ZZ-Telecom Glossary

Add/Drop Allows optical wavelengths to be added or dropped at any line amplifier location.

Add/Drop Multiplexer (ADM) A device that enables data to enter and leave a SONET bit stream without having to demultiplex the stream.

ADSL (Full Rate Asymmetrical DSL) ADSL offers differing upload and download speeds and can be configured to deliver up to six megabits of data per second (6000K), from the network to the customer, that is up to 120 times faster than dial-up service and 100 times faster than ISDN. ADSL enables voice and high-speed data to be sent simultaneously over the existing telephone line. This type of DSL is the most predominant in commercial use for business and residential customers around the world. Good for general Internet access and for applications where downstream speed is most important, such as video-on-demand. ITU-T Recommendation G.992.1 and ANSI Standard T1.413-1998 specify full rate ADSL. ITU Recommendation G.992.3 specifies ADSL2 which provides advanced diagnostics, power saving functions, PSD shaping, and slightly better performance than G.992.1. ITU Recommendation G.992.5 specifies ADSL2Plus, which provides the benefits of ADSL2Plus twice the bandwidth so that bit rates as high as 20 Mb/s downstream can be achieved on relatively short lines.

AIS (Alarm Indication Signal) A signal, transmitted by a system within a communications link, which lets the receiver know that some part of the link has failed. Two types of AIS signals are Alarm Indication Signal Path (AIS-P) and Alarm Indication Signal Line (AIS-L).

American National Standards Institute (ANSI) The coordinating body for voluntary standards groups within the United States. ANSI is a member of the International Standards Organization for Standardization (ISO).

Amplifier Any device that uses a small amount of energy to control a source of a larger amount of energy.

Amplitude Modulation (AM) A form of modulation in which the amplitude of a carrier wave is varied in direct proportion to that of a modulating signal.

Asynchronous Replication A replication technique in which data must be committed to storage at only the primary site and not the secondary site before the write is acknowledged to the host. Data is then forwarded to the secondary site as the network capabilities permit.

Asynchronous Serial Interface (ASI) A serial data transmission method for MPEG-2 that allows packet-based transmission of compressed video in DTV transmission.

Asynchronous Transmission A method of data transmission which allows data bits to be sent at irregular intervals by preceding each with a start bit, and following it with a stop bit.

Asynchronous Transfer Mode (ATM) A cell relay network protocol which encodes data traffic into small fixed-sized (53 byte: 48 bytes of data and 5 bytes of header information) cells instead of variable-sized packets, as found in packet-switched networks such as the Internet Protocol or Ethernet. It is a connection-oriented technology, in which a connection is established between the two endpoints before the actual data exchange begins.

ATM Adaptation Layer (AAL) A process that converts data between higher level protocols such as IP and ATM. At the originating end of an ATM connection, AAL segments the higher level bundles of data into ATM cells. At the destination end of the connection, ATM reassembles the cells into the higher level bundles. AAL is divided into types, which in turn support specific types of data traffic.

ATM Adaptation Layer 1 Circuit Emulation (AAL1) AAL that supports constant bit rate traffic that cannot tolerate delays, such as uncompressed voice and video transmission.

ATM Adaptation Layer 2 Transport (AAL2) AAL that supports variable bit rate traffic such as compressed voice and video.

ATM Adaptation Layer 5 (AAL5) AAL that supports variable bit rate traffic that can be delayed.

ATM Service Access Multiplexer (ATM SAM) An ATM switch used by a service provider to aggregate various forms of subscriber traffic (such as T1 or Ethernet) for transmission over an ATM backbone.

Attenuation (1) Limited Operation (1) The condition in a fiber optic link where operation is limited by the power of the received signal, rather than by bandwidth or by distortion. (2) The decrease in magnitude of power of a signal in transmission between points. A term used for expressing the total losses on an optical fiber consisting of the ratio of light output to light input. Attenuation is usually measured in decibels per kilometer (dB/km) at a specific wavelength. Typical multi-mode wavelengths are 850 and 1300 nanometers (nm); singlemode, at 1300 and 1500 nm. NOTE: When specifying attenuation, it is important to note if it is nominal or average, room temperature, value or maximum over operating range.

Automatically Switched Optical Network (ASON) A network enabling the automatic delivery of transport services, not only leased-line connections but also other transport services, such as soft permanent and switched optical connections.

Automatically Switched Transport Network (ASTN) A network that allows traffic paths to be set up automatically through a switched network. The term ASTN is often used interchangeably with GMPLS (Generalized MPLS).

Availability The amount of time that a system is available during time periods when it is expected to be available. Availability is often measured as a percentage of an elapsed year. For example, 99.95% availability equates to 4.38 hours of downtime in a year (0.0005 * 365 * 24=4.38) for a system that is expected to be available all the time. (Provided by SNIA)


Backup A collection of data stored on (usually removable) non-volatile storage media for purposes of recovery in case the original copy of data is lost or becomes inaccessible. Also called backup copy. To be useful for recovery, a backup must be made by copying the source data image when it is in a consistent state.

Backup Window An interval of time during which a set of data can be backed up without seriously affecting applications that use the data. For example, if an application accesses data from 8AM until midnight, then the window between midnight and 8AM is available for making backup copies. Offline backups require that applications not update data during the backup. Online backups typically use point-in-time copy technology to create consistent images of data for backup.

Backbone (1) The part of a network used as the primary path for transporting traffic between network segments. (2) A high-speed line, or a series of connections, that forms a major pathway within a network.

