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Technical Glossary

Also see the Visionael Glossary for product terms.

Here you will find definitions to many terms and acronyms related to computer networks and telecommunication infrastructure. Feel free to bookmark this page for future reference.

# A B C D E F G H I J K L M N O P Q R S T U V W X

2B1Q
Two Binary, one Quaternary. A line coding technique that compresses two binary bits of data into one time state as a four-level code.

2G
Second generation. 2G (or 2-G) is short for second-generation wireless telephone technology. The main differentiator to previous mobile telephone systems, retrospectively dubbed 1G, is that the radio signals that 1G networks use are analog, while 2G networks are digital. Both systems use digital signaling to connect the radio towers (which listen to the handsets) to the rest of the telephone system.

3G
Third generation

4ESS
A digital phone switching system made by AT&T. It is typically used as a long-distance switch for high-speed national networks.

5ESS
A digital central-office switching system made by AT&T. It is typically used as an "end-office," serving local subscribers.

10Base-2
Ethernet IEEE 802.3 specification requiring thin (50 ohm) coaxial cable (RG-58 A/U) running at 10 Mbps. Maximum distance is 185 meters per segment. This cable is thinner and more flexible than that used for the 10Base5 standard. The RG-58 A/U cable is both less expensive and easier to place.

Cables in the 10Base-2 system connect with BNC connectors. The Network Interface Card (NIC) in a computer has a T-connector where you can attach two cables to adjacent computers. Any unused connection must have a 50 ohm terminator. Also known as thinnet and cheapernet.

10Base-5
IEEE 802.3 baseband (Ethernet) physical layer specification requiring thick coaxial cable running at 10 Mbps. Maximum distance is 500 meters per segment. Also known as yellow cable or thick cable.

10Base-F
10-Mbps baseband Ethernet specification that refers to the 10BaseFB, 10BaseFL, and 10BaseFP standards for Ethernet over fiber-optic cabling.

10Base-FL
10-Mbps baseband Ethernet specification using fiber-optic cabling. 10BaseFL is part of the IEEE 10BaseF specification and, while able to interoperate with FOIRL, is designed to replace the FOIRL specification. 10BaseFL segments can be up to 3280 feet (1000 meters) long if used with FOIRL, and up to 1.24 miles (2000 meters) if 10BaseFL is used exclusively.

10Base-T
IEEE 802.3 physical layer specification for 10 Mbps Ethernet over two pairs of unshielded twisted pair wiring (UTP). The maximum length of a single segment is 500 meters. The cable is thinner and more flexible than the coaxial cable used for the 10Base-2 or 10Base-5 standards. Cables in the 10Base-T system connect RJ-45 connectors. A star topology is common with 12 or more computers connected directly to a hub or concentrator. Also known as UTP, twisted pair, and twisted sister.

100Base-T
Fast Ethernet. An IEEE 802.3 physical layer specification for 100 Mbps Ethernet over different grades of unshielded twisted pair wiring (UTP). Fast Ethernet offers a natural migration from traditional 10 Mbps Ethernet, since it uses the same CSMA/CD access mechanism.

There are several different cabling schemes that can be used with 100BASE-T, including:

100BASE-TX : two pairs of high-quality twisted-pair wires
100BASE-T4 : four pairs of normal-quality twisted-pair wires
100BASE-FX : fiber optic cables.

100Base-T4
An IEEE 802.3 specification for baseband Fast Ethernet over four pairs of Category 3, 4, or 5 unshielded twisted pair (UTP). Segments are limited to 100 meters to ensure the round trip timing specifications are met.

100Base-TX
An IEEE 802.3 specification for Fast Ethernet over two pairs of Category 5 unshielded twisted pair (UTP), or Type 1 shielded twisted pair (STP). The first pair of wires is used to receive data; the second is used to transmit. The maximum distance for a segment is 100 meters in order to ensure round trip timing specifications. Two 100 meter TX segments can be connected together through a single Class I or Class II repeater, thereby providing up to 200 meters between DTEs.

100VG-AnyLan
A relatively new 100 Mbps Fast Ethernet and Token Ring media technology based on the demand priority access; developed by Hewlett-Packard. The IEEE 802.12 VG specification calls for 4-pair UTP category 3, 4, and 5, with 2-pair STP and fiber possibilities in the future.

The Visionael Device Library includes several VG devices and NICs, and the rules engine correctly validates according to this specification. See IEEE 802.12 VG.

AAL5
ATM Adaption Layer 5. AAL5 has been adapted by the ATM Forum for a Class of Service called High Speed Data Transfer.

ACL
Access control list

Activation
Service activation is the actual requested service to the end user, including all agreed configuration, quality of service, features and pricing. For example, VPNs and VLANs are implemented by configuring the service on network routers and switches. Network administrators have different ways of activation different kinds of services on different kinds of network hardware. They may "log in" directly to the devices and enter commands manually. They may create command scripts that send a series of commands to the devices. They may use management systems provided by the device's vendor (See EMS). They may use higher-level activation systems that work with a variety of vendors' equipment.

ADSL
Asymmetic Digital Subscriber Line. A digital line providing asymmetrical bandwidth that uses the existing twisted pair copper telephone network to achieve speeds of up to 6 megabits per second up to 12000 feet, or 1.5 megabits per second up to 18000 feet. The downstream bandwidth going from the network to the subscriber is typically greater than the upstream bandwidth going from the subscriber to the network.

Aerial cable
Telecommunications cable that is attached to poles.

AIX
Advanced Interactive Executive. IBM's implementation of UNIX.

AMI
Alternate Mark Inversion. A line coding technique used to accommodate the ones density requirements of E1 or T1 lines.

AN
Access Node. See Node.

Analog
Representing data in continuously variable physical quantities, in contrast to the digital representation of data in discrete units (the binary digits 1 and 0). Analog systems handle information which is represented by change and flow, such as voltage or current. Analog devices have dials and sliding mechanisms. Digital information, in contrast, is either on or off. An analog is a representation of a pattern by a similar pattern; for example, an analog clock represents the sun circling around the earth.

An analog device converts a pattern such as light, temperature, or sound into an analogous pattern. An example is a video recorder, which converts light and sound patterns into electrical signals with the same patterns. An analog signal such as a sound wave is converted to digital by sampling at regular intervals; the more frequent the samples and the more data recorded, the more closely the digital representation resembles the analog signal. Converting analog signals into digital makes it possible to preserve the data indefinitely and make many copies without deterioration of quality.

Analog signals are described in terms of frequency (Hz, cycles per second), amplitude, and phase relationship. All DSL signals are modulated from digital signals at the modem to analog signals on the telephone lines.

ANSI
American National Standards Institute. A private (nonprofit membership) organization which develops and publishes standards for voluntary use in the United States. Founded in 1918, ANSI is currently composed of over 1,300 members. It is chartered to define, evaluate, and promote industrial standards for product compatibility. ANSI created ASCII, or American Standard Code for Information Interchange. Also used as an adjective to refer to standards established by the Institute or to items conforming to those standards. ANSI is also a member of the International Standards Organization (ISO).

Appletalk
A network protocol family traditionally associated with Macintosh computers, LaserWriter printers, and other products from Apple Computer. Notable for a dynamic and hierarchical service protocol called NBP (Name Binding Protocol).

Assurance
Applications that address fault and performance management and can support near-real-time or real-time traffic management, as well as quality of service and service-level agreements.

Asymmetrical
Transmission where the downstream bit rate and the upstream bit rate differ. 56 K modems are asymmetrical: they offer a maximum speed of 56K for downloading, but only 28.8K or 33.6K for uploading.

ATM
Asynchronous Transfer Mode. An internationally standardized implementation of cell relay technology, ATM represents the first worldwide standard to be embraced by the computer, communications, and entertainment industry with the potential of creating a common "platform" for various types of transmission needs, including LAN interconnections, CAD/CAM, medical imaging, and videoconferencing.

A fixed length 53-byte packet-based transmission technology that includes a 5-byte header and 48 bytes of payload, ATM may be used to transmit data, voice, and video traffic at speeds up to 2.4 Gbps. The small, constant ATM packet size (rather than the random-length "packets" moved by technologies such as Ethernet and FDDI) allows ATM equipment to transmit video, audio, and computer data over the same network, and to assure that no single type of data hogs the line. Current implementations of ATM support data transfer rates of from 25 to 622 Mbps. This compares to a maximum of 100 Mbps for Ethernet, the current technology used for most local-area networks.

Some people think that ATM holds the answer to the Internet bandwidth problem, but others are skeptical. ATM creates a fixed channel, or route, between two points whenever data transfer begins. This differs from TCP/IP, in which messages are divided into packets and each packet can take a different route from source to destination. This difference makes it easier to track and bill data usage across an ATM network, but it makes it less adaptable to sudden surges in network traffic.

ATM is often used as the backbone for the Frame Relay network implemented by telecommunications service providers. It is not uncommon for ATM switches to transmit and receive Frame Relay signals. Point-to-point ATM circuits are, like Frame Relay, referred to as Permanent Virtual Circuits, or PVCs.

Attenuate
The decrease in power of a transmitted signal as it travels across any communications transport medium. Also known as loss (opposite of gain); measured in decibels (dB).

ATU
ADSL Transceiver Unit. The ADSL Forum uses terminology for DSL equipment based on the ADSL model for which the Forum was originally created. The DSL endpoint is known as the ATU-R and the CO unit is known as the ATU-C. These terms have since come to be used for other types of DSL services, including RADSL, MSDSL and SDSL. ATU now represents xDSL services.

ATU-C
ATU - Central Office. An ATU at the carrier's central office in support of DSL-based services.

ATU-R
ATU - Remote. Equipment placed at the customer premises in support of DSL-based services.

AUI
Attachment Unit Interface. An IEEE 802.3 cable connecting the MAU to the networked device. AUI can also refer to the host back panel connector that attaches to an AUI cable. AUI specifies a coaxial cable connected to a transceiver that plugs into a 15-pin socket on the network interface card (NIC). Sometimes an AUI is called a transceiver cable or drop cable.

AWG
American Wire Gauge. A standard American method of classifying wire diameter using the Brown & Sharpe (B&S) gauge. The heavier the gauge, the lower the AWG number and the lower the impedance.

Backbone
The part of a network that acts as the primary path for traffic moving between, rather than within, networks.

Backbone LAN
A transmission facility designed to connect two or more LANs.

B channel
Bearer channel. In ISDN, a full-duplex, 64-kbps channel used to send user data.

Band
A range of frequencies from a lower limit to an upper limit. For example, the voice frequency (VF) band is the range from 300 to 3400 Hz.

Bandpass filter
A device that passes or transmits a given band of frequencies and blocks all other frequencies. In asymmetric digital subscriber line (ADSL), the voice frequency band of 300 Hz is blocked and redirected to a telephone set and all other frequencies are passed through to the modem or transceiver.

Bandwidth
1) The frequency range that a device or medium will pass or respond to. It is computed by subtracting the lower frequency limit from the upper frequency limit and is expressed in hertz (Hz). For example, the typical telephone circuit responds to all frequencies between 300 and 3400 Hz; therefore, its bandwidth is 3100 Hz. In E1/2M, cycles per second (CPS) is still commonly used in lieu of hertz.

2) In informal common usage in digital communications, bandwidth is used to indicate the maximum transmission rate of a facility in bits per second (for example, 4800 bps), or the information carrying capacity of a medium or system. For example, an Ethernet link is capable of moving 10 million bits of data per second. A Fast Ethernet link can move 100 million bits of data per second -- 10 times more bandwidth.

Bearer channel
A communications channel, also known as a B channel. It is usually in an Integrated Services Digital Network (ISDN) system, but possibly in any system that carries user (as opposed to system) data at the standard DS0 data rate of 64 kbps.

BER
Bit Error Rate. Represents the ratio of the number of erred bits to the total number of hits delivered in a digital system. BERs of less than one in one billion are not uncommon in today's digital communication systems.

Bit
Abbreviation for binary digit. A bit is the smallest unit of data that can be processed by a computer; expressed as the presence or absence of a pulse to represent a binary 1 or 0.

Bit error
When the value of a bit has been changed during transmission and is incorrectly received.

Bit rate
The transmission speed of bits, usually expressed as bits per second.

