xDSL Terms

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

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.


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.

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.

ADSL Forum The organization that develops and defines xDSL standards, including those affecting ADSL, SDSL, HDSL, and VDSL. On the Internet, visit at http://www.adsl.com/.

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.

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 http://www.atmforum.com for more info.


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.

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.


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.


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 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.

downstream In xDSL, the communications from the network towards the customer premises.

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.

DS1/DS3 Like T1 or T3 connections.

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.


Enterprise Local Exchange Carrier (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, and ELEC.

enterprise network A term for a widely dispersed, multifaceted telecommunications network for a particular purpose or organization; a term for all of an organization's telecommunications networking services and equipment.

Ethernet A LAN used to connect devices within a single building or campus at speeds up to 10 Mbps. Within the OSI model, Ethernet is defined at layer one (physical) and layer two (data link). Based on Carrier Sense Multiple Access/Collision Detection (CSMA/CD), Ethernet works by simply checking the wire before sending data. Sometimes two stations send at precisely the same time in which case a collision is detected and retransmission is attempted. See also 10BaseT.


Fast Ethernet A LAN used to connect devices within a single building or campus at speeds up to 100 Mbps. Within the OSI model, Fast Ethernet is defined at layer one (physical) and layer two (data link). Like Ethernet, Fast Ethernet uses CSMA/CD.

frame relay (FR) A high-speed packet-switched data communications service, similar to X.25. Frame relay is a leading contender for LAN-to-LAN interconnect services, and is well suited to the bursty demands of LAN environments. See also permanent virtual circuit and switched virtual circuit.


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


High bit-rate Digital Subscriber Line (HDSL) The oldest of the DSL technologies, HDSL continues to be used by telephone companies deploying T1 lines at 1.5 Mbps and requires two twisted pairs.


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.


Incumbent Local Exchange Carrier (ILEC) 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 the 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) ISDN provides standard interfaces for digital communications networks and is capable of carrying data, voice, and video over digital circuits. ISDN protocols are used worldwide for connections to public ISDN networks or to attach ISDN devices to ISDN-capable PBX systems (ISPBXs).

Developed by the International Telecommunications Union (ITU [previously the CCITT]), ISDN includes two user-to-network interfaces: basic rate interface (BRI) and primary rate interface (PRI). Note: See separate entries for basic rate interface and primary rate interface.

An ISDN interface contains one signaling channel (D-channel) and a number of information channels ("bearer" or B channels). The D-channel is used for call setup, control, and call clearing on the B-channels. It also transports feature information while calls are in progress. The B-channels carry the voice, data, or video information.

interface A point of connection between two systems, networks, or devices.

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 Service Provider (ISP) A telecommunications company that provides subscriber access to the Internet.

Internet access The physical telephone circuit connection between the subscriber and the nearest Internet access node.

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


kilobits per second (Kbps) A measure of bandwidth capacity or transmission speed. It represents a thousand bits per second.


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 originating end of a connection and the reception of that unit at the destination end.

Local Exchange Carrier (LEC) A local telephone company (either a Bell Operating Company [BOC] or an independent [e.g., GTE]) that traditionally had the exclusive, franchised right and responsibility to provide local transmission and switching services. Prior to divestiture, the LECs were called telephone companies or telcos. With the advent of deregulation and competition, LECs are now known as ILECs (incumbent LECs). This terminology delineates them from CLECs (competitive LECs) and ELECs (enterprise LECs). See CLEC and ELEC.

local area network (LAN) A data communications network covering a small area, usually within the confines of a building or floors within a building; a relatively high-speed computer communications network for in-building data transfer and applications. Common LAN protocols are Ethernet and Token Ring. See also CAN, MAN, WAN.

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.


megabits per second (Mbps) A measure of bandwidth capacity or transmission speed. It stands for a million bits per second.

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.


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 Operating Center (NOC) The NOC provides the customer's corporate IT staff with access to automated information regarding issues or network outages that affect the customer's teleworkers.


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 over the best available network connection, and then reassembled in their original order at the destination endpoint.

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.

packet switching A switching system that uses a physical communications connection only long enough to transmit a data message; data messages are disassembled into packets and reassembled at the receiving end of the communications link; packets may travel over many diverse communications links to get to the common endpoint. Packet switching is most often contrasted with circuit switching in data communications, where all data messages transmitted during a session are transmitted over the same path for the duration of the session. See also circuit switching .

permanent virtual circuit (PVC) A term found in frame relay and ATM networking in which a virtual connection between two fixed end-points is established through the network. See also switched virtual circuit .

plain old telephone service (POTS) This term commonly refers to standard telephony, as in placing and receiving telephone calls.

Point-to-Point Protocol (PPP) This protocol allows a computer to connect to the Internet with a standard dial-up telephone line and a high-speed modem and enjoy most of the benefits of a direct connection, including the ability to use graphical front ends such as a Mosaic and Netscape. PPP is considered to be better than SLIP, because it features error detection, data compression, and other elements of modem communications protocols which SLIP, the older Internet protocol, lacks. See also SLIP .


