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CompTIA A+ Certification All-in-One Exam Guide

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connects much faster than modems, eliminating that long, annoying mating

call you get with phone modems. The monthly cost per B channel is slightly

more than a regular phone line, and usually a fairly steep initial fee is levied

for the installation and equipment. The big limitation is that you usually need

to be within about 18,000 feet of a central office to use ISDN.

NOTE Another type of ISDN, called a primary rate interface (PRI), is

composed of twenty-three 64-Kbps B channels and one 64-Kbps D channel,

giving it a total throughput of 1.544 megabits per second (Mbps). PRI ISDN

lines are also known as T1 lines.

The physical connections for ISDN bear some similarity to analog

modems. An ISDN wall socket usually looks something like a standard RJ-45

network jack. The most common interface for your computer is a device

called a terminal adapter (TA). TAs look much like regular modems, and like

modems, they come in external and internal variants. You can even get TAs

that connect directly to your LAN.

While 128 Kbps was a nice upgrade over an analog modem in the 1990s,

today with the proliferation of faster connections such as DSL and cable,

finding an ISDN line in the wild has become rare. You will find them in sites

that need rock-solid security, like in military field command centers.

DSL

Digital subscriber line (DSL) connections to ISPs use a standard telephone

line with special equipment on each end to create always-on Internet

connections at speeds much greater than dial-up.

Service levels for DSL can vary widely. At the low end of the spectrum,

speeds are generally in the single digits—less than 1 Mbps upload and around

3 Mbps download. Where available, more recent xDSL technologies can

offer competitive broadband speeds measured in tens or hundreds of

megabits per second.

DSL requires little setup from a user standpoint. A tech comes to the

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