1 / 57

Asymmetrical Digital Subscriber Line (ADSL)

S-72.1130 Telecommunication Systems. Asymmetrical Digital Subscriber Line (ADSL). Asymmetrical Digital Subscriber Line. Overview Historical development PSTN local loop as a high-rate digital transmission channel Physical level (modem technology) Modulation Coding Frame structures

amity
Télécharger la présentation

Asymmetrical Digital Subscriber Line (ADSL)

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. S-72.1130 Telecommunication Systems Asymmetrical Digital Subscriber Line (ADSL)

  2. Asymmetrical Digital Subscriber Line • Overview • Historical development • PSTN local loop as a high-rate digital transmission channel • Physical level (modem technology) • Modulation • Coding • Frame structures • Interoperability issues • Flavors of xDSL • Standardization overview interfaces standards terminals rx-tx channel services xDSL flavors (max DL rates): ADSL lite G.992.2 (1.5 Mb/s – no splitters) ADSL2 (12 Mb/s) HDSL (high-rate,2-4 copper pairs 2 Mb/s) SDSL (single line, 2Mb/s) READSL (Reach-Extended ADSL, 5Mb/s) VDSL (very high rate, 51 Mb/s)

  3. Overview

  4. Development of digital access in PSTN [6] • Through analog voice: • Connecting a voice-band modem (as V.90) • No switch or network • infra changes No modifications in exchange side Example: phones with build-in V.90 modems POTS: Plain Old Telephone Service • Through ISDN switch: • Yields Basic Rate Interface (BRI) • Fixed throughput 2B+D = 2*64 kb/s+16 kb/s ISDN exchange & ISDN modem The ISDN S interfaces can be used for extending ISDN services to locations that do not have ISDN access facilities. Each S interface port operates in full duplex mode over 4-wire twisted pairs at a range of up to 1,000 meters (support for nxBRI , T1 1.544 Mb/s …)

  5. Digital access in PSTN (cont.) • POTS FDM splitters separate voice and DSL channels Digital/analog switch (ADSL G.992.1) Using POTS splitters Requires new in-house wiring here Using digital switch • Intelligent switch recognizes in CO subscriber devices and adjusts its HW parameters (PSTN telephone, voice-band modem, DSL modem) accordingly

  6. History of ADSL Bell Labs develop OFDM to make traditional copper wires to support new digital services - especially video-on-demand (VOD) 1985 -- 1990 -- 1993 -- 1995 -- 1998 -- 1999 -- Phone companies start deploying High-Speed DSL (HDSL) to offer T1 service (1.544 Mb/s) on copper lines without the expense of installing repeaters - first between small exchanges Phone companies begin to promote HDSL for smaller and smaller companies and ADSL forhome internet access Evaluation of three modulation technologies for ADSL: QAM, DMT and CAP. DSL Forum established on 1994 Innovative companies begin to see ADSL as a way to meet the need for faster Internet access DMT adopted by almost all vendors following ANSI T1.413 - issue 2 (in contrast to CAP)ITU-T produced ADSL standards G.992.1 (G.full: 8M/640k) and G.992.2 (G.lite: 1.5M/512k)

  7. … history 2001 -- Number of DSL subscribers 18.7 million worldwide 2002 -- ITU-T completed G.992.3 and G.992.41 standards for ADSL2 2003 -- ADSL2plus released (G.992.5). It can gain up to 20 Mbps on phone lines as long at 1.5 km. 30 million DSL users worldwide 2004 -- VDSL2 standards under preparation in DSL forum 2005 -- VDSL2 standard verified (G.933.2) – symmetrical 100 Mb/s. 115 million DSL users. 2006 -- DSL Forum expects 500 million DSL users by the end of 2010.

