Specifications

at the user and CO side use frequencies above those used for voice telephony to deliver high-speed data.

Several types of DSL have been developed hence the xDSL moniker. The most common types are

Asymmetric DSL (ADSL) and Symmetric DSL (SDSL). Another benefit of DSL to the Telcos is that it

gets long duration data calls off the Public Switched Telephone Network (PSTN). This minimizes the need

to upgrade the expensive circuit switched phone system.

ADSL offers a higher download speed, toward the subscriber, then upload. It has the advantage that it

coexists with Plain Old Telephone Service (POTS). This reduces cost by allowing a single copper pair to be

used for both voice and data service. A residence with a single phone line can be equipped with both

analog POTS line and high-speed data service. Filters split the signals at the CO and inside the residence

low frequencies are delivered to the telephone; high frequencies to the DSL modem.

SDSL is typically marketed as a business service. It requires a separate dedicated copper pair; it does not

coexist with POTS. Being symmetric makes it suitable for use with servers. SDSL is also typically offered

with a static IP address. A static address allows external hosts to more easily connect to servers. A special

case of SDSL is IDSL it offers symmetric speed of 144Kbp/s over longer distances then either ADSL or

SDSL. IDSL uses ISDN signaling allowing it to be used at distances >20K feet.

Speed varies by supplier; it ranges from a low of 144Kbp/s for IDSL up to several megabits per second for

subscribers close to the central office. In our area Verizon consumer ADSL is available at 768/128,

1500/128 and 1500/384 plans. Speed decreases with distance. Subscribers far from the CO are not able to

Multiplexer (DSLAM) combines data from multiple customers into a single high-speed connection. Traffic

from multiple DSLAMs is combined together, along with traffic from other COs on the ISP’s internal

network. From there interexchange carriers, that provide the Internet backbone, carry it.

DSL service is offered by traditional phone companies called Incumbent Local Exchange Carriers (ILEC),

Competitive Local Exchange Carriers (CLEC) and by companies specializing in data services called Data

Local Exchange Carriers (DLEC).

Even though DSL operates over existing t copper wire it requires substantial investment. The subscriber

needs a DSL modem to convert computer data to DSL signals. At the central office a DSLAM multiplexes

individual subscriber lines that are backhauled to the ISP. The ISP routes them to the interexchange carriers

that operate the Internet backbone. Equipment is needed to combine and route the signals from DSL

subscribers to the Internet, provide domain name service (DNS), mail and news server.

Deployment of DSL may require the coordination of three different companies. The ILEC owns the copper

wire. The CLEC or DLEC in turn rents the line and installs the DSLAM at the Telco and the modem at the

subscriber premises. The ISP is the retailer that sells the service to the customer and acts as first line

technical support. Needless to say getting DSL properly installed is sometimes a challenge.

For the latest information on DSL service visit to DSL Reports and the DSL Forum. The USENET news

group comp.dcom.xdsl is another good source of information.

2.2.1 Impairments

DSL is an impressive engineering accomplishment that enables broadband data to be carried on 100-year-

old copper telephone circuits. Not all phone lines are suitable for DSL. Assuming your local central office

is equipped for DSL you may not be able to obtain it for a number of reasons. This section discusses

common problem and where applicable workarounds.

2.2.1.1 Distance

DSL signals decrease as they travel down the wire. Typical practice limits DSL to between 15,000 – 18,000

feet from the CO or remote DSLAM. This is actual cable distance. The route the phone line takes may not