A Internet Dictionary


ADSL (Asymmetric Digital Subscriber Line) is a modem technology that converts existing twisted-pair telephone lines into access paths for multimedia and high-speed data communications. ADSL is called asymmetric because most of its two-way or duplex bandwidth is devoted to sending data downstream, to the user. Only a small portion of bandwidth is available for upstream messages.


The principal feature of analog representations is that they are continuous. In contrast, digital representations are measured at discrete intervals.


The Asynchronous Transfer Mode (ATM) protocol sends information segmented into fixed length cells of 53 bytes. Fixed cells allow the information to be transported in a predictable way, which accommodates different traffic types on the same network. ATM was designed to accommodate the simultaneous transmission of data, audio, and video.


Generally speaking, broadband is the capability of supporting, in both downstream (provider to consumer) and upstream (consumer to provider) directions, at a speed in excess of 200 Kbps per second.


Bandwidth refers to the transmission capacity of an electronic communications line (e.g., a telephone line). Transmission rates are measured by how many bits of data can cross the wire per second. Slower transmission speeds are measured in kilobits per second (1,024 bits, abbreviated Kbps). Faster transmissions are measured in millions of bits, megabits, per second (Mbps) or billions of bits, gigabits, per second (GBps). Note: For analog devices - as opposed to digital ones - bandwidth is expressed in cycles per second, or Hertz (Hz).

Cable Modem

A modem designed to operate over cable TV lines


The client-server model of computer communications features a client computer that calls a server computer to request a service (e.g., the download of a file). The client is the machine that initiates contact. The server is the machine that responds to the request.

Computer Virus

A computer virus is a program that reproduces its own code by attaching itself to other programs in such a way that the virus code is executed when the infected program is executed.


The Dynamic Host Configuration Protocol (DHCP) consists of a protocol for delivering host-specific configuration parameters from a DHCP server to an Internet host and a mechanism for allocation of network addresses to hosts.


Ethernet is one of the most popular protocols for LANs. The terms Ethernet and the IEEE 802.3 standard are often used interchangeably. Ethernet supports data transfer rates of 10 Mbps. A newer version of Ethernet, called 100Base-T (or Fast Ethernet) supports data transfer rates of 100 Mbps. The newest version, Gigabit Ethernet, supports data rates of 1 gigabit (1,000 megabits) per second.


The Fiber Distributed Data Interface (FDDI) is a 100 Megabit technology for sending digital data over fiber optic cable.


A system or combination of systems that enforces a boundary between two or more networks to prevent access by unauthorized persons.


The File Transfer Protocol (FTP) provides the basic elements of file sharing between hosts. FTP uses TCP (Transmission Control Protocol) to create a virtual connection for control information and then creates a separate TCP connection for data transfers. The control connection uses an image of the TELNET protocol to exchange commands and messages between hosts.


Internet Control Message Protocol (ICMP) is an extension to the Internet Protocol (IP) defined by RFC 792. ICMP supports packets containing error, control, and informational messages. The PING command, for example, uses ICMP to test an Internet connection.


Internet Protocol (IP) is the routing layer packet (datagram) delivery service of the TCP/IP suite. IP specifies the addressing scheme. The thirty-two bit, numeric, Internet Protocol (IP) address is an identifier for a computer or another device on a TCP/IP network. Messages are routed based on the IP address.


A Local Area Network (LAN) is one that spans a relatively small area, and is often confined to a building, or a small group of buildings. LANs can be connected over any distance via telephone lines and radio waves. A system of LANs connected in this way is called a Wide Area Network (WAN). Characteristics that distinguish one type of LAN from another include differences in topology (i.e., the geometric arrangement of devices on the network), protocols (i.e., the rules and encoding specifications for sending data), and the media (e.g., twisted-pair wire, coaxial cables, or fiber optic cables) by which devices are connected. Some networks do without connecting media altogether, communicating instead via radio waves.


address The Media Access Control (MAC) address is a hardware address that uniquely identifies each node of a network.


A network is a group of two or more computer systems linked together.


OC3 is an optical fibre line. OC3 (Optical Carrier) speeds are available from 35 Mbps through 155 Mbps.


The OSPF (Open Shortest Path First) protocol is a link-state routing protocol used for routing IP. OSPF is an interior gateway protocol that uses link-state technology for routing within a group of routers.


PCMCIA (Personal Computer Memory Card International Association) is a nonprofit trade association (in Sunnyvale, California) that was created to standardize the connection of peripherals to portable computers. PCMCIA developed the PCMCIA card, a lightweight module about the size of a credit card that adds features to a portable computer.


Primarily used to troubleshoot Internet connections, the Packet INternet Groper (PING) is a utility for determining whether a specific IP address is accessible. PING sends a packet to the specified address and waits for a reply.


Post Office Protocol version 3 (POP3) permits workstations to dynamically access a maildrop on a server host (TCP/IP).


PPP (Point-to-Point Protocol) is designed for simple links that transport packets between two peers. PPP provides a common solution for the easy connection of a wide variety of hosts, bridges and routers.


A protocol is a format for the transmission of data between two devices. The protocol determines the type of error checking to be used, the data compression method (if any), how the sending device will indicate that it has finished sending a message, and how the receiving device will indicate that it has received a message.


