1、,Introduction to DSL,Yaakov J. Stein Chief Scientist RAD Data Communications,PSTN,Original PSTN,UTP,Manual switching directly connected two local loops Due to microphone technology, audio BW was 4 kHz,UTP,Analog switched PSTN,Invention of tube amplifier enabled long distance Between central offices
2、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),Data supported via voice-grade modems,To send data, it is converted into 4 kHz audio (modem) Data rate is determined by
3、 Shannons 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 Shannons days it worked out to about 25 kbps today it is about 35 kbps (V.34 modem - 33.6 kbps),Digital PSTN,“las
4、t mile”,CO SWITCH,“last mile” Subscriber Line,PSTN,CO SWITCH,TDM,TDM,digital,analog,LP filter to 4 kHz at input to CO switch (before A/D),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 subscr
5、iber 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 Shannons theorems 1. Separation theorem 2. Source coding theorem 3. Channel capacity theorem,Voice-grade modems still work over new PST
6、N,UTP subscriber line,CO SWITCH,modem,PSTN,modem,CO SWITCH,But data rates do not increase ! Simulate analog channel so can achieve Shannon rate native 64 kbps rate,Internet,Where is the limitation ?,The digital network was developed incrementally No forklift upgrades to telephones, subscriber lines,
7、 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
8、Is there a better way to use the subscriber line for digital transmissions ?,UTP,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 Twi
9、sted 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.),UTP characte
10、ristics,Resistance per unit distance Capacitance per unit distance Inductance per unit distance Cross-admittance (assume pure reactive) per unit distance,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 effec
11、t: Resistance increases with frequencyTheoretical result R f 1/2 In practice this is a good approximation,UTP capacitance,Capacitance depends on interconductor insulationAbout 15.7 nF per kft Only weakly dependent on gauge Independent of frequency to high degree,UTP inductance,Higher for higher gaug
12、e24 gauge 0.188 mH per kft26 gauge 0.205 mH per kft Constant below about 10 kHz Drops slowly above,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!,Propagation lo
13、ss,Voltage decreases as travel along cable Each new section of cable reduces voltage by a factorSo the decrease is exponentialVa / 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,Attenuation vs. frequency,Why
14、twisted?,from Alexander Graham Bells 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,V = (a+n) - (b+n),n,a,b,Why twisted? - continued,So dont need shieldin
15、g, 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 V1 where Q (Cbc-Cbd) or Q(Lbc-Lad),Loading coils,Long loops have loading coils to prev
16、ent 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 coils88 mH every 6kft starting 3kft,There may also be bridged taps Parallel run of unterminated UTP unused piece left over from old insta
17、llation placed for subscriber flexibilityHigh frequency signals are reflected from the open end A bridged tap can act like a notch filter!,Bridge taps,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
18、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,Other problems,B
19、inder 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 groupWe will see that these pairs interfere with each other a phenomenon called cross-talk (XTALK)
20、,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
21、 more than 2 gaugesIn 1991 more than 60% of US lines met CSA requirements,Present US PSTN,UTP only in the last mile (subscriber line) 70% unloaded 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 powerin
22、g Above 100KHz: severe attenuation cross-talk in binder groups (25 - 1000 UTP) lack of intermanufacturer consistency,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
23、reach - HDSL DA (distribution area) 3-5 kft - VDSL Higher rate - lower reach(because of attenuation and noise!),xDSL,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
24、 price) TIME: 0 (just setup time),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 dont assume anything Use whatever the physics of the UTP
25、allows,xDSL System Reference Model,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 DSLADSLforum/T1E1.4 specified that splitter be separate from modem No interface specification
26、(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,Why is DSL better than a voice-grade modem?,Analog telephony modems are limited to 4
27、 KHz bandwidth Shannons channel capacity theorem gives the maximum transfer rateC = 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,Maximum reach,To use Shannons capacity theorem we need to know how much noise ther
28、e 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 / Hzwhite (i.e. independent of frequency) is a good ap