Rate Adaptive Digital Subscriber Line (RADSL) Using modified ADSL software, RADSL makes it possible for modems automatically and dynamically to adjust their transmission speeds. This often allows for good data rates for customers residing greater distances from the CO.

Regional Bell Operating Company (RBOC) There are seven RBOCs, each of which owns two or more BOCs (Bell Operating Companies). The RBOCs were carved out of the old AT&T/Bell System as a result of the divestiture of the Bell operating companies from AT&T at the end of 1983.

Regional Data Center (RDC) Client traffic from each central office in a region on its regional network, switches the traffic through its RDC, and then delivers the traffic to the corporate network over one or more high-speed wide area network (WAN) circuits. The WAN circuit terminates on a corporate router or switch.

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

router The device that connects multiple computer networks by reading OSI Layer 3 addressing on incoming and outgoing packets. Packet information is read, and the packets are then forwarded to the appropriate end station. While routers are like bridges, they work differently. Routers provide more functionality than bridges. For example, they can find the best route between any two networks, even if there are several different networks in between. Routers provide network management capabilities such as load balancing, partitioning of the network, use statistics, communication priority, and trouble shooting tools that allow network managers to detect and correct problems even in a complex network of networks. Given these capabilities, routers are often used in building wide area or enterprise wide networks. Some routers are protocol-dependent, and some are protocol-independent.


Single-line Digital Subscriber Line or Symmetric Digital Subscriber Line (SDSL) A modified HDSL software technology, SDSL is intended to provide 1.5 Mbps in both directions over a single twisted pair. However, the distance over which this can be achieved is less than 8,000 feet.

switched virtual circuit (SVC) A virtual circuit connection established across a network on an as-needed basis and lasting only for the duration of the transfer. It is the datacom equivalent of a dialed phone call. The specific path provided in support of the SVC is determined on a call-by-call basis and in consideration of both the end points and the level of congestion in the network. SVCs are used extensively in X.25 networks. SVCs also are provided for in Frame Relay network theory, but have not been implemented, because permanent virtual circuits (PVCs) provide the same function and are much less complex to provision. See also permanent virtual circuit .

synchronous transmission Data transmission using synchronization bytes, instead of start/stop bits, to control the transmission. In xDSL, video streams are considered to be synchronous in nature. See asynchronous transmission.


T1 A digital transmission link with a capacity of 1.544 Mbps. 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.

T3 A T3 line consists of 28 T1 lines or 44.736 million bits per second (commonly referred to as 45 Mbps). A T3 line can handle 672 voice conversations. T3 runs on fiber optic and is typically called FT3. See also T1.

telco A generic term for 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 independent telcos, GTE, SNET, and Sprint; in Europe, Asia and elsewhere, the term "telco" generally refers to the incumbent monopoly, but increasingly refers to competing local providers as well.

Transmission Control Protocol/Internet Protocol (TCP/IP) The TCP/IP is a networking protocol that provides communication across interconnected networks, between computers with diverse hardware architectures and various operating systems. TCP (Transmission Control Protocol) and IP (Internet Protocol) are only two protocols in the family of Internet protocols. Over time, however, "TCP/IP" has been used in industry to denote the family of common Internet protocols.

twisted pair (TP) A common form of copper cabling used for telephony and data communications. It consists of two copper lines twisted around each other; the twisting protects the communications from electromagnetic frequency and radio frequency interference. See also unshielded twisted pair .


UAWG . Universal ADSL Working Group The UAWG is a consortium of telecommunications service providers (including Covad), PC and network equipment manufacturers, and ADSL equipment manufacturers. These companies plan to develop a set of technical specifications to be submitted to international standard setting bodies for ADSL products. These specifications will describe ADSL products that will hopefully meet the price points and installation requirements necessary to promote high speed Internet access for the consumer market. The UAWG was initiated by Microsoft, Intel, and Compaq.

unshielded twisted pair (UTP) A cable with one or more twisted copper wires bound in a plastic sheath. Preferred method to transport data and voice to business workstations and telephones. Unshielded wire is preferred for transporting high speed data, because, at higher speeds, radiation is created. If shielded cabling is used, the radiation is not released and creates interference.

upstream In xDSL, the communications from the customer site up into the telecommunications network.


VDSL. Very high-rate Digital Subscriber Line The newest of the DSL technologies, VDSL can offer speeds up to 25 Mbps downstream and 3 Mbps upstream. Similar to SDSL, the gain in speed can be achieved only at short distances. These maximum speeds can be achieved only up to 1,000 feet. Sometimes also called broadband digital subscriber line (BDSL).


Wide Area Network (WAN) A data network typically extending a LAN (local area network) outside the building, over telephone common carrier lines to link to other LANs in remote buildings in possibly remote cities. A WAN typically uses common-carrier lines. A LAN doesn't. WANs typically run over leased phone lines--from an analog phone line to T1 (1.544 Mbps). The jump between a local area network and a WAN is made through a device called a bridge or a router.


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, MDSL, RADSL, SDSL, VDSL.