BNC
Short for British Naval Connector or Bayonet Nut Connector or Bayonet Neil Consulman, a type of connector used with coaxial cables, such as the RG-58 A/U cable used with the 10Base-2 Ethernet system. The basic BNC connector is a male type mounted at each end of a thin Ethernet cable. This connector has a center pin connected to the center cable conductor and a metal tube connected to the outer cable shield. A rotating ring outside the tube locks the cable to any female connector. BNC T-connectors (used with the 10Base-2 system) are female devices for connecting two cables to a network interface card (NIC). A BNC barrel connector makes it possible to connect two cables together.

Bonding
Often referred to as dial-in channel aggregation, takes place at the customer premises through inverse multiplexing. Multiplexing takes a high-bandwidth signal and splits it for transport through the network over multiple lower-bandwidth channels. At the receiving end, the multiple lower-bandwidth signals are recombined into the original high-bandwidth signal. The industry- accepted standards for inverse multiplexing ISDN connections are defined by the Bandwidth On Demand Interoperability Group (BONDING) Consortium and are listed in TIA/EIA 961 and ISO 13871.

BPS
bits per second. Indicates the speed at which bits are transmitted across a data connection.

BRI
Basic Rate Interface. A consumer grade ISDN line consisting of a two-wire interface between the customer premises equipment (CPE) and the telco end office (EO), and a four-wire interface at the customer premises. BRI service provides two clear 64-kbps B channels for voice, video, and data and one 16-kbps D channel used for signaling (known as 2B+D). BRI service is primarily for home or small business use.

Bridge
A Layer 2 (data link layer) device that connects networks using dissimilar protocols. For example, a bridge can connect an Ethernet with a Token-Ring network. When two separate LANs are bridged together, they become one effective LAN. Bridges do not interpret the information they carry, but simply pass it between the two network segments. Consequently, they are faster than routers, but also less versatile.

Bridge tap
Any segment of cable not on a direct path between a central office (CO) and a subscriber. Also referred to as bridged tap.

Broadcast Storms
A state in which a message that has been broadcast across a network results in even more responses, and each response results in still more responses in a snowball effect. A severe broadcast storm can block all other network traffic, resulting in a network meltdown. Broadcast storms can usually be prevented by carefully configuring a network to block illegal broadcast messages.

Broadband
Any system able to deliver multiple channels and/or services to its users or subscribers. Generally refers to cable-television systems. Sometimes called wideband.

Broadband communications
Term characterizing both digital and analog transmission systems. Broadband communications is generally understood to indicate either a fast data-rate digital system or a wide bandwidth analog system.

B-Router
A device incorporating the functionality of a bridge and a router in a single unit. A brouter understands how to route specific types of packets, such as TCP/IP packets (the router function). Any other packets it receives are simply forwarded to other network(s) connected to the device (the bridge function).

BSS
Business Support System (see OSS).

Bus Topology
One of the three principal topologies used in LANs. All devices are connected to a central cable, called the bus or backbone. Bus networks are relatively inexpensive and easy to install. Ethernet systems use a bus topology. See also ring topology and star topology.

Cable
A group of conductors (metallic pairs, coaxial, optical fibers) assembled within a common, protective sheath.

Cable Binder
A cable binder is used to bundle multiple insulated copper pairs together in the telephone network.

Cable Television System (CATV)
A broadband communications system capable of delivering multiple channels of programming from a set of centralized satellite and off-air antennae, generally by coaxial cable, to a community. Many cable-television designs integrate fiber-optic and microwave links.

CAGR
Compound Annual Growth Rate

Call setup
The process of establishing a connection between two communicating entities. In the voice network, call setup involves interpreting the dialed number and establishing a connection between the caller and the destination. The process is similar for data calls, but usually requires more negotiation of parameters.

Call teardown
The procedure of disconnecting a call between the CO and the subscriber.

CAP
1) Carrierless Amplitude and Phase. CAP modulation is a line coding technique that is a variant of quadrature amplitude modulation (QAM). CAP is used in various voice-band modems. Some telecommunications providers utilize CAP technology for transmission. [CAP was the industry standard for early deployment ADSL.]

Capacity Management
A rigorous set of engineering disciplines that plans, predicts and ensures network equipment, elements, products and services will be available regardless of spikes in and planned growth in the number of subscribers and new services.

2) Competitive Access Provider.

Cards
Adapter cards, or Network Interface Cards (NICs), are circuit boards installed in a device's chassis slots to provide network communication capabilities to and from other devices.

Carrier
A telecommunications company that offers communications services to the general public via shared circuits at published tariff rates. In the United States, the Federal Communications Commission or various state public-utility commissions regulate common carriers.

CAT5
Category 5. A level of unshielded twisted pair wiring performance as defined by EIA/TIA-568.

CBR
Committed Bit Rate.

CCITT
Consultative Committee for International Telephone and Telegraph. An international communications committee, based in Geneva, that is responsible for proposing communications standards. The CCITT acronym comes from the French Commite' Consultatif International de Telegraphique et Telephonique. CCIT is now known as ITU (the parent organization). The X.25 protocol was created by the CCITT.

CDDI
Copper Distributed Data Interface. CDDI is a version of FDDI designed to run on either shielded or unshielded twisted pair copper wire. CDDI is a 100-Mbps LAN standard based on token ring arbitration protocols and using copper cable. Like all Token Ring implementations, redundancy is provided by using a dual-ring architecture. CDDI cable lengths are limited to 100 meters.

CDM
Code Division Multiplexing.

CDMA
Code Division Multiple Access

CEU
Commercial End User. See SU, Service User.

Channel
The path network devices use to communicate. A channel can refer to the physical medium (the wires) or to a set of properties that distinguishes one channel from another. In some instances, a multiplexer (MUX) allows multiple channels on a single cable. In telecom terms, a Channel is the smallest subdivision of a communication path on a digital, multiplexed signal for which information can be provided in one direction.

Channel splitter/assembler
A channel splitter transmits channels 1 through 12 over one wire pair and channels 13 through 24 over the other wire pair. A channel assembler performs the reverse process for received information.

Channelized
A transmission option for subscribers of symmetrical services. Channelized service allows a subscriber to assign one device (such as a PC) to a DS0 channel, resulting in many devices being able to transmit and receive simultaneously at 64 kbps.

Chip set
A configuration of two or more microchips for the purpose of a specific function.

Circuit
An object that is used to describe a connection between two or more device ports or locations.

Class 1 Repeater
A type of Fast Ethernet hub that performs translations when transmitting/repeating incoming signals to allow different media types to be connected to a single collision domain. Only one Class I repeater is allowed per collision domain.

Class 2 Repeater
A type of Fast Ethernet hub that only transmits or repeats an incoming signal to other devices on the same media type (no translations are performed). Only two Class II repeaters are allowed per collision domain.

CLEC
Competitive Local Exchange Carrier. A term coined for the deregulated, competitive telecommunications environment, envisioned by the Telecommunications Act of 1996. CLECs compete on a selective basis for local exchange service, as well as long distance, Internet access, and entertainment. They typically lease local loops from the incumbent local exchange carrier (LEC) at wholesale rates and then resells to end users at a discounted price.

CLEI
Common Langauge Equipment Identifier. The first 8 digits from a CLLI + 3 digits detailing the equipment.

CLLI
Common Language Location Identifier.

CLR
Circuit Layout Record. Report detailing a circuit design from Loc A to Loc Z, used for end-to-end install. Compare with DLR.

CMDB
Configuration Management Database

CO
Central Office/Central Site. Refers to the physical facility that contains the telephone switching system, transmission equipment, and other support systems that provide telephone and other telecommunications services to local telephone subscribers. There are numerous types of telephone switching systems, such as 1ESS, 4ESS, 5ESS, DMS 10/100/250/500, EAX2, or GTD5 that can be housed in a central office. The central office is not to be confused with the point of presence (POP) of the interexchange carrier, even though both perform many similar functions. Known as a public exchange outside of North America.

Coaxial Cable (Coax)
Coaxial cables were designed to carry analog or voice signals. The cable is called "coaxial" because it includes one physical channel that carries the signal surrounded (after a layer of insulation) by another concentric physical channel, both running along the same axis. The outer channel of braided wire serves as a ground, minimizing electrical and radio frequency interference. Many of these cables or pairs of coaxial tubes can be placed in a single outer sheathing and, with repeaters, can carry information for a great distance. The most commonly recognized application of coaxial cable is its use for broadband data and cable television delivery. Also known as "coax".

Collision
The situation that occurs when two or more devices attempt to send a signal along the same channel at the same time. The result of a collision is generally a garbled message. All computer networks require some sort of mechanism to either prevent collisions altogether or to recover from collisions when they do occur. Also see Collision Domain.

Collision Detection
CD. The process of detecting simultaneous transmissions on a shared medium. Typically, each transmitting workstation that detects a collision will wait some period of time and try again. Collision detection is an essential part of the CSMA/CD access method. Workstations detect collisions if, after sending data, they fail to receive an acknowledgment from the receiving station.

Collision Domain
In Ethernet, the network area within which frames that have collided are propagated. Repeaters and hubs propagate collisions; LAN switches, bridges, and routers do not.

Community
An SNMP term referring to the administrative relationship between an SNMP agent and its managers to control the access to SNMP MIB (management information base). This relationship is validated every time a SNMP agent wants to transfer data on the Internet.

Access to the SNMP community is regulated by the community name, which is sent in every packet between a management station and an SNMP agent. The community name is usually an ASCII string that validates that the sending SNMP agents are members of the community.

Conduit
A pipe or tube through which cables or inner (sub) ducts are run. A conduit makes placing cable of any type much easier.

Configuration
Deals with the initializing, loading, modifying and tracking of the configuration parameters of required network hardware and software.

Connector
A physical interface on cable ends or port interfaces, such as RJ-45 or RS-232-C, that joins computer and networking equipment. Typically, connectors have male and/or female components.

CoS
Class of Service. Method of classifying different traffic flows into a category and applying a particular quality of service (QoS) for that flow.

COTS
Commercial Off-The-Shelf

CPE
Customer Premises Equipment. Literally, equipment that resides at the customer's site, not at a telecommunications service provider's site. CPE includes PCs, telephones, TVs, scanners, and much more. Managed Network Service Providers often install, configure and manage CPE as a service (for example, managed router services).

CRM
Customer Relationship Management

Crosstalk
Line distortion caused by wire pairs in the same bundle being used for separate signal transmission.

CSA
Carrier Serving Area

CSM
Cluster Systems Management

CSMACD
Carrier Sense Multiple Access with Collision Detection. The means of access control used by Ethernet and the IEEE 802.3 LAN protocol. CSMA/CD works like this: Networked devices wishing to transmit data over a single channel first check for carrier signals (carrier sense). If no carrier is found, transmitting can occur over the channel (multiple access). A collision occurs when two devices transmit at the same time. In this case, the collision is detected by all colliding devices and their re-transmissions are delayed for some period of time (collision detection).

CSP
Communications Service Provider. Also, Chip Scale Package.

CSU/DSU
Channel Service Unit/Data Service Unit. This is a device on a customer's premises (CPE) that terminates a digital circuit from a telecommunications service provider. The CSU/DSU performs coding, conditioning and equalization on the signals it transmits to the service provider, and it responds to loopback commands sent from the service provider's central office. A CSU/DSU is similar to a modem, but it transfers data at much greater rates and doesn't support dial-up functions, it's always "off hook."

DACS
Digital Access & Cross-Connect System. A device that allows DS0 channels to be individually routed and reconfigured.

Datagram
In IP networks, a packet. A way of sending data in which a data message is randomly broken into parts and the parts are correctly reassembled by the receiving machine. Each message part contains information about itself including its destination and source.

Daughter Card
A circuit board that fits into a regular bus adapter card; in essence, a sub-card of the bus card. A daughtercard is similar to an expansion board, but it accesses the motherboard components (memory and CPU) directly instead of sending data through the slower expansion bus. The Device Library contains adapter card compatible daughtercards.

DBS
Direct Broadcast Satellite

DCE
Data Circuit-Terminating Equipment. The devices and connections of a network that connect the communication circuit with DTE devices, such as terminals and computers. DCE establishes, maintains, and terminates the connection in a data conversation. For example, in an RS-232 connection, a modem is considered a DCE device. In an X.25 network, the packet-switching device is considered the DCE.

