Q: What is the bandwidth of one multimode optical fiber for cable television?
Jeff Smith
Classic Cable
Woodward, OK
A: Bandwidth is the measure of the information-carrying capacity of an optical fiber. The information-carrying capacity of a transmission medium is normally referred to in units of MHzkm. Called the bandwidth-distance product, this value is the product of the length of a fiber (in kilometers) and the bandwidth that the fiber can transmit over that length (in megahertz). The amount of information that can be transmitted over any medium changes with distance, and the relationship is not necessarily linear. A 500-MHzkm fiber does not translate to 250 MHz for a 2-km length or 1000 MHz for a 0.5-km length. When comparing media, it is important that the same length for each be considered.
ansi/tia/eia-568a and a vast number of application standards have specified 62.5/125-micron multimode optical-fiber cables with the following characteristics:
- Attenuation of 3.75 decibels per kilometer at an 850-nanometer wavelength and 1.5 dB/km at 1300 nm
- Bandwidth of 160 MHzkm at 850 nm and 500 MHzkm at 1300 nm
However, the cable-TV industry is installing singlemode fiber cable, for which the ansi/tia/eia-568a standard specifies the following attenuation values:
- Outside cable: 0.5 dB/km at 1310 and 1550 nm
- Inside cable: 1.0 dB/km at 1310 and 1550 nm