Analog switch: Switching equipment designed, designated, or used to connect circuits between users for real-time transmission of analog signals.

Associated common-channel signalling: Common-channel signalling in which the signal channel is associated with a specific trunk group and terminates at the same pair of switches as the trunk group.  Note: In associated common-channel signalling, the signalling is usually accomplished by using the same facilities as the associated trunk group.


Asynchronous time-division multiplexing (ATDM): Time-division multiplexing in which asynchronous transmission is used.


Asynchronous transfer mode (ATM): A high-speed multiplexing and switching method utilizing fixed-length cells of 53 octects to support multiple types of traffic. Note: ATM, specified in international standards.


Asynchronous transmission: Data transmission in which the instant that each character, or block of characters, starts is arbitrary; once started, the time of occurrence of each signal representing a bit within the character, or block, has the same relationship to significant instants of a fixed time frame.


Automatic switching system: 1. In data communications, a switching system in which all the operations required to execute the three phases of information-transfer transactions are automatically executed in response to signals from a user end-instrument. Note: In an automatic switching system, the information-transfer transaction is performed without human intervention, except for initiation of the access phase and the disengagement phase by a user. 2. In telephony, a system in which all the operations required to set up, supervise, and release connections required for calls are automatically performed in response to signals from a calling device.


Baseband signalling: Transmission of a digital or analogue signal at its original frequencies; i.e. , a signal in its original form, not changed by modulation.

Bit error ratio (BER): The number of erroneous bits divided by the total number of bits transmitted, received, or processed over some stipulated period. Note 1: Examples of bit error ratio are (a) transmission BER, i.e., the number of erroneous bits received divided by the total number of bits transmitted; and (b) information BER, i.e., the number of erroneous decoded (corrected) bits divided by the total number of decoded (corrected) bits. Note 2: The BER is usually expressed as a coefficient and a power of 10; for example, 2.5 erroneous bits out of 100,000 bits transmitted would be 2.5 out of 105 or 2.5 × 10-5.

Bridge: 1. In communications networks, a device that (a) links or routes signals from one ring or bus to another or from one network to another, (b) may extend the distance span and capacity of a single LAN system, (c) performs no modification to packets or messages, (d) operates at the data-link layer of the OSI--Reference Model (Layer 2), (e) reads packets, and (f) passes only those with addresses on the same segment of the network as the originating user. 2. A functional unit that interconnects two local area networks that use the same logical link control procedure, but may use different medium access control procedures. 3. A balanced electrical network, e.g. , a Wheatstone bridge. Note: A bridge may be used for electrical measurements, especially resistances or impedances. 4. See hybrid coil.

Brouter: A combined bridge and router that operates without protocol restrictions, routes data using a protocol it supports, and bridges data it cannot route.

Bus: One or more conductors or optical fibres that serve as a common connection for a group of related devices.

Busy hour: In a communications system, the sliding 60-minute period during which occurs the maximum total traffic load in a given 24-hour period. Note 1: The busy hour is determined by fitting a horizontal line segment equivalent to one hour under the traffic load curve about the peak load point. Note 2: If the service time interval is less than 60 minutes, the busy hour is the 60-minute interval that contains the service timer interval. Note 3: In cases where more than one busy hour occurs in a 24-hour period, i.e., when saturation occurs, the busy hour or hours most applicable to the particular situation are used. Synonym peak busy hour.


Call 1. In communications, any demand to set up a connection. 2. A unit of traffic measurement.  3. The actions performed by a call originator. 4. The operations required to establish, maintain, and release a connection. 5. To use a connection between two stations. 6. The action of bringing a computer program, a routine, or a subroutine into effect, usually by specifying the entry conditions and the entry point.


Channel-associated signalling: Signalling in which the signals necessary to switch a given circuit are transmitted via the circuit itself or via a signalling channel permanently associated with it. Example of Channel-associated signalling is a T1 channel.

