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10BaseT A 10-Mpbs Ethernet local area network that runs over twisted pair wiring. This network interface was originally designed to run over ordinary twisted pair (phone wiring) but is predominantly used with Category 3 or 5 cabling.

100BaseT A 100-Mbps local area network that maintains backward compatibility with 10BaseT networks running at 10 Mbps.

Access rate The transmission speed of the physical access circuit between the end user location and the local network. This is generally measured in bits per second. Also called "access speed."

Adapter card Circuit board or other hardware that provides the physical interface to a communications network; an electronics board installed in a computer that provides network communication capabilities to and from that computer; a card that connects the DTE to the network. Also called a "network interface card" (NIC). See also data terminal equipment and network interface card.

ADSL See Asymmetric Digital Subscriber Line

ADSL Forum The organization that develops and defines xDSL standards, including those affecting ADSL, SDSL, HDSL, and VDSL. On the Internet, visit at

Algorithm A specific procedure used to modify a signal. For example, the key to a digital compression system is the algorithm that eliminates redundancy.

American Standard Code for Information Interchange (ASCII) Assigns specific letters, numbers, and control codes to the 256 different combinations of 0s and 1s in a byte.

Analog A continuously varying signal or wave. As with all waves, analog waves are susceptible to interference which can change the character of the wave.

ASCII See American Standard Code for Information Interchange

Asymmetric Digital Subscriber Line (ADSL) A term for one-way T1 transmission of signals to the home over the plain old, single twisted-pair wiring already going to homes. ADSL modems attach to twisted pair copper wiring. ADSL is often provisioned with greater downstream than upstream rates (hence "asymmetric"). These rates are dependent on the distance a user is from the central office and may vary from as high as 9 Mbps to as low as 384 Kbps.

Asynchronous transmission Data transmission of one character at a time to the receiving device, with intervals of varying lengths between transmittals, and with start bits at the beginning and stop bits at the end of each character, to control the transmission. In xDSL and in most dial-up modem communications, asynchronous communications are often found in Internet access and remote office applications. See also synchronous transmission.

ATM See Asynchronous Transfer Mode

Asynchronous Transfer Mode (ATM) A protocol that packs digital information into 53-byte cells (5-byte header and 48-byte payload) that are switched throughout a network over virtual circuits. Standardized by the ITU-T in 1988 to create a Broadband Integrated Services Digital Network (B-ISDN). Its ability to accommodate multiple types of media (voice, video, data) makes it a likely player for full service networks based on ADSL and VDSL.

ATM Forum The organization tasked with developing and defining ATM standards. On the Internet, visit at for more info.

ATM25 ATM Forum defined 25.6Mbit/s cell based user interface based on IBM token ring network.

ATU-C and ATU-R ADSL Transmission Unit, Central or Remote: The device at the end of an ADSL line that stands between the line and the first item of equipment in the subscriber premises or telephone switch. It may be integrated within an access node.


B-ISDN (Broadband Integrated Services Digital Network) A digital network with ATM switching operating at data rates in excess of 1.544 or 2.048 Mbps. ATM enables transport and switching of voice, data, image, and video over the same infrastructure.

BDSL See Broadband DSL

Backbone The part of a communications network that handles the major traffic using the highest-speed, and often longest, paths in the network.

Bandwidth This is a reflection of the size or the capacity of a given transmission channel. In digital transmission, bandwidth is normally described in bits per second.

Bit A single unit of data, either a one or a zero, used in digital data communications. When discussing digital data a small "b" refers to bits, and a capital "B" refers to bytes.

