GLOSSARY of Common Telecom Terms and Acronyms

A

AMI - Alternate Mark Inversion. A line encoding scheme for transmitting data bits over T1 transmission systems.

ANI - Automatic Number Identification. A feature in which a series of digits, either analog or digital, are included in the call, thus identifying the telephone number of the caller.

ANSI - American National Standards Institute. This organization organizes committees, made up of computer users, manufacturers etc., to develop and publish industry standards.

ARP - Address Resolution Protocol. An Internet protocol used for mapping an IP address to a physical address on an Ethernet LAN.

ASCII - American Standard Code for Information Interchange. An asynchronous signaling code with character framing. An ASCII character consists of a start bit, seven data bits, a parity bit (even if unused) and a stop bit. In some cases, 1-1/2 or 2 stop bits are used. Thus ASCII is at best 70% efficient with seven data bits out of every ten-bit character. Note that 1-1/2 stop bits can not be used if the data will be carried by modern-day modems. The parity bit is used as an eighth data bit with able characters from 128 to 256. ASCII data can also be transported synchronously (aka clocked async).

Asynchronous transmission - Digital signals are transmitted without precise clocking. The signals have different frequencies and phase relationships. Individual characters contained in control bits (start and stop bits) designate the beginning and end of each character.

ATM - Asynchronous Transfer Mode. A component of ISDN. A switching technology that organizes digital data in 53 byte packets and transmits them using digital signal technology.

Authentication - The process of attempting to ensure that the person using the computer system and performing tasks such as sending or receiving messages is one and the same as the person in whose name the account is registered. Networks require passwords as a means of authentication; however, passwords do not guarantee that the person using the system is the person to whom the password was given.

AWG - American Wiring Gauge. A U.S. measurement standard of the diameter of non-ferrous wire, which includes copper and aluminum. The smaller the number, the thicker the wire. In general, the thicker the wire, the greater the current-carrying capacity and the longer the distance it can span.

Top

B

B8ZS - Binary with 8 Zero Substitution. Line code type, used on T1 and E1 circuits, in which a special code is substituted whenever eight consecutive zeros are sent over the link. Code is then interpreted at the remote end of the connection.

BER - Bit Error Rate. Ratio of received bits that contain errors.

Bipolar Violations - Violations of the electrical characteristic denoting a circuit with both negative and positive polarity.

BERT - Bit Error Rate Tester. A device that calculates the bit error rate on a communication channel.

bps - Bits per second. The speed at which a point-to-point transmission line can convey data.

BRI - Basic Rate Interface. Basic ISDN service level that consists of two 64 Kbps B (bearer) channels that carry voice, data, and video along with one 16 Kbps D (delta) channel for signaling between telephone company switches and for carrying user-network messages. Also see ISDN.

Top

C

CHAP - Challenge Handshake Authentication Protocol. A type of authentication protocol in which the authentication agent sends the client program a key to be used to encrypt the user name and password. CHAP doesn't only require the client to authenticate itself at startup time, but sends challenges at regular intervals to make sure the client hasn't been replaced by an intruder, for instance by switching phone lines. Also see PAP.

CIR - Committed Information Rate. The maximum data rate that a Frame Relay network provider guarantees to transfer under normal network conditions, averaged over a minimum increment of time. CIR is either equal to or less than the full port speed. Measured in bits per second, CIR one of the key negotiated cost factors in setting up a Frame Relay connection. Also see EIR.

CLEC - Competitive Local Exchange Carrier. A company that creates and operates communication networks and provides customers with an alternative to the local telephone company.

CMOS - Complementary Metal Oxide Semiconductor. Technology employing integrated field-effect transistors in a complementary symmetry arrangement, which simulates "Push/Pull" operation because of the placement of opposing-polarity devices (p-channel and n-channel FETs).

CODEC - enCOder/DECoder. Chip used in the voice card to compress or decompress speech or audio signals.

CPE - Customer Premises Equipment. Equipment, such as terminals and modems, supplied by the telephone company, that is installed at customer sites and connected to the telephone company network.

CRC - Cyclic Redundancy Check. A method of checking for errors in data that has been transmitted on a communications link. The dividend polynomial is initially preset to 0, and the 1s and 0s of the data stream become the coefficients of the dividend polynomial. The division uses subtraction modulo 2 (no carries), and the remainder is transmitted as the error check field. The receiving station compares the transmitted remainder with its own computed remainder, and an equal condition indicates that no error has occurred. The polynomial value depends on the protocol and code set being used.

