The term ‘MODEM’ is derived from the words Modulator and Demodulator. A modem contains both modulator and demodulator. The digital data is first converted to the signal which is compatible to the telephone lines. This conversion is done by a special communication box called “MODEM”. It is the box that makes a digital data signal compatible with any non-digital system and medium besides the phone system. An example is a modem which makes the data bits compatible with a microwave radio system. The role of MODEM can be better understood by the fig. below.

DSL:-

DSL stands for digital subscriber line and it had been developed for providing a high-speed access to internet. It supports the high speed digital communication over the existing local loops. DSL is a set of technology differing by first letter and the set is referred as xDSL where x is replaced by A, V, H and 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 supporting up to 768 kbps in each direction. 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. It provides a range of bit rates (25 to 55 Mbps) for upstream communication at distances of 3000 to 10,000 ft. The downstream rate is normally 3.2 Mbps.