29、proximation We can compute the maximum reach from known UTP attenuation,xDSL - Maximum Reach,Other sources of noise,But real systems have other sources of noise, and thus the SNR will be lower and thus will have lower reachThere are three other commonly encountered types of noise RF ingress Near End
30、 Cross Talk (NEXT) Far End Cross Talk (FEXT),Sources of Interference,XMTR RCVR RCVR XMTRFEXTNEXTRCVR XMTR XMTR RCVRRF INGRESS,Ungers 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 empiricall
31、y that cross-talk only increases as N0.6 This is because extra interferers must be further away,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 f 2 /
32、 f 1/2 = f 3/2 Power spectrum of transmission Total NEXT interference (noise power)KNEXT N0.6 f 3/2 PSD(f),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 f 2 Power spectru
33、m of transmission Total FEXT interference (noise power)KFEXT N0.6 L f2 |Hchannel(f)|2 PSD(f),Example - Interference spectrum,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 km6 Mbps 12 Kft 3.5 km (CSA 50% US loops)10
34、Mbps 7 Kft 2 km13 Mbps 4.5 Kft 1.4 km26 Mbps 3 Kft 900 m52 Mbps 1 Kft 300 m (SONET STS-1 = 1/3 STM-1),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 wa
35、ys 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
36、aggregation,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
37、 cancellation mode (ECH) Time Domain Duplexing (requires syncing all binder contents) Frequency Domain 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 da
38、ta stream in 1 direction on N physical lines,(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,xDSL types and history,DSL Flavors,DSL is
39、 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 + telepho
40、ny) ADSL (asymmetric, mainly residential, Internet access) VDSL (very high rate, but short distance),Some xDSL PSDs,F(MHz),PSD(dBm/Hz),IDSL,T1,HDSL,HDSL2,ADSL,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 ADSLG.992.3 ADSL2 G.992.4 splitterl
41、ess ADSL2G.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),ITU xDSL layer model,Transport protocol (ATM, STM, PTM) Transport Protocol Specific - Transmission Convergence (TPS-T
42、C) Physical Medium Specific - Transmission Convergence (PMS-TC) Physical Medium Dependent (PMD) Physical medium,More xDSL flavors,More xDSL flavors (cont.),T1 service (not DSL),1963: Coax deployment of T1 2 groups in digital TDM AMI line code Beyond CSA range should use DLC (direct loop carrier) Rep
43、eaters every 6 Kft Made possible by Bell Labs invention of the transistor 1971: UTP deployment of T1 (but still not DSL) Bring 1.544 Mbps to customer private lines Use two UTP in half duplex mode Requires expensive line conditioning One T1 per binder group,T1 line conditioning,In order for a subscri
44、bers line to carry T1 Single gauge CSA range No loading coils No bridged taps Repeaters every 6 Kft (starting 3 Kft) One T1 per binder group Labor intensive (expensive) process Need something better (DSL),The first true xDSL!,1984,88: IDSL BRI access for ISDN 4B3T (3 level PAM) or 2B1Q (4 level PAM)
45、 modulation Prevalent in Europe, never really caught on in US 144 Kbps over CSA rangeITU-T G.961 describes IDSL There are 4 appendices: Appendix I - 4B3T (AKA MMS43) Appendix II - 2B1Q Appendix III - AMI Time Compression Multiplex (TDD) Appendix IV - SU32 (3B2T + ECH),HDSL - NA improved copy of IDSL
46、,1991: HDSL Replaced T1/E1 service, but full CSA distance w/o line conditioning / repeaters AMI line code replaced with IDSLs 2B1Q line code Use 2 UTP pairs, but in ECH mode (DFE) For T1 784 kbps on each pair For E1, 1, 2, 3 and 4 pair modes (all ECH) Requires DSP for echo cancellation Mature DSL te
47、chnology, now becoming obsolete,HDSL2,With the success of HDSL, customers requested HDSL service that would : require only a single UTP HDSL attain at least full CSA reach be spectrally compatible w/ HDSL, T1, ADSL, etc. The result, based on high order PAM, was called HDSL2 (ANSI) SDSL Symmetric DSL
48、 (ETSI) and is now called SHDSL Single pair HDSL (ITU),SHDSL,Uses Trellis Coded 16-PAM with various shaping options Does not co-exist with POTS service on UTP Can uses regenerators for extended reach single-pair operation 192 kbps to 2.312 Mbps in steps of 8 kbps 2.3 Mbps should be achieved for reac
49、hes up to 3.5 km dual-pair operation (4-wire mode) 384 kbps to 4.608 Mbps in steps of 16 kbps line rate is the same on both pairs Latest standard (G.shdsl.bis - G.991.2 2003 version) bonding up to 4 pairs rates up to 5696 kbps optional 32-PAM (instead of 16-PAM) dynamic rate repartitioning,ADSL,Asym
50、metric - high rate DS, lower rate US Originally designed for video on demand New modulation type - Discrete MultiTone FDD and ECH modes Almost retired due to lack of interest but then came the Internet Studies - DS:US for both applications can be about 10:1 Some say ADSL could mean All Data Subscribers Living,
