DSL Technology:

DSL stands for Digital subscriber line and it has been developed for providing a high speed access to the internet. It allows the user high speed digital communication over the existing local loops.

The DSL technology is classified as follows:

a. ADSL

b. VDSL

c. HDSL

d. SDSL

Thus DSL technology is actually a set of technologies which is referred to as xDSL where x can be A, V, H or S.

ADSL:

The first technology in the set is asymmetric DSL (ADSL), like a 56K modem provides higher speed in the downstream direction than in upstream direction. That is the reason it is asymmetric. Unlike the asymmetry in 56K modems, the designers of the ADSL specially divided the available bandwidth of the local lop unevenly for the residential customer. The local loop is connected to the filter which separates voice and data from each other. The data goes to ADSL modem which modulates it using DMT and creates the upstream and downstream channels.

HDSL:

The high-bit rate digital subscriber line(HDSL) was designed as an alternative to the T-Line uses alternate mark inversion (AMI) encoding, which is very susceptible to attenuation at high frequencies. This limits the length of a T-Line to 3200ft. for longer distances a repeater is necessary which increases the cost. HDSL uses two twisted pairs to achieve full duplex transmission. It uses traditional T1/E1 connections. The lower bit rate allows for a lower frequency range of operation that reduces channel loss and near end crosstalk.

SDSL:

The symmetric digital subscriber line (SDSL) is a one twisted-pair version of HDSL. It provides full-duplex symmetric communication. SDSL, which provides symmetric communication, can be considered an alternative to ADSL. ADSL provides asymmetric communication, with a downstream bit rate that is much higher than the upstream bit rate. Although this feature meets the needs of most residential subscribers, it is not suitable for businesses that send and receive data in large volumes in both directions.

VDSL:

The very high-bit-rate digital subscriber line (VDSL), an alternative approach that is similar to ADSL, uses coaxial, fibre-optic, or twisted-pair cable for short distances. The modulating technique is DMT.

Comparison of DSL schemes: