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Introduction to ADSL Technology

Understand the basics of Asymmetric Digital Subscriber Line technology, its family features, comparison with cable modems, system model, noise cancellation, line codes, and more.

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Introduction to ADSL Technology

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  1. Introduction to ADSL Technology by: Wenmei Zhao May 3, 1999

  2. ADSL — What is it? Why ADSL? ADSL Family Features ADSL vs. cable modem System Model Channel Modeling Noise Echo Canceller Line Codes DMT CAP Frame Structure Bell Atlantic ADSL References Outline

  3. ADSL — What is it? • ADSL — Asymmetric Digital Subscriber Line • High speed communications over twisted pair. • Concurrent with POTS (plain old telephone service). • Secure way of Internet access. • Originally standardized in ANSI (American National Standards Institute) T1.231-1993. • Currently standardized in ANSI T1.413-1998. • Growing really fast.

  4. Why ADSL? • Over the past 15 years, a thousand-fold transmission rate is realized. But it still does not meet today’s need. • Viewing a full-motion movie requires about 5Mbps. • Downloading Netscape requires 10 minutes. • ADSL: • 20 fold faster

  5. DSL Family • ADSL: Asymmetric DSL • It allows 6Mbit/s downstream and 640kbit/s upstream to a distance of 18kft. It uses a modulated analog carrier. • HDSL: High-bit-rate DSL • It uses two twisted pairs of standard subscriber copper telephone lines. It supports 1.544Mbit/s up to 12kft. It uses 2B1Q line code. • VDSL: Very High-bit-rate DSL • It is similar to ADSL, but supports about 26 Mbit/s to 3kft and 51Mbit/s to 1.2kft.

  6. DSL Family (cont.) • RADSL: Rate Adaptive DSL • Usually refer to a proprietary modulation standard designed by Globespan Semiconductor. It uses CAP. • SDSL: Single line DSL • It’s a single-pair version of HDSL. Supports T1/E1 on a single pair to a distance of 11,000ft. • CDSL: Consumer DSL • It’s a proprietary technology from Rockwell International.

  7. DSL Family (cont.) • EtherLoop: Ethernet Local Loop • It’s a proprietary technology from Nortel. • ADSL Lite • It’s a lower data rate version of ADSL. It is known as G.lite. Splitter is not required in the subscriber’s home. It offers up to 1.5Mbps downstream and up to 512Kbps upstream. ATM used as transport protocol.

  8. Features • Appropriate for Internet requirement • downstream data rate of up to 6-8Mbps • upstream data rate of up to 640kbps to 1Mbps • Convenient • always on • Parallel with voice service • no need to unplug telephone line • Secure access to Internet • no need to worry about your noisy neighbors

  9. Pro: Secure. “Point to point connectivity” of ADSL ensures the security of the service. Cable, by contrast, is shared media and is not secure at all. Bigger coverage area. Cheap. ADSL uses existing twisted pair, hence is cheap in installation and also cheap in monthly payment. Cons: Bandwidth. ADSL has about 1.1MHz BW due to loop limitations, while cable modem has about 745MHz BW. Bridge taps, DLCs, load coils can lead to problems. Mutual noise among different DSL lines, T1 lines. ADSL vs. cable modem

  10. System Model

  11. Channel Modeling(characteristic impedance, propagation constant, channel attenuation)

  12. Noise There are three main types of noise that affect DSL system performance: • NEXT (Near End Crosstalk) • FEXT (Far End Crosstalk) • Impulse Noise

  13. NEXT • When a transceiver sends a signal and a nearby transceiver at the same end “hears” the signal, it’s NEXT. A simplified NEXT model for N disturbers:

  14. FEXT • When a transceiver sends a signal and a transceiver at the far end “hears” the signal, FEXT occurs. A simplified FEXT model for N disturbers:

  15. Impulse Noise • Impulse noises are large surges of noise with short duration. The sources of impulse noises are not well understood yet. It is a very devastating noise if not handled well. • A concatenated code, using a 2-dimensional 8-state trellis code and a 4-error-correcting Reed-Solomon code with an interleaving depth of 18 symbols, was found to be suitable for eliminating impulse noise.

  16. Multiple Access • FDM (Frequency Division Multiplexing) • ECH (Echo Canceller with Hybrid)

  17. Line Code Two main contenders: • DMT — Discrete MultiTone • A multi-carrier system using Discrete Fourier Transforms to create and demodulate individual carriers. • CAP — Carrierless Amplitude and Phase • A version of suppressed carrier QAM.