Bandwidth (1) Measure of the information capacity of a transmission channel. (2) The difference between the highest and lowest frequencies of a band that can be passed by a transmission medium without undue distortion, such as the AM band - 535 to 1705 kilohertz. (3) Information carrying capacity of a communication channel. Analog bandwidth is the range of signal frequencies that can be transmitted by a communication channel or network. (4) A term used to indicate the amount of transmission or processing capacity possessed by a system or a specific location in a system (usually a network system).

Best Effort A class of service which does not guarantee delivery of packets, frames, or datagrams, but for which the network, fabric, or interconnect makes every reasonable delivery effort.

Bit Error Rate (BER) (1) Percentage of bits in a transmittal received in error. (2) The number of coding violations detected in a unit of time, usually one second. (3) Specifies expected frequency of errors and compares the ratio of incorrectly transmitted bits to correctly transmitted bits. Also known as Bit Error Ratio.

Bits Per Second (b/ps) (1) The number of bits passing a point every second. The transmission rate for digital information. (2) A measurement of how fast data is moved from one place to another (i.e., a 28.8 modem can move 28,800 b/s).

Block The unit in which data is stored and retrieved on disk and tape devices. Blocks are the atomic unit of data recognition (through a preamble and block header) and protection. A unit of application data from a single information category that is transferred within a single sequence.

Broadband (1) The ability of a system to carry a multitude of signals simultaneously. In data transmission, it denotes transmission facilities capable of handling frequencies greater than those required for high-grade voice communications. The higher frequency allows the carrying of several simultaneous channels. Broadband infers the use of a service provider signal rather than direct modulation (i.e., baseband).

Buffer-to-Buffer Flow Control Flow control that occurs between two directly connected Fibre Channel ports. A port indicates the number of frames buffers that can be sent to it (its buffer credit), before the sender is required to stop transmitting and wait for the receipt of a "ready" indication.


Cable Modem A modem used by a subscriber for high-speed network access over a coaxial cable, such as those used traditionally to provide cable television service.

Cable Modem Termination System (CMTS) Equipment used to provide high-speed data services, such as cable Internet or Voice-over-IP, to cable subscribers.

Carrier A company that provides communications circuits. Carriers are either "private" or "common." A "private" carrier can refuse service. A "common" carrier cannot. Most of the carriers in our industry (the local phone company, AT&T, MCI WorldCom, US, Sprint, etc.) are common carriers. cf. Service Provider

Category 5 Cable (CAT 5) An unshielded, twisted-pair cable designed for high signal integrity. The twisting of the cable reduces electrical interference, and the plastic insulation has low dispersion, so the dielectric constant of the plastic does not depend greatly on frequency.

Central Office (CO) (1) The place where common carriers, or service providers, terminate customer lines and locate the switching equipment that interconnects those lines. (2) A centralized location for the Switching, Transmission and Power equipment that provide telephone service.

Channel (1) A communication path. Multiple channels can be multiplexed over a single cable in certain environments. The term is also used to describe the specific path between large computers and attached peripherals. (2) In the case of fiber optic-based transmission systems, an electrical or photonic communications path, between two or more points of termination. (3) The smallest subdivision of a circuit that provides a type of communication service, usually a path with only one direction. (4) A communications path, or the signal sent over that channel. Through multiplexing, several channels can be transmitted over an optical channel.

Chromatic Dispersion Spreading of a light pulse caused by the difference in refractive indices at different wavelengths.

Circuit A group of electronic components and their interconnections.

Class of Service (CoS) A queuing discipline in which an algorithm compares fields of packets or CoS tags to classify packets and assign them to queues of differing priority. CoS does not ensure network performance or guarantee priority in delivering packets.

Client An intelligent device or system that requests services from other intelligent devices, systems or appliances. cf. Server. An asymmetric relationship with a second party (a server) in which the client initiates requests and the server responds to those requests.

Coarse Wavelength Division Multiplexing (CWDM) WDM systems with fewer than eight active wavelengths.

Common Language Equipment Identification (CLEI) A code used to identify equipment.

Common Language Location Identifier (CLLI) A code used to identify a building or site.

Collocation Multiple service providers sharing a single facility for housing physical network equipment are said to be collocated. In deregulated telecommunications markets, competing service providers often need to share (indeed, may be required by law to share) a number of physical facilities, such as a central office building and long distance trunks. Typically, established incumbent service providers are required to share their existing facilities with newer competitive service providers. The fact of collocation may limit the amount of space available to a service provider in a given facility, making it desirable for the service provider to fit a large amount of equipment in a comparatively small space.

Compression The process of encoding data to reduce its size. Lossless compression utilizes a technique that preserves the entire content of the original data, and from which the original data can be reconstructed exactly.

Concatenation A mechanism for allocating very large amounts of bandwidth for transport of a payload associated with a 'superrate service', which is a service at a transmission rate greater than the normal maximum rate of OC-1. The set of bits in the payload is treated as a single entity, as opposed to being treated as separate bits or bytes or time slots. Therefore, the payload is accepted, multiplexed, switched, transported and delivered as a single, contiguous chunk of payload data.


Data Communications Equipment (DCE) In a data station, the equipment that provides the signal conversion and coding between the data terminal equipment (DTE) and the line. The DCE may be separate equipment, intermediate equipment, or an integral part of the DTE. A DCE may perform other functions usually performed at the network end of the line.