D channel
In an ISDN interface, the D channel is used to carry control signals and customer call data in a packet-switched mode. In the ISDN Basic Rate Interface (BRI), the D channel operates at 16 kbps, and in the ISDN Primary Rate Interface (PRI), the D channel operates at 64 kbps.

DDS
Digital Data Service. Private line digital service that provides digital communication circuits.

Decibel (dB)
The unit of measure for signal loss or gain. It is used to express the ratio of two powers, voltages, currents, or sound intensities.

DEC Net Protocol developed by Digital Equipment Corporation that specifies 16 bit node addresses. The 16 bit addresses break down into a 6 bit area address and a 10 bit node ID. DecNet phase 5 is functionally identical to OSI CLNP.

Demarcation
The point between the wiring that comes in from your local telephone company and the wiring you install to hook up your telephone system to the point where the telco's responsibility to maintain or replace lines or equipment ends and a subscriber's begins. Usually a network interface device (NID) is the demarcation point.

Demodulation
Converting analog signals back into digital signals. A modem is a MOdulator/DEModulator.

Demultiplex
Reversing the process of multiplexing; the separation of the individual channels from the multiplexed signal.

Demux
A demux, or demultiplexer, accepts a large bandwidth signal comprising numerous channels of information and separates it into distinct, individual signals. Demultiplexing is the opposite action of multiplexing.

Device
A generic term referring to network equipment such as hubs, PCs, routers, and so on.

DHCP
Dynamic Host Configuration Protocol. A TCP/IP protocol that provides static and dynamic address management. Enables individual computers on an IP network to extract their configuration from a DHCP server or servers, in particular, servers that have no exact information about the individual computers until they request the information. The overall purpose is to reduce the work necessary to administer a large IP network.

Digital Media
Any system based on discontinuous data or events. Computers are digital machines because at their most basic level they can distinguish between just two values, 0 and 1, or off and on. There is no simple to represent all the values in between, such as 0.25. All data that a computer processes must be encoded digitally, as a series of zeroes and ones.

The opposite of digital is analog. A typical analog device is a clock in which the hands move continuously around the face. Such a clock is capable of indicating every possible time of day. In contrast, a digital clock is capable of representing only a finite number of times (every tenth of a second, for example).

Digital phone
A telephone that converts an analog voice signal to a digital voice signal, so that the signal is in digital format for transmission.

Discovery
The act of exploring a network to determine the types and settings of network devices. Discoveries can ferret out network topologies, router, host, and database information, operating system and memory information, and so on. Discovery also refers to the data or report generated by the software probe. Discoveries are used to document networks, update network designs, and to track network assets.

DLC
Digital Loop Carrier. Network transmission equipment, consisting of a CO terminal and a remote terminal, used to provide a pair gain function.

DLC Address
Short for Data Link Control address. Every network interface card (NIC) has a DLC address or DLC identifier (DLCI) that uniquely identifies the node on the network. Some network protocols, such as Ethernet and Token Ring use the DLC addresses exclusively. Other protocols, such as TCPIP, use a logical address at the Network Layer to identify nodes. Ultimately, however, all network addresses must be translated to DLC addresses. In TCP/IP networks, this translation is performed with the Address Resolution Protocol (ARP).

For networks that conform to the IEEE 802 standards (e.g., Ethernet ), the DLC address is usually called the Media Access Control (MAC) address.

DLCI
Data Link Connection Identifier. DLCI IDs are Frame Relay values that identify a logical connection between two destinations. This number is used as part of the frame relay header. DLCIs are assigned to a permanent virtual circuit (PVC) end point in a Frame Relay network.

DLR
Design (or sometimes Detail) Layout Record. Report issued to a customer detailing the a circuit design from Loc A to Loc Z. Compare with CLR.

DMS100
A digital central-office switching system made by Northern Telecom. It is typically used as an "end office," serving local subscribers.

DMS250
A digital phone switching system made by Northern Telecom. It is typically used as a long-distance switch for high-speed national networks.

DMT
Discrete Multitone. DSL technology using digital signal processors to divide the signal into 256 subchannels. A line code used in some transceivers or modems to transport digital signals. DMT employs rate adaptation and is used in the ANSI standard for xDSL technologies. Currently, ANSI T1.413 Issue 2, International Telecommunications Union (ITU) 992.1 (G.dmt), and ITU 992.2 (G.lite) use DMT as their core modulation scheme.

DNS
Domain Name System (or Service). An Internet service that translates domain names into IP addresses. Because domain names are alphabetic, they're easier to remember. The Internet however, is really based on IP addresses. Every time you use a domain name, therefore, a DNS service must translate the name into the corresponding IP address. For example, the domain name www.visionael.com would be translated to 216.101.250.1. The DNS system is, in fact, its own network. If one DNS server doesn't know how to translate a particular domain name, it asks another one, and so on, until the correct IP address is returned.

Domain
Term referring to the physical or logical divisions of devices on a network. A group of computers and devices on a network that are administered as a unit with common rules and procedures. For example, a physical domain might consist of a separate LAN. A logical domain could be created by simply assigning devices in a particular area their own group name.

Within the Internet, domains are defined by the IP address. All devices sharing a common part of the IP address are said to be in the same domain.

DoS
Denial of Service

Downstream transmission
A telecommunications transmission travelling from the telco CO to the subscriber's location.

DRAM
Dynamic Random Access Memory. Used to store data in PCs and other devices.

Drop Cable
A cable connecting individual network devices, such as PCs, to the main trunk cables of a network.

DSL
Digital Subscriber Line. Another term denoting xDSL. A family of technologies transmitting digital information (and sometimes POTS) over existing copper wire pairs for limited distances or over fiber-optic cables. The "x" in xDSL stands for any number of letters denoting the xDSL family members, commonly IDSL, SDSL, HDSL, ADSL, and VDSL.

DSLAM
Digital Subscriber Line Access Multiplexer. A device that concentrates traffic in DSL implementations through a process of time-division multiplexing (TDM) at the CO or remote line shelf. This device is usually located in the CO for termination of multiple customer DSL devices.

DSLAM-DSL access multiplexer
A MUX installed in a CO that multiplexes digital transmissions from multiple ADSL transceivers or modems and switches them directly to a network, completely bypassing a local router.

DS0
Part of the hierarchy of standard digital transmission rates used in North America and the "quantum unit" of fiber-optic transmission capacity. Digital signal level zero (DS0) represents the 64-kbps single-channel signal (about the capacity historically used to carry a single voice conversation) generated by a T1 terminal device such as a channel bank, multiplexer (MUX), or digital PBX. Each level supports a different number of digital voice-channels:

DS0 is equivalent to 1 digital voice channel and has a line rate 64 kbps.
DS1 (or T1) supports 24 digital voice channels and a line rate of 1.544 Mbps.
DS1C (or T1C) supports 48 digital voice channels and a line rate of 3.152 Mbps.
DS2 (or T2) supports 4 DS1s (96 digital voice channels) and a line rate of 6.132 Mbps.
DS3 (or T3) supports 28 DS1s (7 DS2s, 672 digital voice channels) and a line rate of 44.736 Mbps.
DS4 (or T4) supports 168 DS1s (6 DS3s, 42 DS2s, 4032 digital voice channels and a line rate of 274.176 Mbps.

DS1 circuits are used to carry moderate volumes of traffic by both end users and carriers. DS3 circuits are used most often to carry high volumes of long distance traffic or to provide resold DS1, or voice grade, private line service. Large companies also use DS3s for internal communications.

DSP
Digital Signal Processor. A specialized, programmable computer chip designed to perform speedy and complex operations on digitized waveforms, usually audio or video signals. A DSP-based modem uses a DSP for analog and digital modem communications. The DSP-based modem is programmable for easy upgrades.

DSVD
Digital Simultaneous Voice and Data.

DTE
Data Terminal Equipment. The part of a data station that serves as a data source, data destination, or both. DTE also provides the data communication control functions according to the appropriate protocol. DTE devices are typically user devices, such as terminals and computers and connect to data circuit-terminating equipment (DCE), such as modems and network access nodes or packet-switching nodes. These serial connections are defined by the RS-232C standard. DTE includes workstations and PCs, protocol translators, and multiplexers.

DTMF
Dual Tone Multifrequency. A method for touch-tone phones developed to make dialing an easier process; each digit corresponds to one of 16 combinations of pairs of sine waves chosen from eight different frequencies (example: the 7 digit is defined as the combination of 852 Hz and 1209 Hz).

DTV
Digital Television

Duplex
A characteristic of data transmission, either full-duplex or half-duplex. Full-duplex transmission permits simultaneous, two-way communication. Half-duplex transmission means only one side can transmit at a time.

DWDM
Dense Wave Division Multiplexing. Dense Wavelength Division Multiplexing. DWDM is a transmission technology that allows for increased traffic capacity over fiber-optic media by sending traffic over specific wavelengths of light. DWDM systems currently support the simultaneous transmission of data over 8 to 40 wavelengths of light, and this maximum number of wavelengths continues to increase. Each wavelength can carry up to an OC-48 signal. In some cases, wavelengths can carry up to OC-192, but this limits the number of wavelengths available. So DWDM is used to carry Sonet/SDH circuits, and DWDM is increasingly seen as a media for other transmission protocols such as Gigabit Ethernet. DWDM will therefore replace Sonet/SDH in some cases, and carry Sonet/SDH circuits in other cases. Compare with WDM.

E1
A wideband digital circuit with standardized operating characteristics that operate at a rate of 2.048 Mbps. Defined by the ITU recommendations G.703 and G.704, this standard is widely used in Europe and in submarine cables as the rough equivalent of a DS1 (the E1 provides thirty 64 Kbps channels, six more than a DS1).

E3
A wideband digital circuit with standardized operating characteristics that operate at a rate of 34.368 Mbps. This standard is widely used in Europe for intercarrier communications as the rough equivalent of a DS3.

EDGE
Enhanced Data Rates for Global Evolution

EIA
Electronic Industries Association. A trade association representing the U.S. high technology community. It began in 1924 as the Radio Manufacturers Association. The EIA sponsors a number of activities on behalf of its members, including conferences and trade shows. In addition, it has been responsible for developing some important standards, such as the RS-232, RS-422 and RS-423 standards for connecting serial devices.

EIA-232D
Electronics Industries Association-232D. The official designation for RS-232 (Recommended Standard-232), an Electronics Industries Association standard asynchronous serial line, which is used commonly for modems, computer terminals, and serial printers.

EIA/TIA
Electronic Industries Association/Telecommunications Industry Association. This organization provides standards for the data communications industry to ensure uniformity of the interface between DTEs and DCEs.

EIA/TIA-32E
Interface Between Data Terminal Equipment and Data Circuit-Terminating Equipment Employing Serial Binary Data Interchange (July, 1991). Common physical layer interface standard, developed by EIA and TIA, that supports unbalanced circuits at signal speeds of up to 64 kbps. EIA/TIA 232 E closely resembles the V.24 specification and was formerly called RS-232. This standard is applicable to the interconnection of data terminal equipment (DTE) and data circuit-terminating equipment (DCE) employing serial binary data interchange.

It defines:

Section 2 Signal Characteristics: Electrical and grounding characteristics of the interchange signals and associated circuits.
Section 3 Interface Mechanical Characteristics: Definition of the mechanical characteristics of the interface between the DTE and the DCE.
Section 4 Functional Description of Interchange Circuits: Functional description of a set of data, timing and control interchange circuits group of data communication system applications.
Section 5 Standard Interfaces for Selected Communication System Configurations: Standard subsets of specific interchange circuits are defined for a specific group of data communication system applications.

Element management
A set of procedures, software, equipment and operations procedures designed to maintain, inter-release, monitor, manage and optimize all discrete network elements on a given network.

EMS
Element Management System. A management system that provides functions at the element management layer.

ESF
Extended Super Frame. A T1 framing standard, used for frame synchronization and to locate signaling bits. ESF consists of 24 bits instead of the previous standard 12 bits. ESF allows easy error data storage and retrieval, facilitating network maintenance and performance monitoring.

Ethernet
A baseband LAN specification invented by Xerox Corporation and developed jointly by Xerox, Intel, and Digital Equipment Corporation in 1976. Ethernet standardized the coaxial cables and transceivers used to connect computers. Ethernet networks operate at 10 Mbps using the CSMA/CD access method over coaxial cable. Ethernet is similar to a series of standards produced by the IEEE referred to as IEEE 802.3. Ethernet is one of the most widely implemented LAN standards.

