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College ADSL Presentation


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A presentation on the physical layer basics of ADSL. I presented this to a couple of colleges in the southeast as a guest lecturer, and to the local chapter of the IEEE Signal Processing Society.

A presentation on the physical layer basics of ADSL. I presented this to a couple of colleges in the southeast as a guest lecturer, and to the local chapter of the IEEE Signal Processing Society.

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  • ABOUT THE AUTHOR: Jim Webster is a Senior Staff Engineer in the PCD Systems Group. His background includes over 15 years of experience designing real-time software for various communications applications. Jim graduated from Auburn University with a BS in Computer Engineering in 1984. He currently works on ADSL technology, standards, interoperability, and software design issues at Conexant’s Design Center in Huntsville, Alabama. He can be reached at: [email_address] ACKNOWLEDGEMENTS: Thanks to Yong Song and David Lau for providing some of the graphics. Thanks to Jerry Viviano and Carlos Hall for reviewing and providing inputs for the presentation.
  • This is intended to be a technology training class, focusing on the standardized ADSL technology that is currently being deployed. A prerequisite for the class is general familiarity with communications and modem technology. No prior ADSL knowledge is assumed. For those who are not familiar with ADSL, this class can be an introduction to ADSL. For those who may have already read about or used ADSL, it will describe some of the technology, features, and issues in greater detail. The class is not intended as design engineering training. It does not go into great detail concerning ADSL modulation theory or algorithms. It also does not the details of the hardware or software components that implement ADSL modulation in our products. Rather, it is intended to provide a foundation for those who will be selling and discussing ADSL, setting up demos, and using ADSL in the field. It also does not cover marketing issues such as market size, competitive analysis, deployment, product plans, or pricing.
  • ABBREVIATIONS AND TERMS: Asymmetric: The network-to-user data path is much faster than the user-to-network path. DMT: Discrete Multitone modulation. The standardized ADSL modulation technique that breaks the spectrum into a number of discrete bands (or subcarriers). Each of these subcarriers then uses a QAM method to encode data. ANSI: American National Standards Institute ITU: International Telecommunications Union. An organization associated with the United Nations which generates recommendations on telecommunications standardization. G.dmt: G.992.1 G.lite: G.992.2 NOTES: Technically, ADSL is a form of “Broadband ISDN” as defined by the ITU, and it refers to ADSL as such in its recommendations.
  • TERMS: POTS: Plain Old Telephone Service Data Rates: The ADSL standards actually only require 6.144 Mbps downstream and 640 Kbps upstream. Rate Adaption: Allows the protocol to automatically adjust the data rate based on various factors, such as the line conditions and the maximum configured rates at the transmitter and receiver. ATM: Asynchronous Transfer Mode. A protocol suite for transporting data in discrete “cells” of a fixed length. Intended to be fast, provide for efficient multiplexing of different services (e.g. data, voice, video, etc.) over the same channel, with each service having very finely specified Quality of Service that determines the multiplexing of the cells from each service onto the channel. (More on this later). However, there is no retransmission of data at the ATM layer; cells that are corrupted, or that cannot be transmitted or received, may be discarded. Link Layer: Layer 2 of the the 7-layer ISO protocol model (which ranges from the Physical Layer to the Applications Layer). In ADSL, DMT is the Physical Layer modulation technique. The Link Layer defines how the data medium is accessed, how data is framed, paced, and may contain an error control protocol. Examples of other link layer protocols include HDLC or V.42. Modem: Technically, ADSL transceivers are modems, since they MODulate and DEModulate a signal in order to pass data.
  • TERMS: ATU: ADSL Transceiver Unit RT: Remote Terminal CPE: Customer Premises Equipment Splitter: A special highpass/lowpass filter that separates the high and low frequency bands of the combined ADSL telephone line signal. The low band goes to the POTS telephone set; the high band to the ADSL modem. This prevents either signal from interfering with the other. DSLAM: In the central office, a unit that generally contains a number of ADSL modems in a rack, and concentrates the data to and from each into a high-speed interface such as T1/E1, T3/E3, Ethernet, ATM-25, or OC-3 (fiber optic).
