrical signal. When all components are in place, proper return path alignment is required.

By means of adding an optical RING topography, the cable network affords greater reliability, supports greater bandwidth with the capability to transport more information, and is ready to support two-way operation by the simple addition of requisite components, as illustrated in Figure 22-3.

Figure 22-3 Advanced HFC Network with Ring Topography

Network robustness, scalability, and flexibility is further improved by the introduction of the intermediate hub from which advanced services can ultimately be launched.

The HFC network and topography as outlined become the basic building blocks for developing access transport capabilities needed by the MSOs to compete in the dynamic communication environment.