See also:
Ethernet Specifications

ETSI
European Telecommunications Standards Institute. Organization created by the European post, telephone, and telegraphs (PTTs) and the European community (EC) to propose telecommunications standards for Europe. ETSI's task is to pave the way for telecommunications integration in the EC as part of the single European market program.

EU
European Union. Formerly known as EC, European Commission.

FCC
Federal Communications Commission. The FCC Board of Commissioners regulates all electrical communications that originate in the United States.

FDDI
Fiber Distributed Data Interface. An ANSI-defined LAN standard specifying a 100-Mbps token-passing network using fiber optic cable, with transmission distances of up to 2 km. FDDI uses a dual-ring architecture to provide redundancy. FDDI networks are typically used as backbones for wide-area networks.

An extension to FDDI, called FDDI-2, supports the transmission of voice and video information as well as data. Another variation of FDDI, called FDDI Full Duplex Technology (FFDT) uses the same network infrastructure but can potentially support data rates up to 200 Mbps.

FDI
Feeder Distribution Interfaces. Points where cable bundles from the telephony switch use drop lines extended out to the customer premises.

FDM
Frequency-Division Multiplexing. Consists of separate channels that are assigned to individual frequencies across the frequency bandwidth on a common transmission facility. ADSL technology, for example, has a frequency range of 30 kHz to 1.1 MHz that can be divided into two nonoverlapping areas used for upstream and downstream transmission.

FEP
Front End Processor. A dedicated device usually connected to host computers and mainframes that provides data communications functions and offloads network processing work for the attached computers. In IBM SNA networks, a typical FEP is the IBM 3745 communications controller.

FEXT
Far End CrossTalk. One category of Crosstalk that occurs at the remote end of a link.

Fiber Optic Cable
A technology that uses glass (or plastic) threads (fibers) to transmit data. A fiber optic cable consists of a bundle of glass threads, each of which is capable of transmitting messages at close to the speed of light.

Fiber optic cable has several advantages over traditional metal communications lines:

Fiber optic cables have a much greater bandwidth than metal cables. This means that they can carry more data.
Fiber optic cables are less susceptible than metal cables to interference.
Fiber optic cables are much thinner and lighter than metal wires.
Data can be transmitted digitally (the natural form for computer data) rather than analogically.

The main disadvantage of fiber optics is that the cables are expensive to install. In addition, they are more fragile than wire and are difficult to split.

Fiber optics is a particularly popular technology for local-area networks (LANs). In addition, telephone companies are steadily replacing traditional telephone lines with fiber optic cables. In the future, almost all communications will employ fiber optics.

FIFO
First In, First Out. A type of data buffering that prevents data loss during high-speed communications.

FOIRL
Fiber Optic Inter-Repeater Link. Fiber optic signaling methodology based on the IEEE 802.3 fiber optic specification. FOIRL is a precursor of the 10BaseFL specification, which is designed to replace it.

FRAD
Frame Relay Assembler/Dissembler. A FRAD connects non-frame relay devices to the frame relay network.

Frame Relay
A transport protocol that transmits data in other protocols (usually LAN protocols such as IP) in packets (or frames) that can vary in size. Frame Relay is intended to for LAN-to-LAN internetworking of data communications. It offers cost and performance benefits over dedicated leased lines (T-1s). Frame Relay connections from location to location are defined by building and configuring PVCs (Permanent Virtual Circuits) that identify the locations to be connected. Typically, routers can convert IP LAN traffic to Frame Relay for delivery over the WAN. Each Frame Relay PVC or circuit must be built by configuring the router to understand how the Frame Relay network connects the router to other routers in the WAN. Traffic transported by the Frame Relay PVCs is usually carried by a leased line such as a T-1 to the service provider's central office, where the T-1 is connected to the Frame Relay switch. The Frame Relay switch accepts the frames and delivers them to the proper destination.

Frame Tagging
The marking of packet with special VLAN identifiers. That is, a packets that needs to be forwarded to another switch via an interswitch link or needs to go through a router to another VLAN carries a unique VLAN identifier (VLAN ID) as it leaves its local switch. The VLAN ID allows the VLAN switches and routers to selectively forward the packet to ports with the same VLAN ID. It is the responsibility of the local switch (which receives the packet from the source station) to insert the VLAN ID in the packet header. Similarly, it is the responsibility of the destination switch (on whose port the destination end user is located) to remove the VLAN ID and forward the packet to the appropriate port.

Framing
A method of grouping bits together into meaningful arrangements such that they can be treated as a single entity. Bits are framed not only for convenience but also for purposes of error checking and correction.

Frequency
The number of occurrences per unit time of a complete wavelength (one positive and one negative alternation). Frequency is measured in hertz (Hz), sometimes referred to as CPS.

FRF
Frame Relay Forum.

FRSP
Frame Relay Service Provider.

FTP
File Transfer Protocol. A TCP/IP standard protocol that allows a user on one host to access and transfer files to and from another host over a network.

Fulfillment
A suite of applications that automates and integrates business processes from customer order to configuration, inventory, provisioning and activation of services.

Gateway
A specialty device that converts data protocols across numerous communications layers, permitting two dissimilar networks to communicate. For example, a gateway might be part of a network scheme that bridges a Token Ring and FDDI LAN together.

Gigabit Ethernet
Gigabit Ethernet is a high-speed transmission protocol designed to speed up LAN/campus backbones by carrying Ethernet traffic over fiber-optic cables at speeds of up to 1000 million bits per second. Gigabit Ethernet and ATM are competitive in providing these types of backbones. Gigabit Ethernet is simpler, but is limited to data. ATM is more flexible and versatile, offering support for voice, video and multimedia in addition to data, and it offers more functionality for quality of service guarantees.

GIS
Geographic Information System

GPRS
General Packet Radio Service

GSM
Global System for Mobile Communication (GSM: originally from Groupe Spécial Mobile) is the most popular standard for mobile phones in the world. Its promoter, the GSM Association, estimates that 82% of the global mobile market uses the standard.

GUI
Graphical User Interface.

H channel
A wideband channel of dedicated bandwidth (for example, H0 channel dedicated to 386 kbps of bandwidth).

HDB3
High Density Bipolar Three Zeros Substitution. A line coding technique used to accommodate the ones density requirements of E1 lines.

HDLC
High Level Data Link control. A generic ISO link-level communications protocol that specifies a data encapsulation method over synchronous, serial data links (layer 2). The HDLC protocol embeds information in a data frame that allows devices to control data flow and correct errors. HDLC is an ISO standard developed from the Synchronous Data Link Control (SDLC) standard proposed by IBM in the 1970s.

For any HDLC communications session, one station is designated primary and the other secondary. A session can use one of the following connection modes, which determine how the primary and secondary stations interact:

Normal unbalanced: The secondary station responds only to the primary station.
Asynchronous: The secondary station can initiate a message.
Asynchronous balanced: Both stations send and receive over its part of a duplex line. This mode is used for X.25 packet-switching networks.

The Link Access Procedure-Balanced (LAP-B) and Link Access Procedure D-channel (LAP-D) protocols are subsets of HDLC.

HDM
Hardware Device Module

HDSL
High-bit-rate Digital Subscriber Line. An xDSL technology providing symmetrical transmission at 1.544 Mbps over 24 DS0 channels on two wire pairs.

HDSL-2
An xDSL technology that is a variation of HDSL. HDSL-2 provides symmetrical transmission of 24 DS0 at 1.544 Mbps on a one-wire pair (versus two wire pairs of HDSL). HDSL-2 uses Trellis-coded pulse amplitude modulation (TC-PAM) as a line code.

HDTV/EDTV
High-definition TV (HDTV) and extended-definition TV (EDTV) are types of television transmissions that offer better picture resolution than regular TV. They are generic terms referring to the next generation of broadcast television. For example, HDTV provides approximately twice the resolution of today's national television standard committee (NTSC) and may require from 90 to 900 Mbps for uncompressed transmission.

Hertz
Frequency measurement. 1 Hertz = 1 cycle per second.

Hierarchy
The accumulation of circuits that make up the parent circuit. This structure is referred to as the circuit's hierarchy.

Host
A computer system on a network. In networking terminology, host specifically refers to a computer system, whereas the more general term node refers to any device interacting with the network including computers, routers, printers, and so on. Each host connected to a network has a unique network address.

HP Open View
A network discovery engine developed by Hewlett-Packard. HP OpenView provides integrated network, system, application and database management for multivendor distributed computing environments. HP OpenView can probe IP-, IPX -, or SMS-based networks.

HSP
Host signal processor. In modems, a modem that depends on the host CPU (the Pentium, PowerPC, etc., in the main computer) for part or most of the data processing.

HTML
HyperText Markup Language. An authoring language used on the World Wide Web. HTML is basically ASCII text with HTML commands.

HTU-C
HDSL Terminal Unit - Central. The module at the CO or central site end of an HDSL connection. Also known as a Line Termination Unit (LTU).

HTU-R
HDSL Terminal Unit - Remote. The module at the customer premises end of an HDSL connection. Also known as a Network Termination Unit (NTU).

Hybrid
A device that converts the two-wire local loop to the four-wire central office.

HUB
A device that serves as the center of a star topology network. Hubs are commonly used to connect segments of a LAN. A hub contains multiple ports. When a packet arrives at one port, it is copied to the other ports so that all segments of the LAN can see all packets. Hubs can be active (where they repeat signals sent through them) or passive (where they do not repeat, but merely split, signals sent through them). Intelligent (manageable) hubs include additional features that enable an administrator to monitor the traffic passing through the hub and to configure each port in the hub.

ICMP
Internet Control Message Protocol. An extension to the Internet Protocol (IP) defined by RFC 792 that supports packets containing error, control, and informational messages, and other relevant IP packet processing information.

The ping command, for example, uses ICMP to test an Internet connection.

IDC
Insulation Displacement Connection. A wire connection device.

IDF
Intermediate Distribution Frame. A structure where phone lines are terminated. Similar to a main distribution frame (MDF), an IDF is usually a phone line termination point for a floor's wiring domain, and usually connects to a central MDF somewhere else in the building. For example, a typical office building may have IDFs located on each floor, all cabled back to the MDF in the basement.

IDP
Integrated Data Protection

IDS
Intrusion Detection Software

IDSL
ISDN Digital Subscriber Line. An xDSL technology that is basically a naming convention for an ISDN Basic Rate Interface (BRI), both B-channels and the D-channels permanently bonded for 144 Kbps over a single wire pair. ISDN digital subscriber line (IDSL) uses 2B1Q line coding.

IEEE
Institute of Electrical and Electronic Engineers. Founded in 1884, IEEE is an international professional organization responsible for defining network standards for LANs, including the 802 series.

IEEE 802.12VG
100 Base VG AnyLAN standard approved by the IEEE. 100 VG AnyLAN is a 100 Megabit/sec high speed networking standard that was originally developed to transmit Ethernet or Token Ring packets on existing wiring. The VG in 100 VG AnyLAN stands for Voice Grade, meaning that the 100 VG technology will run standard Ethernet frames at 100 Megabits/sec, utilizing Voice Grade category 3, 4, and 5 Unshielded Twisted pair (UTP) wire.

Data packets are transferred from node to node by a hub, based on the address of the data packet. This ensures an orderly transmission and eliminates collisions. Because of this managed transfer of data, 100 VG AnyLAN can transmit data at peak speeds of 96 Megabits/sec, compared to 4 - 6 Megabits/sec on Ethernet.

100 VG AnyLAN is easily implemented either stand alone or in existing Ethernet networks. Additionally, 100 VG AnyLAN provides a high-speed migration path for Token Ring users and can be bridged to Asynchronous Transfer Mode (ATM) backbones when necessary.

IEEE 802.3
IEEE LAN protocol defining an implementation of the physical layer and MAC sublayer of the data link layer. IEEE 802.3 uses CSMA/CD access method at a variety of speeds over several different media transmission types, such as unshielded twisted pair. IEEE 802.3 is the basis of the Ethernet standard.

