5.5.3 Connectivity

• Conventional Analog Phone Lines

• ISDN: Integrated Services Digital Network

• Cable Modem

• xDSL: Digital Subscriber Line

• Dedicated High Speed Lines

The first link between you and the Internet is the link that connects your computer to an ISP. Home users have several choices of connection technology, depending on where they live. Most of these choices are provided by local telephone companies, although cable companies are now also competing for this business. The following discussion presents a list of options that may be available to you.

Conventional Analog Phone Lines

The word "modem" is a contraction of modulator/demodulator, referring to the modem's function of converting between binary bits and sounds transmitted as analog electrical signals over the telephone line. The fastest modems available for purely analog phone lines run at 56 kilobits per second. Many computers now come with built-in 56-kbs modems that can also be used to send and receive faxes. Internal or external add-on modems are an inexpensive way to connect to an ISP. The principal drawback of analog modems is their limited speed.

Isdn: Integrated Services Digital Network

ISDN is a digital connection carried by standard copper telephone wires, typically providing two channels of 64 kilobits per second each. This allows you to use 128 kilobits per second for data, or to use one channel for data and one for voice, so your ISDN line can be your home phone line. ISDN requires either that you have a special ISDN adapter card for your computer, or that you use your computer's Ethernet interface to attach to an ISDN device. ISDN service and equipment is typically two to three times more expensive than analog modems.

Cable Modem

Cable television companies have begun providing data connections to residential customers over their existing cable lines. Some of the television cable's bandwidth is allocated to data traffic, typically from a few to a few tens of megabits per second. Computers are usually connected to cable modems through Ethernet interfaces so this style of connection is extremely fast. The drawback is that the bandwidth, like the TV cable itself, is shared with your neighbors. If you are the only user, performance is excellent. But, if many subscribers are using the system at the same time, performance declines.

XDsl: Digital Subscriber Line

A variety of digital subscriber line technologies, such as ADSL, HDSL, and VDSL, are in use in limited areas. These services use standard copper telephone lines to carry digital signals. Perhaps the most commonly used is ADSL, or asymmetric digital subscriber line. It is called asymmetric because the speed of upstreamand downstream transmissions are different. Upstream transmissions from the subscriber to the central office are done at low speeds from 16 kilobits per second up to around one megabit per second. Downstream transmissions, from the central office back to the subscriber, go from the megabit per second range up to nearly ten megabits per second. This asymmetry can work very well for individuals running Web browsers, where only a small amount of data travels upstream but large amounts of text and images (and maybe even streaming audio or video) are transferred downstream. Computers usually attach to ADSL modems through their Ethernet interface.