  8. Evolution of digital access Pure Fibre Hybrid Fibre/Copper FTTH Enhanced Copper FTTx, VDSL2, ADSL2plus ADSL ISDN VoicebandModem Fig. from ref [8]

  9. Basic configuration of ADSL Fig. from ref [8]

  10. ADSL – Basic features • ADSL enables faster data communication over conventional copper cables that modems can provide (Telephone modem have moderate rates (56 kb/s) and cable modems have service problems if number of users is large) • Applies frequencies well above 4 kHz (up to 1 MHz) –transmission distance (typically 1 - 2 km) determines bit rate • In exchange side DSLAM (Digital subscriber line access multiplexer) is used to separate voice and data circuits • Basic ADSL (G.992.1) provides asymmetrical full-duplex data paths, higher downlink (DL) rates (DL: 128 kb/s… 8 Mb/s) ; (UL 64 kb/s… 1 Mb/s) • Performance depends on cables at hand! • Conventionally ATM used for data connections meaning that virtual circuits can be provided for different QoS specified service (as for instance high-speed internet, TV and telephone)

  11. ADSL – Basic features… • Nowadays high-speed Ethernet (100 Mb/s, 1000 Mb/s, 10 Gb/s; 100BASE-TX,1000BASE-T, 10 GBASE) techniques can provide cost-effective solutions • xDSL techniques developed to serve both symmetrical and asymmetrical traffic and different rates (STM and ATM supported by G.992.1 ADSL) STM-n: Synchronous Transfer Module (of SDH): DS-1,2: 1.544 Mb/s, 6.312 Mb/s ATM: Asynchronous Transfer Mode DL: Down Link - Down stream

  12. ADSL equipment Standard Telephone Lines Central Office Building ResidentialCustomer ADSL Rackof Line Cards DSLAM ADSL Modem or Gateway Fig. from ref [8]

  13. Applications Fig. from ref [8] Home networking: Home automation as lighting, heating/cooling control, Integrated control of home appliances, security control

  14. Typical ADSL scenario today Video Servers ADSL Internet Broadband Network 1.5 to 8Mbit/s 9.6 to 640kbit/s Telephone Network Live Broadcast Fig. from ref [8]

  15. Working with local loop cables

  16. ADSL challenge: local loop cables • Crosstalk: • Near-end crosstalk (NEXT) appears between TX and RX of the near-end • Far-end crosstalk (FEXT) appears between TX and RX of the far-end • Interference: other lines, overlapping RF-spectra • Bridged taps, loading coils • Weather-conditions (moisture, temperature) effect of crosstalk and line impedance • Attenuation! -Frequency dependent (next slides)

  17. Attenuation of twisted cables • Local loop cables have different wire thicknesses, e.g. 0.016 inch (24 gauge ~ 2r= 0.5 mm) • The longer the cable, the smaller the DSL rate: conductor diameter increases DS-1 DS-2 Twisted cable attenuations DS-1,DS-2: Digital Signal 1,2 Synchronous Digital Hierarchy (SDH) levels STS-1: Synchronous Transport Signal level-1, Synchronous Optical Network’s (SONET) physical level signal 19 gauge ~2r = 0.9 mm, 22 gauge ~2r = 0.64 mm26 gauge ~2r = 0.36 mm Practical xDSL data rates for 24-gauge twisted pair

  18. How ADSL meets local loop challenges? • Restricted bandwidth? • careful allocation of bits for each sub-carrier • Changing circumstances (weather, bridged taps)? • Adaptive setup phase (rate adaptation in setup) • High attenuation? • Usage of relatively high bandwidth for transmission • Compatibility to old POTS? • Own band for POTS by FDM (splitters) • Interference and cross-talk? • Coding • Interleaving • Modulation (OFDM/DMT) • Echo cancellation Note: loading coils mustbe removed from cablesin order for ADSL to work! Performance of G.992.1

  19. ADSL bandwidth allocation in local loop

  20. ADSL rates (DL) and channel frequency band allocation in local loop 2 Mb/s 4 Mb/s 6 Mb/s 8 Mb/s G.992.1 - G.992.2 - Two ways to allocate transmission band in PSTN local loop cables a) Frequency division multiplexing b) Echo Cancellation assigns the upstream band to over-lap the downstream, and separates the two by means of local echo cancellation (same method applied in V.32 and V.34 modems)

