There and back again... how your data travels on the Net

by Alan Zisman (c) 1996. First published in Computer Player, September 13, 1996

When Bilbo Baggins, returned from his adventures, he called the story of his adventures (since reprinted as "The Hobbit") "There and Back Again". When you're looking at a Web page, on the Internet, perhaps a continent or two away, the data you're accessing could tell (if data could talk or write!) a tale of a similarly involved journey.

How Internet data, whether a Web page or an e-mail message gets from here to there is a mysterious-seeming process-but understanding it helps to make sense of other mysteries, like who really pays for the Net, and why your connection sometimes seems so slow.

Let's see what really happens when I, in Vancouver, using my modem at home or at work, try to connect up to a Web page... say Time Magazine's Pathfinder site, in New York City. First, using my modem, I connect to a local Internet Service Provider... one of over 2,300 such services in North America. My modem tries to get a connection at its maximum rate, 28,800 bits per second, but depending on the amount of noise on the phone lines, will often end up settling for a slower connection.

At the Service Provider, I connect to another modem, which is connected to a computer, connected to a local area network-connecting the ISP's bank of computers, modems, and phone lines. There's one modem per phone line-and sometimes all the lines are in use. If I don't get a busy signal, I'm connected to the ISP's Ethernet network connecting all their computers at a rate of 10 million bits per second (Mbps... about 300 times faster than the modem connection). I can check my e-mail or Usenet news groups-in both cases, looking at messages stored on my ISP's network. But to send mail, or use the Web, I've got to go out from my ISP-over the Internet itself.

The Internet is a network of networks. Regardless what sorts of computers they are, they transmit data by a common code-TCP/IP, for Transmission Control Protocol/Internet Protocol. The TCP part describes how data is broken down into small pieces called packets... each packet is transmitted separately, mixed with other data-and sometimes travelling along different routes. At the end, it's all reassembled, hopefully in the correct order. If a packet is missing or corrupted, TCP will request it again.

The IP part gives every data packet its Internet address... a 12-digit number that corresponds to the more human-friendly address that you or I typed. (I know, Internet addresses like http://www.tcp.com doesn't seem that friendly, but try to remember 167.191.045.001 instead). One of the computers at my ISP is the Domain Name Server-storing a large table that translates the Internet address that I typed into its (hopefully) proper numerical equivalent.

To get out to the Internet, my ISP rents a dedicated T1 connection, running at 1.54 Mbps, connecting to a larger, regional ISP. From there, my TCP/IP packets go through a router-a sort of dedicated computer that does nothing but try and figure out the most efficient route to get the data to its destination. From Vancouver, on its way to New York, the data may go via Seattle, or down to Sacramento, travelling along a 45 Mbps T3 connection owned by MCI. Not only is data being exchanged however... despite our sense that the Internet is free, each of these data transmissions is being paid for, as the various Service Providers rent their access from larger companies such as MCI.

While the T3 line sounds massive, much of the so-called backbone-the main transmission route, is overloaded. MCI is currently replacing it with fibre-optic cable capable of 622 Mbps. Moving on the backbone, my data, mixed with packets from thousands of other users on hundreds of other ISPs, may pass through routers in Denver and Kansas City, on its way to New York. There, it passes through a series of progressively smaller Service Providers, on its way to its destination, at Time-Warner's Pathfinder Web site.
(From there, the data needs to get back to me-the same process, though perhaps a different route, in reverse).

Amazingly, this complex process works correctly most of the time. And while there are sometimes bottlenecks along the way (gridlock on the Information Superhighway, if you'll excuse the cliché), most of the traffic problems occur at the destination. Sites like Pathfinder are quite popular, and if several thousand people are all trying to access it at once, it can take a while to connect.

The Web is designed to try to minimize this, however... when I'm reading that Time Magazine article, I'm not actually connected to their site-Web browsers are only actually connected for the few seconds when they are sending or receiving data. Many people can be reading data from a site, taking turns connecting, before performance starts to degrade.

You can see the route your data takes, using the Traceroute utility. Search for 'traceroute' at a search engine like AltaVista (http://www.altavista.digital.com/), to get links to Web sites with this program, which will show you the journey your data takes from There and Back Again.