Circuit switching: 1. A method of routing traffic through a switching centre, from local users or from other switching centres, whereby a connection is established between the calling and called stations until the connection is released by the called or calling station. 2. A process that, on demand, connects two or more data terminal equipments (DTEs) and permits the exclusive use of a data circuit between them until the connection is released.


Common-channel signalling: In a multi-channel communications system, signalling in which one channel in each link is used for signalling to control, account for, and manage traffic on all channels of the link. Note: The channel used for common-channel signalling does not carry user information.

Communications system engineering: The translation of user requirements for the exchange of information into cost-effective and low-risk technical solutions in terms of equipment and subsystems. Note: Communications system engineering encompasses the integration of these parts into a complete entity resulting in a minimum investment for the entire system lifecycle required to satisfy the requirements of a majority of users of the communication system.

Communications system: A collection of individual communications networks, transmission systems, relay stations, tributary stations, and data terminal equipment (DTE) usually capable of interconnection and interoperation to form an integrated whole. Note: The components of a communications system serve a common purpose, are technically compatible, use common procedures, respond to controls, and operate in unison.

Crossbar switch: A switch that has a plurality of vertical paths, a plurality of horizontal paths, and electromagnetical means, i.e., relays, for interconnecting any one of the vertical paths to any one of the horizontal paths.

Datagram: In packet switching, a self-contained packet, independent of other packets, that contains information sufficient for routing from the originating data terminal equipment (DTE) to the destination DTE without relying on prior exchanges between the equipment and the network. Note: Unlike virtual call service, when datagrams are sent there are no call establishment or clearing procedures. Thus, the network may not be able to provide protection against loss, duplication, or mis-delivery.

Dial signalling: Signalling in which dual tone multifrequency (DTMF) signals or pulse trains are transmitted to a switching centre. Note 1: Rotary dials produce pulse trains. Keypads may produce either DTMF signals or pulse trains. Note 2: Dial signalling traditionally refers to pulse trains only.

Digital switch: A switch that performs time-division-multiplexed switching of digitized signals. Note 1: When used with analog inputs, analog-to-digital and digital-to-analogue conversion are required. These functions may be performed by the digital switch. Note 2: Implementation is accomplished by the interchange of time slots between input and output ports on a sequential basis under the direction of a control system.


Distributed switching: Switching in which many processor-controlled switching units are distributed, usually close to concentrations of users, and operated in conjunction with a host switch. Note: Distributed switching provides improved communications services for concentrations of users remote from the host switch, and reduces the transmission requirements, i.e. , the traffic, between such concentrations and the host switch.

Double-ended queue (dequeue): A queue in which the contents may be changed by adding or removing items at either end.

Dual-tone multifrequency (DTMF) signalling: In telephone systems, multifrequency signalling in which standard set combinations of two specific voice band frequencies, one from a group of four low frequencies and the other from a group of four higher frequencies, are used. Synonyms multifrequency pulsing, multifrequency signalling. Note 1: DTMF signals, unlike dial pulses, can pass through the entire connection to the destination user, and therefore lend themselves to various schemes for remote control after access, i.e. , after the connection is established. Note 2: Telephones using DTMF usually have 12 keys. Each key corresponds to a different pair of frequencies. Each pair of frequencies corresponds to one of the ten decimal digits, or to the symbol "#" or "*", the "*" being reserved for special purposes.


Electronic switching system (ESS): 1. A telephone switching system based on the principles of time-division multiplexing of digitized analogue signals. Note: An electronic switching system digitizes analogue signals from subscribers' loops, and interconnects them by assigning the digitized signals to the appropriate time slots. It may also interconnect digital data or voice circuits. 2. A switching system with major devices constructed of semiconductor components. Note: A semi-electronic switching system that has reed relays or crossbar matrices, as well as semiconductor components, is also considered to be an ESS.

