Technology Networking & Internet

How Do Dial-Up Connections Work?

    Modems Modulate and Demodulate

    • On the user's end, the most functional piece of hardware in a dial-up connection is the computer's interface to the telephone network. Known as a modem--short for modulate and demodulate--this piece of hardware may be either an external entity to the computer or, as is more common, built into the machine.

      When a computer connects to another machine or Internet connection, the modem is responsible for picking up the phone line, dialing the proper number and transferring data from the computer to the upstream host. To accomplish this, the modem takes data from the computer, usually from the Internet connection software, and modulates it into audible sounds that can be transmitted across standard telephone lines. When a response is received, the modem demodulates the incoming sound into computer-readable data.

    The Computer Dials a Host

    • In a dial-up configuration, the computer uses its modem to connect to a host machine or point of presence--usually owned by an Internet Service Provider, or ISP--in the local area. This host provides a gateway between telephone lines and the Internet, and may handle hundreds or even thousands of simultaneous connections. In most cases, the host serves only as the gateway, though some ISP companies configure their hosts to provide relevant local news, weather, forums, or items of interest (others even allow users to publish small personal websites on a server connected to the host).

    The Host Has Internet Connectivity

    • To provide a connection between the end user and the Internet, dial-up Internet Service Providers must have a solid and reliable connection to either another upstream provider or to the Internet backbone; in addition, since thousands of users may be accessing the host at the same time, the Internet connection must be of sufficient bandwidth to handle tremendous amounts of traffic. To facilitate this connection, most ISPs connect directly to the Internet backbone on commercial-grade high speed data services like Asynchronous Transfer Mode (ATM), Frame Relay, or Gigabit Ethernet. In some cases, and especially in rural areas, ISPs may be configured to pass traffic through a local telephone company where data traffic to the Internet is switched to powerful high-bandwidth data trunks for transport to the Internet backbone.

    Dial-Up Connections Are Somewhat Slow

    • Although the upstream providers that carry Internet traffic between the backbone and the local host use very fast connections with ample bandwidth, the speed of the connection between the host and the end user is typically limited to about 56,000 bits per second, a speed commonly abbreviated as "56k." This limit is the result of physical limitations in the amount of data that can be modulated into sound and sent across telephone company lines. Originally intended only for voice conversations, the telephone company only samples sound on the line around 8,000 times per second; with this sampling rate, the theoretical maximum speed possible for a data connection across a voice telephone line is about 70,000 bits per second (70Kbps). The V.92 standard set by the Institute of Electronics Engineers in 1999, however, set the standard transfer rates at 48Kbps for upstream data transfer and 56Kbps for downstream (download) transfers in order to accommodate telephone line noise, overhead (connection management) data transfer, and standard error correction. Because of line noise, errors, and other issues, though, many users see a typical connection speed of about 33.6Kbps in actual practice.

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