Broadband A type of transmission that shares the bandwidth of a medium--such as copper or fiber optic cable--to carry more than one signal. Broadband facilities have a bandwidth (capacity) greater than a voice grade line of 3 kHz. Such a broadband facility--typically coaxial cable--may carry numerous voice, video and data channels simultaneously. Each "channel" will take up a different frequency on the cable. "Guardbands" (empty spaces) exist between the channels to make sure that each channel does not interfere with its neighbor. A coaxial CATV cable is the "classic" broadband channel. Simultaneously it carries many TV channels. Broadband cables are used in some office LANs. But more common are the baseband variety, which have the capacity for one channel only. Everything on that cable to be transmitted or received must use that one channel. That one channel is very fast, so each device needs only to use that high speed channel for only a little of the time. (The problem is getting on the channel.) See also baseband.

Broadband DSL (BDSL) Very high bit-rate DSL. The fastest xDSL technology, operating at data rates from 12.9 to 52.8 Mbps with corresponding maximum reach ranging from 4500 feet to 1000 feet of 24 gauge twisted pair. Same as VDSL.

Byte A compilation of bits, seven bits in accordance with ASCII standards and eight bits in accordance with EBCDIC standards.


CAP See Competitive Access Provider

CAP modulation (Carrier-less Amplitude/Phase modulation) Describes a version of QAM in which incoming data modulates a single carrier that is then transmitted down a telephone line. The carrier itself is suppressed before transmission (it contains no information, and can be reconstructed at the receiver), hence the adjective "carrier-less."

Carrier Serving Area (CSA) Area served by a LEC, RBOC or telco, often using Digital Loop Carrier (DLC) technology.

Central Office (CO) A circuit switch that terminates all the local access lines in a particular geographic serving area; a physical building where the local switching equipment is found. xDSL lines running from a subscriber’s home connect at their serving central office.

Channel A generic term for a communications path on a given medium; multiplexing techniques allow providers to put multiple channels over a single medium. See also multiplexer.

CO See central office

Competitive Access Provider (CAP) Now called Competitive Local Exchange Carrier (CLEC)

CPE See Customer Premises Equipment

Crosstalk Interference from an adjacent channel.

CSA See Carrier Serving Area

Customer Covad uses the term "customer" when referring to the employees and subscribers of Covad’s partners. Covad uses the term "partner" when referring to corporations and Internet Service Providers (ISPs) that have contracted with Covad for service.

Customer Premises Equipment (CPE) Equipment located either at the customer (ISP) or end user site.


Data eXchange Interface (DXI) An exchange protocol for interconnecting ATM devices.

Data Link Connection Indicator (DLCI) In Frame Relay, a value that identifies a logical connection.

Data Service Unit/Channel Service Unit. (DSU/CSU) The devices used to access digital data channels are called DSU/CSUs (Data Service Unit/Channel Service Units). At the customer’s end of the telephone connection, these devices perform much the same function for digital circuits that modems provide for analog connections. For example, DSU/CSUs take data from terminals and computers, encode it, and transmit it down the link. At the receive end, another DSU/CSU equalizes the received signal, filters it, and decodes it for interpretation by the end-user.

Dedicated line A transmission circuit that is reserved by the provider for the full-time use of the subscriber. Also called a "private line."

Dial up The process of initiating a switched connection through the network; when used as an adjective, this is a type of communication that is established by a switched-circuit connection.

Digital signal A signal that takes on only two values, off or on, typically represented by "0" or "1." Digital signals require less power but (typically) more bandwidth than analog, and copies of digital signals can be made exactly like the original.

Digital Signal Level 1 (DS1) A digital line at 1.544 Mbps (U. S.) carried on a T1 circuit.

Digital Signal Level 3 (DS3) A digital line at 44.736 Mbps (U. S.) carried on a T3 circuit.

Digital Subscriber Line (DSL) A general term for any local network loop that is digital in nature; technically, DSL equates to ISDN BRI, but this is decreasingly enforced terminology. DSL technology is available in several varieties. See also ADSL, HDSL, IDSL, RADSL, SDSL, VDSL, xDSL.

* ADSL Asymmetric Digital Subscriber Line.

* HDSL High bit-rate Digital Subscriber Line.