  18. DMT • Existing ANSI and ETSI standards • Consists of up to 256 sub-channels, (also called tones or bins), of 4.3125KHz • upstream use 25-163KHz (bins 6 to 38) • downstream use 142KHz-1.1MHz (bins 33 to 255) • bins 16 (69KHz) and 64 (276KHz) are pilot tones. • Outperforms CAP in field trials • More expensive and complex

  19. DMT Line Code

  20. Observations Three Channels: • POTS channel • POTS channel is split off from the digital modem by filters, thus guaranteeing uninterrupted POTS. • High speed downstream channel • Its data rate depends on length of the copper line, its wire gauge, presence of bridged taps, cross talk, etc. • Medium speed upstream channel

  21. DMT Features • Discretely divides the available frequencies into 256 sub-channels or tones. • Incoming data is broken down into a variety of bits and distributed to a specific combination of sub-channels. • To rise above noise, more data resides in the lower frequencies and less in the upper frequencies.

  22. Downstream symbol rate: 4KHz FFT size: 512 Cyclic prefix: 32 Sampling rate: 2.208MHz Transmit power: 20dBm Highpass filter: 62.5kHz Upstream Symbol rate: 4kHz FFT size: 64 Cyclic prefix: 4 Sampling rate: 276kHz Transmit Power:7dBm Lowpass filter: 43.875kHz DMT Transmission Parameters

  23. DMT Block Diagram

  24. PSD of DMTPSD is useful for finding received signal power, thus useful for analyzing NEXT and FEXT noises.Upstream and downstream PSD models are:

  25. Frame Structure

  26. Frame Structure (cont.) A super frame is defined for every 68 IFFT/FFT operations.The super frame has a time duration of 68/4k=17ms for baud rate of 4kHz.

  27. CAP • Initial ADSL implementations were done using CAP • 1996 - 90% of world-wide ADSL implementation based on CAP • Variant of QAM - widely understood • Not yet incorporated in ANSI standards T1.413 or ETSI • Supported by GlobeSpan Technologies

  28. Downstream Constellation size: 64 Baud rate: 266.67KHz Throughput: 1.6 Mbps Sampling rate:1.0667MHz Transmit power: 12dBm Signal spectrum: 170 ~ 410KHz Upstream Constellation size: 16 Baud rate: 6KHz Throughput: 24Kbps Transmit power:-4.8dBm Signal spectrum: 96 ~ 102KHz CAP Transmission Parameters

  29. Bell Atlantic ADSL Packets Product Speeds Price Loop Qual. Infospeed640k d:640 kbps $39.95 12,000ft. u:90 kbps Infospeed1.6M d:1.6 Mbps $59.95 8,000-12,000ft. u:90 kbps Infospeed7.1M d:7.1 Mbps $109.95 8,000ft. u:680 kbps

  30. References • ADSL Tutorial: "ADSL Application Notes", by ADSL Forum at www.adsl.com. • ADSL Tutorial: "Twisted Pair Access to the Information Highway", by ADSL Forum at www.adsl.com. • "ADSL and DSL Technologies", Walter Goralski, 1998, ISBN: 0-07-024679-3. • "ADSL Forum System Reference Model", by ADSL Forum at www.adsl.com.

  31. References (cont.) • "Discrete Multitone vs. Carrierless Amplitude/Phase Line Codes", Aware white paper. • "ADSL: A New Twisted-Pair Access to the Information Highway", by Philip J.Kyees, etc., IEEE Communications Magazine, pp52-59, 1995. • "Evaluation of Near-End Crosstalk Noise Affecting ADSL Systems", by Marco Carbonelli,etc., TELECOM ITALIA.

  32. References (cont.) • "Comparison of Single-Carrier ad Multitone Digital Modulation for ADSL Applications, by Burton R. Saltzberg, IEEE Communications Magazine, Nov., 1998. • "Coded 64-CAP ADSL in an Impulse-Noise Environment --- Modeling of Impulse Noise and First Simulatin Results", by Werner Henkel,etc., IEEE Selected Areas in Comm., December, 1995.

  33. References (cont.) • "Forward Error Correction for Asymmetric Digital Subscriber Lines (ADSL)", by Kenneth J. Kerpez, Bellcore, GLOBECOM'91. • "Frequency Domain Echo Cancellation for Discrete Multitone Asymmetric Digital Subscriber Line Transceivers", by David Jones, IEEE Trans. On Communications, April, 1995. • etc.

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