DDI network consisting of a limited number of directly connected single-attached stations. NXX The current exchange numbering plan, in which N is any digit from 2 to 9 and X is any digit from 0 to 9. OADM (optical add/drop multiplexer) An optical multiplexing device that uses special filters to extract the wavelengths that need to be dropped off at a given location. It eliminates the costly electronics used to convert between light and electricity in a nonoptical multiplexer. OC-1 to OC-768 (Optical Carrier-1 to Optical Carrier-768) Optical carrier levels used for the categories of bandwidth in a SONET fiber-optic network. OC-1 is the base optical carrier transmission speed of 51.840Mbps. To calculate OC-2 to OC-768 speeds, simply multiply the OC-1 base by the desired magnitude. Common OC levels include OC-1 (51Mbps), (155Mbps), OC-12 (622Mbps), OC-48 (2.5Gbps), OC-192 (10Gbps), and OC-768 (40Gbps). Glossary OCAP (OpenCable Applications Platform) OCAP (OpenCable Applications Platform) The OpenCable software specification, which creates a common platform on which interactive services can be deployed. OCAP is intended to enable the developers of interactive TV services and applications to design such products so that they will run successfully on any cable TV system in North America, regardless of the set-top or television receiver, hardware, or operating system software choices. octet 8 bits. An octet is equivalent to a byte, as long as the byte is also 8 bits. Whereas bytes can range from 4 to 10 bits, octets are always 8 bits. odd parity check A test of whether the number of digits in a group of binary digits is odd. OFDM (Orthogonal Frequency Division Multiplexing) A multicarrier modulation scheme that broadcasts on many frequencies, reducing interference from collisions with walls and objects. OFDMA (Orthogonal Frequency Division Multiple Access) A multiple-access scheme for OFDM systems that defines either 2,048 or 4,096 subcarriers. In current OFDM systems, only a single user can transmit on all the subcarri