  21. Example: DMT* frequency allocation with ISDN [2] If no ISDN upstream downstream If 2B1Q ISDN upstream downstream If 4B3T ISDN upstream downstream 1104 kHz 276 kHz 25 kHz 138 kHz Carrier number 0 10 20 50 100 150 200 250 Pilot Sub-carrier spacing is 4.3125 kHz - 256 total sub-carriers Sub-carrier Frequency Meaning 0 0 Hz DC-not used for data 5 25 kHz lower limit for upstream data 18 80 kHz Approx limit for 2B1Q ISDN 28 120 kHz Approx. Limit for 4B3T ISDN 32 138 kHz upper limit for upstream data 64 276 kHz Pilot - not used for data 256 1104 kHz Nyqvist - not used for data POTS 80 kHz 120 kHz 2B1Q ISDN4B3T ISDN line-code types, for instance 2B1Q: 2B: two binary signals encoded by 1Q: 1 quaternary (4-level) signals *Discrete Multi-tone

  22. Modem technology

  23. Basic DMT-transceiver Note: Line transmits analog signal! Ref: Wikipedia

  24. Discrete Multi-tone (DMT) modulation [4] • Transmission band divided into sub-channels (BW = 4 kHz)their frequency difference equals symbol rate => enablesnon-interfering sub-carriers • Tone ordering: On initialization, test-tone determines number of levels in Quadrature Amplitude Modulation (QAM) for each subchannel (each can carry0 - 32 kb/s) • Number of subchannels 256 • Current downstream rates 256 kb/s ... 8 Mb/sdepending on line conditions and operator specifications in ADSL Discrete Multi-tone (DMT) modulation Tone ordering (bit-loading)

  25. Multi-tone modulation (cont.) • In channel activation phase different sub-channels (1-256) are allocated for their optimum rates (by adjusting number of levels in modulation) • DMT-ADSL supports both synchronous transfer modules (STM) of SDH and asynchronous transfer mode (ATM) stream. • ADSL modems offer two data paths: • Fast • low latency (2ms) • real-time traffic • Interleaved • low error rate • Reed-Solomon encoding (concatenated convolutional codes) at the expense of increased latency

  26. Block diagram of an ADSLmodem tone ordering (initialization) OFDM Transmitter Binary input Channel estimation Error correction coding Modulation (QAM) Pilot insertion Serial to Parallel Interleaving Adaptation to burst errors (applied for interleaved data) Adding guard interval Pulseshaping D/A Parallel to serial IFFT RF Tx DMT modulation Multipath & BW adaptation OFDM Receiver Deleting Guard interval Filter A/D Serial to Parallel FFT RF Rx Binary Output Time and frequency synchronisation Error correction coding Demodulation (QAM…) Channel Estimation Parallel to serial Interleaving

  27. Start-up phases of Rate Adaptive ADSL (RADSL) • RADSL modems apply sophisticated hand shaking to initiate transmissions that include • Activation: notice the need for communications • Gain setting/control: Adjust the power for optimum transmission and minimum emission • Channel allocation / bit rate assignment (DMT) • Synchronization: Clocks and frames to the same phases • Echo cancellation:(if used, required for both ends) • Channel identification (ISDN/POTS) and equalization

  28. Pros and cons of OFDM • Pros • enables accurate channel adaptation -> high capacity • resistance against link dispersion • resistance against slowly changing phase distortion and fading • resistance against multi-path using guard interval and cyclic prefix • resistance against frequency response nulls and constant frequency interference • resistance against burst noise • Cons • requires very accurate frequency synchronization in the receiver • requires accurate timing synchronization • signal has high peak-to-average power ratio (PAPR) -> demanding transmitter design and/or compensation methods

  29. ADSL sub-channels and frames

  30. ITU-T G.992.1: Asymmetric Digital Subscriber Line (ADSL) Transceivers • G.992.1 Targets: physical layer characteristics of ADSL interface for two-wire, twisted metallic cable pairs with mixed gauges (no loading coils, but bridged taps are acceptable) (min DL/UL ~ 6.144 Mbs/640 kbs) • A single twisted pair* of telephone wires is used to connect the ADSL transceiver unit (ATU)-C(central office) to the ATU-R(remote). • Transmission unit can simultaneously convoy: • downstream(C->R) simplex (broadcasting) high speed bearers, • low speedduplex bearers, • a baseband analogue duplex channel (POTS compatibility), • ADSL line overheads take capacity for • framing, • error control • operations and maintenance (O&M) • Bearer channels can coexist with voice band & ISDN (G.961 :Appendices I and II) that is separated with filtering (POTS splitter) / echo cancellation *ADSL2 offers bundling of cables for increased capacity