CSU - Channel Service Unit. A device used to connect a digital phone line coming in from the phone company to network access equipment located on the customer premises. CSU may also be built into the network interface of the network access equipment.

Top

D

D4 - Super Frame. A common framing type used on T1 circuits. It consists of twelve frames of 192 bits each, with the 193rd bit providing error checking and other functions. It is superseded by ESF but still widely used.

DAC - Digital to Analog Converter. A device for converting digital signals into continuous analog signals. The converter usually buffers the input so that the output remains the same until the input changes. A typical converter provides two analog output channels ranging from 0 to +10 volts with eight bits per channel resolution; also provides two logic level outputs for external device control.

DB Connector - A family of plugs and sockets widely used in communications and computer devices. DB connectors come in 9, 15, 25, 37 and 50-pin sizes. The DB connector defines the physical structure of the connector, not the purpose of each line.

dBm - A unit of energy level measurement.

DCE - Data Circuit-Terminating Equipment. Device and connections of a communications network that comprise the network end of the user-to-network interface. It provides physical connection to the network, forwards traffic and provides clocking signals used to synchronize data transmission between DTE devices and itself.

DCS - Digital Cross-Connect System. A network element providing automatic cross-connection of a digital signal or its constituent.

DDS - Dataphone Digital Service (previously Digital Data Service). A digital network service available from AT&T, the Bell Operating Companies and independent telephone companies. Non-switched synchronous data rates of 2400, 4800, 9600 and 56,000 bps and switched synchronous 56,000 bps are available.

DHCP - Dynamic Host Configuration Protocol. Dynamically assigns IP addresses to client stations located on a TCP/IP network at startup, and allows the addresses to be re-used when no longer needed. This allows a larger group of stations to share a limited pool of addresses.

DLCI - Data Link Connection Identifier. The number of a private or switched virtual circuit in a frame relay network. Located in the frame header, the DLCI field identifies which logical circuit the data travels over, and each DLCI has a committed information rate (CIR) associated with it. The DLCI number is local to the frame access device and frame relay switch it connects to, and it is generally changed by the switch within the network, because the receiving switch uses a different DLCI for the same connection.

DNS - Domain Name System. The manner in which the Internet locates and translates domain names into IP addresses.

Drop and Insert - A process of adding data (insert) to a T1 data stream, or terminating data (drop) from a T1 data stream to other devices connected to the drop and insert equipment.

DS0 - Digital Signal Level 0. The base for the digital signal X series; transmits digital signals over a single channel at 64 Kbps.

DS1 - Digital Signal Level 1. Framing specification used in transmitting digital signals at 1.544-Mbps on a T1 facility (United States) or at 2.108-Mbps on an E1 facility (Europe).

DS3 - Digital Signal Level 3. Framing specification used in transmitting digital signals at 44.736-Mbps on a T3 facility (United States) or at 34.368-Mbps on an E3 facility (Europe).

DSL - Digital Subscriber Line. Technology that brings high-bandwidth information to homes and small business over copper telephone lines. It can carry both data and voice signals.

DSU - Data Service Unit. A device used in digital transmission that adapts a physical interface on a DTE device to a transmission facility such as T1 or E1. It is also responsible for functions such as timing.

DTE - Data Terminal Equipment. A device at the user end of a user-network interface that serves as a data source, destination, or both. It connects to a data network through a DCE device (such as a modem) and typically uses clocking signals generated by the DCE. It includes devices such as computers, protocol translators and multiplexers.

DTMF - Dual Tone Multi Frequency. The signal a telephone company receives when a telephone's touch keys are pressed. Also known as Touchtone.

Top

E

E1 - A digital transmission link with a capacity of 2.048 Mbps, used predominantly in Europe. E1 is channelized into 32 DS0s, each capable of carrying a single voice conversation or data stream.

EBCDIC - Extended Binary Coded Decimal Interchange Code. A synchronous signaling code, commonly used by IBM, with no character framing (start/stop) bits and no parity bit. An EBCDIC character consists of eight data bits. It is used as one of many contiguous characters within a large block. The block contains start and stop bit sequences and an error control bit sequence. Also see Synchronous.