Data-over-Cable Service Interface Specification (DoCSIS) An international standard which defines the communications and operation-support interface requirements for a data-over-cable system. Also permits the addition of high-speed data transfer to an existing cable TV system.

Data Rate The number of bits of information transmitted per second, as in a data transmission link. Typically expressed as megabits per second (Mb/s).

Data Terminating Equipment (DTE) The part of a data station that serves as a data source (originates data for transmission), a data sink (accepts transmitted data), or both.

Decibel (dB) Unit for measuring the relative strength of a signal. Power level referenced in decibels to a microwatt.

Degrade The condition where one or more established performance parameters fall outside of predetermined limits, resulting in lower quality performance.

Demultiplexing A process applied to a multiplexed signal for recovering signals combined within it and for restoring the distinct individual channels of these signals.

Dense Wavelength Division Multiplexing (DWDM) WDM systems with more than eight active wavelengths per fiber.

Deregulation The removal of regulatory authority to control certain activities of telephone companies.

Digital Cross Connect System (DCS) A device that provides switching services for various private lines by setting up the necessary connections prior to the corresponding calls. The connections are specified explicitly by an administrator or by the person making the call.

Digital Signal 3 (DS3) A digital signal level 3 T-carrier for which the data rate is 44.736 Mb/s. This level of carrier can transport 28 DS1 level signals and 672 DS0 level channels within its payload.

Digital Subscriber Line (DSL) A method of providing high-speed data services over the twisted pair copper wires traditionally used to provide POTS. Types of DSL include ADSL (asymmetric digital subscriber line), HDSL (high data rate digital subscriber line), SDSL (single line digital subscriber line), and VDSL (very high data rate digital subscriber line).

Digital Wrapper A structured text file that binds digital object content files and their associated metadata together, and that specifies the logical relationship of the content files.

Disaster Recovery (DR) The recovery of data, access to data and associated processing through a comprehensive process of setting up a redundant site (equipment and work space) with recovery of operational data to continue business operations after a loss of use of all or part of a data center. This involves not only an essential set of data but also an essential set of all the hardware and software to continue processing of that data and business. This may involve down time to perform the recovery.

Dispersion The cause of bandwidth limitations in a fiber. Dispersion causes a broadening of input pulses along the length of the fiber. Three major types of dispersion include: (a) mode dispersion, caused by differential optical path lengths in a multimode fiber; (b) material dispersion, caused by a differential delay of various wavelengths of light in a waveguide material; and (c) waveguide dispersion, caused by light traveling in both the core and cladding materials in single-mode fibers.

Dual Link A 3 Gb/s nominal interface used in applications (such as digital cinema) requiring greater fidelity and resolution than standard HDTV can provide.


E1 The European version of the T1 digital transmission link with a line bit rate of 2.048 megabits per second (as used by European Conference of European and Postal Telecommunication [CEPT] Administrations service providers).

Element Management System (EMS) A platform supporting multiple WDM spans to provide Network Management services.

Erbium Doped Fiber Amplifier (EDFA) EDFAs differ from the normal method of regenerative or electro-optic repeaters in that light does not have to be converted to an electrical signal, amplified, and then converted back to light. Optical amplifiers contain a length of fiber that is doped with erbium (a rare earth substance) that provides the gain medium, and a LASER source that "pumps" the doped fiber with light at the correct frequency to thereby amplify an input optical signal. One of the most important features, after the fact that EDFAs are not frequency-dependent, is that they allow bandwidth upgrades (within limits) without replacing the entire transmission system.

Errored Seconds (ES) A performance monitoring parameter. ES "Type A" is a second with exactly one error, ES "Type B" is a second with more than one and less than the number of errors in a severely errored second (SES) for the given signal. ES by itself means the sum of the type A and B ESs.

Enterprise Systems Connection A 200 Mb/s serial I/O bus used on IBM Corporation's Enterprise System 9000 data center computers. Abbreviated ESCON. Similar to Fibre Channel in many respects, ESCON is based on redundant switches to link computers and storage subsystems using serial optical connections.

Ethernet The predominant local area networking technology, based on packetized transmissions between physical ports over a variety of electrical and optical media. Ethernet can transport any of several upper-layer protocols, the most popular of which is TCP/IP. Ethernet standards are maintained by the IEEE 802.3 committee. The unqualified term Ethernet usually refers to 10 Mb/s transmission on multi-point copper. Fast Ethernet is used to denote 100 Mb/s transmission, also on multipoint copper facilities. Gigabit Ethernet (GbE) and 10 Gigabit Ethernet (10 GbE) utilize optical fiber transmission.


Fiber In The Loop (FITL) Fiber optical technology from central office (CO) to customer premises.

Fiber Optic Cable A transmission medium composed of glass or plastic fibers, rather than copper wire, used to transport data, video and voice signals simultaneously. The signal is imposed on the fiber via pulses (modulation) of light from a LASER or a light-emitting diode (LED). Because of its high bandwidth and lack of susceptibility to interference, fiber optic cable is used in long haul or noisy applications.

Fibre Channel A set of Storage Area Networking (SAN) standards for a serial I/O bus capable of transferring data between two ports at 100 Mb/s (FC100), 200 Mb/s (FC200), 400 Mb/s (FC400) with standards proposals to go to 1000 Gb/s. Note that 1000 Mb/s is equivalent to 1 Gb/s). Fibre Channel supports point to point, arbitrated loop, and switched topologies. Fibre Channel was completely developed through industry cooperation, unlike SCSI, which was developed by a vendor and submitted for standardization after the fact.