  31. ADSL subchannels [3] • ITU-T G.992.1 specifies DMT modem for ASDL applications • Downstream: • 2.208 MHz sampling rate, 256 subchannels at 0 … 1.104 MHz (simplex) • DMT symbol rate 4000 symbols /s. Each sub-channel is 4.3 kHz wide • max rate 32 kb/s per subchannel (compare to V.90 modem!) • Upstream: 275 kHz sampling rate, 32 tones 0 … 138 kHz • For instance, for ATM AS0 and LS0 usually applied as dictated by ATM QoS fast data path ASx: high-speed,downstream simplex nx1.54 Mb/s LSx: low-speed, duplex channels 160…576 kb/s crc: cyclic redundancy checkFEC f,i: (fast,interleaved): forward error correctionscram f,i: scrambling ATU-C: ADSL transmitter unit - central office interleaved data path ATU-C transmitter V-C interface

  32. ADSL frame structure [3] super frame boundaryidentification 68 DMT data symbols, ->symbol rate ~4000/sec - bearer channel allocation during initial setup determines ratio of interleaved and fast data frames (Nf,Ns) - fast byte takes care of CRC, O&M and sync. control: - 8 crc bits (crc0-7) supervise fast data transmission - 24 indicator bits (ib0-ib23) assigned for O&M functions

  33. Fast byte [3] Crc: Cyclic Redundancy Check for error detection/correction in super frameib: Indicator Bits assigned for error detection, loss of signal, remote defects, … eoc: Embedded Operations Control for O&M functions

  34. ADSL system total data rate • Total data rate = Net data rate + System overheads • The net data rate is transmitted in the ADSL bearer channels • ADSL system overheads: • an ADSL embedded operations channel, eoc (O&M) • an ADSL overhead control channel, aoc • crc check bytes • fixed indicator bits for O&M* • Reed-Solomon FEC redundancy bytes • These data streams are organized into ADSL frames and super-frames for the downstream and upstream data O&M: error detection, corrected errors, loss of signal, remote defects ...

  35. Reference models

  36. Generic DSL reference model (DSL Forum) • CP: Customers premises - local loop connects to switch (CO) • TE: Terminal equipment - PC or telephone • NT: Network terminal - DSL modem at CP • NID: Network interface device - all customer’s installation reside right from this point and telephone company's to the left in the diagram – over voltage protection, test access • CO: Central office • MDF: Main distribution frame - wire cross-connection field connects all loops to CO • LT: Line termination eg DSL modem • repeater: signal regeneration for transmission introduced impairments • local loop: in ADSL 2-wire connection between CO and CP CO CP MDF NID repeater Switch or multiplexer LT ADSL transceiver units repeater NT TE Local loop ATU-R ATU-C ATU: ADSL transceiver unit, -C : Central office, -R: Remote unit

  37. G.992.1 (ITU-T)/T1.413 (ANSI) reference model ATU ADSL Transceiver Unit ATU‑C ATU at the central office end (i.e. network operator) ATU‑R ATU at the remote terminal end (i.e. CP) ATU-x Any one of ATU-C or ATU-R NT1,2 Network terminals TA Terminal adapter SM Service module -The V-C and T-R interfaces are defined only in terms of their functions but they are not technically specified - T/S not defined

  38. Example 10 base T Ethernet 45 or 155 Mbit/s ATM orEthernet ISP Ethernet Hub/Switch CPE or PC ADSL Remote Router ADSL Linecard 10 base T, 100 base T... 10 base T Ethernet CPE: Customer premises equipment ISP: Internet service provider Fig. from ref [8]