  • POTS: Plain Old Telephone Service. Note that Each band is separated by an unused section of the spectrum, to prevent interference. This also makes echo canceling not essential. ADDITIONAL INFO: An alternative approach that can be used is called “Overlapped Spectrum”, in which the downstream and upstream bands partially overlap. This option allows additional bandwidth to be allocated to the downstream path, but requires very robust echo cancellation. Another variation on the above spectrum allocation is for Basic Rate ISDN (BRI) over ADSL (also known as “Annex B”, referring to the ITU G.dmt specification. This approach is used in Europe but generally not in North America. It allows ISDN (which has a frequency range of 0 - 80 kHz) to coexist on an ADSL line instead of POTS.
  • Each subcarrier is individually modulated with a QAM signal, that allows up to 15 bits (4 to 32kbps) SNR: Signal-to-Noise Ratio During Training, each subcarrier’s SNR is measured, and that is used to determine how many bits to allocate per subchannel. “ Full Rate” refers to T1.413 and G.dmt, since they can carry rates up to 8 Mbps. G.lite consumes less power and was intended to be less expensive to implement than “full-rate” ADSL; therefore, it does not use any bandwidth above subchannel 127 (550.2 kHz). Technically, this limits its downstream rate to around 4 Mbps, though often it is further restricted to 1.5 Mbps.
  • This diagram shows how the frequency spectrum of the telephone line is divided into “subcarriers”, also known as “subchannels” or “bins”, which are equally-spaced frequency bands of 4.3125 kHz. They are numbered from 0-255, for up to 256 bins. (256 * 4.3125 = 1.104 MHz)
  • TERMS: Local Loop: Telephone line to the central office. PSTN: Public Switched Telephone Network The ATM Access switch routes ATM cells between various servers (including the DSLAM), and other LAN and WAN connections.
  • ATU-R: ADSL Transceiver Unit - Remote (Client modem) The internal wiring to the ATU-R is standard twisted pair (POTS) wiring. After going through the splitter, the ADSL and the POTS lines each require one pair of twisted wires. In North America, the installer will either wire the ADSL pair to the outer (extra) two wires of the existing POTS jacks, keeping the inner pair dedicated to the existing telephones. Or, in some cases, a new jack will be installed for the ADSL, with the signal appearing on either the inner or outer pairs. Some ADSL products have the ability to automatically sense which pair of wires in the jack (inner or outer) the ADSL appears on.
  • Even though in the diagram both POTS phones are shown with micro-filters, not all POTS equipment requires them. Typically, in a G.lite application, the ADSL appears either on the outer (extra) two wires of the existing telephone jack (if an existing POTS line remains), or may appear on the inner (normally used) pair of wires on installations where it replaces the existing POTS service.
  • The training sequence is the most complex and critical during ADSL operation, since it determines the rates, features, etc. It normally takes about 10-15 seconds; though if unsuccessful on the first attempt, most ATU's will continuously retry until a connection is established. G.hs (handshake) uses a relatively simple modulation scheme to pass data frames that contain parameters. These parameters are used to negotiate the operating modes and options to be used during training and operation of the unit.
  • The number of bits allocated to each channel is determined by the SNR measurement made. This way, each channel may contain the optimum number of bits, and interference in a particular band (e.g. from a nearby radio station) only limits the number of bits on those subchannels, rather than interfering with the entire spectrum. On longer loops, as the high frequencies are attenuated, the SNR on the higher subcarriers is low, and few or no bits may be allocated to them, resulting in a lower data rate. Another factor that determines the data rate is the SNR Margin that is specified by the ATU's. This margin is the average SNR above what is needed to carry the data if there is no additional interference. The higher the margin is set, the more robust the data, but the few number of bits that can be sent on each subchannel. For example, if the margin is set to 4db, more bits may be allocated on a channel than if the margin is set at 6db, but the signal will be more susceptible to noise. The ATU-C (CO) ultimately determines the maximum data rate. It may be set to artificially limit the data rate; for example, to provide a more economical ADSL service offering. “ Showtime” is the term used for the normal ADSL data mode. Perhaps this reference originated when the primary use for ADSL was envisioned to be the delivery of video on demand.
