Asymmetrical Digital Subscriber Line (ADSL): Asymmetrical Digital Subscriber Line (ADSL)


Asymmetrical Digital Subscriber Line: rx-tx channel standards Asymmetrical Digital Subscriber Line Background motivation for developing ADSL historical development DSL end-to-end environment and reference model Line environment - characteristics of local loop (LL) Why conventional modems don’t work so well in LL? Modem technology in ADSL DSL flavors: ISDN (!), HDSL, ADSL, VDSL ... Standards terminals interfaces services


Motivation for developing ADSL: Motivation for developing ADSL Need for high-speed Internet access - also telephone modem speeds have peaked and cable modems have turned out DSL means methods to transmit high speed data to local loop by using unshielded 2-wire twisted pairs DSL allows rates varying from 160 kb/s up 50 Mb/s on down link (DL) depending on technology used! In the most popular commercial ADSL 512 kbit/s upstream and 2048 kb/s downstream Different operation modes developed to serve symmetric and asymmetric traffic requirements and different rates (STM and ATM supported by ADSL)


History of digital access in PSTN: History of digital access in PSTN Through analog voice: Connecting a voice-band modem (as V.90) No switch or network infra changes Through ISDN switch: Yields basic rate access (BRI) fixed throughput 2B+D The first DSL technique


Digital access in PSTN (cont.): Digital access in PSTN (cont.) POTS FDM splitters separate voice and DSL channels Requires new in-house wiring here Using POTS splitters Digital/analog switch Using digital switch Next generation intelligent switch recognizes subscriber devices and adjusts its HW parameters (PSTN telephone, voice-band modem, DSL modem)


Comparing modem technologies: Comparing modem technologies


Short history of ADSL: Short history of ADSL 1985 -- 1990 -- 1995 -- 1998 -- 1999 -- Bell Labs discovers a new way to make traditional copper wires support new digital services - especially video-on-demand (VOD) Phone companies start deploying High-Speed DSL (HDSL) to offer T1 service 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 for home internet access Innovative companies begin to see ADSL as a way to meet the need for faster Internet access DMT was adopted by almost all vendors following ANSI T1.413 - issue 2 (in contrast to CAP) ITU-T produced UADSL G.992.2 (G.lite) and G.922.1 (G.full) 1993 evaluation of three major technologies for ADSL: QAM, DMT and CAP


Generic DSL reference model: Generic DSL reference model CO: Central office 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 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 Switch or multiplexer LT MDF repeater NID NT TE CO CP repeater Local loop ATU-R ADSL ATU-C


ADSL and ADSL-litereference model: ADSL and ADSL-lite have the major difference in the missing FDM splitter This causes lower rates for ADSL-lite but makes it cheaper to install ADSL and ADSL-lite reference model POTS phones POTS phones


DSL access multiplexer (DSLAM): DSL access multiplexer (DSLAM) DSLAM provides access to LANs, WANs and other services DSLAM consists of subscriber links (ATU-R to ATU-C) connections to other DSL/broadband-circuits interfaces to ISDN exchange


Using DSLAM: Using DSLAM


Slide12: Using ADSL


What is specified in ADSL standard?ANSI T1.413 ADSL reference model:: What is specified in ADSL standard? ANSI T1.413 ADSL reference model: Standard specifies interfaces and units as for example ATU-R: ADSL transceiver unit - remote terminal ATU-C: ADSL transceiver unit - central office terminal U-C (2), U-R (2) In T1.413 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 by T1.413 Cross connections PC SM: service module


ADSL challenge: bad quality local loop cables: ADSL challenge: bad quality local loop cables Attenuation Crosstalk Near-end crosstalk (NEXT) appears when same frequency band used for UL and DL - between A-A Far-end crosstalk (FEXT) appears in the link A-B Interference: other lines, overlapping RF-spectra Bridged taps, loading coils Weather-conditions (moisture, temperature) affect crosstalk and line impedance


Modeling the loop cable: Modeling the loop cable


Cable attenuation: Cable attenuation


DSL Data rates vrs distance: DSL Data rates vrs distance 0.4 mm (26 AG) twisted pair


ADSL meets local loop challenges: ADSL meets local loop challenges Restricted bandwidth careful allocation of bits for each sub-carrier Changing circumstances (whether, bridged taps) Adaptive setup phase High attenuation Usage of relatively high bandwidth for transmission Compatibility to old POTS Own band for POTS by splitters Interference and cross-talk Coding Interleaving Modulation (OFDM/DMT) Echo cancellation Note: loading coils must be removed from cables in order to ADSL to work


Reference: A baseband system: Reference: A baseband system


Modem parts: Modem parts Analog parts analog transmit and receiver filters DAC, automatic gain control, ADC Digital parts modulation/demodulation coding/decoding Reed-Solomon Trellis bit packing/unpacking (compressed transmission) framing scrambling


Modem technology: Modem technology Conventional modem modules Constellation mapping Interleaving (convolutional) Symbol/bit conversion Timing recovery Advanced techniques for DSL Carrierless AM/PM (CAP) or QAM line codes (97% of USA installations apply this method) Fast Fourier Transforms for Discrete Multi-Tone Modulation (DMT) - the dominant method tone ordering -> peak-to-average ratio (PAPR) decrease channel equalization water pouring bit allocations guard intervals Turbo - coding Adaptive echo canceller


