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Digital subscriber line
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<p>Digital subscriber line (DSL; originally digital subscriber loop) is a family of technologies that are used to transmit <a href="/facts/Digital_data/JChckAvr">digital data</a> over <a href="/facts/Telephone_line/YdKUAka9">telephone lines</a>. In telecommunications marketing, the term DSL is widely understood to mean <a href="/facts/Asymmetric_digital_subscriber_line/Y5wsglVy">asymmetric digital subscriber line</a> (ADSL), the most commonly installed DSL technology, for <a href="/facts/Internet_access/EMbwSrnK">Internet access</a>. </p><p>In ADSL, the data throughput in the <a href="/facts/Upstream_(networking)/PtHgaPtx">upstream</a> direction (the direction to the service provider) is lower, hence the designation of <i>asymmetric</i> service. In <a href="/facts/Symmetric_digital_subscriber_line/7YwchjIc">symmetric digital subscriber line</a> (SDSL) services, the downstream and upstream data rates are equal. </p><p>DSL service can be delivered simultaneously with <a href="/facts/Plain_old_telephone_service/7uEc2wFY">wired telephone service</a> on the same telephone line since DSL uses higher <a href="/facts/Frequency_band/94Rq1PkQ">frequency bands</a> for data transmission. On the customer premises, a <a href="/facts/DSL_filter/1AzAy1eU">DSL filter</a> is installed on each telephone to prevent undesirable interaction between DSL and telephone service. </p><p>The <a href="/facts/Bit_rate/ppUNMmVd">bit rate</a> of consumer ADSL services typically ranges from 256 <a href="/facts/Kilobit_per_second/Nv9VoT7T">kbit/s</a> up to 25 <a href="/facts/Megabit_per_second/Nv9VoT7T">Mbit/s</a>, while the later VDSL+ technology delivers between 16 Mbit/s and 250 Mbit/s in the direction to the customer (<a href="/facts/Downstream_(networking)/n9EHXVSN">downstream</a>), with up to 40 Mbit/s upstream. The exact performance is depending on technology, line conditions, and service-level implementation. Researchers at <a href="/facts/Bell_Labs/fKP5Gk0H">Bell Labs</a> have reached SDSL speeds over 1 Gbit/s using traditional copper telephone lines, though such speeds have not been made available for the end customers yet. </p>