How the Internet works

Internet Access

The term Internet accessrefers to the communication between a user and an Internet service provider (ISP) that connects to the Internet. Access falls into two broad categories: dedicated and dial-up. With dedicated access, a subscriber’s computer remains directly connected to the Internet at all times by a permanent, physical connection. Most large businesses have high-capacity dedicated connections; small businesses or individuals who desire dedicated access choose technologies such as digital subscriber line (DSL) or cable modems, which both use existing wiring to lower cost. A DSL sends data across the same wires that telephone service uses, and cable modems use the same wiring that cable television uses. In each case, the electronic devices that are used to send data over the wires employ separate frequencies or channels that do not interfere with other signals on the wires.

Dial-up is the least expensive access technology, but it is also the least convenient. Fortunately it is becoming obsolete nowadays. To use dial-up access, a subscriber must have a telephone modem, a device that connects a computer to the telephone system and is capable of converting data into sounds and sounds back into data. The user’s ISP provides software that controls the modem. To access the Internet, the user opens the software application, which causes the dial-up modem to place a toll-free telephone call to the ISP. Unlike dedicated access technologies, a dial-up modem does not use separate frequencies, so the telephone line cannot be used for regular telephone calls at the same time a dial-up modem is sending data.

How Information Travels Over the Internet

All information is transmitted across the Internet in small units of data called packets. Software on the sending computer divides a large document into many packets for transmission; software on the receiving computer regroups incoming packets into the original document. Each packet has two parts: a packet header specifying where the data came from, and the computer to which the packet should be delivered; and a packet payload containing the data being sent. The header also specifies how the data in the packet should be combined with the data in other packets by recording which piece of a document is contained in the packet.

A series of rules known as computer communication protocols specify how packet headers are formed and how packets are processed. The set of protocols used for the Internet are named TCP/IP after the two most important protocols in the set: the Transmission Control Protocol and the Internet Protocol. Hardware devices that connect networks in the Internet are called IP routers because they follow the IP protocol when forwarding packets. A router examines the header in each packet that arrives to determine the packet’s destination. The router either delivers the packet to the destination computer across a local network or forwards the packet to another router that is closer to the final destination. Thus, a packet travels from router to router as it passes through the Internet.

TCP/IP protocols enable the Internet to automatically detect and correct transmission problems. For example, if any network or device malfunctions, protocols detect the failure and automatically find an alternative path for packets to avoid the malfunction. Protocol software also ensures that data arrives complete and intact. If any packets are missing or damaged, protocol software on the receiving computer requests that the source resend them. Only when the data has arrived correctly does the protocol software make it available to the receiving application program, and therefore to the user.