Back Channel: A means of communication from users to content providers. Examples include a connection between the central office and the end user, an Internet connection using a modem, or systems where content providers transmit interactive television (analog or digital) to users while users can connect through a back channel to a web site, for example.

BB-I: Abbreviation for Broadband Interactive Services. The delivery of all types of interactive video, data, and voice services over a broadband communications network.

Backreflection (BR): A term applied to any process in the cable plant that causes light to change directions in a fiber and return to the source. Occurs most often at connector interfaces where a glass-air interface causes a reflection.

Backscattering: The return of a portion of scattered light to the input end of a fiber; the scattering of light in the direction opposite to its original propagation.

Bandwidth (BW): 1) The difference between the lower and upper frequencies of a band of electromagnetic radiation. 2)The amount of data that can be passed along a communications channel in a given amount of time.

Bandwidth-limited Operation: The condition in a fiber optic link when bandwidth, rather than received optical power, limits performance. This condition is reached when the signal becomes distorted, principally by dispersion, beyond specified limits.

Baseband: A method of communication in which a signal is transmitted at its original frequency without being impressed on a carrier.

Baud: A unit of signaling speed equal to the number of signal symbols per second, which may or may not be equal to the data rate in bits per second.

Beamsplitter: An optical device, such as a partially reflecting mirror, that splits a beam of light into two or more beams. Used in fiber optics for directional couplers.

Bel (B): The logarithm to the base 10 of a power ratio, expressed as: B = log₁₀(P₁/P₂), where P₁ and P₂ are distinct powers. The decibel, equal to one-tenth bel, is a more commonly used unit.

Bending Loss: Attenuation caused by high-order modes radiating from the outside of a fiber optic waveguide which occur when the fiber is bent around a small radius. See also macrobending, microbending.

Bend Radius: The smallest radius an optical fiber or fiber cable can bend before excessive attenuation or breakage occurs. Bending a fiber less than the minimum recommended bend radius will shorten it's service life and cause excessive attenuation. Most fibers can tolerate a minimum bend radius of 2".

BER (Bit Error Rate): The fraction of bits transmitted that are received incorrectly. The bit error rate of a system can be estimated as follows:

Where N₀ = Noise power spectral density (A²/Hz).
I_MIN = Minimum effective signal amplitude (Amps).
B = Bandwidth (Hz).
Q(x) = Cumulative distribution function (Gaussian distribution).

BIDI: Abbreviation for bidirectional transceiver, a device that sends information in one direction and receives information from the opposite direction.

Bidirectional: Operating in both directions. Bidirectional couplers operate the same way regardless of the direction light passes through them. Bidirectional transmission sends signals in both directions, sometimes through the same fiber.

Binary: Base two numbers with only two possible values, 0, or 1. Primarily used by communication and computer systems.

Birefringent: Having a refractive index that differs for light of different polarizations. Several crystalline materials such as calcite exhibit birefringence.

Bit: The smallest unit of information upon which digital communications are based; also an electrical or optical pulse that carries this information.

Bit Depth: The number of levels that a pixel might have, such as 256 with an 8-bit depth or 1,024 with a 10-bit depth.

BITE: Abbreviation for built-in test equipment. Features designed into a piece of equipment that allow on-line diagnosis of failures and operating status. Status LEDs are one example.

Bit Period (T): The amount of time required to transmit a logical one or a logical zero.

BNC: Popular coax bayonet style connector, Often used for baseband video.

Bragg Grating: A technique for building optical filtering functions directly into a piece of optical fiber based on interferometric techniques. Usually this is accomplished by making the fiber photosensitive and exposing the fiber to deep UV light through a grating. This forms regions of higher and lower refractive indices in the fiber core.

Broadband: A method of communication where the signal is transmitted by being impressed on a high-frequency carrier.

Buffer: 1) In optical fiber, a protective coating applied directly to the fiber (illustrated). 2) A routine or storage used to compensate for a difference in rate of flow of data, or time of occurrence of events, when transferring data from one device to another.

Bus Network: A network topology in which all terminals are attached to a transmission medium serving as a bus. Also called a daisy-chain configuration.

Butt Splice: A joining of two fibers without optical connectors arranged end-to-end by means of a coupling. Fusion splicing is an example. A mechanical splice is shown.

Bypass: The ability of a node to isolate itself optically from a network while maintaining the continuity of the cable plant.

Byte: A unit of eight bits.