  • Each data frame is actually 68/69 * 250 microseconds, since there are 69 total frames per Superframe (exactly 17 ms). The Sync Frame allows the receiver to synchronize on the incoming frame by detecting a predefined bit pattern. Forward Error Correction is a technique for adding enough redundant information to the data such that a certain number of bit errors may be corrected by the receiver, and an even greater number of bit errors may be detected. In ADSL, a variable number of FEC bytes may be reserved, depending on the desired tradeoff between error correction/detection and the amount of overhead. Reed-Solomon is a form of Forward Error Correction which is used in DMT.
  • CRC: Cyclic Redundancy Check. Additional (overhead) bits that are added to a data stream to allow the receiver to determine if any bits in the data has been corrupted. The CRC value that the receiver calculates on the transmitted data should match the CRC value in the CRC byte; else the frame is invalid, and this constitutes a “CRC Error”. CRC works by statistics; it can identify most errors in a data stream, but not all. The more CRC bits are allocated vs. data bits, the greater the probability that an error can be detected. The number of physical layer CRC errors (indicated during a data error at the physical layer) over time is one of the primary measurements of the quality of the connection. Most test standards specify that less than 1 in 10e-7 bits may be in error for a connection to be considered adequate. In T1.413 and G.dmt, the data may be sent on either the Fast or Interleaved channel. In G.lite, only the interleaved channel is used.
  • In the U.S., ANSI is the national body that is designated by the ITU to provide technical contributions that may then become or affect ITU Recommendations. Each country that is an ITU member may have a similarly-designated body for filtering and voting on recommendations before they go to the ITU. One disadvantage of the ITU is that it can be slow, since recommendations must pass through a two-stage process and also be voted on by all ITU member bodies. The ATM Forum and DSL Forum each address issues that affect their respective technologies, and also promote those technologies. They typically move more quickly than the ITU, and their technical recommendations may become de-facto standards or affect future ITU recommendations. Conexant is a member of all of the above standards bodies.
  • “ Full Rate” generally means capable of operation up to 6 or 8 Mbps. Issue 1 was conceived before the internet, and its possible data rates were multiples of the T1, E1, and H0 data rates. It had a maximum rate of 6.1 Mbps downstream and 640 kbps upstream. It was also oriented towards Synchronous Transfer Mode or STM, rather than ATM.
  • Splitterless: both the ADSL and POTS share the same line all the way to each CPE (phones and ADSL modems). The advantage is that less wiring changes are required; the disadvantage is that steps must be taken to prevent or minimize the interference between ADSL and POTS. G.hs allows parameters such as operational modes, maximum data rates, optional features to be supported, and so forth to be negotiated. The negotiation takes the “least common denominator” subset of features that both ends support when determining the connection parameters. G.test specifies ADSL operational and loop (performance) tests for ADSL. Most of these test scenarios are very similar to those specified in the appendices of T1.413. G.ploam (ploam = “Physical Layer OAM”), refers to the Operations and Maintenance channel of ATM. In ATM, the lowest levels of OAM are done by sending bits of information at the physical layer; therefore, the lower layer OAM “flows” (called F1 and F2) are specific to each media type (T1, E1, T3, ATM-25… and ADSL). G.ploam specifies how the F1 and F2 OAM flows are conveyed within the ADSL superframe.
  • G.dmt is actually specified in ITU Recommendation G.992.1.
