Thomas William
Nathan Garcia, special to PCWorld.com
Dial-up networking: the set of protocols and software used to connect a computer to an Internet service provider, an online service, or a remote computer through an analog modem and POTS (plain old telephone system).
Dial-up networking is the most widely used type of computer connection to the Internet. By the end of 2000, over a quarter of a billion subscribers were dialing in to the Internet--more than four times the number of users of other popular access methods, namely broadband DSL, cable, and ISDN modems. Here are the facts you need to know:
- Dial-up networking uses a modem as the interface between a single PC and a network such as the Internet; the modems are typically capable of speeds up to 56 kbps.
- Dialing up with a modem is still the cheapest and most widely available way to connect to the Internet, but because it offers comparatively slow connection speeds, graphics-intensive Web sites can take a long time to load.
- The maximum speed at which you can download data using dial-up networking is limited by the telephone system's analog bandwidth, the line quality, and the Internet traffic load.
- Dial-up networking usually communicates with the ISP using the Point to Point Protocol standard.
While broadband services such as DSL, cable modems, and satellite Internet are rapidly becoming available to more areas around the nation, dial-up networking continues to grow. It's estimated that wireless connectivity will pose the biggest challenge to dial-up networking in terms of subscribers sometime in the future. But analysts predict that, as of the end of 2001, more than twice as many people will continue to use dial-up networking than will use broadband services to connect to the Internet. In 2003 the gap will close, but dial-up networking will still be the most popular method by one and one-half times.
What's in a Handshake?
Dial-up networking is the simplest way to connect to the Internet: You just connect over the phone line using your modem, once you've subscribed to an ISP. For the most part, the software tools you need come built into Windows; some ISPs, such as EarthLink, furnish a user interface that makes setup even easier. Whichever method is used, dial-up customers usually only need to enter a user name and password, as well as the ISP's phone number, into a dialog box. The dial-up networking software uses that information to make the connection with the ISP and does all the rest of the work. After the initial setup, all that most customers need to do to make a connection is double-click an icon.
When you initiate the process, dial-up networking first directs your modem to dial the ISP's phone number, which is answered by another modem at the other end. For a few seconds the modems send control signals back and forth to determine how fast each can connect. The familiar screeching you hear when your modem first connects is the sound of your modem and the ISP's modem harmonizing the connection and deciding on a speed to use.
Once the connection is established, your modem silences its internal speaker, and dial-up networking sends your user name and password to the ISP using a process called CHAP--the challenge handshake authentication protocol. At the ISP's end, a computer checks your user name and password against a database of active customers. Once the ISP authenticates your information, the dial-up networking status window disappears, and you are free to surf, check and send e-mail, download files, and so forth. The process can take anywhere from 30 seconds to 2 minutes to complete.
PPP: Slightly Slower, but More Fault-Tolerant
Dial-up networking also breaks down your data into chunks, encoding and packaging the data before it goes on its way.
Dial-up networking uses a method called PPP (Point to Point Protocol) to package data for transmission over phone lines. As with ethernet, the PPP data packet, often called a frame, includes several parts. There are beginning and ending flags (called "wrappers") that bound the packet. Just like ethernet packets, PPP frames contain wrapper upon wrapper. The wrappers help direct the data within the packet to its destination using different protocols, such as TCP/IP, and also determine the type of data compression used on the packet.
One important distinction between PPP frames and ethernet packets is the capacity of damaged PPP packets to be recovered using a process called the frame check sequence. A packet occasionally gets lost or damaged en route; when it arrives at its destination, a damaged ethernet packet gets thrown out and a request goes back to the computer that sent the packet to resend it--a very time-consuming process if you use a dial-up connection, which is much slower than broadband.
In contrast, one part of a PPP wrapper contains a chunk of data called the validation value, which is verified at the destination. In some cases, a damaged PPP frame can be restored through the validation value, so it does not need to be resent.
While this process saves valuable time that would otherwise be used to resend every damaged packet, PPP's error-recovery features tend to make PPP run a bit more slowly than the simpler protocols that lack such error correction. But the Internet is a dangerous place for a data packet; errors or other problems can cause dozens of packets to drop each second. In the long run, PPP is far better suited to use over the Internet than other, less stringent protocols, so it has survived and flourished.
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