 frequency is 15.75 kHz. Finally, the color subcarrier has a frequency of 3.58 MHz. Nippon Telephone and Telegraph. Nx384 N-by 384. The ITU-T's approach to developing a standard algorithm for video codec interoperability that was expanded into the Px64 or H.261 standard, and approved in 1990. It was based on the ITU-T's HO switched digital network standard. Nyquist's Theorem A formula that defines the sampling frequency in analog to digital conversions that states that a signal Glossary of Terms must be sampled at twice its bandwidth in order to be digitally characterized without distortion. For a sine wave, the sampling frequency must be no less than twice the maximum signal frequency. Optical carrier, as defined in the SONET specification. OC-1 or Optical Carrier Level 1 is defined as 51.84 Mbps. OC-3 is defined as 155.52 Mbps. octet An eight-bit datum representing one of 256 binary values. Also known as a BYTE operating system A fixed program used for operating a computer. An os accepts system commands and manages space in memory and on storage media. optical disk drives Peripheral storage disks. Used in video communications to store and play back motion image sequences, often accompanied by sound. optical fiber Very thin glass filaments of extremely high purity onto which light from lasers or LEDs is modulated in such a way that the transmission of digitally encoded information takes place at very high speeds. Fiber optics systems are capable of transmitting huge amounts of information. See fiber-optic cable. Open Systems Interconnection. An international standard that describes seven layers of communication protocols that allows dissimilar information systems and equipment to be interconnected. out-of-band signaling Network signaling (addressing, supervision and status information) that traverses a network using a separate path, channel or network than the information (call or transmission) itself. One well- known type of out-of-band signaling is the ITU-T's Signaling System #7. output devices Electrical com