  • WEB SITES: (xdsl news and commercial information site) (DSL Forum) [formerly] BOOKS: (in order of increasing technical complexity): “ ADSL: Standards, Implementation, and Architecture” by Charles K. Summers (1999) “ Implementing ADSL” by David Ginsburg (1999) “ ATM The New Paradigm for Internet, Intranet, and Residential Broadband Services and Applications”, by Timothy Kwok (1998) “ DSL: Simulation Techniques and Standards Development for Digital Subscriber Line Systems” by Walter Chen (Macmillan, 1998) “ ADSL/VDSL Principles: A Practical and Precise Study of Asymmetric Digital Subscriber Lines and Very High Speed Digital Subscriber Lines” by Dennis Rauschmayer, (1999) WHITE PAPERS: ATM Tutorial by Jim Webster (email me if you’d like a copy) “ An Interoperable End-to-End Broadband Service Architecture over ADSL Systems” (UAWG White Paper, June, 1997, V1.0) (email me if you’d like a copy)
  • Transcript

    • 1. ADSL Physical Layer Basics Overview IEEE Eastern NC Signal Processing Society Sept 12, 2007 Jerry Viviano
    • 2. Introduction
      • Goals
        • Training on ADSL Technology Basics
      • Scope
        • Will include standard ADSL modes
        • Will describe physical layer
        • Will not cover applications
        • Will not cover marketing issues
        • Will not cover other types of DSL
    • 3. ADSL
      • Asymmetric Digital Subscriber Line
      • Modulation Scheme: Discrete Multi-tone
      • Types of DMT ADSL:
        • Proprietary
        • ANSI T1.413
          • Issue 1 and Issue 2 - both are “full rate”
            • Issue 1 rarely used today
        • ITU G.dmt (“full-rate”)
        • ITU G.lite (“consumer” - lower max rate)
          • Rarely used
    • 4. ADSL Features
      • Coexists with POTS on the same line
      • Data Rates:
        • Max. ~8 Mbps down, 1 Mbps up
        • Auto Rate Adaptation depending on line
        • Rate limits can be, and often are intentionally limited by Central site modem
      • Typical Maximum reach: 18 kFt (~5.5 km)
      • ATM now typically used as the link layer
    • 5. ADSL Features (Cont’d)
      • ATU-R (Remote, or Client)
        • Also called “RT” or “CPE”
        • High receive rate, low transmit rate
        • May require μ Filters for separating POTS
      • ATU-C (Central Office)
        • Also called “CO”
        • High transmit rate, low receive rate
        • Splitter required for POTS interface
        • Typically concentrated in a ‘ DSLAM ’
          • D igital S ubscriber L ine A ccess M ultiplexer
    • 6. ADSL Spectrum Usage 30 134 1104 Frequency, kHz 0 4 Downstream ADSL Channel Upstream ADSL Channel POTS 138
    • 7. DMT Modulation Features
      • 4.3125 kHz per subcarrier (subchannel)
      • Up to 255 subchannels (aka bins)
      • Capacity per subchannel is 2-15 bits
      • Number of bits/channel depends on SNR
      • Bandwidth allocation:
        • POTS: DC to channel 1 + 3 Guard Channels
        • Upstream: channels 5-31 – 21:134 kHz
        • Downstream:
          • Full Rate (T1.413 & G.dmt): Ch. 32-255 138:1104 kHz
          • G.lite: Ch. 32-127 138:548 kHz
    • 8. ADSL Modulation: Subcarriers 1.1 MHz DC 4312.5 Hz 255 Subcarriers Upstream Downstream
    • 9. Simple ADSL Network Diagram DSLAM ATM Access Switch Internet Broadband Access Server Local Loop PSTN
    • 10. What Happened to the 64 kbps limit?
      • With old voiceband modems,we were asymptotically creeping towards a 64 kbps limit for years. Now we’ve suddenly jumped to 8 mbps!
      • What happened???
    • 11. What Happened? (cont.)
      • The 64 kbps limit was not a limit of the telephone wires themselves.
      • The 8-bit Analog-to-Digital and Digital-to-Analog converters on one or both ends were sampling at 8000 Hz.
      • 8 bits/sample * 8000 samples/sec = 64,000 bits/sec
      • ADSL only uses the wire, not the ADC’s and DAC’s
      • ADSL supplies its own ADC’s and DAC’s, typically 14 bits/sample running at 2.2 Msamples / second.
    • 12. What Happened? (cont) 64 kbps Voiceband ADC/DAC ADSL CO Modem “ DSLAM” ADSL RT Modem 64 kbps Telco Switch 64 kbps Voiceband ADC/DAC To ISP / Internet Twisted 26 Gauge to Your House Telco V.90 56 kbps V.90 56 kbps ADSL 8 mbps!