Extensions to the IEEE 802.3 standard specify implementations for Fast Ethernet. Physical variations of the original IEEE 802.3 specification include 10Base-2, 10Base-5, 10Base-F, 10Base-T, and 10Broad36. Physical variations for Fast Ethernet include 100Base-T, 100Base-T4, and 100BaseX.

IEEE 802.3Z
A supplement to Information Technology - Local and Metropolitan Area Networks - Part 3: Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Access Method and Physical Layer Specifications ? Media Access Control Parameters, Physical Layers, and Repeater and Management Parameters for 1,000 Mb/s Operation. Defines Carrier Sense Multiple Access with Collision Detection (CSMA/CD) Media Access Control (MAC) parameters and minimal augmentation of its operation, physical layer characteristics, repeater functions and management parameters for transfer of 802.3 and Ethernet format frames at 1,000 Mb/s.

The IEEE 802.3z standard was created to extend the 802.3 protocol to an operating speed of 1,000 Mb/s in order to provide a significant increase in bandwidth, while maintaining maximum compatibility with the installed base of CSMA/CD nodes, previous investment in research and development, and principles of network operation and management.

IEEE 802.5
IEEE LAN protocol defining an implementation of the physical layer and MAC sublayer of the link layer. 802.5 uses the token passing access method at 4 or 16 Mbps, over shielded twisted pair (STP) cabling. See also Token Ring.

IETF
Internet Engineering Task Force. The primary working body developing TCP/IP standards for the Internet.

ILEC
Incumbent Local Exchange Carrier. Refers to the primary existing central office carrier, as distinguished from new competitive carriers established after deregulation.

IMS
IP Multimedia Subsystem

INA
Information Not Available

Interoperability
The ability of equipment from multiple vendors to communicate using standardized protocols.

Inventory Management
Tracks and manages all service objects, software and assets of installed and "on-hand" services for efficient inventory, procurement, availability and reuse.

IP
Internet Protocol. The most important protocol upon which the internet is based, IP is a standard describing software that keeps track of internet addresses for different nodes, routes outgoing messages, and recognizes incoming messages. It allows a packet of data to traverse multiple networks on its way to its final destination. IP was originally developed by the US Dept of Defense as part of the development of DOD's ARPAnet network in the 1970's. IP works in conjunction with another protocol, TCP, and is usually referred to jointly as TCP/IP. IP is a connectionless protocol that operates at the networking layer (layer 3) of the OSI model.

IP is the Internet's most basic protocol. In order to function in a TCP/IP network, a network segment's only requirement is to forward IP packets. In fact, a TCP/IP network can be defined as a communication medium that can transport IP packets. Almost all other TCP/IP functions are constructed by layering atop IP. IP is documented in RFC 791, and IP broadcasting procedures are discussed in RFC 919.

IP is a datagram-oriented protocol, treating each packet independently. This means each packet must contain complete addressing information. Also, IP makes no attempt to determine if packets reach their destination or to take corrective action if they do not. Nor does IP checksum the contents of a packet, only the IP header.

IP provides several services:

Addressing: IP headers contain 32-bit addresses which identify the sending and receiving hosts. These addresses are used by intermediate routers to select a path through the network for the packet.
Fragmentation: IP packets may be split, or fragmented, into smaller packets. This permits a large packet to travel across a network which can only handle smaller packets. IP fragments and reassembles packets transparently.
Packet timeouts: Each IP packet contains a Time To Live (TTL) field, which is decremented every time a router handles the packet. If TTL reaches zero, the packet is discarded, preventing packets from running in circles forever and flooding a network.
Type of Service: IP supports traffic prioritization by allowing packets to be labeled with an abstract type of service.
Options: IP provides several optional features, allowing a packet's sender to set requirements on the path it takes through the network (source routing), trace the route a packet takes (record route), and label packets with security features.

IP Address
Internet Protocol address; identifies a network node. IP addresses are 32-bit numeric addresses, expressed as four fields separated by decimal points (e.g., 128.10.1.1). In an IP address, each of the four numbers separated by decimal points can be zero to 255. IP Addresses are site-dependent and usually assigned by a network administrator. IP address have five classes, three of which (A, B, and C) are primary. IP itself is a Layer 3 network protocol containing addressing information and some control information that allows datagrams to be routed across the transmission path.

IPsec
IP Security

IPT
IP Telephony

IPTV
IP television

IP-VPN
A Virtual Private Network implemented over the public Internet (IP). The term "virtual" implies that the connections between nodes are not dedicated, rather connections are established as needed between points. Further, "virtual private" means that private "tunnels" are established over a public network (the Internet). Tunneling involves the encapsulation of encrypted data inside IP packets. Additional security is provided through firewalls at sites that participate in the VPN. VPNs allow for the prioritization of network traffic to enhance performance.

IPX
Internetworked Packet eXchange. A Novell NetWare LAN based protocol that delivers data to nodes on the network and manages networks, routes and services. Software layers above IPX can use this protocol to deliver messages for a variety of LAN applications including databases, file services, e-mail, and print spooling. The successor to IPX is the NetWare Link Services Protocol (NLSP).

ISA
Industry Standard Architecture. A standard for connections of personal computer bus architecture.

ISDN
Integrated Services Digital Network. Telecommunication service that uses digital transmission and switching technology to provide voice and data communications on a bearer channel while sending signaling on a data channel. Capable of speeds from 57.6 K to 128 K, ISDN provides two data channels, each with its own phone number, making simultaneous voice and data possible.

ISDN provides access to several integrated networks, where services such as POTS, X.25 service, and Frame Relay, as well as many others, are accomplished via a single connection. ITU-T officially defines ISDN as "a network, in general evolving from a telephony integrated digital network, that provides end-to-end digital connectivity to support a wide range of services, including voice and nonvoice services, to which users have access by a limited set of standard multipurpose user network interfaces."

ISO
International Standards Organization. A worldwide federation of national standards bodies from over 110 countries, based in Geneva. Its mission is to promote the development of standardization and related activities throughout the world, with a view to facilitating the international exchange of goods and services, and to developing cooperation in the spheres of intellectual, scientific, technological and economic activity.

ISP
Internet Service Provider. A company that provides access to the Internet through modems, ISDN, T1s, etc.

ISV services
Services provided by independent software vendors (ISVs) in support of their licensed software products. Examples include IT consulting, systems and network design, software development, integration, education and training, business management, data extraction, and support and maintenance services.

ITU
International Telecommunications Union. An intergovernmental organization headquartered in Geneva, Switzerland, the ITU is an international organization within which governments and the private sector coordinate global telecom networks and services. With membership from virtually every government in the world, ITU specializes in three main activities: defining and adopting telecommunications standards, regulating the use of the radio frequency spectrum, and furthering telecommunications development around the world, particularly in developing countries.

The ITU was founded in 1865 and became a United Nations agency in 1947. The standardization functions were formerly performed by a group within the ITU called CCITT, but after a 1992 reorganization the CCITT no longer exists as a separate body.

ITV
Interactive TV

IXC
Inter-eXchange Carrier. A carrier providing services between exchanges, term usually means "interLATA carrier." AT&T, MCI, Sprint, and all the long-distance carriers are called inter-exchange carriers, in contrast to a local exchange carrier (LEC); also known as Other Common Carriers.

J2EE
Java 2 Platform, Enterprise Edition. The platform was known as Java 2 Platform, Enterprise Edition or J2EE until the name was changed to Java EE in version 1.5. Java EE is a widely used platform for server programming in the Java programming language. The Java EE Platform differs from the Standard Edition (SE) of Java in that it adds additional libraries which provide functionality to deploy fault-tolerant, distributed, multi-tier Java software, based largely on modular components running on an application server.

Jumper
A pair of wires that connects the outside plant connectors on the vertical side or main distribution frame (MDF) to the equipment blocks on the horizontal side or intermediate distribution frame (IDF).

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Kbps
Short for kilobits per second, a measure of data transfer speed. Modems, for example, are measured in Kbps. Note that one Kbps is 1,000 bits per second, whereas a KB (kilobyte) is 1,024 bytes. Data transfer rates are measured using the decimal meaning of K whereas data storage is measured using the powers-of-2 meaning of K. Technically, kbps should be spelled with a lowercase k to indicate that it is decimal but almost everyone spells it with a capital K.

KHz/kilohertz
One thousand hertz (Hz) or cycles per second.

L2F
Layer 2. A forwarding transport protocol that enables tunneling through the Internet for the establishment of virtual private networks.

L2TP
Layer 2 Tunneling Protocol. A transport protocol that enables tunneling through the Internet for the establishment of virtual private networks. This protocol was developed as a new standard for tunneling taking the best features of the Cisco L2F protocol and Microsoft's point-to-point tunneling protocol (PPTP).

LAN
Local Area Network. Generally, a network spanning a small geographic area, such as a floor or a building. LANs are often privately held by an enterprise, whereas WANs are usually furnished by a third party (e.g., phone company). Also, LANs are less error-prone than WANs, since the media covers less terrain. Timers and other facilities inherent in LAN components place definite restrictions on how far LANs can extend. Nowadays, LANs are primarily implemented using Ethernet, a transmission protocol that operates at layer 2 of the OSI model (Data Link) and carries the TCP/IP packets. Token Ring was a competitor to Ethernet, but is used less and less.

The following characteristics differentiate one LAN from another:

Topology: The geometric arrangement of devices on the network.
Protocols: The rules and encoding specifications for sending data. The protocols also determine whether the network uses a peer-to-peer architecture or client/server architecture.
Media: Devices can be connected by twisted-pair wire (UTP , STP), coaxial cables, or fiber optic cables. Some networks even use radio waves instead of cables.

LANE
LAN Emulation. Typically used in LANE over ATM.

LAPB
Link Access Procedure Balanced. The balanced mode, bit-oriented protocol, an enhanced version of HDLC. Used in X.25 packet-switching networks.

LAPD
Link Access Procedure, D-Channel. Protocol that operates at the data-link layer (level 2) of OSI. LAPD was derived from the LAPB protocol and is designed primarily to satisfy the signaling requirements of ISDN basic access (ISDN BRI). The D channel carries signaling information for circuit switching. LAPD is defined by ITU-T Recommendations Q.920 and Q.921.

LAP-F
Link Access Protocol - Frame Relay. Q-922 Framing.

Last mile
The cable connection between the CO and a subscriber. Every call has a "last mile" at each end; not a literal mile; a "last mile" might be only a hundred feet or it could be several miles.

LATA
The USA has about 200 Local Access and Transport Areas that roughly reflect a population densisty (California has 11, Wyoming has 1). LECs generally provide service within LATAs, IXCs generally across LATAs.

Layer
OSI reference model. Each layer performs certain tasks to move the information from the sender to the receiver. Protocols within the layers define the tasks for the networks.

Layer 1 VLAN (Port Based)
Layer 1 VLANs are the class of VLANs that the IEEE 802.1Q VLAN committee has approved so far. In this type of VLAN, VLAN members are switch ports, and all users attached to a switch port inherit the VLAN assigned to that port.

Note that a port can be in more than one VLAN.

Layer 2 VLAN (MAC Address Based)
MAC address-based VLANs permit users on the same link to belong to different VLANs. Although this type of VLAN is more complicated than the port-based VLAN, it is more flexible than the port-based VLAN.

In this case, each switch has to examine each arriving packet for its VLAN membership, and this involves more bookkeeping than the port-based VLAN. However, it provides a greater flexibility in defining VLAN membership, since members of a VLAN can be located anywhere in the network.

Layer 3 VLAN (protocol based)
Layer 3 VLANs operate at the layer 3 of the OSI reference model, and thus are defined according to layer 3 protocols, such as IP, IPX, AppleTalk, and DECnet. They are usually referred as virtual subnets.

Notice that, in the figure below, the users labeled X and Y belong to the two IP subnets, and can be servers to which members of the two subnets need to have access.

Virtual subnets are essentially implemented with proprietary protocols because they are not covered by the IEEE 802.1Q specification. The argument against its inclusion in the specification is that the IEEE 802 LAN specifications deal only with layer 2 issues, not layer 3.

Layout
A circuit has a path or layout. It proceeds from end to end through device ports and/or other circuits.

Leased Line
A permanent digital or analog telephone connection between two points set up by a telecommunications common carrier. Typically, leased lines are used by businesses to connect geographically distant offices. Unlike normal dial-up connections, a leased line is always active. The fee for the connection is a fixed monthly rate. The primary factors affecting the monthly fee are distance between end points and the speed of the circuit. Because the connection doesn't carry anybody else's communications, the carrier can assure a given level of quality.