Echo - Async terminals use an "echo" to display entered characters on the screen. The echo can be remote (provided by the host computer) or local (within the terminal). A remote echo requires a full-duplex transmission facility while a local echo does not. Consequently, many terminals use the "FDX" terminology to configure for remote echo and the "HDX" terminology to configure for local echo. Failure to configure the host to complement the terminal will result in either no screen display (no echo) or double characters (two echoes). Also see Full-Duplex and Half-Duplex.

EIR - Excess Information Rate. The amount of data in excess of the CIR that can be transferred, and is used for burst traffic support. Also see CIR.

EIA - Electronic Industries Association. A membership organization that sets standards for consumer products and electronic components.

E&M - Receive and Transmit. A trunking arrangement used for two-way switch-to-switch or switch-to-network connections. It is used with E1 and T1 interfaces.

ESF - Extended Superframe. A framing format for T1 applications that consists of twenty four frames of 192 bits each with the 193rd bit providing timing and other functions. It includes provisions for continuous monitoring by both the user and the telephone company central office. ESF provides a 4 Kbps link control channel (transparent to user data) which allows the telephone company to monitor the local loop, transmit and receive test messages and retrieve performance data, all without interrupting normal operations.

Ethernet - The most widely-used standard for local area networks (LANs).

Top

F

FCC - Federal Communications Commission. United States government agency that supervises, licenses and controls electronic and electromagnetic transmission standards.

File Server - In a local area network, a computer that has been set aside to store frequently accessed files.

Frame - At the data link layer in a point-to-point transmission line, the technique used to demarcate the data so that it can be received in an orderly and meaningful way.

Frame Relay - At the data link layer in a wide area network (WAN), a protocol for transferring packets at speeds up to 1.544 Mbps, depending on the physical medium being used. Frame Relay is designed for noise-free digital lines, and therefore omits the error correction facilities. The result is increased bandwidth.

FTP - File Transfer Protocol. Application protocol used for transferring files between network nodes.

Full-Duplex - Also known as "FDX" and "Duplex." See Half-Duplex, Simplex, and Echo.
1) The ability of a transmission facility to transport data in both directions simultaneously. Modems (or DSUs) at both ends of point-to-point full-duplex facilities and at the master location in multipoint full-duplex facilities use "constant carrier." Modems at remote locations in multipoint full-duplex facilities use "controlled carrier" (carrier controlled by RTS with data transmission controlled by CTS). Also see Point-to-Point and Multipoint.
2) The ability of a data processing device or protocol to simultaneously transmit and receive data. Note that some devices and protocols (e.g., BiSync) require full-duplex facilities but actually operate half-duplex.

FXO - Foreign Exchange Office. An interface that connects to the Public Switched Telephone Network's (PSTN) central office and is the interface offered on a standard telephone. Example: RJ-11 connector that allows analog connection to the central office.

FXS - Foreign Exchange Subscriber. An interface that connects directly to a standard telephone and supplies ring, voltage, and dial tone. Example: RJ-11 connector allows connections to basic telephone equipment.

Top

H

Half-Duplex - Also known as "HDX." See Full-Duplex, Simplex and Echo.
1) The ability of a transmission facility to transport data in both directions but not simultaneously. Half-duplex facilities use modems (or DSUs) configured for "controlled carrier," also known as "switched carrier," (carrier controlled by RTS with data transmission controlled by CTS) at all locations.
2) The ability of a data processing device or protocol to transmit and receive data but not simultaneously. Note that some devices and protocols (e.g., BiSync) require full-duplex facilities but actually operate half-duplex.

HDLC - High-Level Data Link Control. An ISO communications protocol used in X.25 packet switching networks. It provides error correction at the data link layer.

HDSL - High-bit rate Digital Subscriber Line. The earliest variation of DSL. It is used for wideband digital transmission within a business and between the telephone company and a customer. HDSL is symmetrical meaning an equal amount of bandwidth is available in both directions.

Host - A computer that can function as the endpoint of a data transfer. The computer can be a single-user personal computer or workstation that is part of a local area network (LAN), a minicomputer, or a mainframe computer. The term "node" is synonymous with host.

Top

I

IC - Integrated Circuit. An integrated circuit (IC), sometimes called a chip or microchip, is a semiconductor wafer on which thousands or millions of tiny resistors, capacitors, and transistors are fabricated. An IC can function as an amplifier, oscillator, timer, counter, computer memory, or microprocessor. A particular IC is categorized as either linear (analog) or digital, depending on its intended application.

