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Introduction to DSL

Introduction to DSL. Yaakov J. Stein Chief Scientist RAD Data Communications. PSTN. Original PSTN. UTP. UTP. Manual switching directly connected two local loops Due to microphone technology, audio BW was 4 kHz. Analog switched PSTN.

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Introduction to DSL

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  1. Introduction toDSL Yaakov J. Stein Chief ScientistRAD Data Communications

  2. PSTN

  3. Original PSTN UTP UTP Manual switching directly connected two local loops Due to microphone technology, audio BW was 4 kHz

  4. Analog switched PSTN Invention of tube amplifier enabled long distance Between central offices used FDM spaced at 4 kHz (each cable carrying 1 group = 12 channels) Developed into hierarchical network of automatic switches (with supergroups, master groups, supermaster groups)

  5. UTP modem modem Data supported viavoice-grade modems • To send data, it is converted into 4 kHz audio (modem) • Data rate is determined by Shannon's capacity theorem • there is a maximum data rate (bps) called the "capacity" • that can be reliably sent through the communications channel • the capacity depends on the BW and SNR • In Shannon's days it worked out to about 25 kbps • today it is about 35 kbps (V.34 modem - 33.6 kbps)

  6. Digital PSTN CO SWITCH “last mile” TDM analog digital PSTN TDM “last mile” Subscriber Line CO SWITCH LP filter to 4 kHz at input to CO switch (before A/D)

  7. Digital PSTN Sample 4 kHz audio at 8 kHz (Nyquist) Need 8 bits per sample = 64 kbps Multiplexing 64 kbps channels leads to higher and higher rates Only the subscriber line (local loop) remains analog (too expensive to replace) Can switch (cross connect) large number of channels Noise and distortion could be eliminated due to Shannon's theorems 1. Separation theorem 2. Source coding theorem 3. Channel capacity theorem

  8. network/ ISP router Voice-grade modemsstill work over new PSTN CO SWITCH PSTN UTP subscriber line modem CO SWITCH But data rates do not increase ! Simulate analog channel so can achieve Shannon rate < native 64 kbps rate modem Internet

  9. Where is the limitation ? The digital network was developed incrementally No forklift upgrades to telephones, subscriber lines, etc. Evolutionary deployment meant that the new network needed to simulate pre-existing analog network So a 4 kHz analog channel is presented to subscriber The 4 kHz limitation is enforced by LP filter at input to CO switch (before 8 kHz sampling) The actual subscriber line is not limited to 4 kHz Is there a better way to use the subscriber line for digital transmissions ?

  10. UTP

  11. What is UTP? The achievable data rate is limited by physics of the subscriber line The subscriber line is an Unshielded Twisted Pair of copper wires • Two plastic insulated copper wires • Two directions over single pair • Twisted to reduce crosstalk • Supplies DC power and audio signal • Physically, UTP is • distributed resistances in series • distributed inductances in series • distributed capacitances in parallel so the attenuation increases with frequency • Various other problems exist (splices, loading coils, etc.)

  12. UTP characteristics • Resistance per unit distance • Capacitance per unit distance • Inductance per unit distance • Cross-admittance (assume pure reactive) per unit distance

  13. UTP resistance Influenced by gauge, copper purity, temperature Resistance is per unit distance • 24 gauge 0.15 W/kft • 26 gauge 0.195 W/kft Skin effect: Resistance increases with frequency Theoretical result R ~ f 1/2 In practice this is a good approximation

  14. UTP capacitance Capacitance depends on interconductor insulation About 15.7 nF per kft Only weakly dependent on gauge Independent of frequency to high degree

  15. UTP inductance Higher for higher gauge 24 gauge 0.188 mH per kft 26 gauge 0.205 mH per kft Constant below about 10 kHz Drops slowly above

  16. UTP admittance Insulation good so no resistive admittance Admittance due to capacitive and inductive coupling Self-admittance can usually be neglected Cross admittance causes cross-talk!