Erlang: A dimensionless unit of the average traffic intensity (occupancy) of a facility during a period of time, usually a busy hour. Note 1: Erlangs, a number between 0 and 1, inclusive, is expressed as the ratio of (a) the time during which a facility is continuously or cumulatively occupied to (b) the time that the facility is available for occupancy. Note 2: Communications traffic, measured in erlangs for a period of time, and offered to a group of shared facilities, such as a trunk group, is equal to the average of the traffic intensity, in erlangs for the same period of time, of all individual sources, such as telephones, that share and are served exclusively by this group of facilities. Synonym traffic unit.

Exchange: 1. A room or building equipped so that telephone lines terminating there may be interconnected as required. Note: The equipment may include manual or automatic switching equipment. 2. In the telephone industry, a geographic area (such as a city and its environs) established by a regulated telephone company for the provision of local telephone services.


Fast packet switching: A packet switching technique that increases the throughput by eliminating overhead. Note 1: Overhead reduction is accomplished by allocating flow control and error correction functions to either the user applications or the network nodes that interface with the user. Note 2: Cell relay and frame relay are two implementations of fast packet switching.

Frame relay: An interface protocol for statistically multiplexed packet-switched data communications in which (a) variable-sized packets (frames) are used that completely enclose the user packets they transport, and (b) transmission rates are usually between 56 kb/s and 1.544 Mb/s (the T-1 rate). Note 1: In frame relay, (a) there is neither flow-control nor an error-correction capability, (b) there is information-content independence, (c) there is a correspondence only to the ISO Open systems Interconnection--Reference Model Layers 1 and 2, (d) variable-sized user packets are enclosed in larger packets (frames) that add addressing and verification information, (e) frames may vary in length up to a design limit, usually 1 kilobyte or more, (f) one frame relay packet transports one user packet, (g) implementation of fast-packet technology is used for connection-oriented frame relay services, and (h) there is a capability to handle time-delay insensitive traffic, such as LAN interworking and image transfer.

Free-space loss: The signal attenuation that would result if all absorbing, diffracting, obstructing, refracting, scattering, and reflecting influences were sufficiently removed so as to have no effect on propagation. Note: Free-space loss is primarily caused by beam divergence, i.e. , signal energy spreading over larger areas at increased distances from the source.


Frequency-division multiple access (FDMA): 1. [Satellite Engineering] The use of frequency division to provide multiple and simultaneous transmissions to a single transponder.

Frequency-division multiplexing (FDM): The deriving of two or more simultaneous, continuous channels from a transmission medium by assigning a separate portion of the available frequency spectrum to each of the individual channels.


In-band signalling: Signaling that uses frequencies or time slots within the bandwidth or data stream occupied by the information channel.


In-slot signalling: Signalling performed in the associated channel time slot.

International Telecommunication Union (ITU): A civil international organization established to promote standardized telecommunications on a worldwide basis. Note: The ITU-R (R for Radio) and ITU-T  (T for telecommunications) are committees under the ITU. The ITU headquarters is located in Geneva, Switzerland. While older than the United Nations (established in Paris 1865 and International Telegraph Union and took its present name in 1932), it is recognized by the U.N. as the specialized agency for telecommunications.


Local call: 1. Any call using a single switching facility. 2. Any call for which an additional charge, i.e. , toll charge, is not made to the calling or called party. Note: Calls such as those via "800" numbers do not qualify as local calls, because the called party is charged.


Message switching: A method of handling message traffic through a switching center, either from local users or from other switching centers, whereby the message traffic is stored and forwarded through the system.

Mobile services switching center (MSC): In an automatic cellular mobile system, the interface between the radio system and other public switched telephone networks (fixed or mobile or data (read internet). Note: The MSC performs all signalling functions that are necessary to establish calls to and from mobile stations.


Multiparty line: In telephone systems, an arrangement in which two or more user end instruments, usually telephones, are connected to the same loop. Note: If selective ringing is not used, individual users may be alerted by different ringing signals, such as a different number of rings or a different combination of long and short rings. Party lines remain primarily in rural areas where loops are long. Privacy is limited and congestion often occurs. Synonym party line.


Nonassociated common-channel signalling: A form of common-channel signalling where the signaling channel serves one or more trunk groups, at least one of which terminates at a point other than the signal transfer point at which the signalling channel terminates.