ICMP - Internet Control Message Protocol. A protocol used between a host server and a gateway to the Internet to send message control and error-reporting messages.

ILEC - Incumbant Local Exchange Carrier.

Interface - One "side" of a pair of connected devices, i.e., 1) The DCE side or the DTE side. 2) The signaling specification used by a pair of connected devices.

IOC - Independent Operating Company.

IP - Internet Protocol. The method by which data is sent from one computer to another on the Internet. It is a connection less protocol, meaning there is no established connection between the two computers. Data is sent as independent packets, also known as data units or data grams, which are contained in the IP address, a 32-bit number that identifies each sender or receiver of information. When the packets are received, they are put into the correct order by the Transmission Control Protocol (TCP).

IP Datagram-  The basic unit of the information passed accross an IP Internet. It contains address information and data.

IPX - Internetwork Packet Exchange. A NetWare communications protocol used to route messages from one node to another. IPX packets include net-work addresses and can be routed from one network to another.

ISDN - Integrated Service Digital Network. A system that provides simultaneous voice and high speed data transmission through a single channel to the user's premises. ISDN is an international standard for end-to-end digital transmission of voice, data and signaling.

ITU-T - International Telegraph and Telephone Consultative Committee. International body that develops worldwide standards for telecommunications technologies.

IXC - Inter-exchange Carrier. A common carrier providing long distance connectivity between local telephone areas. Includes AT&T, MCI and Sprint.

Top

K

Kbps - Kilobits per second or one thousand bits per second.

L

LAN - Local Area Network. A computer network that links personal computers and workstations within a limited geographic area, such as a building or several contiguous buildings. Linked by cables such as coaxial cables or twisted pair, the computers connected to the LAN can access resources on other computers and shared peripheral devices. If there is a central network device, it is a file server that includes resources of use to all. To keep two workstations from accessing the LAN at the same time, LANs employ a Medium Access Control (MAC) protocol; ethernet is one such protocol.

Leased Line - A permanently connected telephone line that links a host to a service provider. Typically, leased lines can handle between 56,000 and 64,000 bits per second of data. Larger host systems require T1 lines (1.544 Mbps) or T3 lines (45 Mbps).

LED - Light Emitting Diode. A display technology that uses a semiconductor diode that emits light when charged.

Loopback - A method of checking the accuracy of data transmission in which the transmitted data stream is returned, or looped back, to its source for comparison with the original data. Loopbacks can be performed on data in an analog or digital state.

Top

M

MAC Header - In the Open Systems Interconnection (OSI) Reference Model, a sub layer that was added between the physical layer and the data link layer; this sub layer describes the protocols for broadcast networks (i.e., LANs).

Mbps - Megabits per second or one million bits per second.

MIB - Management Information Base. A directory listing information that used and maintained by a network's management protocol, such as SNMP.

Modem - Modulator-Demodulator. A device that adapts a terminal or computer to a voice-grade telephone line. At the source, a modem converts digital signals to a form suitable for transmission over analog communication facilities. At the destination, the analog signals are returned to their digital form. Modems are available that are synchronous or asynchronous for either 2-wire dial or 4-wire leased line applications. Also see Asynchronous and Synchronous.

Top

N

NAT - Network Address Translation. A method for translating internal IP addresses or network addresses into a single globally unique IP address. It permits a nearly unlimited number of users of one class C Network address because global addresses are required only when a user is connected to the Internet. It also serves as a fire wall by keeping individual IP addresses hid-den from the outside world. NAT is configured by defining address pools and specifying whether a port is "Inside" or "Outside". Also see IP.

NEBS - Network Equipment Building Systems. Adhering to standards from Bellcore for equipment used in Telco central offices (COs). It provides stringent specifications for durability, grounding, cables and hardware interfaces.

Top

O

OSPF - Open Shortest Path First. OSPF is a routing protocol that determines the best path for routing IP traffic over a TCP/IP network. It uses less router-to router update traffic than the RIP protocol that it has been designed to replace.

Overhead - Framing, error control, addressing, idle code, or any other characters or bit sequences in a data transmission other than actual end-user data.

Top

P

Packet - Also known as a data unit or a datagram. A unit of data that has been broken down into a small enough size so that the Internet Protocol can handle it and the Internet transport it.

