ADSL: Short for asymmetric digital subscriber line, a new technology that allows more data to be sent over existing copper telephone lines. ADSL supports data rates from 1.5 to 9 Mbps when receiving data (known as the downstream rate) and from 16 to 640 Kbps when sending data (known as the upstream rate). ASDL requires a special modem.

bandwidth: The amount of data that can be transmitted in a fixed amount of time. For digital services, the bandwidth is usually expressed in bits or bytes per second (bps). For analog devices, bandwidth is expressed in cycles per second, or Hertz (Hz).

cable modem: A modem designed to operate over cable TV lines. Because the coaxial cable used by cable TV provides much greater bandwidth than telephone lines, a cable modem can be used to achieve speeds of up to 2 Mbps.

dedicated line: A line reserved exclusively for one type of communication, such as voice or data. Also called a dedicated channel.

dial-up access: Connecting a computer device to a network via a modem and a public telephone line. Dial-up access is just like a phone connection, except the parties at both ends are computer devices rather than people.

frame relay: a protocol for connecting devices on a WAN. Frame relay networks support data transfer rates at T1 (1.544 Mbps) and T3 (45 Mbps) speeds.

ISDN: Short for integrated services digital network, an international communications standard for sending voice, video, and data over digital telephone lines. ISDN requires special metal wires and supports data transfer rates of 64 Kbps. Most ISDN lines offered by phone 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 128 Kbps. Because ISDN lines send data digitally, they do not require a modem.

ISP: Short for internet service provider, a company that provides access to the internet. For a monthly fee, an ISP gives you a username, password, and access phone number. Equipped with a modem, you can then browse the web and send and receive eMail.

LAN: Short for local-area network, a network of computers that are geographically close together (usually within a single building). There are three main topologies, or shapes, for a LAN. In a bus topology, all devices are connected to a central cable, called the bus or backbone. Bus networks are fairly inexpensive and easy to install. In a star topology, all devices are connected to a central hub. Star networks are easy to install and manage, but bottlenecks can occur because all data must pass through the hub. In a ring topology, all devices are connected to one another in the shape of a closed loop. Ring networks are fairly expensive and difficult to install, but they offer high bandwidth and can span large distances.

leased line: A permanent telephone connection between two points set up by a telecommunications carrier. Unlike normal dial-up connections, a leased line is always active. The fee for the connection is a fixed monthly rate.

modem: Acronym for modulator-demodulator, a device or program that enables a computer to transmit data over telephone lines. Computer information is stored digitally, while information is transmitted over telephone lines in the form of analog waves. A modem converts between these two forms.

network: A group of two or more computer systems that are linked together to communicate and share resources. Computers on a network are sometimes called nodes. Computers and devices that allocate resources for a network are called servers.

POTS: Short for plain old telephone service, which refers to the standard phone service you use in your home. In contrast, telephone services based on high-speed, digital communications lines, such as ISDN, are not POTS. The main distinction between POTS and non-POTS are speed and bandwidth.

PPP: Short for point to point protocol, a method of connecting a computer to the internet. PPP is more stable than the older SLIP protocol and provides error-checking features.

SLIP: Short for serial line internet protocol, one of two methods of connecting to the internet (the other is PPP). SLIP is an older and simpler protocol, but from a practical perspective, there’s not much difference between connecting to the internet through SLIP or PPP. ISPs generally offer one or the other, although some support both protocols.

SMDS: Short for switched multimegabit data service, a high-speed data communications service offered by phone companies that allows you to connect geographically separate LANs into a single WAN. Before SMDS, the only way to connect LANs was through a dedicated private line. SMDS is an attractive alternative because it is more flexible and often more economical.

T1 line: A dedicated line supporting data rates of 1.544Mbps. A T1 line actually consists of 24 individual channels, each supporting 64Kbps.

UTP: Short for unshielded twisted pair, a popular type of cable that consists of two unshielded wires twisted around each other. Because of its low cost, UTP cable is used extensively for LANs and telephone connections, yet it does not offer as high bandwidth or as good protection from interference as coaxial or fiber optic cables.

WAN: Short for wide-area network, a network that spans a relatively large geographic area. Typically, a WAN consists of two or more local area networks. Computers are connected to a WAN by telephone lines, leased lines, or satellites. The largest WAN in existence is the internet.