  17. Propagation loss Voltage decreases as travel along cable Each new section of cable reduces voltage by a factor So the decrease is exponential Va / Vb = e -g x = H(f,x) where x is distance between points a and b We can calculate g, and hence loss, directly from RCLG model 1v 1/2 v 1/4 v

  18. 24 AWG 26 AWG Attenuation vs. frequency

  19. Why twisted? from Alexander Graham Bell’s 1881 patent To place the direct and return lines close together. To twist the direct and return lines around one another so that they should be absolutely equidistant from the disturbing wires n a V = (a+n) - (b+n) b

  20. Why twisted? - continued So don't need shielding, at least for audio (low) frequencies But at higher frequencies UTP has cross-talk George Cambell was the first to model (see BSTJ 14(4) Oct 1935) Cross-talk due to capacitive and/or inductive mismatch |I2| = Q f V1where Q ~ (Cbc-Cbd) or Q~(Lbc-Lad)

  21. Loading coils Long loops have loading coils to prevent voice distortion What does a loading coil do? Flattens response in voice band Attenuates strongly above voice frequencies loops longer than 18 kft need loading coils 88 mH every 6kft starting 3kft

  22. Bridge taps There may also be bridged taps Parallel run of unterminated UTP • unused piece left over from old installation • placed for subscriber flexibility High frequency signals are reflected from the open end A bridged tap can act like a notch filter!

  23. Other problems Splices Subscriber lines are seldom single runs of cable In the US, UTP usually comes in 500 ft lengths So splices must be made every 500 ft Average line has >20 splices Splices are pressure connections that add to attenuation Over time they corrode and may spark, become intermittent, etc. Gauge changes US binder groups typically start off at 26 AWG Change to 24 AWG after 10 kft In rural areas they may change to 19 AWG after that

  24. Binder groups UTP are not placed under/over ground individually In central offices they are in cable bundles with 100s of other UTP In the outside plant they are in binder groups with 25 or 50 pairs per group We will see that these pairs interfere with each other a phenomenon called cross-talk (XTALK)

  25. CSA guidelines 1981 AT&T Carrier Service Area guidelines advise as follows for new deployments • No loading coils • Maximum of 9 kft of 26 gauge (including bridged taps) • Maximum of 12 kft of 24 gauge (including bridged taps) • Maximum of 2.5 kft bridged taps • Maximum single bridged tap 2 kft • Suggested: no more than 2 gauges In 1991 more than 60% of US lines met CSA requirements

  26. Present US PSTN UTP only in the last mile (subscriber line) • 70% unloaded < 18kft • 15% loaded > 18kft • 15% optical or digital to remote terminal + DA (distribution area) PIC, 19, 22, 24, 26 gauge Built for 2W 4 KHz audio bandwidth DC used for powering Above 100KHz: • severe attenuation • cross-talk in binder groups (25 - 1000 UTP) • lack of intermanufacturer consistency

  27. Present US PSTN - continued We will see, that for DSL - basically four cases • Resistance design > 18Kft loaded line - no DSL possible • Resistance design unloaded <18 Kft <1300 W - ADSL • CSA reach - HDSL • DA (distribution area) 3-5 kft - VDSL Higher rate - lower reach (because of attenuation and noise!)

  28. xDSL

  29. Alternatives for data services Fiber, coax, HFC COST: $10k-$20k / mile TIME: months to install T1/E1 COST: >$5k/mile for conditioning TIME: weeks to install DSL COST: @ 0 (just equipment price) TIME: @ 0 (just setup time)

  30. xDSL Need higher speed digital connection to subscribers Not feasible to replace UTP in the last mile Older voice grade modems assume 4kHz analog line Newer (V.90) modems assume 64kbps digital line DSL modems don’t assume anything Use whatever the physics of the UTP allows

  31. Analog modem CO SWITCH PSTN POTS-C POTS-R network/ UTP ISP POTS POTS PDN SPLITTER SPLITTER DSLAM xTU-R router WAN xTU-C x = H, A, V, ... POTS xDSL frequency DC 4 kHz xDSL System Reference Model

  32. Splitter Splitter separates POTS from DSL signals • Must guarantee lifeline POTS services! • Hence usually passive filter • Must block impulse noise (e.g. ring) from phone into DSL ADSLforum/T1E1.4 specified that splitter be separate from modem No interface specification (but can buy splitter and modem from different vendors) Splitter requires installation • Costly technician visit is the major impediment to deployment • ADSL has splitterless versions to facilitate residential deployment

  33. N S Why is DSL better than a voice-grade modem? Analog telephony modems are limited to 4 KHz bandwidth Shannon’s channel capacity theorem gives the maximum transfer rate C = BW log2 ( SNR + 1 ) So by using more BW we can get higher transfer rates! But what is the BW of UTP? for SNR >> 1 C(bits/Hz)  SNR(dB) / 3

  34. Maximum reach To use Shannon's capacity theorem we need to know how much noise there is One type of noise that is always present (above absolute zero temperature) is thermal noise Maximum reach is the length of cable for reliable communications ASSUMING ONLY THERMAL NOISE Bellcore study in residential areas (NJ) found • -140 dBm / Hz • white (i.e. independent of frequency) is a good approximation We can compute the maximum reach from known UTP attenuation

