A Digital Subscriber Line (DSL) uses the same wires as a telephone line to generate high-speed internet connection. The wires used in telephone installation can handle a greater range of frequencies than those used for voice. DSL takes advantage of this without having to disturb the line’s ability to carry a conversation.
ADSL, or asymmetric DSL, works on the assumption that users browse more information than they send. ADSL is sensitive to distance, limited to 18,000 feet. The maximum downstream speed is 8 megabits/second and maximum upstream speed is 640 kilobits/second. ADSL can be disqualified when loading coils, which boost voice signals, are used. Bridge taps, fiber-optic cables
, and distance can also compromise ADSL signals.
Two standards for ADSL are the discrete multitone system (DMT) and the carrier less amplitude/phases system (CAP). DMT divides data signals into 247 channels, each 4 KHz wide. The system switches between channels to find the optimum for transmission and reception. CAP divides signals into three bands. Voices are carried in the 0 to 4 KHz band, the upstream channel is carried between 25 to 160 KHz, and the downstream channel is carried between 240 KHz and 1.5 MHz. Low-pass filters are used to block signals above 4 KHz. These prevent data signals from interfering with phone calls.
The DSLAM, DSL access multiplexer, consolidates connections from many users onto a single connection to the internet. DSLAM supports multiple types of DSL in a central office and different varieties of protocol and modulation in the same type of DSL. ADSL connections trace back to the DSLAM which prevents performance decrease as users increase.
Other types of DSL include: very high-bit rate DSL (VDSL), symmetric DSL (SDSL), rate-adaptive DSL (RADSL), ISDN DSL (IDSL), and universal DSL (Uni-DSL). DSL alternatives include cable and wireless.
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