Nonblocking switch: A switch that has enough paths across it that an originated call can always reach an available line without encountering a busy condition.


Out-of-band signalling: 1. Signaling that uses a portion of the channel bandwidth provided by the transmission medium, e.g. , the carrier channel, which portion is above the highest frequency used by, and is denied to, the speech or intelligence path by filters. Note: Out-of-band signalling results in a lowered high-frequency cutoff of the effective available bandwidth. 2. Signalling via a different channel (either FDM or TDM) from that used for the primary information transfer. Contrast with common-channel signalling, in-band signalling, out-slot signalling.

Optical switch: A switch that enables signals in optical fibers or integrated optical circuits (IOCs) to be selectively switched from one circuit to another. Note 1: An optical switch may operate by (a) mechanical means such as physically shifting an optical fiber to drive one or more alternative fibers, or (b) electro-optic effects, magneto-optic effects, or other methods. Note 2: Slow optical switches, such as those using moving fibers, may be used for alternate routing of an optical transmission path, e.g., routing around a fault. Fast optical switches, such as those using electro-optic or magneto-optic effects, may be used to perform logic operations.

PABX: Abbreviation for private automatic branch exchange. A telephone system usually owned by an organization for its internal communication. It is usually connected to one or more public telephone lines and has many internal telephone lines which are referred to as extensions. Note: Use of the term "PBX" is more common than "PABX," regardless of automation.


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.


Public switched network (PSN): Any common carrier network that provides circuit switching among public users. Note: The term is usually applied to public circuit-switched telephone networks, but it could be applied more generally to other switched networks, e.g. , packet-switched public data networks.


Quality assurance (QA): 1. All actions taken to ensure that standards and procedures are adhered to and that delivered products or services meet performance requirements. 2. The planned systematic activities necessary to ensure that a component, module, or system conforms to established technical requirements. 3. The policy, procedures, and systematic actions established in an enterprise for the purpose of providing and maintaining a specified degree of confidence in data integrity and accuracy throughout the lifecycle of the data, which includes input, update, manipulation, and output.


Quality of service (QOS): 1. The performance specification of a communications channel or system. Note: QOS may be quantitatively indicated by channel or system performance parameters, such as signal-to-noise ratio (S/N), bit error ratio (BER), message throughput rate, and call blocking probability. 2. A subjective rating of telephone communications quality in which listeners judge transmissions by qualifiers, such as excellent, good, fair, poor, or unsatisfactory.


Queue traffic: 1. A series of outgoing or incoming calls waiting for service. 2. In a store-and-forward switching centre, the outgoing messages awaiting transmission at the outgoing line position.


Queue: A set of items, such as telephone calls or packets, arranged in sequence. Note: Queues are used to store events occurring at random times and to service them according to a prescribed discipline that may be fixed or adaptive.


Router: In data communications, a functional unit used to interconnect two or more networks. Note 1: Routers operate at the network layer (layer 3) of the ISO Open Systems Interconnection Reference Model. Note 2: The router reads the network layer address of all packets transmitted by a network, and forwards only those addressed to another network.


Shadow loss: 1. The attenuation caused to a radio signal by obstructions in the propagation path. 2. In a reflector antenna, the relative reduction in the effective aperture of the antenna caused by the masking effect of other antenna parts, such as a feed horn or a secondary reflector, which parts obstruct the radiation path.


Signalling: 1. The use of signals for controlling communications. 2. In a telecommunications network, the information exchange concerning the establishment and control of a connection and the management of the network, in contrast to user information transfer.  3. The sending of a signal from the transmitting end of a circuit to inform a user at the receiving end that a message is to be sent.


SONET: Acronym for synchronous optical network. An interface standard for synchronous 2.46-Gb/s optical-fiber transmission, applicable to the Physical Layer of the OSI Reference Model. Note 1: SONET uses a basic data rate of 51.840 Mb/s, called OC1 (optical carrier 1). The SONET hierarchy is defined in multiples of OC1, up to and including OC48, for a maximum data rate of 2.48832 Gb/s. Note 2: SONET was developed by the Exchange Carriers Standards Association (ECSA) in the USA.


