DSL Glossary

ADSL Asymmetric Digital Subscriber Line Modems attached to twisted pair copper wiring that transmit from 1.5 Mbps to 9 Mbps downstream (to the subscriber) and from 16 kbps to 800 kbps upstream, depending on line distance.

ATM Asynchronous Transfer Mode This high speed network protocal is composed of 53 byte "cells" having 5 byte headers and 48 byte payloads. Because of its short packet length, it is especially good for real time voice and video.

ATU-C ADSL Termination Unit - Central Office - The device at the end of an ADSL line that stands between the line and the first item of equipment in the telephone switch. It may be integrated within an access node.

ATU-R ADSL Termination Unit - Remote The device at the end of an ADSL line that stands between the line and the first item of equipment in the subscriber's premises. It may be integrated within an access node.

AWG American Wire Gauge A measure of the thickness of copper, aluminum and other wiring in the U.S. and elsewhere. Copper cabling typically varies from 18 to 26 AWG. The higher the number, the thinner the wire. The thicker the wire, the less suceptible it is to interference. In general, thin wire cannot carry the same amount of electrical current the same distance that thicker wire can.

BERT Bit Error Rate Test A test that reflects the ratio of errored bits to the total number transmitted. Usually shown in exponential form (10^-6) to indicate that one out of a certain number of bits are in error.

bps Bits Per Second - A measurement of transmission speed

BRI Basic Rate Interface This is an Integrated Services Digital Network (ISDN) interface typically used by smaller sites and customers. This interface consists of a single 16 Kbps Data (or "D") channel plus 2 Bearer (or "B") channels for voice and/or data. Also known as Basic Rate Access, or BRA

BRIDGE TAP - an accidental connection of another local loop to the primary local loop. Generally it behaves as an open circuit at DC, but becomes a transmission line stub with adverse effects at high frequency. It is generally harmful to DSL connections and should be removed. Extra phone wiring within one's house is a combination of short bridge taps. A POTS splitter isolates the house wiring and provides a direct path for the DSL signal to pass unimpaired to the ATU-R modem.

CAP - Carrierless Amplitude - 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 "carrierless."

CATV - Cable TV

CBR - Constant Bit Rate

CCITT - Consultative Committee for International Telegraph and Telephone

CLEC - Competitive Local Exchange Carrier

CO - Central Office 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 subscriberís home connect at their serving central office.

CODEC - An abbreviation for coder/decoder. Specifically it converts a voice grade analog signal to u-law or A-law encoded samples at an 8KHz sampling rate. DSL bypasses the CODECs at the central office by separating the frequencies in a POTS splitter and passing the DSL signal to a DSLAM, the DSL equivalent of a CODEC.

CPE - Customer Premise (or Provided) Equipment A wide range of customer-premises terminating equipment which is connected to the local telecommunications network. This includes telephones, modems, terminals, routers, settop boxes, etc.

CSU - Channel Service Unit

DCE - Data Communication (or Circuit-Terminating) Equipment

DMT - Discrete Multi-tone

DSL - Digital Subscriber Line - Modems on either end of a single twisted pair wire that delivers ISDN Basic Rate Access.

DSLAM - Digital Subscriber Line Access Multiplexer

DSU - Data Service Unit A digital interface device that connects end user data communications equipment to the digital access lines, and which provides framing of sub-64Kbps customer access channels onto higher rate data circuits. A DSU may be combined with a CSU into a single device called a CSU/DSU. See Channel Service Unit/Data Service Unit.

DTE - Data Terminal (or Termination) Equipment Typically the device that transmits data such as a personal computer or data terminal.

ECHO SUPPRESSOR/ECHO CANCELLER - These are active devices used by the phone company to suppress positive feedback (singing) on the phone network. They work by predicting and subtracting a locally generated replica of the echo based on the signal propagating in the forward direction. Modems deactivate these devices by sending the 2100Hz answer tone with 180 phase reversals every 450msec at the beginning of the connection.

FDM - Frequency Division Multiplexing

FTTC - Fiber To The Curb - Network where an optical fiber runs from the telephone switch to a curbside distribution point close to the subscriber where it is converted to copper pair.

FTTH - Fiber To The Home - Network where an optical fiber runs from the telephone switch to the subscriber's premises.

HDSL - High bit-rate Digital Subscriber Line - Modems on either end of one or more twisted wire pair that deliver T1 speeds. At present, this requires two lines.

HFC - Hybrid Fiber-Coax

IEC - Inter-Exchange Carrier

ISDL - ISDN Digital Subscriber Line - Uses ISDN transmission technology to deliver data at 128 kbps in an IDSL modem bank connected to a router.

ISDN - Integrated Services Digital Network - Gives a user up to 56 kbps of data bandwidth on a phone line that is also used for voice, or up to 128 kbps if the line is only used for data.

ISO - International Organization for Standards

ISP - Internet Service Provider - An entity that provides commercial access to the Internet. These can range in size from someone operating dial-up access with a 56 kilobit line and several dozens of customers to providers with multiple pops in multiple cities and substantial backbones and thousands or even tens of thousands of customers.

