Some definitions and acronyms you might want to know:
AAL ATM Adapation Layer: The standards layer that allows multiple applications to have data converted to and from the ATM cell. A protocol used that translates higher layer services into the size and format of an ATM cell.

AAL-1 TM Adaptation Layer Type 1: AAL functions in support of constant bit rate, time-dependent traffic such as voice and video.

AAL-2 ATM Adaptation Layer Type 2: This AAL is still undefined by the International Standards bodies. It is a placeholder for variable bit rate video transmission.

AAL-3/4 ATM Adaptation Layer Type 3/4: AAL functions in support of variable bit rate, delay-tolerant data traffic requiring some sequencing and/or error detection support. Originally two AAL types, i.e. connection-oriented and connectionless, which have been combined.

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

ABR Available Bit Rate: ABR is an ATM layer service category for which the limiting ATM layer transfer characteristics provided by the network may change subsequent to connection establishment. A flow control mechanism is specified which supports several types of feedback to control the source rate in response to changing ATM layer transfer characteristics. It is expected that an end-system that adapts its traffic in accordance with the feedback will experience a low cell loss ratio and obtain a fair share of the available bandwidth according to a network specific allocation policy. Cell delay variation is not controlled in this service, although admitted cells are not delayed unnecessarily.

ACM Address Complete Message: A BISUP call control message from the receiving exchange to sending exchange indicating the completion of address information.

ACR Attenuation to Crosstalk Ratio: One of the factors that limits the distance a signal may be sent through a given media. ACR is the ratio of the power of the received signal, attenuated by the media, over the power of the NEXT crosstalk from the local transmitter, usually expressed in decibels (db). To achieve a desired bit error rate, the received signal power must usually be several times larger than the NEXT power or plus several db. Increasing a marginal ACR may decrease the bit error rate.

AIMUX ATM Inverse Multiplexing: A device that allows multiple T1 or E1 communications facilities to be combined into a single broadband facility for the transmission of ATM cells.

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 - 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 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.

Attenuation - The dissipation of the power of a transmitted signal as it travels over a wire.

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.

BER - Bit Error Ratio. Measure of transmission quality indicating the number of bits incorrectly transmitted in a given bit stream compared to the total number of bits transmitted in a given duration of time.

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."

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

CBR - Constant Bit Rate

CCITT - Consultative Committee for International Telegraph and Telephone

CLEC - Competitive Local Exchange Carrier

CEU - Commercial End User. See SU, Service User

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.

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

CSA - Carrier Serving Area.

CSU - Channel Service Unit

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

DCE - Data Communication (or Circuit-Terminating) Equipment

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

DLCI - Data Link Connection Identifier. The virtual circuit number corresponding to a particular connection between two destinations. This number is used as part of the frame relay header.

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.

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

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

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

Frame Relay - A high-speed connection-oriented packet switching WAN protocol using variable-length frames.

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

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).

IEC - Inter-Exchange Carrier

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

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. The telecommunications agency of the United Nations, established to provide standardized communications procedures. Formerly known as CCITT.

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.

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.

Last mile - Refers to the local loop and is the difference between a local telephone company office and the customer premises; a distance of about 3 miles or 4 kilometers.

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.

LOF - Loss of Frame (UNI Fault Management)

LOP - Loss of Pointer (UNI Fault Management)

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

Mbps - Megabits Per Second

MDF - Main Distribution Frame. The point where all local loops are terminated at a CO.

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.

MUX - Multiplexer. A device that can send several data streams over a single physical line.

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.

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

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.

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 System. A set of requirements for the reliability and usability of equipment, established by Bellcore.

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.

Node - A connection or switching point on the network.

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

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

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. A network shared among many users who can use telephones to establish connections between two points. Also known as the dial 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. Modulation technique using variations in signal amplitude.

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

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

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

RVCI - Remote Virtual Channel Identifier

RVPI - Remote Virtual Path Identifier

SDH - Synchronous Digital Hierarchy. Based in part on SONET, SDH is an

ITU standard for the interworking of ANSI and ITU transmission techniques.

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 Multiplexer. A device that enables the simultaneous transmission of multiple independent data streams into a single high-speed data stream.

TIMS - Transmission Impairment Measurement Set.

UBR - Unspecified Bit Rate

UNI - User to Network Interface.

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

VPI - Virtual Path Identifier

VCI - Virtual Channel Identifier

VC - Virtual Circuit. A logical connection or packet-switching mechanism established between two devices at the beginning of a transmission.

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.