Fibre Connect (FICON) IBM Corporation's implementation of ESCON over Fibre Channel.

Forward Error Correction (FEC) A system of error control for data transmission specifically designed to allow the receiver to correct some errors without having to request a re-transmission of data.

Frame Relay An efficient data-transmission technique used to send digital information quickly and cheaply in a relay of frames to one or many destinations from one or many end-points.

Fiber-to-the-x (FTTX) Refers to several different forms of optical fiber architectures, including:

  1. Fiber-to-the-Building (FTTB) A telecommunications system based on fiber-optic cable carrying network data from an Internet service provider to a customer's physical building.
  2. Fiber-to-the-Cabinet (FTTCab) A telecommunications system using passive optical networking as an infrastructure going from an Internet service provider's central office, or headend, to a remote cabinet, bringing multiplexers closer to the service areas.
  3. Fiber-to-the-Curb (FTTC) A telecommunications system based on fiber-optic cables run to a platform serving several customers. Each of these customers has a connection to this platform via coaxial cable or twisted pair.
  4. Fiber-to-the-Exchange (FTTEx) A telecommunications system based on copper wiring run to a customer located close to the Internet service provider's central office.
  5. Fiber-to-the-Home (FTTH) or Fiber-to-the-Premises (FTTP) A broadband telecommunications system based on fiber-optic cables and associated optical electronics for delivery of multiple advanced services, such as the Triple Play of telephone broadband Internet and television to homes and businesses.
  6. Fiber-to-the-Node/Neighborhood (FTTN) A broadband architecture that provides high-speed Internet and other services to the home by running fiber to the node and VDSL over the existing telephone copper plant to the home. Data rates are limited to 25-30 Mb/s.


G-series Automatic Switched Optical Network (G.ASON) Part of the ITU-T's broad charter G-series recommendations pertaining to Transmission Systems and Media, Digital Systems and Networks. G-series recommendations include ITU-T G.8080/Y1304, frequently referred to as G.ASON, or simply ASON, which describes the control plane components used to provide fast and reliable connection set-up, maintenance and tear down within the OTN, in response to both signaled and management-driven requests.

Generalized Multi-Protocol Label Switching (GMPLS) Allows traffic paths to be set up automatically through a switched network. The term ASTN is often used interchangeably with GMPLS.

Gigabit Ethernet (GbE) Describes various technologies for implementing Ethernet networking at a nominal speed of one gigabit per second defined by the IEEE 802.3z and 802.3ab standards. Gigabit Ethernet has recently been overtaken by 10 Gigabit Ethernet (10GbE), which was ratified by the IEEE in 2002, and provides data rates 10 times greater than that of Gigabit Ethernet.

Gigabit Interface Converter (GBIC) A transceiver that converts between electrical signals used by Ethernet, Fibre Channel and FICON devices and either electrical or optical signals suitable for transmission. Gigabit interface converters allow designers to design one type of device and adapt it for either copper or optical applications. GBICs can be hot swapped and software programmed in the field to various protocols.

Gigabits Per Second (Gb/s or Gbps) Billion bits per second. A measure of transmission speed.


High Definition Serial Digital Interface (HD-SDI) A digitized video interface, standardized in SMPTE-292M, used for broadcast-grade transmission of uncompressed, unencrypted digital television signals.

High Definition Television (HDTV) Broadcast of television signals with a higher resolution than traditional formats (NTSC, SÉCAM, PAL) allow.

Hot Swap The ability to remove and replace components of a machine, usually a computer, while it is operating. Once the appropriate software is installed on the computer, one can plug and unplug the hot-swap component without rebooting.

Hub A communications infrastructure device to which nodes on a multi-point bus or loop are physically connected. Commonly used in Ethernet and Fibre Channel networks to improve the manageability of physical cables. Hubs maintain the logical loop topology of the network of which they are a part, while creating a "hub and spoke" physical star layout. Unlike switches, hubs do not aggregate bandwidth. Hubs typically support the addition or removal of nodes from the bus while it is operating.

Hybrid Fiber Coaxial (HFC) A network incorporating both optical fiber and coaxial cable to create a broadband network. By using frequency division multiplexing, an HFC network may carry a variety of signal types, including analog TV, digital TV, telephone and data.

In-band (transmission) Transmission of a protocol, other than the primary data protocol, over the same medium as the primary data protocol. Management protocols are a common example of in-band transmission.

Information Technology (IT) All aspects of information creation, access, use, storage, transport and management. The term 'information technology' addresses all aspects of computer and storage systems, networks, users and software in an enterprise.

Infrastructure The basic facilities, service and installations needed for the functioning of a community or society, such as transportation and communications systems.

Interexchange Carrier (IXC) (1) Any individual, partnership, association, joint-stock company trust, governmental entity or corporation engaged for hire in interstate or foreign communication by wire or radio, between two or more exchanges. (2) A long-distance telephone company offering circuit-switched, leased-line or packet-switched service or some combination thereof.

Internet Group Management Protocol (IGMP) A communications protocol used to manage the membership of Internet Protocol multicast groups. IGMP is used by IP hosts and adjacent multicast routers to establish multicast group memberships. It is an integral part of the IP multicast specification.