PAP - Password Authentication Protocol. PAP is the most basic form of authentication for logging into a network. A user's name and password are transmitted over a network and compared to a table of name-password pairs.Typically, the passwords stored in the table are encrypted. Also see CHAP.

Parity - A character-level error-control mechanism used with asynchronous signaling codes. A parity bit is set to either a "one" (mark) or a "zero" (space) to ensure that the total quantity of "ones" within a character are either odd (odd parity) or even (even parity). Start and stop bits are excluded from the parity calculation. Character-level error-control has not been successful, and today, parity is rarely used. Asynchronous hardware, however, requires that the parity bit be present (usually forced to "one") even if unused.

PCM - Pulse Coded Modulation. Transmission of analog information in digital form through sampling and encoding the samples with a fixed number of bits.

PCS - Personal Communications Service. A network that provides personal, terminal and service mobility. Includes broadband and narrowband in the United States.

PING - Packet Internet Groper. A diagnostic utility program that indicates whether a remote host is actually connected to the network.

PLAR - Private Line Auto Ring. A leased voice circuit that connects two single endpoints together. When the telephone handset is removed from the hook, the remote telephone automatically rings.

Point-to-Point - A communications link connecting two and only two stations. See Full-Duplex.

Point-to-Point Network - A network in which a message originates from one node and travels to one or more destination nodes, but not to every node on the network. Wide area networks (WANs) are point-to-point networks, while most local area networks (LANs) are broadcast networks that send all network data to every computer connected to the LAN.

Polling - Interrogation of devices for purposes such as to avoid contention, to determine operational status, or to determine readiness to send or receive data.

Port-  A destination point used by transport level protocols to distinguish among multiple destinations within a given host computer .

PP - Point-to-Point Protocol. A protocol at the data link layer used for dialup IP access. It establishes a temporary but direct connection in which packets of data can travel directly from and to a node. PPP offers data compression, data negotiation and error correction.

Protocol - A standard that governs the operation of a network communications function by providing a clear-cut set of rules for its operation as well as the specific programs that are designed to implement these rules. Specific protocols apply to each layer in a network's architecture.

Top

R

Reverse Channel - The RS232 specification assumes that normal transmit and receive data (pins #2 and #3) will be at identical rates. However, the specification also includes a full complement of secondary data and control signals (but no clocks) for additional data transport, typically at rates from 5 to 75 bps. The need for these secondary channels no longer exists but, when it did, a half-duplex facility was configured to operate as simplex facility with a secondary channel in the opposite, or reverse, direction (hence, "reverse channel"). Also see Half-Duplex and Simplex.

RFC - Request for Comments. The document series maintained by the Inter-net community that records the protocols within the Internet and gives other information.

RIP - Routing Information Protocol. A simple routing protocol that is part of the TCP/IP protocol suite. It determines a route based on the smallest hop count between source and destination. RIP is a distance vector protocol that routinely broadcasts routing information to its neighboring routers and is known to waste bandwidth.

Router - An electronic device that connects a local area network (LAN) to a wide area network (WAN) and handles the task of routing messages between the two networks.

RS232 - RS232 is an electrical signaling specification published by the Electronic Industries Association (EIA). Although not identified in the specification, the 25-pin (DB25) connector, with specific pin assignments, is commonly accepted as "the RS232 connector or the serial connector."

RS530, RS449, RS422/422A - RS530, RS449, RS422 and RS422A are signaling specifications published by the Electronic Industries Association (EIA). RS422 defines electrical signaling characteristics. It references a companion standard RS449 that defines signal functions placed on a 37-pin (DB37) mechanical connector with specific pin assignments. Because of the cost of the DB37 connectors, manufacturers generally ignored that portion of the specification and placed a subset of the RS449 signals on a 25-pin (DB25) connector. The EIA subsequently legitimized this practice by publishing RS422A/RS530. As a practical matter, RS530 is simply "RS449 on DB25."

RSVP-TE - Reservation Protocol for Traffic Engineering.

Top

S

SCADA - (System Control and Data Acquisition, Supervisory Control and Data Acquisition, Security, Control and Data Acquisition). A common process control application that collects data from sensors on the shop floor or in remote locations and sends them to a central computer for management and control.

