An optical cable comprises a tight-buffered optical cable and a protective sleeve which surrounds the tight-buffered optical cable. An exemplary receives units of circuitry from a plurality of business entities, and determines if access to a unit is blocked by lack of a primary password. The graphic symbol is then identified by matching a profile of the image intensity to a predetermined image intensity profile associated with a given graphic symbol. The head or bottom element forms guide paths for the printed circuit boards of the plug-in modules between the rows of guide strips of the guide rails.