    • 13. Actual Central Office DSLAM LOW PASS FILTERS LINE TERM. CARDS
    • 14. ATU-R (Client) Wiring Detail Splitter ATU-R to Local Loop
    • 15. “ Splitterless” Client Wiring Detail ATU-R to Local Loop  Filter  Filter
    • 16. DMT in Layman’s Terms
      • It’s like having 230 singers in your living room at the same time, all singing different songs. There are also 230 guests in the room. Each guest hears only one singer clearly and efficiently, and noe of the others.
    • 17. Sending Digital Data - What Are Symbols?
      • A symbol is a “Doo-Dad” sent from the transmitter to the receiver to imply a specific set of bits.
      • Symbols could be anything -
        • Voltages
        • Frequencies
        • Smoke Signals
        • Fruits
        • Animals
    • 18. Sending Digital Data - What Are Symbols?
      • Symbols could be animals. Let’s send the bits ‘1100’.
      Transmitter Encodes and Sends Symbol 0010 -> Chihuahua 1001 -> Chicken 1100 -> Monkey Symbol Travels to Receiver Receiver Receives and Decodes Symbol Monkey ?? Oh! That means someone sent 1100 !!!
    • 19. Sending Digital Data - What Are Symbols?
      • But Cary has city ordinances against animals.
    • 20. Sending Digital Data - What Are Symbols?
      • So, they use complex numbers as symbols instead.
      Transmitter Encodes and Sends Symbol 0010 -> 1+ j1 1001 -> -5 + j1 1100 -> 3 – j1 Symbol Travels to Receiver Receiver Receives and Decodes Symbol 3 – j1 ?? Oh! That means someone sent 1100 !!!
    • 21. Sending Digital Data - What Are Symbols?
      • The complex number symbol is further encoded into a sinusoid of a specific amplitude and phase.
      • This is QAM!
      • I + jQ -> A * sin ( ω t + θ )
      • Sqrt ( I 2 + Q 2 ) * sin{ ω t + tan -1 (Q/I)}
      • =
      • I * cos( ω t ) + Q * sin( ω t )
      • Or something like that!
    • 22. Single QAM Transmitter/Receiver I 1 + jQ 1 I 1 Q 1 cos ( ω 1 t) sin ( ω 1 t) cos ( ω 1 t) - sin ( ω 1 t) I 1 Q 1 I 1 + jQ 1 Lo-Pass Filter Lo-Pass Filter
    • 23. Double QAM Transmitter/Receiver I 1 + jQ 1 I 1 Q 1 cos ( ω 1 t) sin ( ω 1 t) cos ( ω 1 t) - sin ( ω 1 t) I 1 Q 1 I 1 + jQ 1 I 2 + jQ 2 I 2 Q 2 cos ( ω 2 t) sin ( ω 2 t) cos ( ω 2 t) - sin ( ω 2 t) I 2 Q 2 I 2 + jQ 2 Lo-Pass Filter Lo-Pass Filter Lo-Pass Filter Lo-Pass Filter
    • 24. DMT QAM Transmitter/Receiver IFFT FFT Transmitter Receiver I 1 + jQ 1 I 2 + jQ 2 I 255 + jQ 255 I 1 + jQ 1 I 2 + jQ 2 I 255 + jQ 255
    • 25. Actual 16 and 64 QAM Constellations
    • 26. Actual 128 and 4096 QAM Constellations
    • 27. Bit Allocation Tables
      • Display how many bits are assigned to each of the 255 channels.
      • A channel with many bits indicates that there is plenty of clean energy received on that channel.
      • A channel with few bits indicates that either there is little energy received, the signal is not clean, or both.
    • 28. Power Spectral Density @ 0 kFt Note strong lobe of low freq. energy from our transmitter.
    • 29. Bit Allocation Table @ 0 kFt
    • 30. Power Spectral Density @ 10 kFt Note strong lobe of low freq. energy from our transmitter.
