'''High Data Rate Digital Subscriber Line''' ('''HDSL''') bola prvá [[DSL]] technológia používajúca širšie [[frekvenčné spektrum]] medeného vedenia, [[krútená dvojlinka|krútenej dvojlinky]]. HDSL bola vyvinutá v [[USA]] ako lepšia technológia pre vysokorýchlostné synchrónne obvody, ktoré sa zvyčajne používali na prepájanie systémov miestnych ústrední a ako nosič pre vysokorýchlostné firemné dátové a hlasové kanály použitím liniek [[Digital_Signal_1|T1]].
T1 circuits operate at 1.544 [[Mbit/s]]. These circuits were originally carried using a [[line code]] called [[AMI]]-Code ([[Alternate Mark Inversion]]). AMI did not have sufficient range, and required the application of [[repeaters]] over long circuits, consuming a great deal of power.
The first attempts to use DSL technology to solve the problem were done in the USA , using the line code [[2B1Q]]. This modulation allowed for a 784 [[kbit/s]] data rate over a single [[twisted pair]] cable. With two [[twisted pair]] cables, the full 1.544 Mbit/s was achieved. The new technology attracted the attention of the industry, but could not be directly used worldwide, due to the differences between the T1 and E1 standards. A new standard was then developed by the [[ITU]] for HDSL, using the CAP ([[Carrierless Amplitude Phase Modulation]]) line code, that reached the maximum bandwidth of 2.0 Mbps using two pairs of copper.
HDSL gave the telcos a greater distance reach when delivering a T-1 circuit. It was marketed originally as a Non Repeated T-1, with a distance of 12k feet over 24 gauge cable. The cable gauge affects the distance. To allow for longer distances, a repeater can be used. The repeater actually terminates the circuit and regenerates the signal. Up to four repeaters can be used for a reach of 60k feet (about 20 km). This reduced the cost of maintenance when compared with AMI-based repeaters that had to be used at every 35 db of attenuation (about 1 mile).