data communications

Broadly speaking, data communications is the transfer of data between computers and their users. At its most abstract level, data communications requires two or more comput-ers, a device to turn data into electronic signals (and back again), and a transmission medium. Telephone lines, fiber optic cable, network (Ethernet) cable, video cable, radio (wireless), or other kinds of links can be used. Finally, there must be software that can manage the flow of data.

Until recently, the modem was the main device used to connect personal computers to information services or networks (see modem). In general, data being sent over a communications link must be sent one bit at a time (this is called serial transmission, and is why an external modem is connected to a computer’s serial port). However most phone cables and other links are multiplexed, meaning that they carry many channels (with many streams of data bits) at the same time.

To properly recognize data in a bit stream coming over a link, the transmission system must use some method of flow control and have some way to detect errors (see error cor-rection). Typically, the data is sent as groups or “frames” of bits. The frame includes a checksum that is verified by the receiver. If the expected and actual sums don’t match, the recipient sends a “negative acknowledgment” message to the sender, which will retransmit the data. In the original system, the sender waited until the recipient acknowledged each frame before sending the next, but modern protocols allow the sender to keep sending while the frames being received are waiting to be checked.

The actual transmission of data over a line can be con-sidered to be the lowest level of the data communications scheme. Above that is packaging of data as used and inter-preted by software. Unless two computers are directly con-nected, the data is sent over a network, either a local area network (LAN) or a wide-area network such as the global Internet. A network consists of interconnected nodes that include switches or routers that direct data to its destina-tion (see network). Networks such as the Internet use packet-switching: Data is sent as individual packets that contain a “chunk” of data, an address, and an indication of where the data fits within the message as a whole. The packets are routed at the routers using software that tries to find the fastest link to the destination. When the pack-ets arrive at the destination, they are reassembled into the original message.

Applications

Data communications are the basis both for networks and for the proper functioning of servers that provide ser-vices such as World Wide Web pages, electronic mail, online databases, and multimedia content (such as audio and streaming video). While Web page design and e-com-merce are the “bright lights” that give cyberspace its char-acter, data communications are like the plumbing without which computers cannot work together. The growing demand for data communications, particularly broadband services such as DSL and cable modems, translates into a steady demand for engineers and technicians specializing in the maintenance and growth of this infrastructure (see broadband).

Besides keeping up with the exploding demand for more and faster data communications, the biggest chal-lenge for data communications in the early 21st century is the integration of so many disparate methods of com-munications. A user may be using an ordinary phone line (19th-century technology) to connect to the Inter-net, while the phone company switches might be a mix-ture of 1970s or later technology. The same user might go to the workplace and use fast Ethernet cables over a local network, or connect to the Internet through DSL, an enhanced phone line. Traveling home, the user might use a personal digital assistant (PDA) with a wireless link to make a restaurant reservation (see wireless comput-ing). The user wants all these services to be seamless and essentially interchangeable, but today data communica-tions is more like roads in the early days of the automo-bile—a few fast paved roads here and there, but many bumpy dirt paths.