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Asymmetric vs symmetric DSL; ADSL modems

Short answer

Asymmetric DSL has higher downstream than upstream, for homes; symmetric DSL has equal rates, for businesses. ADSL modems are ATU-R at the customer and ATU-C inside the DSLAM, with splitters separating voice and data. DMT is the standard modulation, dividing the band into 256 subchannels with bit loading; early ADSL used CAP.

Differentiate between asynchronous and synchronous DSL. Explain the modems used in ADSL.

Here "asynchronous / synchronous DSL" is read as the usual asymmetric and symmetric DSL.

Asymmetric vs symmetric DSL

PointAsymmetric DSLSymmetric DSL
Data ratesDownstream much higher than upstreamSame rate both ways
ExamplesADSL, ADSL2+, RADSL, VDSLHDSL, SDSL, SHDSL
Typical rate8 Mbps down / 640 kbps up1.544 or 2.048 Mbps each way
UsersHomes: browsing, videoBusinesses: servers, T1/E1 lines
Voice on same pairYes (splitter)Usually no (HDSL)

Modems used in ADSL

An ADSL link has an ATU-R (ADSL Transceiver Unit – Remote) modem at the customer and an ATU-C (Central) modem inside the DSLAM at the exchange. A splitter at each end separates voice (0–4 kHz) from data.

PC--[ATU-R]--+--copper loop--+--[ATU-C in DSLAM]--ISP
Phone--------+   (splitter)  +--------------PSTN

Two modulation methods have been used:

1. CAP (Carrierless Amplitude/Phase)

  • A form of QAM in which the carrier is not sent; the signal is built digitally by two orthogonal filters.
  • Bands: voice 0–4 kHz, upstream about 25–200 kHz, downstream about 250 kHz–1.1 MHz.
  • Simple and cheap, but one wide band is badly hit by noise at any frequency. Used in early ADSL.

2. DMT (Discrete Multitone) – the ITU-T G.992.1 standard

  • The 0–1.104 MHz band is divided into 256 sub-channels of 4.3125 kHz.
  • Channel 0 is voice; channels 1–5 are guard band; about 25 channels (≈26–138 kHz) carry upstream and the rest (≈138 kHz–1.1 MHz) carry downstream, some kept for control.
  • Each sub-channel uses QAM. During training the modem measures SNR on each sub-channel and loads more bits on good ones and fewer or none on noisy ones (bit loading).
  • Implemented with FFT/IFFT (like OFDM).
amplitude
 |V|    upstream   |      downstream
 |o|  |||||||||||  ||||||||||||||||||||||
 +-+--+---------+--+--------------------+--> f
 0 4k 26k     138k                   1104k

DMT adapts to each line, so it gives higher and more reliable rates than CAP and is now the standard ADSL modem technique.

✓ Answer checked by Claude Opus 5.5 (high effort)

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