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Open Systems Interconnection (OSI)—Protocol Specification: The lowest level of abstraction within the OSI standards scheme. Note: Each OSI—Protocol Specification operates at a single layer. Each defines the primitive operations and permissible responses required to exchange information between peer processes in communicating systems to carry out all or a subset of the services defined within the OSI—Service Definitions for that layer. Protocol Definitions
Packet switching: The process of routing and transferring data by means of addressed packets so that a channel is occupied during the transmission of the packet only, and upon completion of the transmission the channel is made available for the transfer of other traffic.
Physical topology: The physical configuration, i.e., interconnection, of network elements, e.g., cable paths, switches, concentrators. Note: Physical topology is in contrast to logical topology. For example, a logical loop may consist of a physical star configuration, or a physical loop. Also see Bus Topologies.
Plesiochronous: That relationship between two signals such that their corresponding significant instants occur at nominally the same rate, any variations being constrained within a specified limit. Note: There is no limit to the phase difference that can accumulate between corresponding significant instants over a long period of time.
Protocol: A formal set of conventions governing the format and control of interaction among communicating functional units. Note: Protocols may govern portions of a network, types of service, or administrative procedures. For example, a data link protocol is the specification of methods whereby data communications over a data link are performed in terms of the particular transmission mode, control procedures, and recovery procedures. In layered communications system architecture, a formal set of procedures that are adopted to facilitate functional inter-operation within the layered hierarchy. [Protocols]
Serial: Pertaining to a process in which all events occur one after the other; for example, the serial transmission of the bits of a character according to the CCITT V.25 protocol. Pertaining to the sequential or consecutive occurrence of two or more related activities in a single device or channel. 3. Pertaining to the sequential processing of the individual parts of a whole, such as the bits of a character or the characters of a word, using the same facilities for successive parts.
Shielded twisted pair: A transmission line composed of a twisted 2-wire metallic transmission line surrounded by a sheath of conductive material that protects it from the effects of external fields and confines fields produced within the line. Read more Twisted Pair Cable.
Synchronization bit: A bit used to achieve or maintain synchronism. Note: The term “synchronization bit” is usually applied to digital data streams, whereas the term “synchronization pulse” is usually applied to analog signals.
Throughput: The number of bits, characters, or blocks passing through a data communication system, or portion of that system. Note 1: Throughput may vary greatly from its theoretical maximum. Note 2: Throughput is expressed in data units per period of time; e.g., in the DDN, as blocks per second. The maximum capacity of a communications channel or system. 3. A measure of the amount of work performed by a system over a period of time, e.g., the number of jobs per day.
Time-Division Multiplexing (TDM): Digital multiplexing in which two or more apparently simultaneous channels are derived from a given frequency spectrum, i.e., bit stream, by interleaving pulses representing bits from different channels. Note: Successive pulses represent bits from successive channels, e.g., voice channels in a T1 system.
Token: A Token is the permission passed to a node on a bus allowing the node to begin communication on the bus [if required]. A Token is nothing more than a grant to talk on the bus.
Transmission block: A group of bits or characters transmitted as a unit and usually containing an encoding procedure for error control purposes. In data transmission, a group of records sent, processed, or recorded as a unit. Note: A transmission block is usually terminated by an end-of-block character (EOB), end-of-transmission-block character (ETB), or end-of-text character (EOT or ETX).