Step-by-step (SXS) switching system: An automatic dial telephone system in which calls are switched by a succession of switches that move a step at a time, from stage to stage, each step being made in response a group of pulses corresponding to a dialled number.


Switchboard: Equipment used for manual switching operations.


Synchronous TDM: A multiplexing scheme in which timing is obtained from a clock that controls both the multiplexer and the channel source.


Synchronous transfer mode: In a Broadband Integrated Services Digital Network (B-ISDN), a proposed transport level technique in which time-division multiplexing and switching is to be used across the user's network interface.


Tactical Automatic Digital Switching System (TADSS): A transportable store-and-forward message-switching system used for rapid deployment in support of tactical forces.


Telecommunication: 1. Any transmission, emission, or reception of signs, signals, writing, images and sounds or intelligence of any nature by wire, radio, optical or other electromagnetic systems. 2. Any transmission, emission, or reception of signs, signals, writings, images, sounds, or information of any nature by wire, radio, visual, or other electromagnetic systems.


Telephony: 1. The branch of science devoted to the transmission, reception, and reproduction of sounds, such as speech and tones that represent digits for signaling. Note 1: Transmission may be via various media, such as wire, optical fibers, or radio. Note 2: Analog representations of sounds may be digitized, transmitted, and, on reception, converted back to analog form. Note 3: "Telephony" originally entailed only the transmission of voice and voice-frequency data. Currently, it includes new services, such as the transmission of graphics information. 2. A form of telecommunication set up for the transmission of speech or, in some cases, other sounds.

Time-division multiple access (TDMA): A communications technique that uses a common channel (multipoint or broadcast) for communications among multiple users by allocating unique time slots to different users. Note: TDMA is used extensively in satellite systems, local area networks, physical security systems, and combat-net radio systems.

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.


Time-division switching: Switching of time-division multiplexed (TDM) channels by shifting bits between time slots in a TDM frame.


Traffic analysis: 1. In a communications system, the analysis of traffic rates, volumes, densities, capacities, and patterns specifically for system performance improvement.  2. [The] study of communications characteristics external to the text.  3. The analysis of the communications-electronic environment for use in the design, development, and operation of new communications systems.


Traffic capacity: The maximum traffic per unit of time that a given telecommunications system, subsystem, or device can carry under specified conditions.


Traffic engineering: The determination of the numbers and kinds of circuits and quantities of related terminating and switching equipment required to meet anticipated traffic loads throughout a communications system.


Traffic intensity: A measure of the average occupancy of a facility during a specified period of time, normally a busy hour, measured in traffic units (erlangs) and defined as the ratio of the time during which a facility is occupied (continuously or cumulatively) to the time this facility is available for occupancy. Note: A traffic intensity of one traffic unit (one erlang) means continuous occupancy of a facility during the time period under consideration, regardless of whether or not information is transmitted. Synonym call intensity.


Traffic load: The total traffic carried by a trunk or trunk group during a specified time interval.


Traffic: 1. The information moved over a communication channel.  2. A quantitative measurement of the total messages and their length, expressed in erlangs, CCS or other units, during a specified period of time.


Transmission loss: The decrease in power that occurs during transmission from one point to another. Note: Transmission loss is usually expressed in dB.


Trunk: 1. In a communications network, a single transmission channel between two points that are switching centers or nodes, or both. 2. between switchboards or other switching equipment, as distinguished from circuits which extend between central office switching equipment and information origination/termination equipment. Note: Trunks may be used to interconnect switches, such as major, minor, public and private switches, to form networks.

Wavelength-division multiplexing (WDM): In optical fibre communications, any technique by which two or more optical signals having different wavelengths may be simultaneously transmitted in the same direction over one fiber, and then be separated by wavelength at the distant end.

Prof. James Kulubi May-Aug 2015