To use the Internet, businesses, schools, and other organizations first must connect their own computers together using a LAN technology, such as Ethernet. One of the nodes on the LAN is the gateway. A gateway translates from one kind of network to another. In this case, the gateway translates from Internet protocols to those of the LAN.
Then the gateway must be connected to the Internet. Because all the computers on the LAN must communicate with the Internet through this one connection, the connection needs a lot of bandwidth. A common solution is a T-line, a line leased from a service provider that comes in different levels for different bandwidth needs.
A T1 line is usually a twisted pair and provides about 1.5 megabits per second, or about 190 kilobytes per second. That means it could transmit the contents of a CD-ROM in about an hour. At one time a T1 line was considered fast, but that is no longer the case.
A T3 line is a bundle of T1 lines and provides about 44 megabits per second, or about 5 megabytes per second. That means it could transmit the contents of a CD-ROM in about two and a half minutes.
Home users originally connected to the Internet through ordinary phone lines, known as voice lines in the industry, and often still do. The phone system works on analog principles: As a person talks on one end, the voice vibrates a speaker coil, which produces an electrical level that varies as the vibrations vary.
On the other end, the electricity is applied to another speaker coil, which produces the same vibrations in the air at that location, reproducing the sound of the original voice. Except for using electricity, the process is the same as the childhood game where two tin cans are connected by a string. Because computers communicate digitally and phone lines are analog, a conversion is required.
A modem is a device that can convert a digital signal to an analog signal and vice versa. The name comes from MOdulator/DEModulator; modulation is digital-to-analog conversion, and demodulation is analog-to-digital conversion.
When someone uses a dial-up connection, a modem must exist in the computer system and at the other end of the connection. An Internet Service Provider, or ISP, actually provides the connection to the Internet. Each message sent over a dial-up connection is digital until it gets to the sender's modem, where it is converted to analog, transmitted over the phone line to another modem, which converts it back to a digital message, reformats it to the Internet's protocol, and starts it on its way to its ultimate destination.
Phone lines do not support a high range of frequencies, which is why people heard over a phone once sounded like they were in a box. This low-frequency range results in low bandwidth. Dial-up connections can only support a speed of 56 kilobits per second, which means it would take more than a day to transmit the contents of a CD-ROM. Even this speed is only available in one direction - from the Internet to the user's computer. In the other direction, the speed is only 33 kilobits per second.
The term downstream refers to the direction from the Internet to the user's computer, and upstream refers to the direction from the user's computer to the Internet. In many methods of communicating with the Internet, the downstream bandwidth is much higher than the upstream bandwidth. In many applications this is no problem, but some applications, such as video conferencing, require the same bandwidth in both directions.
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