* IDSL ISDN Digital Subscriber Line.

* RADSL Rate Adaptive Digital Subscriber Line.

* SDSL Single-line Digital Subscriber Line or Symmetric Digital Subscriber Line.

* VDSL Very high-rate Digital Subscriber Line.

* xDSL A generic term for the suite of digital subscriber line (DSL) services.

Digital Subscriber Line Access Device (DSLAD) usually located at the end user site.

Digital Subscriber Line Access Multiplexer (DSLAM) The technical description of the Hitchhiker system. Also called "service access multiplexer." The DSLAM uses digital subscriber line (xDSL) and asynchronous transfer mode (ATM) technologies to deliver high speed data rates over the existing copper network. See also Hitchhiker, xDSL, and ATM. For an extensive description of the Hitchhiker system, refer to The Hitchhiker System manual from Diamond Lane Communications Corporation.

Discrete Multi-Tone (DMT) Describes a version of multicarrier modulation in which incoming data is collected and then distributed over a large number of small individual carriers, each of which uses a form of QAM modulation. DMT creates these channels using a digitial technique known as Discrete Fast-Fourier Transform. DMT is the basis of ANSI Standard T1.413.

Discrete Wavelet Multi-Tone (DWMT) Describes a version of multicarrier modulation in which each carrier is created by Wavelet transform rather than Fourier Transform.

DLCI See Data Link Connection Indicator

DMT See Discrete Multi-Tone

DNS See Domain Name System

Domain Name System (DNS) The protocol used for assigning text addresses (such as for specific computers and computer accounts on the Internet.

* Downstream In xDSL, the communications from the network towards the customer premises.
* Upstream In xDSL, the communications from the customer site up into the telecommunications network.

DS1 See Digital Signal Level 1

DS3 See Digital Signal Level 3

DSL See Digital Subscriber Line

DSLAD See Digital Subscriber Line Access Device

DSU/CSU See Data Service Unit/Channel Service Unit.

DSLAM See Digital Subscriber Line Access Multiplexer

DWMT See Discrete Wavelet Multi-Tone

DXI See Data eXchange Interface


E1 A communications circuit for voice and data transmission utilized primarily in Europe operating at 2.054 Mbps equal to 30 separate 64 Kbps channels.

E3 European designation for T3: A point-to-point communications circuit service created operating at 44 Mbps equal to 672 channels of 64 Kbps.

ELEC See Enterprise Local Exchange Character

Enterprise Local Exchange Character (ELEC) Generally, a larger corporation or organization (e.g., university) that operates as its own local exchange carrier (LEC) as a means of obtaining better carrier rates for itself, possibly selling services to others for a profit. ELECs could be considered a subset of CLECs. See also LEC, ILEC, ELEC, and enterprise network. A term for a widely dispersed, multifaceted telecommunications network for a particular purpose or organization; a term for of an organization’s telecommunications networking services and equipment.

Ethernet A protocol used to connect different devices.


Frame relay A high-speed packet switching protocol used in wide area networks (WANs), often to connect local area networks (LANs) to each other, with a maximum bandwidth of 44.725 megabits per second.

Frequency The number of oscillations in an alternating current that occur within one second, measured in Hertz (Hz).


Gbps (Gigabits Per Second) A measure of bandwidth capacity or transmission speed. It represents a billion bits per second.

Gigabyte 1,000,000,000 bytes, or 1,000 megabytes (see byte).


HDSL See High bit-rate Digital Subscriber Line

High bit-rate Digital Subscriber Line (HDSL) An xDSL technology that is symmetric, providing the same amount of bandwidth both upstream and downstream. Due to its speed-1.544 Mbps over two copper pairs and 2.048 Mbps over three copper pairs-telcos sometimes deploy HDSL as an alternative to T1/E1.

High Speed Serial Interface (HSSI) A physical port to interface devices.