For example, a T-1 channel is a type of leased line that provides a maximum transmission speed of 1.544 Mbps. You can divide the connection into different lines for data and voice communication or use the channel for one high speed data circuit. Dividing the connection is called multiplexing.

Increasingly, leased lines are being used by companies, and even individuals, for Internet access because they afford faster data transfer rates and are cost-effective if the Internet is used heavily.

LEC
Local Exchange Carrier. A carrier that provides local telephone service within a particular LATA, such as a Bell Operating company or independent telephone company (i.e., GTE). LECs are also referred to as Telcos.

LEN
Local Exchange Node.

Life Cycle Management
Includes software, systems and equipment that manages, designs and tests the creation and delivery of new services from the laboratory to retirement.

Line
A medium for transferring electrical energy between two points, such as a transmission line. A subscriber's line or a communication channel.

Line coding
A set of rules for mapping the voltage changes of a digital signal to usable 1 bit or 0 bits. Both transmitting and receiving devices need to employ the same line code to transfer meaningful information.

Line condition
The state of cable (usually copper wire pairs) accounting for such factors as varying wire gauges, wire temperature, and amount of bridge tap.

Line driver
That part of a modem, data service unit (DSU), or terminal unit that prepares signals to meet the required electrical specifications for transmission over a cable pair or network facility.

LMDS
Local Multipoint Distribution Service

Load coil
Inductors placed equally spaced over a local-loop circuit of a telephone system for the purpose of reducing line capacitance and increasing the distance of signal transmission.

Local loop
The physical wires that run from the subscriber's telephone set, PBX, or key telephone system, to the telephone company's central office.

Logical Protocol
Protocols are formal sets of rules governing how network devices exchange information. Logical protocols generally define data link details, such as packet-switching technology in X.25 or frame relay frames.

LSR
Label Switch Router. The core device that switches labeled packets according to precomputed switching tables. This device can also be a switch or a router. See MPLS.

MAC
Media-specific Access Control. Protocol for controlling access at the data link Layer 2.

Mac Address
This is a data link layer address associated with a particular network device; it is often called a hardware address or physical address because of its device association. Contrast this to an IP protocol address for instance, which is a network layer address that defines a network connection. You can set MAC addresses for devices by using the Table View window.

MAN
Metropolitan Area Network. A network that extends to 50-kilometer range, spanning an entire metropolitan area. Generally, MANs span a larger geographic area than LANs, but a smaller area than WANs. MANs operate at speeds from 1 Mbps to 200 Mbps, and provide an integrated set of services for real-time data, voice, and image transmission. MANs are usually characterized by very high-speed connections using fiber optical cable or other digital media.

MAU
Multistation Access Unit (also MSAU). MAUs are wiring concentrators to which Token Ring stations attach. MAUs connect network computers in a star topology, while retaining the logical ring structure. Patch cables are used to connect MAUs to adjacent MAUs, and network adapter/lobe cables are used to connect eight or more stations to a MAU. Up to 12 MAUs can be interconnected. MAUs include bypass relays for removing powered-down or failed stations from the ring in order to preserve the integrity of the ring.

One of the problems of the token-ring topology is that a single non-operating node can break the ring. The MAU solves this problem because it has the ability to short out non-operating nodes and maintain the ring structure.

Mbps
Megabits per second. An abbreviation for a unit of measurement, one million bits or one megabit per second, that expresses a data transfer rate. Also known as Mb/s. Often used as a standard measure of LAN transmission capacities.

MDF
Main Distribution Frame. A structure where phone lines are terminated. For example, a typical office building may have a single MDF located in the basement where all office phone lines terminate. Some large companies may employ multiple MDFs for redundancy; this is especially true in investment firms.

Also, the hardware component in the CO, which provides an interface between outside lines (subscriber lines and trunks) and the switching equipment. The vertical side of the mainframe where the outside plant cables are terminated on connectors/protectors. Also known as mainframe.

Media
In computer networks, media refers to the cables linking workstations together. There are many different types of transmission media, the most popular being twisted-pair wire (normal electrical wire ? STP, UTP), coaxial cable (the type of cable used for cable television), and fiber optic cable (cables made out of glass).

Megabit/Mb
One million bits. Also abbreviated as Mbit. For example, ten megabits = 10 Mb = ten million bits.

MIB
Management Information Base. A directory listing the logical names of all information resources residing in a network and pertinent to management of the network. MIB variables on network devices are polled using SNMP.

Microsoft SMS
The Systems Management Server^™ network discovery engine developed by Microsoft Corporation for Windows NT. The SMS discovery engine can interface either with an SMS site or specific domains in an SMS site. SMS uses an SQL database to store network information.

MNS
Managed Network Services. These are services typically offered by large consulting firms (IBM Global Services, EDS, Worldcom, AT&T Solutions, etc.) that provide design, installation and management of all or part of a customer's network.

Modem
A contraction of the words modulator and demodulator (MOdulator/DEModulator). A device consisting of a modulator and demodulator that can encode digital signals from a computer into analog signals that can be transmitted over analog lines, and vice versa. Used in a data link to modulate the carrier waveform at the transmitting end and demodulate the carrier at the receiving end. Also, equipment that converts signals into a suitable format in order to be transported over a network (traditionally over an analog telephone network, but may also include digital networks).

Modulation
Converting digital signals into analog signals. A modem is a MOdulator/DEModulator. Also, the process of varying the characteristics of the carrier wave (for example, amplitude, frequency, phase) in accordance with the instantaneous value or samples of a complex waveform, for the purpose of transmitting information.

Modulate
To change the characteristics of a carrier signal in order to carry information to be transmitted.

Monitoring
Encompasses the use of equipment and software that can capture, record and analyze details of service, equipment and traffic patterns. Normally used in conjunction with problem management.

MPEG
Motion Picture Experts Group. Group developing ISO standards for full motion video.

MPLS
Multi-protocol Label Switching. An evolving standard intended for Internet-based services. MPLS is designed to improve the speed and performance of IP network traffic. Using this technology, the router that initiates the traffic over the Internet adds a "label" to each packet. This label allows the traffic to be transported across the Internet backbone much more quickly to the destination. This speed increase is enabled because the traffic can be "switched" rather than "routed" through the Internet backbone. Usually this switching is done by ATM switches that carry the IP traffic. Switching is faster than routing because routing requires the examination of each packet followed by the consulting of routing tables and the determination of the "next" node to receive the packet. This lookup and determination is not required when "switching" traffic. MPLS is one of the technologies that is used in enabling more sophisticated VPN service offerings. It allows managed network service providers to enforce, manage and assure the quality of the VPN service.

MSDSL
Multirate SDSL.

MSO
Multiple System Operator. A cable company that operates more than one TV cable system.

MTBF
Mean Time Between Failures.

MTSO
Mobile Telephone Switching Office. A generic name for the main cellular switching center which supports multiple base stations.

Multicast
The broadcast of messages to a selected group of workstations on a LAN, WAN, or the Internet. For example, an IPub router might address a series of packets associated with a routing table update to a number of other routers in a LAN internetwork.

Multiplexer
A device that combines two or more signals into one bandwidth for transmission on a single transmission facility. Also known as a channel bank in North America.

MUX
Multiplex. Used to combine multiple signals (analog or digital) for transmission over a single line or media. A common type of multiplexing combines several low-speed signals for transmission over a single high-speed connection.

The following are examples of different multiplexing methods:

Frequency Division Multiplexing (FDM): Each signal is assigned a different frequency
Time Division Multiplexing (TDM): Each signal is assigned a fixed time slot in a fixed rotation
Statistical Time Division Multiplexing (STDM): Time slots are assigned to signals dynamically to make better use of bandwidth
Wavelength Division Multiplexing (WDM): Each signal is assigned a particular wavelength; used on optical fiber.

MVL
Multiple Virtual Lines. New local loop access technology developed by Paradyne. Designed and optimized for multiple concurrent services for residential, SOHO and small business markets. MVL transforms a single copper wire loop into multiple virtual lines supporting multiple and independent services simultaneously.

MVNO
Mobile Virtual Network Operator

Nanometer
One billionth of a meter. Common unit of measurement for wavelength.

NAP
Network Access Provider. The provider of the physical network that permits connection of service subscribers to NSPs.

NDIS
Network Driver Interface Specification. Used for all communication with network adapters. NDIS works primarily with LAN manager and allows multiple protocol stacks to share a single NIC.

NEBS
Network Equipment Building Systems. In Operations Support System (OSS), the Bellcore requirement for equipment deployed in a central-office environment. Covers spatial, hardware, crafts person interface, thermal, fire resistance, handling and transportation, earthquake and vibration, airborne contaminants, grounding, acoustical noise, illumination, Electromagnetic Certification (EMC), and electrostatic discharge (ESD) requirements. NEBS compliance is often required by telecommunications service providers such Bell operating companies (BOCs) and IXCs for equipment installed in their switching offices. NEBS defines everything from fire spread and extinguishability test to Zone-4 earthquake tests to thermal shock, cyclic temperature, mechanical shock, and ESD. Conforming to NEBS is not inexpensive.

NEP
Network Equipment Provider

NetFlow
Netflow is a feature of Cisco IOS® software used to monitor packet flows across a router. NetFlow Manager software allows the customer to collect the packet flow data and present it in a report format. NetFlow can be used to monitor a particular IP address in such a way as to actually see where that address originated, where it ended, and the time it took to get there and back. This information can be used to bill customers for differenciated services or QoS.

Network
An interconnected group of computers or terminals. A series of points connected by communications channels. The public switched network composed of the telephone lines normally used for dialed telephone calls.

Network management
A set of procedures, software, equipment and operations techniques designed to maintain and optimize efficiency.

Network mediation
Provides raw network data collection, aggregation and enhancement for upstream business-critical systems like billing, reporting and analytics.

NEXT
Near End CrossTalk. Crosstalk in which the interfering signal is traveling in the opposite direction as the desired signal.

NGN
Next-Generation Network

NGOSS
Next-Generation OSS

NIC
Network Interface Card. An adapter card that installs in network device slots, providing port access to other nodes on the network. Most NICs are designed for a particular type of network, protocol, and media, although some can serve multiple networks.

NID
Network Interface Device. Defined by the Federal Communications Commission (FCC) as the demarcation point on the U-loop, marking where the telco's service responsibility ends and the customer's responsibility begins. The NID provides U-loop circuit protection and a grounding point, as well as a line testing location. The NID is functionally invisible.

NMS
Network Management System. A windows-based system that is responsible for managing a network. Implements functions at the Network Management Layer using a network management protocol, such as SNMP.

NNI
Network to network Interface. In an ATM network, the interface between one ATM switch and another, or an ATM switch and a public ATM switching system.

Node
Term referring to any device that can access a network, such as computers, printers, and so on. Every node has a unique network address, sometimes called a Data Link Control (DLC) or Media Access Control (MAC) Address. For example, a network consisting of four computers and a printer would be a five node network.

Nominal span
The distance from a CO at which a one-milliwatt reference tone reaches an attenuation point of 35 dB.

Nonchannelized
Bandwidth that is not broken up into individual smaller channels.

NRP
Node Route Processor. Receives traffic from OC-3 interface ports via the node switch processor (NSP) ATM switch. The NRP reassembles the ATM cells into packets, processes (routes, bridges, and so on) the packets, segments the packets, and sends them back to the ATM switch for transmission out of another OC-3 interface.

NSN
Network Service Node.

NSP
Network Service Provider. A vendor, such as an ISP, local telephone company, CLEC or corporate LAN, that provides network services to subscribers.

NT
Network Termination. A term borrowed from ISDN definitions and means network termination of the network terminal. In ISDN, different NT numbers identify specific types of network terminations, such as voice/phone, data/phone, combined voice/data phone, and so on. NT1 is the ISDN device responsible for terminating the ISDN connection at the customer premises. B-NT1 refers to broadband ISDN NT1. NT2 is the active switching or call distribution equipment on the customer premises (for example, PBX, LAN, host computer, router).

NTU
Network Termination Unit. Equipment at the customer premises which terminates a network access point.

Nx64
Describes a continuous bit stream to an application at the Nx64 kbps rate.

