Introduction to xDSL Part I

1
IntroductiontoxDSL Part I

• Yaakov J. Stein
• Chief ScientistRAD Data Communications

2
Introduction to xDSL

• I Background
• history, theoretical limitations
• II Modems
• line codes, duplexing, equalization,
• error correcting codes, trellis codes
• III xDSL - What is x?
• xI,A,S,V - specific technologies
• competitive technologies

3
What is DSL?

• Drinking Straw Line
• A sophisticated method that enables used drinking
  straws to be
• employed as fire hoses under certain
  circumstances
• Can this work?
• If you know enough about drinking straws
• If you dont apply to much pressure
• If you use a lot of tricks
• Why not buy a new fire hose?

4
Timeline of UTP 1800-1876

• Early 1800s first telegraph experiments
• 1832-3 Henry, Gauss, Weber set up communications
  systems
• 1836 Salva and Steinheil demostrate that a
  single wire suffices
• 1837 Samuel Morse receives US patent for
  telegraph
• Wheatstone demostrates 5 needle telegraph in
  London
• 1843 Morse sends What hath God wrought? to
  Alfred Vail
• 1844 First commercial telegraph line - 2 wires
  on cross-piece
• 1850s Morses patent expires
• Western Union connects US with single steel
  wires
• 1858 First subatlantic telegraph cable connects
  US with Europe

5
Timeline of UTP 1876-1877

• Feb 14 1876 Alexander Graham Bells 29th
  birthday
• Bell files for patent on telephone
• Elisha Gray files for caveat two hour later
• Mar 7 1876 Patent 174,465 issued to Bell
• Mar 10 1876 Bell spills acid on his pants
• Mr. Watson come here, I want you
• 1877 Long distance telephone experiments (using
  telegraph wires)
• 1878 Telephone exchange in New Haven Conn
• Theodore Vail becomes general manager of Bell
  Telephone

6
Timeline of UTP 1877-1899

• 1879 Four 7-conductor cables laid over Brooklyn
  bridge
• Technician reports on cross-talk
• Bell Telephone establishes patent division
• 1881 Bell receives patent for metallic circuit
• 1888 Western Electric establishes standard cable
  
• 1891 Paper pulp insulation standard cable

7
Timeline of UTP 1900-1918

• 1900 Michael Pupin invents loading coil
• 1912 New standard cable
• 1915 First use of amplifiers
• First use of repeaters
• Transcontinental long distance line (6 gauge)
• 1918 Carrier system (5 calls) Baltimore-Pittsburg
  h

8
The importance of Theodore Vail

• Theodore Who?
• Son of Alfred Vail (Morses coworker)
• Ex-head of US post office
• First general manager of Bell Telephone Company
• Why is he so important?
• Made telephone service into a business
• Organized PSTN and COs (Bell sold telephones!)
• Established principle of reinvestment in RD
• Established Bell Telephones IPR division
• Executed merger with Western Union to form ATT
• Solved the four main problems

9
Problem I - the metal to use

• Galvanized iron inexpensive, good outdoors
• Steel stronger but didnt
  conduct well
• Silver good conductor but
  too expensive
• Copper good conductor but too
  soft and weak
• Vail saw that none were perfect
• Decided to invest in improving the strength of
  copper
• Thomas Doolittle makes hard-drawn copper wire
• Vail tests around the country
• First commercial use Boston - New York

10
Problem II - silencing the martians

• Original deployments used single telegraph wires
• Customers complained of strong babble noise
• Watson joking remarked
• they must be picking up conversations from
  Mars
• Experts claimed it must be induction
• (but didnt know what that meant)

11
Problem II - continued

• Vail brought Bell back from retirement
• Bell invents the metallic circuit (UTP)
• Vail claimed it was too expensive (need two
  wires!)
• 1883 JJ Carty put in UTP line from Providence to
  Boston
• Customers claimed that the improvement was magic
• Took 20 years to migrate entirely to UTP

12
Problem II - continued

• from 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

n
a
V (an) - (bn)
b
13
Problem II - continued

• But even UTP has some cross-talk
• George Cambell models UTP crosstalk (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)

