A block in which a bit pattern or a flag identifies the beginning or end of a block.

See SEED.

In the optical regime, the property of an element (rod or slab of material) that (a) has a refractive index gradient that is a defined function of the radial distance from its optical axis, and (b) by virtue of which gradient, is capable of focusing light in a manner after that of a conventional simple lens. Note 1: For example, a self-focusing slab (i.e., a plane-parallel, usually circular) element that is analogous in function to a convex simple lens, is relatively thin with respect to its diameter. By virtue of the fact that its refractive index is at a maximum along its optical axis (which is perpendicular to the faces of the slab), and decreases radially as a function of distance from the optical axis, it is able to bring a beam of light to a focus. If the thickness of the slab is increased greatly, so that it becomes a rod having a length very many times its diameter, it is able to relay a beam or image in a cyclical fashion in much the same manner as a series of discrete conventional lenses and bring it to a final focus, e.g., at or just outside the endface of the rod. (This phenomenon should not be confused with the relaying of images by means of a spatially coherent bundle of a large number of individual optical fibers, each having a diameter of microscopic proportions, and each of which in effect relays a single pixel, and within which bundle no focusing takes place.) Note 2: Within a self-focusing element, the rays traverse curved paths, unlike the straight internal paths that characterize conventional lenses or lens elements that are made of homogeneous materials having a uniform refractive index. In a conventional single- or multi-element lens, the rays may change direction abruptly at a refractive discontinuity, such as an air-glass boundary, or the boundary between the crown and flint elements of an achromatic lens, but within each such element, the ray paths are straight. Note 3: Self-focusing elements do not produce images of a quality that may be obtained with the best conventional multi-element lenses.

In computer programming, a partial computer address that uses the base address of the computer program, in which it appears, as the ellipsed component of the partial address. Note 1: A program that uses self-relative addresses will be portable, in that all related programs and subroutines can be moved, together, to a different computer or subdirectory, and all components will still be accessible. Note 2: An example would be a Web page with a complete address of http://www.example.com/menu.html. If that page links to another page, at that same website, with the complete address of http://www.example.com/index.html, that link could be written as a self-relative address, as index.html. A browser, reading this link, would “assume” that the complete address of that page must include the base address of the first page (i.e., http:///www.example.com/).

A code in which the symbol stream formed by a portion of one code word, or by the overlapped portion of any two adjacent code words, is not a valid code word. Note 1: A self-synchronizing code permits the proper framing of transmitted code words provided that no uncorrected errors occur in the symbol stream. Note 2: External synchronization is not required. Note 3: High-level data link control (HDLC) and Advanced Data Communication Control Procedures (ADCCP) frames represent self-synchronizing code words.

A circuit that uses the data it is retransmitting to perform tests on itself to ensure that it is transmitting properly, i.e., to ensure that all circuit components are functioning properly. [After Weik ’96]

A connection established via a service order or via network management. [T1.627-1993]

A machine-aided command and control system in which human intervention is required in varying degrees to operate the system.

In telephone systems, a switching system in which telephone operators receive call instructions orally from users and complete them by automatic equipment. 2. At tape-relay intermediate stations, the manual routing or rerouting of taped messages without rekeying them.

Synonym injection laser diode.