OC3, OC12, OC48, OC192
Optical Carrier Level N. A unit of synchronous transmission capacity measured in 51.84 Mbps increments (about 16% more capacity than a DS3) and used with SONET (Synchronous Optical Network). An OC3 is a fiber optic line capable of 155 megabits per second (155,000K), while an OC48 is capable of 2400 megabits per second (2,400,000K).

Ocn
Optical Carrier level n signal. The fundamental transmission rate for SONET. For example, OC3 represents a transmission rate of about 155 Mbps.

Octet
A fixed group of information bits specified as the number of bits that a digital electronic device is designed to receive, store, transfer, or otherwise process. While byte length is generally 8 bits, byte length can vary. To avoid confusion over byte size, the ITU-T and others use the term "octet" to refer to an 8-bit byte. Note: These terms are not necessarily interchangeable; there are some subtle differences. The bits of a byte are normally numbered from 0 to 7. The bits of an octet are normally numbered from 1 to 8; therefore, "bit 3" refers to a different bit in an octet that in a byte. A second important, though subtle, difference is bit order. Bits are most often written from the highest numbered bit to the lowest numbered bit, but there are several circumstances where it is not true, so care must be exercised.

OE
Operating Environment

OEM
Original Equipment Manufacturer

OMSG
optical Multiservice Gateways

OMSN
Optical Multiservice Nodes

OS
Operating System

OSI
Open System Interconnection. The only internationally accepted framework of standards for communication between different systems made by different vendors. OSI was established by ISO (the International Standards Organization) with the goal of creating an open systems networking environment where any vendor's computer, connected to any network, can freely share data with any other computer system on that network or on a linked network. OSI is a model, not a set of protocols. The OSI model organizes network communication into 7 layers. Layers 1 through 3 deal with networked communications. In general, lower level protocols can encapsulate and deliver higher-level protocols. For example, IP traffic can be delivered through ATM, which can be delivered over a SONET transport facility. The 7 layers are:

Physical - Deals with the physical means of data communications (optical-electro-mechanical transport)
Data Link - Concerned with procedures and protocols for operating communications lines, including error detection and correction (SONET, Ethernet, DWDM, Token Ring)
Network - Determines how data is transferred between networks and computers. (ATM, Frame Relay, IP)
Transport - Defines rules for complete information exchange and manages end-to-end delivery within and between networks. (TCP)
Session - Concerned with dialog management
Presentation - Packages communications services, masking the differences of varying implementations and data formats
Application - Contains functions for particular application services such as file transfer (FTP), remote file access, and virtual terminals, among others

OSS
Operations Support System. Methods and procedures (automated or not) that directly support the daily operations of a telecommunications service provider. Telecom service providers can have hundreds of OSSs, among them, systems for order processing, service design, activation, testing, billing, and ongoing service assurance. Often these systems are referred to collectively as the OSS.

Overhead bits
A bit other than an information bit. Overhead bits are used primarily for error checking, framing, control, routing, and transport of tributary data.

Packet
A block of bits containing all or part of a data message that is carried across a network. Each packet contains information about itself, including its destination and source.

The definition of packet size varies, but it is generally defined as a maximum length. Data messages exceeding the defined packet size are sent in multiple packets.

In IP networks, packets are often called datagram.

PBX
Private Branch Exchange. A switch used to connect telephones to central office lines (trunks) and route calls within the customer's telephone system. A small version of the phone company's larger central switching office, a PBX is far more flexible than a central-office based Centrex. A PBX has more features and can change them easier and faster.

PCI bus
Peripheral Component Interconnect. A 64-bit local bus on a motherboard that is capable of transferring data at speeds up to 132 Mbps.

PCM
Pulse Code Modulation. A method of encoding an audio signal in digital format.

PCMCIA
Personal Computer Memory Card International Association. A standard for miniturized laptop expansion cards for modems, storage, and other devices. Often called PC cards.

Performance and reporting
Tools that support monitoring and performance issues when developing real-time or near-real-time information for trends and reporting.

Performance Analysis and Reporting
A suite of tools for measurement, analysis, trending and support to improve the performance of network, sub-networks and network elements. These tools deals more with performance issues than with "hard" faults.

Physical Protocol
Protocols are formal sets of rules governing how network devices exchange information. Physical protocols generally define the physical layer, or Layer 1 of the OSI reference model. The physical layer defines the mechanical, electrical, and physical interfaces to the network and aspects of the network medium. For example, V.35 and RS-232 are physical protocols that use serial media for transmission.

Ping
Packet Internet Groper. An ICMP echo message and its reply. One of the most useful network debugging tools available. Pings are used to discover network devices and to check their accessibility to the network. It takes its name from a submarine sonar search - you send a short sound burst and listen for an echo - a ping - coming back.

In an IP network, ping sends a short data burst - a single packet - and listens for a single packet in reply. This tests the most basic function of an IP network (delivery of single packet).

Pipe
A colloquial term for a communications channel transporting higher-speed digital signals.

Planning and Engineering
Includes the steps from network planning to construction (for example, budgeting, procurement, and line and service testing).

POP
Point of Presence. A local dialin point for an Internet Service Provider.

Port
A plug or receptacle on a PC, router, bridge, multiplexer, and so on. Ports allow external connections to be made to cards within a device.

Almost all PCs come with a serial RS-232C port or RS-422 port for connecting a modem or mouse and a parallel port for connecting a printer. On PCs, the parallel port is a Centronics interface that uses a 25-pin connector. SCSI (Small Computer System Interface) ports support higher transmission speeds than do conventional ports and enable you to attach up to seven devices to the same port.

See also: Logical Protocol and Physical Protocol.

POTS
Plain Old Telephone Service. Regular analog phone service, as opposed to ISDN, ADSL, and other digital phone services. Usually associated with residential or business subscribers. See PSTN.

POTS splitter
The component within a transceiver or modem containing a bandpass filter. The POTS splitter is where incoming analog POTS calls are split off to a separate line and other higher-frequency digital signals continue on through the transceiver.

PPP
Point-to-Point Protocol. Provides router-to-router and host-to-network connections over synchronous and asynchronous circuits. PPP is more stable than the older SLIP protocol and provides error checking features.

PPPoA
PPP over ATM. The transport of PPP frames over ATM.

PPPoE
PPP over Ethernet. The transport of PPP frames over Ethernet.

PPV
Pay-Per-View

PRI
ISDN Primary Rate Interface. An international communications standard that consists of twenty-three (T1) 64 Kbps B channels carrying voice and data, and one 64 Kbps D channel used for signaling (known as 23B+D).

Problem Management
Includes the ability to track and capture network problems that are reproducible, identified as defects, or anomalies with a product or service, and potentially to identify feature enhancements.

Processing
A set of administrative functions, invisible to a subscriber, that occupies an ADSL medium-speed subchannel.

Provisioning
The act of supplying a service to an end user.

PSC
Public Service Commission. State-level regulators of the local phone company in the United States.

PSTN
Public Switched Telephone Network. The normal dialup telephone system that is available for use by the general public. PSTN is the general term referring to the variety of voice telephone networks and services in place worldwide. In the U.S., it is sometimes called basic telephone service, or POTS.

PTO
Public Telecommunications Operator. Frequently called PTT (Post, Telephone, and Telegraph) in the past. These are the "telephone companies" in many countries outside of North America. Historically government owned, many have been, or are in the process of being, sold to private investors.

PTT
Postal Telephone & Telegraph. Provider of access services. A governmental agency in many countries.

PVC
Permanent Virtual Circuit. "Permanent" because the circuit is configured from router to router and remains as a part of the router's configuration, "virtual" because the connection uses no network resources unless users and applications send traffic (in the form of packets or cells or frames) down the circuit. This distinguishes a PVC from a dedicated leased line (T-1, T-3, etc.), which are always "on" and always consume network resources. See Frame Relay and ATM above.

QAM
Quadrature Amplitude Modulation. Modulation technique using variations in signal amplitude.

Quality Analysis
A process facilitated by software, reporting and analysis tools that can collect information and determine by defined measures, rules and agreements the quality provided to an end user.

QoS
Quality of Service. Measure of performance for a transmission system that reflects its transmission quality and service availability.

RADSL
Rate-Adaptive Digital Subscriber Line. A version of ASDL where the modems test the line at startup and adapt their operating speed to the fastest the line can handle.

Rate adaptation
The process of assessing line condition, determining the maximum bit rate a subscriber can receive, and then adjusting the asymmetrical transmission to realize that maximum downstream bit rate. DMT features rate adaptation.

RBOC
Regional Bell Operating Company. Local or regional telephone company that owns and operates telephone lines and switches in one of seven U.S. regions. The RBOC was created by the divestiture of AT&T. Also called BOC.

Receive
To convert incoming electromagnetic waves into visible or audible signals.

Repeater
A Layer 1 (physical layer) device that regenerates and propagates electrical signals between two network segments. These subnetworks may use different protocols or cable types. Repeaters increase the reach of a LAN connection.

Repeaters placed on a digital line may regenerate signals by resquaring an arriving digital signal, returning the signal to its original form, and transmitting the signal to its destination. This process reduces noise and the possibility of transmitting errors. A repeater cannot do the intelligent routing performed by bridges and routers.

RFI
Radio Frequency Interference. All computer equipment generates radio waves. Levels are regulated by the FCC.

Ring Topology
One of the three principal topologies used in LANs. All devices are connected to one another in the shape of a closed loop, so that each device is connected directly to two other devices, one on either side of it. Ring topologies are relatively expensive and difficult to install, but they are robust (one failed device does not usually make the entire network fail). See also bus topology and star topology.

RJ-11
Registered Jack-11, a four- or six-wire connector used primarily to connect telephone equipment in the United States. RJ-11 connectors are also used to connect some types of local-area networks (LANs), although RJ-45 connectors are more common.

RJ-45
Registered Jack-45, an eight-wire connector used commonly to connect computers onto local-area networks (LANs), especially Ethernets. RJ-45 connectors look similar to the RJ-11 connectors used for connecting telephone equipment, but they are somewhat wider.

Router
A highly intelligent device that connects like and unlike networks. Routers can act as the connection between separate corporate TCP/IP networks, or between a corporate network and a service provider's ATM or Frame Relay network. Routers vary in size from small devices that connect the corporate LAN to the Internet, to huge devices that connect offices of Internet backbone providers. Routers operate primarily at the bottom three layers of the OSI model, reaching up to layer 4 (transport) to ensure end-to-end reliability of data transfer.

Routers are similar to bridges, but provide additional functionality, such as the ability to filter messages and forward them to different places based on various criteria. There are also B-Routers, which combine routing and bridging functions.

RS-232C
Recommended standard-232C, a standard interface approved by the Electronic Industries Association (EIA) for connecting serial devices. In 1987, the EIA released a new version of the standard and changed the name to EIA-232-D. In 1991, the EIA teamed up with Telecommunications Industry Association (TIA) and issued a new version of the standard called EIA/TIA-232-E. Many people, however, still refer to the standard as RS-232C, or just RS-232.

Almost all modems conform to the EIA-232 standard, and most personal computers have an EIA-232 port for connecting a modem or other device. In addition to modems, many display screens, mice, and serial printers are designed to connect to a EIA-232 port. In EIA-232 parlance, the device that connects to the interface is called a Data Communications Equipment (DCE), and the device to which it connects (e.g., the computer) is called a Data Terminal Equipment (DTE).

The EIA-232 standard supports two types of connectors -- a 25-pin D-type connector (DB-25) and a 9-pin D-type connector (DB-9). The type of serial communications used by PCs requires only 9 pins, so either type of connector will work equally well.

Although EIA-232 is still the most common standard for serial communication, the EIA has recently defined successors to EIA-232 called RS-422 and RS-423. The new standards are backward compatible so that RS-232 devices can connect to an RS-422 port.

RSVP
Resource reSerVation setup Protocol. Provides priority data transmissions based on reservation protocol.

RT Remote Terminal. The local loop can be terminated at RTs. RTs are intermediate points closer to the customer premises used to improve service reliability.

RTP
Real-Time Transport Protocol. RFC 1889 -- Part of the ITU-T H.323 specification for streaming real-time applications.

RTU Remote Termination Unit. A device installed at the customer premises that connects to the local loop. Also referred to as the ATU-R.