Internet Protocol (IP) A protocol that provides connectionless best effort delivery of data across heterogeneous physical networks. Data is broken down into a number of small bundles known as packets, and each packet gets transmitted to the destination separately, possibly along a different route than other packets from the same message. Packets are often retransmitted utilizing TCP when data is dropped due to over constrained routing.

Internet Protocol Television (IPTV) A system where a digital television service is delivered to subscribers using Internet Protocol over a broadband connection. This service is often provided in conjunction with Video on Demand and may also include Internet services such as Web access and VoIP, where it may be called Triple Play, and is typically supplied by a broadband operator using the same infrastructure.

Internet Protocol version 6 (IPv6) A network layer standard used by electronic devices to exchange data across a packet-switched Internetwork. It follows IPv4 as the second version of the Internet Protocol to be formally adopted for general use. IPv6 is intended to provide more addresses for networked devices, allowing, for example, each cell phone and mobile electronic device to have its own address.

Internet Small Computer Systems Interface (iSCSI) A transport protocol that provides for the SCSI protocol to be carried over a TCP-based IP network. Standardized by the Internet Engineering Task Force and described in RFC 3720.

International Telecommunication Union (ITU) An international organization established to standardize and regulate international radio and telecommunications. The ITU allocates call sign prefixes for radio and television stations of all types. These prefixes are agreed upon internationally, and are a form of country code. A call sign can be any number of letters and numerals but each country must only use call signs that begin with the characters allocated for use in that country. The call signs may be viewed on a chart or a grid.


Jitter Timing Short-term variations of the significant instants of a digital signal from their ideal positions in time.


LAN-free backup A backup methodology that moves data over a SAN without using LAN resources.

LASER (Light Amplification by Stimulated Emission of Radiation) An optical source that emits photons in a coherent beam.

Latency Synonym for I/O request execution time, the time between the making of an I/O request and completion of the request's execution. Latency contributors are light delay through the fiber optic line over distance as well as network element delays caused by excessive protocol conversions or network routing complexities.

Leased Line A physical line that a single subscriber leases from a service provider, giving the subscriber exclusive rights to that line's capacity.

Line Amplifier Also called an Optical Line Amplifier (OLA). It does not multiplex or demultiplex signals but instead amplifies signals; it does so to maintain signal strength over long distances.

Line Terminating Equipment (LTE) Network elements that originate and/or terminate line signals.

Local Area Network (LAN) A communications infrastructure intended for the local transport of data, video, and voice. Designed to use dedicated wiring over a limited distance (typically a diameter of less than five kilometers) to connect a large number of intercommunicating nodes. Ethernet is the most popular of LAN technologies. LANs are interconnected over distance through Metropolitan Area Networks (MANs) and Wide Area Networks (WANs) that utilize carrier-class transport and switching equipment.

Local Exchange Company (LEC) A telephone company that provides customer access to the worldwide public switched network through one of its central offices.

Loss Of Frame (LOF) An indication on network devices or software indicating that one or more network frames did not get through the networking device.

Loss Of Signal (LOS) An indicator on a networking device to indicate that a network signal or connection has been lost.


Media Access Control (MAC) is the lower sub-layer of the OSI data link layer, the interface between a node's Logical Link Control and the network's physical layer. The MAC differs for the various physical media (such as Ethernet, token ring, WLAN). The MAC sub-layer is primarily concerned with:

  1. recognizing where frames begin and end in the bit-stream received from the physical layer (when receiving)
  2. delimiting the frames (when sending), inserting information (e.g. some extra bits) into or among the frames being sent so that the receiver(s) are able to recognize the beginning and end of the frames
  3. detecting transmission errors by means of inserting a checksum into every frame sent, recalculating, and comparing them on the receiver side
  4. inserting the source and destination MAC addresses into every frame transmitted
  5. filtering out the frames intended for the station by verifying the destination address in the received frames
  6. controlling access to the physical transmission medium (which of the stations attached to the wire or frequency range has the right to transmit)

Megabits per Second (Mb/s or Mbps) A digital transmission speed of millions of bits per second.

Management Information Base (MIB) The specification and formal description of a set of objects and variables that can be read and possibly written using the SNMP protocol. Various standard MIBs are defined by the IETF.

Metropolitan Area Network (MAN) A network that connects nodes distributed over a metropolitan (city-wide) area as opposed to a local area (campus) or wide area (national or global).

Moving Picture Experts Group (MPEG) A working group of the International Organization for Standardization (ISO) and the International Electrotechnical Commission (IEC), charged with the development of video and audio encoding standards.

Multicast The simultaneous transmission of content to a subset of more than one of the ports connected to a communication facility.

Multi-protocol Label Switching (MPLS) A method used to direct data traffic in networks in which IP over ATM is being used. In MPLS, IP routers at the edge of the network label packets in a way that greatly facilitates their handling by ATM switches at the network core.

Multiple System Operator (MSO) A cable service provider that operates in more than one geographic area, thus having multiple headend facilities.

Multimode Fiber Optical fiber that supports propagation of multiple modes of light. Multimode fibers have a larger core diameter than single mode fibers.

Multiplex The combination of several signals onto a single communications channel.

Multiplexer (MUX) (1) Equipment that enables several data streams to be sent over a single physical line or fiber. (2) A function by which one connection from an ISO layer is used to support more than one connection to the next higher layer.

Multiplexing In data transmission, a function that permits two or more data sources to share a common transmission medium such that each data source has its own channel. Methods of multiplexing include time division multiplexing, and wavelength division multiplexing.