HSSI See High Speed Serial Interface

HTML See HyperText Markup Language

HTTP See HyperText Transfer Protocol

Hypertext Documents or other information with embedded links that enable a reader to access tangential information at specific points in the text.

HyperText Markup Language (HTML) The computer language used to create hypertext documents, allowing connections from one document or Internet page to numer-ous others. HTML is the primary language used to create pages on the World Wide Web.

HyperText Transfer Protocol (HTTP) The first part of an address (URL) of a site on the Internet, signifying a document written in Hypertext Markup Language (HTML).


IDSL See ISDN Digital Subscriber Line

ILEC See Incumbent Local Exchange Carrier

Incumbent Local Exchange Carrier (ILEC) e.g. Bell Altantic, Southwestern Bell or other RBOCs. A new term that describes traditional local telephone companies that, prior to deregulation of the telephone industry, had the exclusive right and responsibility to provide local telephone service. ILEC delineates these service providers from new competitive providers (CLECs) and enterprise providers (ELECs). The term "local exchange carrier" (LEC) is used as the generic term for all three. See LEC, CLEC, and ELEC.

Integrated Services Digital Network (ISDN) A digital subscriber line network with circuit and packet switching capabilities for voice and data communications. Typycally run at "2B+D" with two 64 Kbps data channels and a 16 Kbps control channel.

Internet The world’s largest computer network. The Internet originated from a research effort initiated by the U.S. Government and was initially used to connect defense contractors and U.S. universities. Today, its nature is more commercial, and it is becoming the preferred method of linking businesses and individuals’ computers to one another.

Internet protocol (IP) The standard signaling method used for all communication over the Internet.

Internet Service Provider (ISP) An organization offering and providing Internet services to the public and having its own computer servers to provide the services offered.

Intranet A local network, for example, an office network, where there are internal web servers accessible to computers in the office, but not accessible from outside the company. Many Intranets are protected from exterior access by various security devices, like firewalls.

ISDN See Integrated Services Digital Network

ISDN Digital Subscriber Line (IDSL) IDSL provides up to 144-Kbps transfer rates in each direction and can be provisioned on any ISDN capable phone line. Unlike ADSL and other DSL technologies, IDSL can be deployed regardless of the distance the user is from the central office.

ISP See Internet Service Provider


Joint Photographic Experts Group (JPEG) A committee formed by the International Organization of Standardization to set standards for digital compression of still images. Also refers to the digital compression standard for still images created by this group.

JPEG See Joint Photographic Experts Group


Kbps Abbreviation for Kilobits per second, or thousands of bits per second.

Kilobit One thousand bits (see bit).

Kilobyte One thousand bytes (see byte).


LAN See Local Area Network

Latency A measure of the temporal delay. Typically, in xDSL, latency refers to the delay in time between the sending of a unit of data at the end of a connection and the reception of that unit at the destination end.

LEC See Local Exchange Carrier

Local Area Network (LAN) A network connecting a number of computers to each other or to a central server so that the computers can share programs and files.

Local Exchange Carrier (LEC) One of the new U.S. telephone access and service providers that have grown up with the recent U.S. deregulation of telecommunications.

Local loop A generic term for the connection between the customer’s premises (home, office, etc.) and the provider’s serving central office. Historically, this has been a wire connection; however, wireless options are increasingly available for local loop capacity. Also colloquially referred to as "the last mile" (even though the actual distance can vary).

Long distance The communication of information to a destination outside the local calling area. Also called "long haul" traffic.


Mbps Abbreviation for Megabits per second, or millions of bits per second.

Megabit One million bits.

Minimum Point Of Entry (MPOE) The ILEC demarcation port at the End User or Customer premises.

Modem (MOdulator/DEModulator) Equipment that converts digital signals to analog signals and vice versa. Modems are used to send data signals (digital) over the telephone network, which is usually analog. The modem modulates the 1s and 0s into tones that can be carried by the phone network. At the other end, the demodulator part of the modem converts the tones back into digital 1s and 0s.