14
Problem III - where to put the wires

• Originally overhead with cross-bars
• NY nightmare

15
Problem III - continued

• To place wires underground
• Insulate the wires from each other
• Keep moisture out
• Original solution
• Wrap wires in cotton and drench in oil
• 1888 Vail started experiments
• John Barrett discovered how to economically twist
  wires
• and mold lead into tight moisture lock
  around cable
• JJ Carty heard of technique to wrap wire in paper
  for hats
• Created pulp-insulated UTP
• 1890 Philadelphia trial resulted in best-sounding
  line yet

16
Problem IV - the price

• 25 of revenue went to copper mines
• Standard was 18 gauge
• Long distance required even heavier wire
• Higher gauge was too lossy and too bassy
• Interim solutions
• 1900 Jacobs (UK) and JJ Carty invented the
  phantom circuit
• Party lines shared same subscriber line
• Vail realized that needed to use thinner wires

17
Problem IV - continued

• 1900 Michael Pupin invents the loading coil
• flattens spectrum by low-pass filtering
• placed between the wires in pair every km
• 1906 Lee DeForest invents the audion
• triode vacuum tube amplifier
• deployed 1915
• 1918 First carrier system (FDM)
• 5 conversations on single UTP
• later extended to 12 (group)

18
Problem IV - continued

• WWII Invention of coax
• Enabled supergroups, master groups, supermaster
  groups,
• 1950s plastic insulated copper (PIC)
• Use of polyolefin/polypropylene insulation
• Neighboring pairs have different pitch
• Usually multiple of 25 pairs
• 1977 Deployment of fiber optic cables
• 30,000 conversations on 2 fiber strands
• entire PSTN converted to fiber, except the last
  mile

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Problem IV - continued

• 1963 Coax deployment of T1
• 2 groups in digital TDM
• RZ-AMI line code
• Beyond CSA range should use DLC (direct loop
  carrier)
• Repeaters every 6 Kft
• Made possible by Bell Labs invention of the
  transistor
• 1971 UTP deployment of T1
• Bring 1.544 Mbps to customer private lines
• Use two UTP in half duplex
• Requires expensive line conditioning
• One T1 per binder group

20
Line conditioning

• In order for a subscribers 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)
• Europeans already found something better

21
Problem IV - continued

• 1984,88 IDSL
• BRI access for ISDN
• 2B1Q (4 level PAM) modulation
• Prevalent in Europe, never really caught on in US
• 144 Kbps over CSA range
• 1991 HDSL
• Replace T1 line code with IDSL line code (2B1Q)
• 1 UTP (3 in Europe for E1 rates)
• Full CSA distance without line conditioning
• Requires DSP

22
Resistance design rules

• ATT 1954 guidelines
• maximum resistance 1300 W
• no finer than 26 gauge
• loops longer than 18 Kft need loading coils
• 88 mH every 6Kft starting 3Kft
• less than 6Kft of bridged taps

23
CSA guidelines

• 1981 Carrier service area guidelines
• 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 met CSA requirements

24
Present US PSTN

• UTP only in the last mile (subscriber line)
• 70 unloaded lt 18Kft
• 15 loaded gt 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

25
Present US PSTN - continued

• For DSL - basically four cases
• Resistance design gt 18Kft loaded line - no DSL
  possible
• Resistance design unloaded lt18 Kft lt1300 W ADSL
• CSA reach HDSL
• DA (distribution area) 3-5 kft VDSL
• Higher rate - lower reach
• (because of
  attenuation and noise!)

26
DSL - another definition

• Need high speed digital connection to subscribers
• Too expensive to replace UTP in the last mile
• Voice grade modems assume lt4KHz analog line
• Newer (V.90) modems assume 64Kbps digital line
• DSL modems dont assume anything
• Use whatever the physics of the UTP allows

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Line loss vs. frequency
28
UTP characteristics

• Resistance per unit distance
• Capacitance per unit distance
• Inductance per unit distance
• Cross-admittance (assume pure reactive) per unit
  distance

29
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

30
UTP capacitance

• Capacitance depends on interconductor insulation
• About 15.7 nF per Kft
• Only weakly dependent on gauge
• Independent of frequency to high degree

31
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

32
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!

