The DSL Forum was formed in December 1994 to promote the DSL concept and facilitate developement of DSL system architectures, protocols, and interfaces for major DSL applications. The ANSI, working group TIE1.4, approved the first ADSL in 1995. It supported data rates upto 6.1 Mbps. The ETSI contributed an annex to T1.413 to reflect European requirements. The ITU-T standards are most commonly referred to as G.lite (G.992.2) and G.dmt (G.992.1). The ATM Forum has recognised ADSL as a physical layer transmission protocol for unshielded twisted pair media.

ADSL, a modem technology, converts existing twisted pair telephone lines (subscriber loop) into access paths for multimedia and high speed data communications. ADSL can transmit up to 6 Mbps to a subscriber, and as mush as 832 kbps or more in both directions.

ADSL Frquency spectrum. ADSL divides the bandwidth of a twisted pair cable into three bands. The twisted pair cable used in telephone wire has a frequency spectrum of ADSL. Fig. 10.9 shows the frequency spectrum of ADSL.

ADSL uses various encoding methods to divide the available bandwidth of the channel into multiple subchannels. Earlier, FDM or Echo cancellation are used to divide the available channels. Presently, ADSL uses DMT encoding methods, which use QAM to divide the bandwidth of the channel into multiple subchannels with each channel transmitting information using QAM modulation. DMT uses the frequency spectrum from 26 kHz to 1.1 MHz for broad band data.

POTS 4 kHz

26 kHz upstream

138 K downstream 1.1 MHz

Fig. 10.9. Frequency spectrum of ADSL.

The frequency spectrum from 26 kHz to 138 kHz is used for upstream transmission, and the frequency spectrum from 1.38 kHz to 1.1 MHz is used for down stream transmission. The lower 4 kHz channel is separated by an analog circuit and used in POTS. The frequency spectrum above 26 kHz is divided into 249 independent subchannels, each containing 4.3 kHz bandwidth. 25 channels are used for upstream transmissions and 224 channels are used for downstream transmissions.

Topology For ADSL System. ADSL modem is connected to each end of twisted pair, one at the subscriber end and other at the central office. The ADSL modem at the exchange is called ATU-C (ADSL-Terminal Unit Central office) and the ADSL modem at the subscriber end is called ATU-R (ADSL-Remote). Fig. 10.10 shows the ADSL connection between exchange and subscriber. ADSL modems with various speed ranges and capabilities available. Fig. 10.10 shows the modem with 1.5 Mbps to 9 Mbps downstream bit rate and 16 to 640 kbps duplex channel.

ADSL modem

1.5 to 9 Mbps 16 to 640 kbps

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