Moving Pictures Experts Group (MPEG) A committee formed by the ISO to set stan-dards for digital compression of full-motion video. Also stands for the digital compression standard created by this committee.

MPEG See Moving Pictures Experts Group

MPOE See Minimum Point Of Entry

Multicast The transmission of information over the Internet to two or more users at the same time.

Multiplexing Transmitting multiple signals over a single communications line or computer channel. The two common multiplexing techniques are frequency division multiplexing, which separates signals by modulating the data onto different carrier frequencies, and time division multiplexing, which separates signals by interleaving bits one after the other.


NAP See Network Access Provider

Narrowband A designation of bandwidth less than 56 kilobits per second.

NCC See Network Control Center

Network Access Provider (NAP) Another name for the provider of networked telephone and associated services, usually in the U.S.

Network Control Center (NCC) The Network Management and Network Operations functions.

Network Interface Card (NIC) The circuit board or other form of computer hardware that serves as the interface between a computer (or other form of data terminal equipment) and the communications network; in ADSL, a common NIC is an Ethernet NIC, which serves as the interface to the ADSL modem from the computer. See also adapter.

Network Interface Device (NID) A device that terminates a copper pair from the serving central office to the user’s destination. The NID is typically found installed on the exterior premises of the destination location.

Network Operations Center (NOC) The Network Management and Network Operations functions.

NIC See Network Interface Card

NID See Network Interface Device

NOC See Network Operations Center


Packet A sub-unit of a data stream; a grouping of information that includes a header (containing information such as address destination) and, in most cases, user data.

Partner Covad uses the term "partner" when referring to corporations and Internet Service Providers (ISPs) that have contracted with Covad for service. Covad uses the term "customer" when referring to the employees and subscribers of these partners.

Packet Competitive Local Exchange Carrier (PCLEC) An LEC coined this term to align itself with the rapid innovation and service deployment in the PC industry, and strives to provide new services at speeds and customer satisfaction levels not found in the current telecommunications industry.

Packet switched network A network that does not establish a dedicated path through the network for the duration of a session but, instead, transmits data in units called packets in a connectionless manner. Data streams are broken into packets at the front end of a transmission, sent the best available network connection, and then reassembled in their original order at the destination endpoint.

PCLEC See Packet Competitive Local Exchange Carrier

Peripheral An external device that increases the capabilities of a communication system.

Plain Old Telephone Service (POTS) i.e. an analog phone line - what you have in your home today.

Point Of Presence (POP) The physical point of connection between a data network and a telephone network.

POP See Point Of Presence

POTS See Plain Old Telephone Service

Private Virtual Circuit (PVC) A logical connection in the ATM cloud.

PVC See Private Virtual Circuit


RADSL See Rate-Adaptive Digital Subscriber Line

Rate-Adaptive Digital Subscriber Line (RADSL) An xDSL technology that adjusts dynamically to varying lengths and qualities of twisted-pair local access lines, by testing the line at start up and adapting the connection speed to the fastest the line can handle. R-ADSL makes it possible to connect over different lines at varying speeds.

RJ-11 A phone jack commonly used in phones, modems and fax machines.

RJ-45 An 8 pin connector used to attach data transmission devices to standard telephone wiring. Commonly used in 10BaseT connections.

Router The central switching device in a packet-switched computer network that directs and controls the flow of data through the network.


SCSI See Small Computer System Interface

SDSL Symmetric Digital Subscriber Line

Small Computer System Interface (SCSI) [pronounced "scuzzy"] A type of interface between computers and peripherals that allows faster communication than most other interface standards, often used to connect PCs to external disk drives.