33
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

1v
1/2 v
1/4 v
34
Other problems

• What does a loading coil do?
• Flattens response in voice band
• Attenuates strongly above voice frequencies

35
Other problems - continued

• I forgot to mention bridged taps!
• Parallel run of unterminated UTP
• unused piece left over from old installation
• placed for subscriber flexibility
• Signal are reflected from end of a BT
• A bridged tap can act like a notch filter!

36
Other problems - continued

• Subscriber lines are seldom single runs of cable
• US UTP usually comes in 500 ft lengths
• Splices must be made
• Average line has gt20 splices
• Splices corrode and add to attenuation
• Gauge changes
• Binders typically 26 AWG
• Change to 24 after 10 Kft
• In rural areas change to 19 AWG after that

37
Is that all?

• We know the signal loss
• as a function of frequency and distance
• Are we ready to compute the capacity of a DSL?
• NO
• What didnt find out about the noise.
• We forgot about cross-talk!
• and there are two kinds!
• And there is RF ingress too!

38
What noise is there?

• First there is thermal noise
• (unless its very cold outside)
• Bellcore study in residential areas (NJ) found
• -140 dBm / Hz
• white (i.e. independent of frequency)
• is a good approximation
• The range a DSL can attain with only this noise
• is called maximum reach.

39
Sources of Interference

• XMTR RCVR
• RCVR XMTR
• FEXT
• NEXT
• RCVR XMTR
• XMTR RCVR
• RF INGRESS

40
Interference for xDSL
ISDN
DSL
AM BROADCASTRADIO
THERMAL NOISE
41
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 empirically that cross-talk only increases
  as N0.6
• This is because extra interferers must be further
  away

42
NEXT

• Only close points are important
• Distant points twice attenuated by line
  H(f,x)2
• Unger dependence on number of interferers
• Frequency dependence
• Transfer function I2Campbell / R f 2 / f 1/2
  f 3/2
• Power spectrum of transmission
• Total NEXT interference (noise power)
• KNEXT N0.6 f 3/2 PSD(f)

43
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 spectrum of transmission
• Total FEXT interference (noise power)
• KFEXT N0.6 L f2 Hchannel(f)2
  PSD(f)

44
What do we do now?

• We now know the loss and the interference
• We have all the needed ingredients
• The time has come to learn what to do with them!
• Once again the breakthrough came from Bell Labs

45
Shannon - Game plan

• Claude Shannon (Bell Labs) 1948
• No loss in going to digital communications
• All information can be converted to bits
• Source channel separation theorem
• Source encoding theorems
• Channel capacity theorems
• All information should be converted to bits

46
Shannon - Separation Theorem

• Source channel separation theorem
• Separate source coding from channel coding
• No efficiency loss
• The following are NOT optimal !!!
• OSI layers
• Separation of line code from ECC

47
Shannon - Channel Capacity

• Every bandlimited noisy channel has a capacity
• Below capacity errorless information reception
• Above capacity errors
• Shocking news to analog engineers
• Previously thought
• only increasing power decreases error rate
• But Shannon didnt explain HOW!

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Channel Capacity (continued)

• Shannons channel capacity theorem
• If no noise (even if narrow BW)
• Infinite information transferred instantaneously
• Just send very precise level
• If infinite bandwidth (even if high noise)
• No limitation on how fast switch between bits
• If both limitations
• C BW log2 ( SNR 1 )

49
Channel Capacity (continued)

• The forgotten part
• All correlations introduce redundancy
• Maximal information means nonredundant
• The signal that attains channel capacity
• looks like white noise filtered to the BW

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Channel Capacity (continued)

• That was for an ideal low-pass channel
• What about a real channel (like DSL)?
• Shannon says ...
• Simply divide channel into subchannels and
  integrate

51
Water pouring

• How can we maximize the capacity?

52
Next time ...

• In lecture 2
• We will learn how to build modems
• that get close
• to the Shannon channel capacity
• for a given range
• OR
• that get close
• to the maximum range
• for a given information rate