High Data Rate Digital Subscriber Line: Rozdiel medzi revíziami
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Verzia z 16:13, 4. december 2006
High Data Rate Digital Subscriber Line (HDSL) bola prvá DSL technológia používajúca širšie frekvenčné spektrum medeného vedenia, 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 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).
HDSL can be used either at the T1 rate (1.544 Mbps) or the E1 rate (2 Mbps). Slower speeds are obtained by using multiples of 64 kbps channels, inside the T1/E1 frame. This is usually known as channelized T1/E1, and it's used to provide slow-speed data links to customers. In this case, the line rate is still the full T1/E1 rate, but the customer only gets the limited (64 multiple) data rate over the local serial interface.
HDSL gave way to two new technologies, called HDSL2 and SDSL. HDSL2 offers the same data rate over a single pair of copper; it also offers longer reach, and can work over copper of lower gauge or quality. SDSL is a multi-rate technology, offering speeds ranging from 192 kbps to 2.3 Mbps, using a single pair of copper. SDSL is used as a replacement (and in some cases, as a generic designation) for the entire HDSL family of protocols.