What is the ADSL and HDSL?

High performance transmission over subscriber loops has historically required considerable engineering in order to accommodate the wide range of loop configurations encountered across the access loop. Recently, self-adaptive filtering techniques have been applied to transmission over subscriber loops. This technology, previously too complex and expensive for general use, can now be implemented in VLSI devices, and allows more robust and cost effective transmission over the Access loop

Two systems are being proposed to provide high data rates across the access loop which the future services require. The two systems are called ADSL (Asymmetric Digital Subscriber Line) and HDSL (High-Rate Digital Subscriber Line). HDSL provides a bidirectional high data rate service which is able to provide services, such as video conferencing, where high data rates are required in both directions. HDSL provides a bidirectional data rate of 1.544 Mb/s over the unmodified subscriber line for a maximum distance of about 3.66 km from the exchange. A variant of HDSL is VHDSL (Very High-Rate Digital Subscriber Line) which provides an HDSL service at a data rate in excess of 10 Mb/s. With the increased data rate of VHDSL there is a dramatic decrease in the radius which such a system could service. For this reason a VHDSL system would be implemented with fiber optics transporting the data to a remote sight near the subscribers, and use VHDSL is bridge the last few tens of meters to the subscriber.

ADSL unlike HDSL provides a high data rate only in one direction. A low speed full duplex channel and a low speed digital maintenance and control channel are superimposed with the ADSL high data rate stream. ADSL is engineered to overlay the existing analogue telephone service (POTS), and basic rate ISDN service. To achieve this ADSL avoids the use the frequencies in the range of 0 to 50 kHz where POTS and ISDN reside. ADSL is proposed for future services, like video on demand, where the main flow of data is unidirectional.

ADSL systems have been defined at three different data rates of 1.544, 3.072, and 6.144 Mb/s . These different data rates can only be supplied from a local switch to a limited area. As the data rate increases the service radius decreases. The service distances proposed are 5.49, 3.66, and 2.44 km respectively for the different rates. The area which can operate the 3.072 Mb/s ADSL service is called the carrier service area (CSA), and the 6.144 Mb/s service area is known as the Distribution Area (DA).

The introduction of HDSL and ADSL allows the existing infra-structure to be used to provide a number of new services which demand high data rates. HDSL and ADSL provide a bridging technology which allows the service provide to better utilise their large investment in the copper loop, and allow a gradual transition to an all fiber network [21]. Two such networks are fiber to the curb (FTTC) where the fiber extends from the local switch to a curb side distribution point with copper over the last few tens of meters to the subscribers. The second is fiber to the home (FTTH) where the entire subscriber loop is fiber. To directly change from the existing copper loop to one which is all fiber would require a huge investment. The ADSL/HDSL system allows a gradual deployment of fiber into the subscriber loop as demand for higher data rates increases, or as the copper requires replacement. This gradual deployment allows a gradual investment in fiber and maximises revenue from the investment in the existing copper loop.

The ADSL and HDSL systems could be used to provide a number of new services to current telephone users.