Protocols Another common problem, especially for new modems, is that
TCP/IP or other protocols have not been properly configured. Dial-up
connections typically require the PPP protocol. Verify that the protocols
are enabled and that the proper bindings are set for the protocols.
Remote response It could be that the NAS you are attempting to connect
to is down or having problems. Call the ISP to check.
Telephone company problems If there is sufficient static or crosstalk on
the telephone line, it can cause the modem to disconnect very soon after
completing the connection or cause enough data retransmissions that the
line appears exceptionally slow.Digital Subscriber Lines (DSL)
DSL transmits high-speed Internet data over a standard telephone line. Depending on the
type of DSL service subscribed to, data speeds can range from 128 Kbps (for IDSL—ISDN
over DSL) to 1.1 Mbps (for SDSL—symmetrical DSL). The most common DSL service is
ADSL (asymmetrical DSL), which is available to many more homes and businesses than
the other forms of DSL.
DSL service is very distance dependent. All DSL services emanate from a telephone
company (called the ILEC or Incumbent Local Exchange Carrier) central office. The
central office (CO) is where the telephone switching takes place in a town or a portion of a
city; the CO is where you connect to other exchanges and long distance service. Your
distance from the CO (as measured along the path of the copper wiring that runs from the
CO to your house or office) determines the type of DSL service you are able to get. Although
distances vary by ILEC, in general SDSL is available u pto about 10,000 feet away; ADSL
is available u pto about 18,000 feet away; and IDSL u pto 24,000 feet away. ADSL is a
best-effort service, which means that although a data speed is given, the speed realized
will depend on a number of network factors. SDSL and IDSL are committed-information-
rate (CIR) services that carry guarantees of data transmission rates.
DSL is available only to those locations that have twisted pair copper wiring all the
way from the CO. In many situations, the phone companies have used fiber optic cable to
run to a distribution box in an area and then run copper wire to each home or building.
Any circuit that has fiber optic cabling is not eligible for DSL service—at least, not yet.
One difference between DSL and a legacy dial-u pconnection is that DSL is always on.
The connection is a permanent circuit back to the CO and then through the DSL pro-
vider’s DSLAM (DSL Access Multiplexer—pronounced “dee-slam”) and out to the
Internet. The bad news about it being always on is that is it is also always open, which is
why a PC connected to a DSL connection should be running firewall software. A firewall
protects the PC from unwanted outside intrusions.
When DSL is installed in your home or business, there may be as many as three com-
paniesinvolvedintheinstallation:thelocalphonecompany(theILEC),theDSLprovider
(the company that placed the DSLAM in the CO), and a local or regional ISP. The phone(^532) PC Hardware: A Beginner’s Guide