RADSL start-up phases: RADSL start-up phases RADSL (rate adaptive DSL) 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 Synchronization: Clocks and frames to the same phases Echo cancellation Channel identification and equalization. In DMT modulation during the handshaking active channels are decided and bit rates assigned for them


Multi-tone modulation (cont.): Multi-tone modulation (cont.) In channel activation phase different sub-channels are allocated for their optimum rates (by changing number of levels in modulation) DMT-ADSL supports both synchronous transfer mode (STM) and asynchronous transfer mode (ATM, AS0 used for primary cell stream) DMT defines two data paths: fast and interleaved Fast low latency (2ms) real-time traffic Interleaved low error rate Reed-Solomon encoding (convolutional codes) at the expense of increased latency


ADSL is based on OFDM/DMT: ADSL is based on OFDM/DMT Error correction coding Interleaving Modulation (QAM) Pilot insertion Parallel to serial Pulse shaping D/A Adding Guard interval RF Tx Binary input Error correction coding Interleaving Demodulation (QAM…) Channel Estimation Serial to Parallel Filter A/D Deleting Guard interval RF Rx Binary Output Serial to Parallel IFFT FFT Parallel to serial Time and frequency Synchronisation OFDM Transmitter OFDM Receiver Channel estimation DMT modulation Multipath & BW adaptation Adaptation to burst errors


Discrete multi-tone (DMT) modulation: Discrete multi-tone (DMT) modulation ANSI T1.413 specifies DMT modem for ASDL applications Downstream: 2.208 MHz sampling rate, 256 tones 0 … 1.104 MHz Symbol rate 4000 symbols /s. Each sub-channel is 4.3 kHz wide max rate 32 kb/s per channel (compare to V.90 modem) Upstream: 275 kHz sampling rate, 32 tones 0 … 138 kHz ASx: high-speed,downstream simplex nx1.54Mb/s LSx: low-speed, duplex channels 160…576 kb/s crc: cyclic redundancy check FEC f,i: (fast,interleaved): forward error correction scram f,i: scrambling ATU-C: ADSL transmitter unit - central office V-C interface ATU-C transmitter


DMT spectra / ISDN linecodes: DMT spectra / ISDN linecodes upstream downstream upstream downstream upstream downstream If no ISDN If 2B1Q ISDN If 4B3T ISDN 0 10 20 50 100 150 200 250 carrier 25 kHz 120 kHz 1104 kHz 276 kHz POTS 138 kHz 80 kHz 2B1Q ISDN 4B3T ISDN 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


ADSL system total data rate: 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 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 ...


ADSL frames: ADSL frames 68 DMT data symbols, -> symbol rate ~4000/sec super frame boundary identification - bearer channel allocation during initial setup determines ratio of interleaved and fast data frames (Nf,Ns) - 8 crc bits (crc0-7) supervise fast data transmission - 24 indicator bits (ib0-ib23) assigned for OAM functions see next slide


Fast sync - byte: Fast sync - byte crc: cyclic redundancy check eoc: embedded operations channel (O & M of ATU-C and ATU-R) ib: indicator bits (O & M)


ATU-C transmitter reference model for STM* transport: ATU-C transmitter reference model for STM* transport *Synchronous transfer mode Asx: any one of the simplex bearer channels AS0, AS1, AS2 or AS3 LSx: any one of the duplex bearer channels LS0, LS1 or LS2 NTR: Network Timing Reference: 8 kHz reference transmitted downstream aoc: ADSL overhead control channel eoc: embedded operations channel Indicator bits


xDSL- systems: xDSL- systems HDSL -- High Bit Rate DSL 1.544 Mbps (T1) or 2.048 Mbps (E1) symmetrical channel associated signaling 2- or 4-wire connections ADSL -- Asymmetric DSL up to 8 Mbps downstream and 640 Kbps upstream ATM / STM compatible 2-wire compatible requires splitter and separate phone line from box to wall CDSL -- Consumer DSL/ADSL-lite ATM (Q.2931) signaling only up to 1.555 Mbps downstream and 512 Kbps upstream reduced options, performance, cost, easy to install


xDSL- systems (cont.): xDSL- systems (cont.) RDSL -- Rate-Adaptive DSL adjusts transmission rates in both directions to obtain the best speed under prevailing conditions otherwise like ADSL SDSL -- Symmetric DSL one pair of copper wire used, 774 kbps channel associated signaling or Q.921 VDSL -- Very-High-Bit-Rate DSL speeds up to 13- 52 Mbps DL, 1.5-2.3 Mbps UL, but for only short distances, applies ATM


xDSL systems (cont.): xDSL systems (cont.) BRI ISDN (DSL) uses existing ISDN equipment, but in 'always on' mode instead of as a dial-up service. Yields 2B+D up to 128 kbps + 16 kbps or X.25 with 160 kbps signaling Q.921/Q.931 designed for speech networks V.90 56 kbps DL, 33.6 kbps UL signaling analog for speech network


xDSL systems and applications: xDSL systems and applications


xDSL systems compared (cont.): xDSL systems compared (cont.)


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


Peak to T1.413 table of contents: Peak to T1.413 table of contents


References: References T. Starr, J.M. Cioffi, P.J. Silverman: Understanding Digital Subscriber Line Technology, Prentice-Hall W.Y. Chen: DSL Simulation Techniques and Standards - Development for Digital Subscriber Line Systems, MacMillan Tech. Publishing C.K. Summers: ADSL - Standards, Implementation and Architecture, CRC Press ANSI T1.413, issue 2 standard