    • 31. Bit Allocation Table @ 10 kFt
    • 32. Bit Allocation Table @ 18 kFt
    • 33. Typical Bit Allocation Table
    • 34. Time Domain Equalizer (TEQ)
      • The job of the TEQ is to adaptively invert the overall impulse response of the following elements:
        • Far End Transmit Filter Impulse Response
        • Channel Impulse Response
        • Near End Receive Filter Impulse Response
      • Reduced impulse response results in less inter-symbol interference –> Better data rates.
    • 35. Typical Adapted TEQ
    • 36. Frequency Domain Equalizer (FEQ)
      • The job of the FEQ is to compensate for the gain and phase characteristics resultant of the combination of the following elements:
        • Far End Transmit Filter
        • Channel Filtering Effects
        • Near End Receive Filter
        • Time Domain Equalizer
    • 37. Typical Adapted FEQ @ 0 kFt
    • 38. Typical Adapted FEQ @ 10 kFt
    • 39. ADSL Training Sequence
      • Idle mode
      • Activation and Acknowledgement
        • For initial activation, synchronization, and parameter exchange
          • T1.413: uses a tone-based protocol
          • G.dmt and G.lite: Use G.hs (handshake)
      • Transceiver Training
        • Setting analog gains
        • Clock recovery
        • Equalizer training
    • 40. ADSL Training Sequence (Cont’d)
      • Channel Analysis
        • SNR measurements of each channel
        • Bit allocation per channel
      • Exchange
        • Negotiate data rates and other parameters
        • Exchange of bit and gain information
        • Entry into normal data mode
      • After ~7 seconds, “Showtime”
    • 41. ADSL Data Frame
      • Approx. 250  s per frame (constant)
      • 68 frames per superframe (17 ms)
        • Frame 0 to 67 contain data
        • Plus, frame 68 is sync frame (no user data)
      • Data Frames contain data plus overhead:
        • FEC - Forward Error Correction (Reed-Solomon)
        • Synchronization Control
        • Embedded Operations Channel (EOC)
        • ADSL Overhead Channel (AOC)
        • Indicator Bits (e.g. for line conditions)
    • 42. ADSL Data Frame (Cont’d)
      • Frame 0 contains an error detection CRC
      • Two options for data transfer:
        • Fast:
          • sequential data, low latency
          • susceptible to noise bursts
        • Interleaved:
          • data bytes spread over several frames
          • noise immunity, but higher latency
      • Frames can contain both Fast and Interleaved data (2 separate channels)
    • 43. ADSL Standards Bodies
      • ANSI: T1E1 committee
        • Physical Layer specs, testing
      • ITU
        • Physical Layer, testing, maintenance
      • ATM Forum
      • DSL Forum (formerly ADSL Forum)
        • Testing and Interoperability
        • Auto Configuration
        • Management
    • 44. ANSI T1.413
      • “ Full-Rate”, requiring a splitter or μ Filters
      • Issue 1: 1995
        • Telephony-centric rates, sync mode data
        • Designed with Video-on-Demand in mind
      • Issue 2: 1998
        • Added rate multiples down to 32 kbps
        • More Data-centric for Internet applications
        • Added explicit support for ATM mode
        • 8 Mbps down, 1 Mbps up
    • 45. ITU ADSL Standards
      • G.dmt (G.992.1):
        • full rate, splitter required
      • G.lite (G.992.2):
        • reduced rates and power, splitterless
      • G.hs (G.994)
        • handshake protocol for parameter negotiation during training sequence
        • Used in both G.dmt and G.lite
      • Others: G.test, G.ploam
    • 46. G.dmt
      • Similar to T1.413
        • Full Rate (up to 8 mbps)
        • Requires POTS splitter or μ Filters
      • BUT: Uses the G.hs handshaking protocol
        • Added flexibility
        • Easier Interoperation with G.lite equipment
      • Will eventually replace T1.413
    • 47. Summary
      • ADSL Technology is standardized
      • Work is ongoing to address issues
      • Demand and Potential is very great
      • Additional Information (see notes):
        • Web
        • Books
        • Papers
      • QUESTIONS?