Frequency Allocation
Cable operators will be distributing both analog and TV programs over HFC networks as long as analog TV is broadcast over the air. Analog distribution is required throughout the period of transition from analog to digital over-the-air broadcasting to protect the viability of the embedded base of analog/cable-ready TV sets. Channel capacity will be divided between the analog and digital channels according to the availability of digital programming and management preferences of the MSOs. The analog channels will be carried from 54 MHz up to roughly 350 to 400 MHz. Digital channels will start at this cut-off frequency and proceed to 550 or possibly 750 MHz. Table 3-2 shows a representative frequency allocation in the United States after fiber and return path upgrades are applied to a cable network.
| Frequency Range (MHz) | Direction | Primary Application | Comment |
|---|---|---|---|
| 5 to 42 | Upstream | Return path data, telephony, network management, pay-per-view billing | Low frequency; supports longer distances without amplification compared with forward traffic, but is subject to ingress noise. |
| 54 to 350 | Downstream | Analog TV | Upper bound will vary by operator. As over-the-air TV goes digital, this range will be reduced. |
| 350 to 750 | Downstream | Digital TV | Frequency range will vary by operator. |
| 750 to 1000 | Upstream | Additional return path | Need to augment the limited capacity of 5 to 42 MHz. High frequency is subject to greater attenuation. Requires new types of return path amplifier. Expensive and problematic. |
Frequency use will vary in Europe because of 8 MHz channelization and analog TV stations that begin above 65 MHz.
The proliferation of fiber and return path systems are key elements in the improvement in the physical capability of cable systems. But a better cable plant alone is not sufficient to support RBB service. Another key factor has been the develoment of standards for digital TV, data service, and telephony over cable systems. Cable's practical, proactive approach to standardization has enabled equipment suppliers, semiconductor desigers, and content developers to plan and execute on a known physical plant. The key standards in the U.S. are OpenCable™ for digital television, Data-over-Cable System Interface Specification (DOCSIS) for data service, and PacketCable, for cable telephony.