Protocol stack Also called the ISO/OSI (International Organization for Standardization/Open System Interconnection) reference model, a protocol stack is a framework consisting of seven layers that work together to transmit data from one network device to another.


Essentially, RFCs (Requests for Comments) are a collection of notes about many aspects of computing and computer communication.


(Requests for Proposals) Companies will issue this in request for contract work.


A router is a device that connects any number of LANs (Local Area Networks) and routes data among them. Routers use headers and a forwarding table to determine where data packets go.


Modeled on FTP, Simple Mail Transfer Protocol (SMTP) is a protocol for transfering email messages between servers and providing delivery status.


T1 is a high speed (1.544 Mbps) digital network developed by AT&T to support long-haul pulse-code modulation (PCM) voice transmission. The primary innovation of T1 was to introduce digitized voice and to create a network fully capable of digitally representing what was up until then, a fully analog telephone system. (T1-C operates at 3.152 Mbps, T-2 at 6.312 Mbps, T-3 at 44.736 Mbps, and T-4 at 274.176 Mbps.)


Transmission Control Protocol (TCP) provides a reliable stream delivery and virtual connection service to applications, through the use of sequenced acknowledgement with retransmission of packets when necessary (TCP/IP).


TELNET is the terminal emulation protocol of TCP/IP (Transmission Control Protocol/Internet Protocol). TELNET allows a personal computer to connect to a server or a mainframe and transmit commands as if the PC were a hardwired terminal.


ISDN (Integrated Services Digital Network) is an all digital communications line that allows for the transmission of voice, data, video, and graphics at very high speeds, over standard communication lines. ISDN may be transmitted via telephone networks, packet switched networks, telex, CATV networks, and more. (Using built-in data compression with a theoretical 4:1 compression ratio, one ISDN line can provide 512 Kbps throughput.) Most ISDN lines offered by telephone companies give you two lines at once, called B channels. You can use one line for voice and the other for data, or you can use both lines for data to give you data rates of 128 Kbps, three times the data rate provided by today's fastest modems. The original version of ISDN employs baseband transmission. Another version, called B-ISDN, uses broadband transmission and is able to support transmission rates of 1.5 Mbps. B-ISDN requires fiber optic cables and is not widely available.


A dedicated phone connection supporting data rates of about 43 Mbps. A T-3 line actually consists of 672 individual channels, each of which supports 64 Kbps. T-3 lines are used mainly by Internet Service Providers (ISPs) connecting to the Internet backbone and for the backbone itself. T-3 lines are sometimes referred to as DS3 lines.


Short for Domain Name System (or Service), an Internet service that translates domain names into IP addresses. Because domain names are alphabetic, they're easier to remember. The Internet however, is really based on IP addresses. Every time you use a domain name, therefore, a DNS service must translate the name into the corresponding IP address. For example, the domain name www.example.com might translate to DNS system is, in fact, its own network. If one DNS server doesn't know how to translate a particular domain name, it asks another one, and so on, until the correct IP address is returned. Short for digital nervous system, a term coined by Bill Gates to describe a network of personal computers that make it easier to obtain and understand information.


Short for Hyper Text Transfer Protocol, the underlying protocol used by the World Wide Web. HTTP defines how messages are formatted and transmitted, and what actions Web servers and browsers should take in response to various commands. For example, when you enter a URL in your browser, this actually sends an HTTP command to the Web server directing it to fetch and transmit the requested Web page. HTTP is called a stateless protocol because each command is executed independently, without any knowledge of the commands that came before it. This is the main reason that it is difficult to implement Web sites that react intelligently to user input. This shortcoming of HTTP is being addressed in a number of new technologies, including ActiveX, Java, JavaScript and cookies. Currently, most Web browsers and servers support HTTP 1.1. One of the main features of HTTP 1.1 is that it supports persistent connections. This means that once a browser connects to a Web server, it can receive multiple files through the same connection. This should improve performance by as much as 20%. The other main standard that controls how the World Wide Web works is HTML, which covers how Web pages are formatted and displayed. (HTTP) is an application-level protocol with the speed necessary for distributed, collaborative, hypermedia information systems. Messages are passed in a format similar to that used by Internet Mail and the Multipurpose Internet Mail Extensions (MIME).


Short for Optical Carrier, used to specify the speed of fiber optic networks conforming to the SONET standard. These are speeds for common OC levels. OC-1 = 51.85 MbpsOC-3 = 155.52 MbpsOC-12 = 622.08 MbpsOC-24 = 1.244 GbpsOC-48 = 2.488 GbpsOC-192=9.952 Gbps


Short for Synchronous Optical Network, a standard for connecting fiber-optic transmission systems. SONET was proposed by Bellcore in the middle 1980s and is now an ANSI standard. This standard defines the interface at the physical layer of the OSI seven-layer model. The standard defines a hierarchy of interface rates that allow data streams at different rates to be multiplexed. SONET establishes Optical Carrier (OC) levels from 51.8 Mbps (about the same as a T-3 line) to 2.48 Gbps. Prior rate standards used by different countries specified rates that were not compatible for multiplexing. With the implementation of SONET, communication carriers throughout the world can interconnect their existing digital carrier and fiber optic systems.

Revised 2013 by Larry Gentleman