Network An interconnect that enables communication among a collection of attached nodes. A network consists of optical or electrical transmission media, infrastructure in the form of hubs and/or switches, and protocols that make message sequences meaningful. Networks are typically characterized by large numbers of nodes that act as peers, large inter-node separation and flexible configurability.

Network Attached Storage (NAS) Storage elements that connect to a LAN and provide file access services to computer systems. A NAS Storage Element consists of an engine, which implements the file services, and one or more devices, on which data is stored. Much like a SAN, a NAS is used to share storage resources across multiple servers; however, NAS technology does not provide LAN traffic relief.

Network Element (NE) (1) Any device which is part of a communications path and serves one or more of the section, line or path terminating functions. (2) Used for an addressable set of equipment, either an OTS End Terminal or an OTS Repeater. The terms 'OTS' and 'system' are used for a set of network elements that can be addressed through a common communications link (and are connected by a common Optical Service Channel) either by an EMS or a CIT.

Network Management System (NMS) A system responsible for managing at least part of a network. NMSs communicate with agents to help keep track of network statistics, resources and performance.

Network Interface Card (NIC) An I/O adapter that connects a computer or other type of node to a network. The term NIC is universally used in Ethernet contexts. In Fibre Channel contexts, the terms adapter and NIC are used in preference to host bus adapter (HBA).

Next-Generation Network (NGN) A packet-based network able to provide services, including Telecommunication Services, and to make use of multiple broadband, QoS-enabled transport technologies. In an NGN, service-related functions are independent from underlying transport-related technologies. An NGN offers users generalized mobility for consistent and ubiquitous service-provision and unrestricted access to different service providers.

Next-Generation Network Architecture (NGNA) Network architecture that allows decoupling of the network's transport and service layers so that providers can enable new services by defining them directly at the service layer.

Open Transport Network (OTN) A network technology that aims at transporting a number of communication protocols over an optical fiber. This includes serial protocols (e.g. RS232) as well as telephony (POTS/ISDN), audio, Ethernet and video (via M-JPEG, MPEG2/4).

Optical Add/Drop Multiplexer (OADM) An optical amplifier-like network element that allows the extraction/insertion of one or more wavelengths to/from the multi-wavelength signal as it is passed through the amplifier.

Optical Amplifier (OA) A device used to amplify the signal optically without any conversion to an electrical signal. It can be used as a Booster Amplifier, In-Line Amplifier or Pre-Amplifier.

OC-n A data rate that is a multiple of the fundamental SONET STS-1 rate of 51.84 Mb/s. OC-3 (155 Mb/s), OC-12 (622 Mb/s), OC-48 (2488 Mb/s) and OC-192 (9953 Mb/s) are currently in common use.

Open Systems Interconnection (OSI) A logical structure for network operations standardized by the International Standards Organization (ISO). The OSI model organizes the communications process into seven different categories and places the categories in a layered sequence based on their relationship to other users. Layers seven through four deal with end-to-end communications between the message source and the message destination, while layers three through one deal with network access.

Operations Support System (OSS) A network management system used for a single specific purpose, such as billing or alarm monitoring.

Optical Channel The wavelength allocation for a particular signal.

Optical Control Plane Within an ASON, the control plane sets up and releases connections and may restore a connection in case of a failure.

Optical Line Amplifier (OLA) cf. Line Amplifier

Optical Path The optical transmission medium made up of the individual fiber sections traversed by the multi-wavelength optical signal.

Optical Service Channel (OSC) An optical maintenance channel linking the OTS Repeaters (OLAs) to each other and to the OTS End Terminals. It is multiplexed onto the same fiber as the OC-48 channels. All telemetry, data and voice traffic originating and/or terminating at OTS Repeater sites are routed over the OSC.

Out-of-band (transmission) Transmission of management information outside of the data channel network, typically over Ethernet or through the use of a separate Optical Supervisory Channel (OSP).

Outside Plant (OSP) The part of the LEC telephone network that is physically located outside of telephone company buildings. Outside Plant includes the local loops from the LEC's switching centers to the customers' premises, and all facilities which serve to interconnect the various switches (e.g., central office and tandem) in the service provider's internal network.

Oversubscription Scheduling a network line to carry a greater volume of data than the line is designed to carry at any one time. Oversubscribing a line assumes that it is unlikely that any one subscriber (or group of subscribers) will use all of the line's capacity at any one time, and relies on methods such as quality of service to prioritize subscriber traffic during periods of congestion.

PacketCable™ Multimedia PacketCable is an organization started by CableLabs®. The purpose of the organization is to define standards for the Cable TV industry. Built on top of the industry's DOCSIS™ 1.1 (Data-over-Cable Service Interface Specifications) cable modem infrastructure, PacketCable networks use Internet Protocol (IP) to enable a wide range of multimedia services, such as IP telephony, multimedia conferencing, interactive gaming, and general multimedia applications. A DOCSIS 1.1 network with PacketCable extensions enables cable operators to deliver data and voice traffic efficiently using a single high-speed, quality-of-service (QoS)-enabled broadband architecture.

Packet-over-SONET/SDH (POS) A communications protocol for transmitting packets over circuit-switched protocols SDH or SONET.

Packetize The process by which IP breaks a file/message into numerous small bundles for transmission to a destination. Each packet is independent from the others; it has the destination's address, and may reach the destination by a different route than other packets for the same file/message.