Symmetric Digital Subscriber Line (SDSL)
An xDSL technology that supports symmetrical T1/E1 transmissions, with two exceptions. It uses a single copper-pair wire, and it has a maximum operating range of 10,000 feet, and operates over POTS lines. SDSL provides a constant, reliable and cost-efficient connection between the ISP and the end-user. Within its distance limitation, SDSL is capable of accommodating applications that require identical downstream and upstream speeds, such as videoconferencing or collaborative computing.


T Carrier A hierarchy of digital circuits, designated T1, T2, T3, and T4.

T1 A 1.544 Mbps digital circuit. T1 uses two pairs of normal twisted wires, the same as found in most residences. T1 normally handles 24 voice conversations, each one digitized at 64 Kbps. But, with more advanced digital voice encoding techniques, it can handle more voice channels. T1 is a standard for digital transmission in the United States. T1 lines are used to connect networks across remote distances. Bridges and routers are used to connect LANs over T1 networks. There are faster services available. T1 links can often be connected directly to new PBXs and many new forms of short haul transmission, such as short haul microwave systems. The same as DS1.

T2 A 6.312 Mbps digital circuit, equal to four T1 circuits.

T3 A 44.736 Mbps digital circuit, equal to 28 T1 circuits. A T3 line can handle 672 voice conversations. T3 runs on fiber optic and is typically called FT3. The Same as DS3. See also T1.

T4 A 274.176 Mbps digital circuit, equal to 178 T1 circuits.

TCP/IP See Transmission Control Protocol/Internet Protocol

Telco The local telephone company operator in a given area. In the U.S., the major telcos are the seven regional Bell operating companies and the leading independents, GTE, SNET, and Sprint.

Telecommuting The practice of using telecommunication technologies to facilitate work at a site away from the traditional office location and environment. teleconference - Interactive, electronic communication among three or more people at two or more sites. Includes audio-only, audio and graphics, and video-conferencing.

Terabyte 1,000,000,000,000 bytes, or 1,000 gigabytes (see Byte).

Transmission Control Protocol/Internet Protocol (TCP/IP) A method of packet-switched data transmission used on the Internet. The protocol specifies the manner in which a signal is divided into parts, as well as the manner in which "address" information is added to each packet to ensure that it reaches its destination and can be reassembled into the original message.

TT Trouble Ticket.

Twisted pair The set of two copper wires used to connect a telephone customer with a switching office, loosely wrapped around each other to minimize interference from other twisted pairs in the same bundle.  Synonymous with 2-wire line.


UNI See User to Network Interface

Uniform Resource Locator (URL) A text-based address used to identify specific resources on the Internet, such as web pages. URLs are arranged in a hierarchical form that specifies the name of the server on which a resource is located (such as and the name of the file on that server (

Universal Serial Bus (USB) A computer interface with a maximum bandwidth of 1.5 Megabytes per second used for connecting computer peripherals such as printers, keyboards and scanners.

Upstream See Downstream/Upstream

URL See Uniform Resource Locator

USB See Universal Serial Bus

User to Network Interface (UNI) In ATM and Frame Relay networks, the interface between an end system (such as a router) and a switch.


Variable Bit Rate (VBR) A data transmission that can be represented by an irregular grouping of bits or cell payloads followed by unused bits or cell payloads.

VBR See Variable Bit Rate

Very high bit-rate DSL (VDSL) The fastest xDSL technology, operating at data rates from 12.9 to 52.8 Mbps with corresponding maximum reach ranging from 4500 feet to 1000 feet of 24 gauge twisted pair. Same as BDSL.

VDSL See Very high bit-rate DSL

V.35 A physical port to network interface devices.


WAN See Wide Area Network

Wide Area Network (WAN) Private network facilities, usually offered by public telephone companies but increasingly available from alternative access providers (sometimes called Competitive Access Providers, or CAPs), that link business network nodes.


xDSL A generic term for the suite of digital subscriber line (DSL) services, where the "x" can be replaced with any of a number of letters. See also DSL, ADSL, HDSL, IDSL, RADSL, SDSL, VDSL.




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