SDSL - Single-line Digital Subscriber Line. One of four DSL technologies. SDSL delivers 1.544 Mbps both downstream and upstream over a single cop-per twisted pair. The use of a single twisted pair limits the operating range of SDSL to 10,000 feet (3048.8 meters).

Simplex - A data processing device or protocol capable of transmitting or receiving but not both. Simplex devices are rare in data processing applications. One example is printing via the obsolete "202" modem. In dial mode, the 202 was Half-Duplex and printer flow control could not get back to the computer fast enough. The printer was configured as Simplex with a reverse channel for flow control. Simplex examples in other industries include commercial radio and television. See Full-Duplex, Half-Duplex, and Reverse Channel.

SLIP - Serial Line Internet Protocol. A serial protocol used for providing communication between a remote user and a TCP/IP Network. Commonly used to provide dial-up or private line access to the Internet. Also see PPP.

SNMP - Simple Network Management Protocol - Network management protocol used in TCP/IP networks. SNMP monitors and controls network devices, and manages configurations, statistics collection, performance and security.

SONET - Synchronous Optical NETwork. A high speed (up to 2.5 Gbps), fiber-optic transmission system for digital traffic. SONET uses time division multiplexing (TDM) to send multiple data streams simultaneously. STS1 is the basic building block of SONET.rs).

STS1 - Synchronous Transport Signal level 1. Basic building block signal of SONET, operating at 51.84 Mbps. Faster SONET rates are defined as STS-n, where n is a multiple of 51.84 Mbps.

Subnet Address- An extension of the IP addressing scheme which enables an IP site to use a single IP address for multiple physical networks. Subnetting is applicable when a network grows beyond the number of hosts allowed for the IP address class of the site.

Switchover - an action that occurs when a redundant device replaces a primary device that failed.

Synchronous transmission - Digital signals are transmitted with precise clocking. Signals have the same frequency. Individual characters contained in control bits (start and stop bits) designate the beginning and end of each character.

Top

T

T1 - A high-bandwidth telephone trunk line with a capacity of 1.544 Mbps, used in North America. T1 is channelized into 24 DS0s, each capable of carrying a single voice conversation or data stream.

TCP - Transmission Control Protocol. The method used with the Internet Protocol (IP) to send data in the form of packets between computers over the Internet. TCP is responsible for dividing a message into packets that the IP can use, and for reassembling packets into complete message.

TDM - Time Division Multiplexing. A technology that transmits multiple signals simultaneously over a single transmission path. Each lower-speed signal is time sliced into one high speed transmission.

Telnet - A user command and an underlying TCP/IP protocol for accessing remote computers.

TFTP - Trivial File Transfer Protocol. Network application that is simpler than the File Transfer Protocol (FTP) but less capable. Uses the User Datagram Protocol (UDP).

TIMS - Transmission Impairment Measurement Sets. Used to measure the power of a test signal amplitude of a sinewave voltage across a standard resistive termination.

Top

U

UDP - User Datagram Protocol. A connection less, communication transport method that offers a limited amount of service when messages are exchanged over the Internet Protocol. It is an alternative to TCP. Unlike TCP, UDP does not acknowledge or guarantee delivery, nor does it provide sequencing of packets.

Top

V

V.35 - V.35 is an electrical signaling specification published by CCITT. Although not identified in the specification, the M-series, 34-pin (Winchester-type) connector, with specific pin assignments, is commonly accepted as "the V.35 connector." V.35 signaling is also legitimately supported on DB25 connectors but the pin assignments have not been standardized within the industry.

V.54 - V.54 is an ITU standard for various loopback tests that can be incorporated into modems for testing the telephone circuit and isolating transmission problems. Operating modes include local and remote digital loopback and remote analog loopback.

Virtual Tributary (VT) - A signal designed for transport and switching of pay-loads smaller than the STS1 rate. All services below DS3 rate are trans-ported in the VT structure. T1 payloads are equivalent to a VT1.5, while E1 payloads are equivalent to a VT-2.

Top

W

WAN - Wide Area Network. A communications network that is capable of spanning a geographic area larger than a metropolitan area.

Workstation - A computer that is connected to a network. A workstation has its own processor, processes applications locally and may access data and resources located elsewhere on the network.

Top

X

X.21 - X.21 is an electrical standard published by the CCITT that includes a 15-pin (DB15) mechanical connector identification with specific pin assignments.

Top