DSL (Digital Subscriber Line)
A technology for high-speed network or Internet access over voice lines. There are various types, including asymmetric DSL (ADSL), high-bit-rate DSL (HDSL), symmetric DSL (SDSL) and very-high-bit-rate DSL (VDSL). The whole group is sometimes referred to as “xDSL.”
DSL technology has its roots in work done by Bell Communications Research, Inc., in the late 1980s to explore the feasibility of sending broadband signals over the American telecommunications network. The first efforts in this area resulted in another high-speed Internet technology, called integrated services digital network (ISDN). In the early 1990s the first variety of DSL, high-bitrate DSL (HDSL), was rolled out with the intent of being used for on-demand television. Initial efforts looked promising, but the rapidly growing number of channels provided by cable television companies made setting up an on-demand service financially less attractive. Soon after, DSL was repurposed for connecting devices to the Internet. Other varieties of DSL soon followed the creation of HDSL, including the most common type: asynchronous DSL (ADSL). Asynchronous refers to the way that more bandwidth is given to downstream traffic, which comes to the user from the Internet, than to upstream traffic, which goes from the user to the Internet. Traffic on DSL is transmitted over normal telephone lines through a DSL terminal adapter, also known as a DSL modem, which connects a computer or local computer network to the DSL line.
Each DSL user has a dedicated telephone line. However, DSL is limited by distance. A user has to be within a few miles of a telephone switching office for DSL to work, and signal strength degrades even within that distance. In addition, DSL speeds—typically, 5 Mbps (5 million bits per second) upstream and 2 Mpbs downstream, though premium business services may be faster—are much slower than cable and fibre-optic service. Cable, which is generally less expensive than DSL, offers speeds in excess of 20 Mbps upstream and 150 Mbps downstream, and fibre-optic service is even faster. Beginning in the 2010s, DSL subscriptions declined in the face of competition from cable and fibre-optic services.
ISDN, in full integrated services digital network, all-digital high-speed network provided by telephone carriers that allowed voice and data to be carried over existing telephone circuits.
In the early 1980s ISDN was developed as an offshoot of efforts to upgrade the telephone network from analog to digital using fibre optics. The expense of connecting every home with fibre-optic cables, however, led to changes in the ISDN standard. ISDN ran on ordinary copper wire, which lowered the cost but also lowered the speed. The ISDN standard divided a telephone line into separate data channels, which, along with a slower signaling channel, could be grouped together into “interfaces” for more speed. The two main interfaces were basic rate interface (BRI), which used up to two data channels and was meant for home users, and primary rate interface (PRI), which used up to 23 channels (up to 30 in Europe) and was meant for businesses that needed more speed.
Despite its advantages over standard dial-up—faster speed and the ability to employ telephones and computers on the user’s network at the same time—telephone companies had trouble selling ISDN services, which were much more expensive than dial-up. Because of this, there were only about one million ISDN users in the United States five years after it became available. Another problem for ISDN was the growth of digital subscriber line (DSL) and asymmetric digital subscriber line (ADSL) over copper telephone wires and the introduction of Internet services by cable television providers. These technologies were rolled out much faster and widely supplanted ISDN for home users, though some organizations still used ISDN. For example, many radio stations relied on ISDN lines to connect studios because of their clear sound quality. In many countries ISDN services were decommissioned in the 2010s and ’20s.
local area network (LAN), any communication network for connecting computers within a building or small group of buildings. A LAN may be configured as (1) a bus, a main channel to which nodes or secondary channels are connected in a branching structure, (2) a ring, in which each computer is connected to two neighbouring computers to form a closed circuit, or (3) a star, in which each computer is linked directly to a central computer and only indirectly to one another. Each of these has advantages, though the bus configuration has become the most common.
Even if only two computers are connected, they must follow rules, or protocols, to communicate. For example, one might signal “ready to send” and wait for the other to signal “ready to receive.” When many computers share a network, the protocol might include a rule “talk only when it is your turn” or “do not talk when anyone else is talking.” Protocols must also be designed to handle network errors.