SAM
Software Asset Management

S-HDSL
Single-Pair High-bit-rate DSL. Another term for HDSL2, which provides symmetrical transmission at 1.544 Mbps over a one-wire pair (versus HDSL's two wire pairs).

Satellite communications
Using orbiting satellites to relay transmissions from one earth station to one or more other earth stations.

SDH
Synchronous Digital Hierarchy

SDLC
Synchronous Data Link Control, a protocol used in IBM's SNA networks. SDLC is similar to HDLC, an ISO standard.

SDSL
Single-pair Digital Subscriber Line. An xDSL technology providing symmetrical bandwidth over a single-wire pair. Also referred to as symmetrical digital subscriber line.

Server
A device that "serves" multiple users on a LAN by allowing shared access to file systems, printers, electronic mail systems, and the like.

Service provider
Any business selling telecom services. Includes regulated (for example, telcos) and nonregulated businesses.

SI
System Integration or System Integrator

SID
NGOSS Shared Information/Data Model. SID provides the industry with a common vocabulary and set of information/data definitions and relationships used in the definition of NGOSS architectures. The SID is an object model, which uses UML, a data and process modelling language, to define entities and define the relationships between them, as well as the attributes and processes (termed methods) which make up the entity or object.

Signal
A variable electrical parameter, such as voltage or current, that is used to convey information through an electronic circuit or system.

Signaling
The use of electrical or electronic signals for communication. A method of conveying to the receiving end of a circuit that transmission of information is going to occur. The exchange of electrical information other than by speech. It is specifically concerned with the establishment and control of connections in a communication network.

Simplex mode
A communications circuit that is used to transmit information in one direction only.

SIP
SMDS Interface Protocol. A three-layer protocol implemented in SMDS networks. Also Session Initiation Protocol.

SLA
Service-Level Agreement

SLA Management
Software tools that provide a means to manage the service levels agreed to in a service-level agreement and that are capable of analyzing performance and generating defined reports.

SMDS
Switched Multimegabit Data Service. A public, connectionless, switched data service, offering access rates from sub-T1 through DS3. Though based largely on IEEE 802.6 standards, the SMDS service offerings available from many carriers do not implement all features and functions of IEEE 802.6. Generally used for LAN to LAN interconnections, SMDS provides reasonable security, multicast capability, network management through SNMP, and can meet a variety of needs.

SNA
Systems Network Architecture. A description of the logical structure and protocols that transmit information and control the operation on an IBM network.

SNMP
Simple Network Management Protocol. Originally developed for the Department of Defense. SNMP is the standard management protocol for TCP/IP networks.

SNMP is essentially a request-reply protocol running over UDP (ports 161 and 162). SNMP is an asymmetric protocol, operating between a management station (smart) and an agent (dumb). The agent is the device being managed - all its software has to do is implement a few simple packet types and a generic get-or-set function on its MIB variables. The management station presents the user interface. Simple management stations can be built with UNIX command-line utilities. More complex (and expensive) ones collect MIB data over time and use GUIs to draw network maps.

SNMP can now manage virtually any network type and has been extended to include non-TCP devices such as 802.1 Ethernet bridges. In November 1993, Cisco Systems announced that its internetwork routers would support Version 2 of SNMP and licensed the SNMP v2 spec to SNMP Research for development.

SOA
Service-Oriented Architecture is an architectural design pattern that concerns itself with defining loosely-coupled relationships between producers and consumers. While it has no direct relationship with software, programming, or technology, it is often confused with an evolution of distributed computing and modular programming.

SOHO
Small Office, Home Office

Sonet/SDH
Sonet stands for Synchronous Optical Network. SDH stands for Synchronous Digital Hierarchy. Sonet and SDH are the same thing, with slightly different "packaging" and terminology. Sonet is used in North America, SDH everywhere else. Sonet and SDH are a family of signals with transmission rates from 51.84 million bits per second up to 13.27 gigabits (thousand million) per second delivered over fiber optic media. Sonet and SDH are standards that allow internetworking of equipment from multiple vendors. Sonet and SDH define the physical aspects of the communication at a series of transmission rates. The base rate is 51.84 mbs, named OC-1 and STS-1. An OC-1/STS-1 can carry a DS-3 (see above) digital signal over fiber optic media. Remember that a DS-3 has a rate of 44.736 mbs. The next supported higher-level rate is OC-3/STS-3, which at 155 mbs can be channelized to carry 3 OC-1s/STS-1s or 3 DS-3s. This rate of 155 mbs is the starting rate for SDH. In SDH, this rate is named STM-1, and STM-1 is then the same thing as OC-3 and STS-3. Simple, right? The primary supported transmission rates above OC-3 are: OC-12 (STS-12, STM-4), OC-48 (STS-48, STM-16), and OC-192 (STS-192, STM-64).

Sonet and SDH are often deployed in a ring topology, with multiple fibers for redundancy. If one fiber fails, the redundant fiber takes over to maintain the connection. Sonet and SDH are also implemented as linear, point-to-point topologies, but underneath it's a ring consisting of two nodes. The nodes of a Sonet/SDH ring are devices called Add/Drop Multiplexors, or ADMs. Add/Drop refers to their ability to add and drop lower level circuit with transmission rates of DS-3, OC-3, etc. onto the higher-level transmission ring operating at OC-12, OC-48 or OC-192. Sonet and SDH are the high-speed backbone that carries voice and data traffic across cities, regions and the world. Sonet/SDH can transport ATM, Frame Relay, dedicated private lines, and IP traffic.

SP
Service Provider. A company that offers a variety of connections to the Internet. These connections may include corporate virtual private dialup network (VPDN) home gateways, wholesale dial for ISPs, leased-line access to corporations, and individual dial access. See also ISP.

SPID
Service Profile Identifiers

Star Topology
One of the three principal topologies used in LANs. All devices are connected to a central hub. Star networks are relatively easy to install and manage, but bottlenecks can occur because all data must pass through the hub. See also bus topology and ring topology.

SS7
Signaling System #7 (SS7) is a set of telephony signalling protocols which are used to set up the vast majority of the world's public switched telephone network telephone calls. The main purpose is to set up and tear down telephone calls.

SSM
Self Service Manager

STP
Shielded Twisted Pair. A pair of shielded insulated wires that are twisted around each other (spiral pattern) to reduce electrical noise. One wire carries the signal, while the other wire is grounded and absorbs signal interference. Commonly found in office buildings. STP cable contains an outside wrap-around conductor to reduce noise further.

STS-1
Synchronous Transport Signal 1. The fundamental SONET standard for transmission over optical fiber at 51.84 Mbps.

SU
Service User. The end user at the customer premises.

Subchannel
In asymmetrical transmission, the component channels that together compose a main channel.

Subnet
Refers to a method of partitioning TCP/IP networks by using a portion of the host address to represent a subset of the local network. The subnet partitioning method creates an additional address hierarchy layer. Dividing a network into subnets is useful for both security and performance reasons. On TCP/IP networks, subnets are defined as all devices IP addresses have the same prefix. For example, all devices with IP addresses that start with 100.100.100. would be part of the same subnet. IP networks are divided using a subnet mask.

Subscriber
An individual customer or user of telecommunications services or equipment. Also known as customer.

SVC
Switched Virtual Circuit. A virtual circuit that is dynamically established on demand and is torn down when transmission is complete. SVCs are used in situations where data transmission is sporadic. Called a switched virtual connection in ATM terminology.

SWC
Service Wire Center.

Switch
A Layer 2 device that filters and forwards packets (frames) to destination ports. Switches fall somewhere between repeaters (Layer 1) and routers (Layer 3), and therefore support any packet protocol. A switch has more internal bandwidth than a repeater and almost as much as a high-end router. Generally, switches can forward packets faster than a router, but not as fast as a repeater.

In telephony, a switch is a device that selects or connects circuit paths or interconnects circuits, lines, or trunks.

Symmetrical
Transmission where both the downstream and upstream bit rates are the same. Compare with asymmetrical.

System
Circuits with a hierarchy.

System integrator services
Encompasses services purchased from system integrators for planning throughout the implementation of information systems. Examples include IT consulting, systems and network design, software development, integration, education and training, business management and data extraction services.

T1/DS1
DS-1 stands for Digital Signal, level 1. In North America, a digital carrier for a DS1-formatted signal used to provide digital service over copper facilities. A T1 circuit differs from a DS-1 in certain respects. Nonetheless, "T1" is frequently used as a synonym for "DS1." T1/DS1 is a North American standard. The rest of the world has standardized on a signal that transmits 2.048 Mbps referred to as E1, which can be channelized into 32 DS-0s.

T1, as used in the United States, is a 1.544 Mbps digital carrier facility provided by long distance communications carriers for voice/data transmissions. Full T1 lines are divided into 24 channels (64 Kb each) known as DS-0s. 24 times 64,000 is actually 1,536,000, so 8,000 bits per second were used to provide information allow for error detection and signal control. T1 lines are a popular leased line option for businesses connecting to the Internet.

A circuit that transmits data as a DS1 (or DS3) signal is referred to as a private line or a dedicated circuit. These circuits are dedicated, always "on" and always available, and are referred to as circuit-switched, as opposed to signals that divide data into packets and transmit the packets over a carrier.

T3/DS3
In North America, a digital carrier for a DS3-formatted signal. Digital Signal, level 3 is a 44.736 Mbps signal that can be channelized into 28 DS-1s, each of which contains 24 DS-0s, or a total of 672 DS-0s within the DS-3. Outside of North America and Japan, the world has standardized on the E-3 signal, which transmits data at a rate of 34.368 Mbps. The E-3 is channelized into 16 E-1s, yielding 512 DS-0s (16 * 32).

TA
Terminal Adapter. Converts non-ISDN signals from the terminal equipment 2 (TE2) into an ISDN format. The connection between the TE2 and the TA is configured according to manufacturers' specifications and is called the R interface.

T-Connector
T-shaped device with two female and one male BNC connector. Used to connect a computer to thin coax cable.

TCP
Transmission Control Protocol. Developed as a part of the ARPAnet project, this is a transport layer (layer 4 in the OSI model) protocol. Whereas the IP protocol deals only with packets, TCP enables two hosts to establish a connection and exchange streams of data. TCP guarantees delivery of data and also guarantees that packets will be delivered in the same order in which they were sent. TCP is typically implemented as a family of Internet protocols collectively referred to as TCP/IP.

TCP/IP
Transmission Control Protocol/Internet Protocol. TCP/IP uses several protocols, the two main ones being TCP and IP, the two most widely used Internet protocols mistakenly thought of as one protocol. TCP/IP was developed by the Department of Defense to support the Internet and is now the de facto standard for transmitting data over networks. TCP is Layer 4, the transport layer, and provides for the reliable transmission of data. IP is Layer 3, and provides connectionless datagram service.

TDM
Time-Division Multiplexing. A process for simultaneous transmission of two or more information signals (voice, data, or video) over a single network link. Pieces or samples from each signal (bits or bytes) are interleaved into a single link transmission. The signals are reconstructed from the samples at the destination.

TDM bus
Time-Division Multiplexing Bus. A technique for transmitting a number of separate data, voice, or video signals simultaneously over one communications medium by quickly interleaving a piece of each signal one after another. Many of the Cisco access servers and routers are based on this architecture for the backplane of the device. The TDM bus provides a multigigabit data path for transmission between cards in the chassis.

Telco
A generic American term that refers to any telephone company.

Telnet
Virtual terminal protocol in the Internet suite of protocols. Allows the user of one host computer to log into a remote host computer.

TIMS
Transmission Impairment Measurement Set.

TMN
Telecommunications Management Network. A network management model defined in ITU-T specifications. This model is organizes networks into a series of layers in similar to OSI. These layers are:

Element management (EML) - Management of individual network elements or nodes
Network management (NML) -Management of the entire network
Service management (SML) - Management of services over the defined network
Business management (BML) - Management of business processes enabled by the services provisioned on the network

Once management is defined at the lower layers, additional management applications can be built on this foundation.

The TMN model applies directly to telecommunications service providers, while the OSI model applies to any network, but is most often associated with enterprise data networks.

Token Ring
A 4/16 Mbps local area network access mechanism and topology in which all stations actively attached to the bus listen for a broadcast token or supervisory frame. Stations wishing to transmit must receive the token before doing so. After a station finishes transmission, it passes the token to the next node in the ring.