ITU - International Telecommunications Union

IXC - Inter-exchange Carrier - Post-1984 name for long distance phone companies in the United States. AT&T is the largest, followed by MCI and Sprint, but several more small IXCs exist.

Kbps - Kilobits Per Second

LATA - Local Access and Transport Area - This was created by the 1984 divestiture and defines the geographic area over which the LEC may provide toll calls. The area is often smaller than that covered by a long distance area code. Even though ten or twenty LATAs are normally to be found within the territory of a LEC, the LEC may not provide calls that cross LATA boundaries. Such inter-LATA traffic is the exclusive domain of the IXC.

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

LOADING COIL - A device used to extend the range of a local loop for voice grade communications. They are inductors added in series with the phone line which compensate for the parallel capacitance of the line. They benefit the frequencies in the high end of the voice spectrum at the expense of the frequencies above 3.6KHz. Thus, loading coils prevent DSL connections.

LOCAL LOOP - A pair of wires, moderately twisted for the entire length between the telephone company's end office and the user premises (the common telephone set) form a loop, so it is referred to as the local loop. This loop provides a user with access to the global telecommunications infrastructure that is installed all over the world. The local loop has been historically designed to provide voice grade audio service. The circuit is powered from the central office with 48V (open circuit voltage) limited in current to a value somewhat higher than 20mA. This current is used for signaling phone access, burning off moisture, breaking through metalic oxides caused by corrosion, and powering a carbon microphone. The original telephone equipment contained no active electronics. The actual wiring of the local loop may be considered to be a lossy transmission line. DSL uses whatever frequencies will propagate on this line for purposes of digital data transmission. T1 modulation (alternate mark inversion) has been doing this for years. DSL extends the capability by using modern technology to increase the data rates and distances spanned.

Mbps - Megabits Per Second

MDF - Main Distribution Frame

MODULATION - Is a prescribed method of encoding digital (or analog) signals on a different waveform (the carrier signal). Once encoded, the original signal may be recovered by an inverse process, demodulation. Modulation is performed to adapt the signal to a different frequency range (and medium) than that of the original signal.

MVL - Multiple Virtual Line

NAT - Network Address Translation is the translation of an Internet Protocol address (IP address) used within one network to a different IP address known within another network. One network is designated the inside network and the other is the outside. Typically, a company maps its local inside network addresses to one or more global outside IP addresses and unmaps the global IP addresses on incoming packets back into local IP addresses. This helps ensure security since each outgoing or incoming request must go through a translation process that also offers the opportunity to qualify or authenticate the request or match it to a previous request. NAT also conserves on the number of global IP addresses that a company needs and it lets the company use a single IP address in its communication with the world.

NEBS - Network Equipment Building Standards

NEXT - Near-end Crosstalk - Interference between pairs of lines at the telephone switch end.

NID - Network Interface Device A device that terminates copper pair from the serving central office at the userís destination and which is typically located outside that location.

PCM - Pulse Code Modulation

POP - Point of Presence - A node of an ISP containing a DSU-CSU, terminal server and router and sometimes one or more hosts, but no network information center or network operations center.

POTS - Plain Old Telephone Service - Basic voice service available in residences throughout the United States.

PPP - Point to Point Protocol

PRI - Primary Rate Interface
This is an Integrated Services Digital Network (ISDN) interface typically used by larger customers. This interface consists of a single 64 Kbps Data (or "D") channel plus 23 or 30 Bearer (or "B") channels for voice and/or data. Also known as Primary Rate Access, or PRA.

PSTN - Public Switched Telephone Network

PTT - Postal, Telegraph and Telephone - Generic European name usually used to refer to state-owned telephone companies.

PVC - Permanent Virtual Circuit - Connection-oriented circuit that may be set up by software between any two nodes of a switched network.

QAM - Quadrature Amplitude Modulation

QoS - Quality of Service

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

RBOC - Regional Bell Operating Company - One of the seven U.S. telephone companies that resulted from the break up of AT&T

SDSL - Symmetrical Digital Subscriber Line - HDSL plus POTS over a single telephone line. This name has not been adopted by a standards group but is being discussed by ETSI. It is important to distinguish, however, as SDSL operates over POTS and would be suitable for symmetric services to premises of individual customers.

SNR - Signal-to-Noise Ratio

SVC - Switched Virtual Circuit A term found in frame relay and ATM networking in which a virtual connection, with variable end-points, is established through an ATM network at the time the call is begun; the SVC is de-established at the conclusion of the call. See also Permanent Virtual Circuit.

TELCO - Telephone Company - Generic name for telephone companies throughout the world which encompasses RBOCs, LECs and PTTs.

TDM - Time Division Multiplexing

UBR - Unspecified Bit Rate

UTP - Unshielded Twisted Pair 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.

VBR - Variable Bit Rate

VDSL - Very high bit-rate Digital Subscriber Line - Modem for twisted pair access operating at data rates from 12.9 to 52.8 Mbps with corresponding maximum reach ranging from 4500 to 1000 feet of 24-gauge twisted pair.