Payload The data in an ATM cell or Ethernet/IP packet that subscribers want to access (the message, conversation, file, etc.). Payload is used to distinguish the subscriber's data from the "overhead," which is data in an ATM cell or IP packet that network equipment tacks on to the payload to help guide its transmission across the network.

Performance Monitoring (PM) Measures the quality of service and identifies degrading or marginally-operating systems (before an alarm would be generated).

Point of Presence (POP) A facility used by a network access provider to house physical equipment that enables subscribers to access the network. The term is used to describe the location where a long distance service provider connects to a local service provider, and also the location where an Internet service provider houses equipment that enables dial-up subscribers to access the Internet.

Polarization Mode Dispersion (PMD) Dispersion (spreading of light pulses) arising from differences in the speed of the two polarization modes of light (vertical and horizontal) in a fiber. The asymmetric nature of fiber causes the two polarization modes to experience slightly different conditions and travel along the fiber at slightly different speeds. Primarily an issue for communications systems operating at rates faster than 2.5 Gb/s.

Port A port can be an entrance to or exit from a network element. It can be a connection point for a peripheral device or an application program. It can be logical, physical or both. Examples include Ethernet, IP, Fibre Channel, and SCSI Ports.

  • Ethernet uses Media Access Control identifiers (commonly referred to as MAC addresses) to distinguish between separate logical channels connecting two ports on the same physical transport network interface.
  • Transmission Control Protocol (TCP) and User Datagram Protocol (UDP) of the IP Suite use logical ports as communication endpoints, including client-side user ports (source of application requests) and server-side well-known ports for service access. Examples of well-known server-side ports include: Internet Small Computer Systems Interface (iSCSI - 3260); File Transfer Protocol (FTP Data - 20, FTP Control - 21); Simple Mail Transfer Protocol (SMTP - 25); Hypertext Transfer Protocol (HTTP - 80), and Network File System (NFS - 2049).
  • Fibre Channel ports provide physical interface attachment to other Fibre Channel ports. A Fibre Channel port includes the transmitter, receiver and associated logic at either end of a link within a Node. Ports can be implemented on Host Bus Adapters (HBAs), Storage Adapters (SAs), routers, switches, bridges, gateways, etc.
  • SCSI Bus physical ports provide the means which allow a device to connect drivers and receivers to the SCSI parallel bus cable. An SCSI logical port is either an SCSI initiator port or an SCSI target port; it is the logical entity that originates or processes SCSI commands (including data transfer) and task management requests. For example, the SCSI initiator port enables SCSI operations to flow to and from a server operating system device driver.

Plain Old Telephone Service (POTS) Analog voice transmission over twisted pair copper wires, without any of the more recently added amenities such as caller ID or call waiting. POTS was invented over a century ago, and is still used widely in public telephone networks.

Protocol A set of rules for using an interconnect or network so that information conveyed on the interconnect can be correctly interpreted by all parties to the communication. Protocols include such aspects of communication as data representation, data item ordering, message formats, message and response sequencing rules, block data transmission conventions, and timing requirements.

Provider A company that provides an interface between the teleservices platform and an installed telephone device, such as a telephone line or fax machine.

Public Network A network operated by common service providers or telecommunications administrations for the provision of circuit-switched, packet-switched and leased-line circuits to the public.

Public Switched Telephone Network (PSTN) The traditional voice network infrastructure, including both local service and long distance service, that has been in use in various parts of the world for up to a century or so.


Quadrature Amplitude Modulation (QAM) A modulation scheme which conveys data by changing (modulating) the amplitude of two carrier waves. These two waves, usually sinusoids, are out of phase with each other by 90° and are thus called quadrature carriers.

Quality of Service (QoS) A set of guidelines for prioritizing subscriber data traffic on an ATM network, and for establishing a scale of fees for carrying that traffic, based on specific graduated guarantees of network availability and performance.


Radio Frequency (RF) Refers to that portion of the electromagnetic spectrum in which electromagnetic waves can be generated by alternating current fed to an antenna.

Reconfigurable Optical Add/Drop Multiplexer (ROADM) A new form of add/drop multiplexer that essentially adds the ability to switch individual networks. The key differentiating feature of a ROADM is the ability to switch traffic and both the wavelength and SONET/SDH layers. ROADM functionality is not new; indeed most long-haul DWDM equipment has built-in ROADM functionality. What's new is the appearance of these features in purpose-built metro optical gear, an application that has been long hyped but is only now beginning to gain support.

Regional Bell Operating Company (RBOC) Local telephone companies created in 1984 as part of the break-up of AT&T. The six RBOCs were Ameritech, Bell Atlantic, Bell South, NYNEX, Southwestern Bell and U.S.West. Ameritch and US West became Qwest; Southwestern Bell became AT&T, and Bell Atlantic and NYNEX became Verizon.

Regeneration In the telecommunications industry, regeneration has the following meanings:

  1. In a regenerative repeater, the process by which digital signals are amplified, reshaped, retimed and retransmitted. A synonym for this meaning is "positive feedback".
  2. In a storage or display device, the restoration of stored or displayed data that have deteriorated. For example, conventional cathode ray tube displays must be continually regenerated for the data to remain displayed.

Remote Authentication Dial In User Service (RADIUS) RADIUS is an authentication and accounting protocol used by many Internet Service Providers (ISPs). Information, such as username and password, is entered when a connection is made. This information is passed to a RADIUS server that verifies the information in order to authorize access to the system. Radius is defined in RFC 2865.

