Glossary of Key Terms
Glossary of Key Terms
| Term / Acronym | Definition |
| ADPCM | Adaptive Differential Pulse Code Modulation. A voiceband signal encoding method mentioned in Recommendation G.721. |
| AIC | Application Identification Channel. A function of the C11 bit in the 44 736 kbit/s structure for C-bit Parity application. |
| AIS | Alarm Indication Signal. A special signal transmitted to maintain transmission continuity and indicate an upstream failure. |
| CCITT | The predecessor organization to the ITU-T (Telecommunication Standardization Sector). |
| CMB | CRC Message Block. A sequence of serial bits that is used for calculating a Cyclic Redundancy Check. |
| CRC | Cyclic Redundancy Check. A procedure for error monitoring. G.704 specifies CRC-6 for 1544 kbit/s, CRC-5 for 6312 kbit/s, and CRC-4 for 2048 kbit/s interfaces. |
| DL | Data Link. A channel used for transmitting maintenance messages, performance reports, or other operational data. For 1544 kbit/s, it is a 4 kbit/s channel derived from the F-bit. |
| F-bit | A bit used for frame alignment, performance monitoring, and providing a data link in 1544 kbit/s and 6312 kbit/s structures. |
| FAS | Frame Alignment Signal. A specific bit pattern used by receiving equipment to locate the beginning of a frame or multiframe. |
| FEAC | Far-End Alarm and Control. A bit used in the 44 736 kbit/s structure to send alarm or control signals to the far-end terminal. |
| IDN | Integrated Digital Network. A type of network for which the interfaces in G.704 are defined. |
| ISDN | Integrated Services Digital Network. A type of network for which the interfaces in G.704 are defined. |
| ITU-T | International Telecommunication Union – Telecommunication Standardization Sector. A specialized agency of the United Nations responsible for studying technical questions and issuing recommendations on them to standardize telecommunications globally. |
| LAPD | Link Access Procedure on the D-channel. A data link protocol referenced for path maintenance messages in the 44 736 kbit/s structure. |
| LFA | Loss of Frame Alignment. An alarm condition declared when the framing alignment signal is lost. |
| Multiframe | A sequence of a specific number of consecutive frames in which the position of each frame can be identified. Multiframe structures are defined for all hierarchical levels to support signalling, CRC, and other overhead functions. |
| NNI | Network Node Interface. An interface at which specific performance values for C/R and TEI are defined. |
| PCM | Pulse Code Modulation. A method of digital representation of an analog signal, such as a voiceband signal conforming to Recommendation G.711. |
| QL | Quality Level. Refers to the Synchronization Quality Level described by Synchronization Status Messages (SSM). |
| RAI | Remote Alarm Indication (also known as a “yellow” signal). A signal sent to the remote end to indicate that the local end has detected a fault (e.g., loss of signal or loss of frame alignment). |
| ROA | Recognized Operating Agency. Any individual, company, corporation, or governmental organization that operates a public correspondence service. |
| SGC | Signalling Grouping Channel. A channel used for the transmission of channel-associated signalling information in 32 kbit/s grouping structures. |
| SMF | Sub-Multiframe. A division of a multiframe. In the 2048 kbit/s CRC-4 multiframe, it consists of eight frames. |
| SSM | Synchronization Status Message. A message, carried in Sₐn bits at 2048 kbit/s, that indicates the quality of the synchronization network source. |
| SSU | Synchronization Supply Unit. A timing source within a synchronization network, as defined in Recommendation G.812. |
| UNI | User-Network Interface. An interface at which specific performance values may be used by local access protocols. |
| WTSC | World Telecommunication Standardization Conference. A conference held every four years by the ITU-T to establish topics for study and produce Recommendations. |