FEC Forward error correction

© 2004 Cisco Systems, Inc. All rights reserved. BCRAN v2.1—4-12

The following are important cable technology terms about RF signal handling:

■ Carrier: Also RF carrier. An electromagnetic signal on which another, lower-frequency signal (usually baseband, such as analog audio, analog video, or digital data) is modulated to transport the lower-frequency signal to another location.

■ Spectrum reuse: The most fundamental concept of cable TV is spectrum reuse. Historically, the over-the-air spectrum has been assigned to many uses: two-way radio, broadcasting, cellular phones, and pagers. Much of the spectrum is therefore not available for the carriage of just TV. The result is an inadequate supply of spectrum to serve viewer needs. Cable operators can reuse spectrum that is "sealed" in the coaxial cables of their networks.

■ Frequency-division multiplexing (FDM): An RF transmission method in which a number of transmitters share a transmission medium. Each transmitter occupies a different frequency.

Quadrature phase shift keying (QPSK): A digital modulation method in which the phase of the RF carrier is varied to transmit data. There are 2 bits per symbol.

Quadrature amplitude modulation (QAM): A digital modulation method in which the phase and amplitude of an RF carrier are varied to transmit data. Typical QAM types are 16-QAM (4 bits per symbol), 64-QAM (6 bits per symbol), and 256-QAM (8 bits per symbol).

■ Carrier-to-noise (C/N): Also carrier-to-noise ratio (CNR). The difference in amplitude between the desired RF carrier and the noise in a defined bandwidth.

■ Signal-to-noise (S/N): Also signal-to-noise ratio (SNR). Similar to C/N but relates to a baseband signal.

■ Ingress noise: Over-the-air (OTA) signals that are coupled into the nominally-closed coaxial cable distribution system, generally via damaged cable, other network components, or poorly shielded TVs and VCRs. Difficult to track down and intermittent in nature.

Forward error correction (FEC): In data transmission, a process by which data is added that is derived from the payload by an assigned algorithm. It allows the receiver to determine if certain classes of errors have occurred in transmission and, in some cases, allows other classes of errors to be corrected.

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