Repeater (1) A device that regenerates and propagates electrical signals between two network segments. (2) A device that restores a degraded digital signal for continued transmission; also called a regenerator. (3) A device which consists of a transmitter and a receiver or transceiver, used to regenerate a signal to increase the system length.

Resilient Packet Ring (RPR) A standard designed for the optimized transport of data traffic over fiber rings. Provides the resilience found in SONET/SDH networks (50ms protection) using a packet-based transmission rather than circuit-oriented connections to increase the efficiency of Ethernet and IP services. Also known as IEEE 802.17

Restoration The copying of a backup to on-line storage for application use. Restoration normally occurs after part or all of an application's data has been destroyed or become inaccessible.

Route A series of network elements that include multiple end-nodes. From a logical and visual viewpoint, a route consists of one or more paths.

Router A device that directs bundles of data being transmitted between nodes on different networks.

Serial Digital Interface (SDI) A digitized video interface used for broadcast-grade video. Standardized in ITU-R BT.656 and SMPTE-259M.

Signal-to-Noise Ratio (SNR) The ratio of signal power to noise power. Measured in dB.

Signaling (1) The process of sending a transmission signal over a physical medium for purposes of communications. (2) A method of communications, between network components, providing control, management and performance monitoring.

Simple Network Management Protocol (SNMP) An IETF protocol for monitoring and managing systems and devices in a network. The data being monitored and managed is defined by a Management Information Base (MIB). The functions supported by the protocol are the request and retrieval of data, the setting or writing of data, and traps that signal the occurrence of events.

Single Mode Fiber Used to describe optical fiber that allows only one mode of light signal transmission. Single mode fiber has a narrow core. Such fiber has a higher bandwidth than mulitmode fiber, but requires a light source with a narrow spectral width (for example, a LASER).

Small Computer System Interface (SCSI) A collection of ANSI standards and proposed standards which define I/O buses primarily intended for connecting storage subsystems or devices to hosts through host bus adapters. Originally intended primarily for use with small (desktop and desk-side workstation) computers, SCSI has been extended to serve most computing needs, and is arguably the most widely implemented I/O bus in use today.

Small Form-factor Pluggable (SFP) A specification for a new generation of optical modular transceivers. The devices are designed for use with small form factor (SFF) connectors, and offer high speed and physical compactness, and are hot-swappable. SFP transceivers are expected to perform at data speeds of up to five gigabits per second (5 Gb/s), and possibly higher. Because SFP modules can be easily interchanged, electro-optical or fiber optic networks can be upgraded and maintained more conveniently than has been the case with traditional soldered-in modules. Rather than replacing an entire circuit board containing several soldered-in modules, a single module can be removed and replaced for repair or upgrading. This can result in a substantial cost savings, both in maintenance and in upgrading efforts.

SONET/SDH SONET is a contraction of synchronous optical network, the ANSI (North American) standard for transmitting data over fiber optic lines; SDH is an abbreviation of synchronous digital hierarchy, the ITU-T (European) standard for transmitting data over fiber optic lines.

Standard Definition Television (SDTV) Television systems that have a lower resolution than HDTV systems. The term is usually used in reference to digital television, in particular when broadcasting at the same (or similar) resolution as analog systems.

Storage Area Network (SAN) A network whose primary purpose is the transfer of data between computer systems and storage elements and among storage elements. A SAN consists of a communication infrastructure, which provides physical connections, and a management layer, which organizes the connections, storage elements, and computer systems so that data transfer is secure and robust. The term SAN is usually (but not necessarily) identified with the Fibre Channel protocol and block I/O services rather than file access services.

Statistical Multiplexing A method for combining multiple calls onto a single line by giving priority to subscribers according to the volume of data they're trying to transmit at any given time. For example, a subscriber who typically transmits a large volume of data at a certain time of day (say, 5:00PM), but who transmits relatively little data during the rest of any given day, may ordinarily have a low priority in the overall multiplexing scheme for the line, but may be given a much higher priority at 5:00 when the subscriber transmits the bulk of its traffic.

Switch A network infrastructure component to which multiple nodes attach. Unlike hubs, switches typically have internal bandwidth that is a multiple of link bandwidth, and the ability to rapidly switch node connections from one to another. A typical switch can accommodate several simultaneous full link bandwidth transmissions between different pairs of nodes. A switch filters, forwards and directs frames or circuits based on a destination address.

Synchronous Digital Hierarchy (SDH) ITU-TSS International standard for transmission over optical fiber.

Synchronous Optical Network (SONET) (1) A set of standards for transmitting digital information over optical networks. "Synchronous" indicates that all pieces of the SONET signal can be tied to a single clock. (2) A CCITT standard for synchronous transmission up to multi-gigabit speeds. (3) A standard for fiber optics.

Synchronous Replication A replication technique in which data must be committed to stable storage at both the primary site and the secondary site before the write is acknowledged to the host. With flow control techniques and low latency transport, Synchronous Replication can be utilized up to 300 km. Examples of Synchronous Disk Mirroring applications are Global Mirroring (IBM), TrueCopy (Hitachi), and SRDF (EMC).

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.

Synchronous Transport Signal (STS, STS-n) (1) SONET standard for transmission over OC-1 optical fiber at 51.84 Mb/s. (2) A SONET frame including overhead and payload capacity. The basic SONET frame is the STS-1. STS-1s can be multiplexed or concatenated with no additional overhead.