  35. xDSL - Maximum Reach

  36. Other sources of noise But real systems have other sources of noise, and thus the SNR will be lower and thus will have lower reach There are three other commonly encountered types of noise • RF ingress • Near End Cross Talk (NEXT) • Far End Cross Talk (FEXT)

  37. THERMAL NOISE Sources of Interference XMTR RCVR RCVR XMTR FEXT NEXT RCVR XMTR XMTR RCVR RF INGRESS

  38. Unger’s discovery What happens with multiple sources of cross-talk? Unger (Bellcore) : 1% worst case NEXT (T1D1.3 185-244) • 50 pair binders • 22 gauge PIC • 18 Kft Found empirically that cross-talk only increases as N0.6 This is because extra interferers must be further away

  39. NEXT Only close points are important • Distant points are twice attenuated by line attenuation |H(f,x)|2 Unger dependence on number of interferers Frequency dependence • Transfer function ~ I2Campbell / R ~ f2 / f1/2= f3/2 • Power spectrum of transmission Total NEXT interference (noise power) KNEXT N0.6 f3/2 PSD(f)

  40. FEXT Entire parallel distance important • Thus there will be a linear dependence on L Unger dependence on number of interferers Frequency dependence • Transfer function ~ I2Campbell ~ f2 • Power spectrum of transmission Total FEXT interference (noise power) KFEXT N0.6 L f2 |Hchannel(f)|2 PSD(f)

  41. Example - Interference spectrum

  42. Examples of Realistic Reach More realistic design goals (splices, some xtalk) • 1.5 Mbps 18 Kft 5.5 km (80% US loops) • 2 Mbps 16 Kft 5 km • 6 Mbps 12 Kft 3.5 km (CSA 50% US loops) • 10 Mbps 7 Kft 2 km • 13 Mbps 4.5 Kft 1.4 km • 26 Mbps 3 Kft 900 m • 52 Mbps 1 Kft 300 m (SONET STS-1 = 1/3 STM-1)

  43. Bonding (inverse mux) If we need more BW than attainable by Shannon bounds we can use more than one UTP pair (although XT may reduce) This is called bonding or inverse multiplexing There are many ways of using multiple pairs: • ATM - extension of IMA (may be different rates per pair) ATM cells marked with SID and sent on any pair • Ethernet - based on 802.3(EFM) frames are fragmented, marked with SN, and sent on many pairs • Time division inverse mux • Dynamic Spectral Management (Cioffi) • Ethernet link aggregation

  44. POTS US DS frequency DC 4 kHz Duplexing Up to now we assumed that only one side transmits Bidirectional (full duplex) transmission requires some form of duplexing For asymmetric applications we usually speak of DS downstream and US upstream Four methods are in common use: • Half duplex mode (4W mode) (as in E1/T1) • Echo cancellation mode (ECH) • Time Domain Duplexing (requires syncing all binder contents) • Frequency Domain Duplexing

  45. inverse multiplexing multiplexing data streams physical line data stream physical lines duplexing Muxing, inverse muxing, duplexing Duplexing = 2 data streams in 2 directions on 1 physical line Multiplexing = N data streams in 1 direction on 1 physical line Inverse multiplexing = 1 data stream in 1 direction on N physical lines

  46. modulator 4W to 2W HYBRID demodulator (Adaptive) echo cancellation Signal transmitted is known to transmitter It is delayed, attenuated and distorted in the round-trip Using adaptive DSP algorithms we can • find the delay/attenuation/distortion • subtract

  47. xDSL types and history

  48. DSL Flavors DSL is often called xDSL since there are many varieties (different x) e.g. ADSL, HDSL, SHDSL, VDSL, IDSL, etc. There were once many unconnected types but now we divide them into three main families The differentiation is by means of the application scenario • HDSL (symmetric, mainly business, data + telephony) • ADSL (asymmetric, mainly residential, Internet access) • VDSL (very high rate, but short distance)

  49. Some xDSL PSDs PSD(dBm/Hz) T1 IDSL HDSL HDSL2 ADSL F(MHz)

  50. ITU G.99x standards • G.991 HDSL (G.991.1 HDSL G.991.2 SHDSL) • G.992 ADSL (G.992.1 ADSL G.992.2 splitterless ADSL G.992.3 ADSL2 G.992.4 splitterless ADSL2 G.992.5 ADSL2+) • G.993 VDSL (G.993.1 VDSL G.993.2 VDSL2) • G.994 HANDSHAKE • G.995 GENERAL (INFO) • G.996 TEST • G.997 PLOAM • G.998 bonding (G.998.1 ATM G.998.2 Ethernet G.998.3 TDIM)

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