  39. Interoperability issues

  40. Using ADSL • DSLAM provides access to LANs, WANs and other services at CO • ADSL (G.992.1 ) supports traffic over • ATM • STM • ISDN • Indirect support for example for • Ethernet • X.25 • Frame relay FRAD: Frame Relay Access Distributor DSU: Digital Subscriber Unit (Packet switching) CSU: Circuit Switching Unit STM: Synchronous Transport Module (in SDH) PRI ISDN: Primary rate ISDN: 30B+D (30x64k+64k)

  41. Standardization

  42. -examples: ITU: International Telecommunications Union - yields recommendations that may be adapted by companies International level Regional/national level -examples: ANSI (American Standards Institute) /ETSI (European Technical Standards Institute) Hierarchy of standards Multi-corporate level -examples: ADSL forum/ATM forum Corporate level -open or proprietary standard created by a company Standardization bodies Company based ADSL standards G.full International/national standardization G.lite Similar to IETF UAWG: Universal ADSL working group - strives to make ADSL more commercially adaptable SNAG: Service network architecture group See also: http://www.ktl.com/testing/telecoms/xdsl-standards.htm

  43. Topics of ITU-T G.992.1 • Basic capabilities specified in G992.1: • combined options and ranges of the simplex and full-duplex bearer channels • spectral composition of the ATU-C and ATU-R signals • electrical and mechanical specifications of the network interface • organization of transmitted and received data intoframes • functions of the operations channel • ATU-R to service module(s) interface functions • ATM support (Transmission Convergence Sub-layer) • Optional capabilities: echo cancellation, trellis coded modulation, transport of a network timing reference, transport of STM and/or ATM, reduced overhead framing modes

  44. ITU-T DSL standards Reference: DSL Forum: DSL Anywhere - Issue 2, Sep. ‘04

  45. xDSL flavors and performance comparison

  46. 8 Overview to xDSL techniques • -ATM / STM compatible • -2-wire compatible • - G.992.2 requires splitter and separate phone line from box to wall For short distances, applies ATM

  47. Overview to xDSL techniques (cont.) • -Channel associated signaling • -2- or 4-wire connections • -performance increase by cable bundling

  48. Performance comparison ADSL2 (12 Mb/s) ADSL (8 Mb/s) VDSL1 (very high rate, 51 Mb/s) HDSL (high-rate,2-4 copper pairs 2 Mb/s) SDSL (single line, 2Mb/s) READSL (Reach-Extended ADSL, 5Mb/s) 0 0.6 1.2 1.8 2.5 3.0 3.6 4.3 4.9 5.5 6.0 Distance/km Reference: DSL Forum: DSL Anywhere - Issue 2, Sep. ‘04

  49. Review questions • Briefly summarize methods that ADSL modems use to meet challenging local loop transmission • List the pros and cons of OFDM technology • Sketch and explain the two basic approaches to allocate transmission band in PSTN local loop cables • Summarize the development of digital (local loop) access in PSTN by starting from POTS and ending to digital switching • Explain the basic principle of discrete multi-tone modulation (DMT) • ADSL frame structure • Sources of ADSL system overheads • Describe the start-up phases of Rate Adaptive ADSL

  50. References [1] T. Starr, J.M. Cioffi, P.J. Silverman: Understanding Digital Subscriber Line Technology, Prentice-Hall [2] W.Y. Chen: DSL Simulation Techniques and Standards - Development for Digital Subscriber Line Systems, MacMillan Tech. Publishing [3] C.K. Summers: ADSL - Standards, Implementation and Architecture, CRC Press, page 55-66 from Edita [4] William Stallings: Data and Computer Communications (7th Ed), Prentice Hall [5] ANSI T1.413, issue 2 standard [6] Y. Chen: DSl – Simulation techniques and Standards, MacMillian Technical Pub. [7] ABCs_home_networking_final[1].pdf, available at www.dslforum.org/mktwork/mr/ABCs_home_networking_final.pdf [8] ADSL forum: Tutorial slideshow Note: - Informative tutorial of DSL at:www2.rad.com/networks/2005/adsl/main.htm -Matlab tutorial on DMT principle:http://cnx.rice.edu/content/m11710/latest

More Related