dsl intro-1
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Stein Intro xDSL 1.1
Introductionto
xDSL
Part I
Yaakov J. Stein
Chief Scientist
RAD Data Communications
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Stein Intro xDSL 1.2
Introduction to xDSL
I Background
history, theoretical limitations
II Modems
line codes, duplexing, equalization,
error correcting codes, trellis codes
III xDSL - What is x?
x=I,A,S,V - specific technologies
competitive technologies
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Stein Intro xDSL 1.3
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?
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Stein Intro xDSL 1.4
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
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Stein Intro xDSL 1.5
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
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Stein Intro xDSL 1.6
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
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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-Pittsburgh
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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 serviceinto a business
Organized PSTN and COs (Bell sold telephones!)
Established principle of reinvestment in R&D
Established Bell Telephones IPR division
Executed merger with Western Union to form AT&T
Solved the four main problems
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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
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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)
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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
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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
V = (a+n) - (b+n)
n
a
b
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Problem II - continued
But even UTP has somecross-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)
a
d
c
b
Cbc
Cbd
Lbc
Lad
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Problem III - where to put the wires
Originally overhead with cross-bars NY nightmare
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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
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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
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Stein Intro xDSL 1.17
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)
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Stein Intro xDSL 1.18
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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Stein Intro xDSL 1.19
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
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Stein Intro xDSL 1.20
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
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Stein Intro xDSL 1.21
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
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Stein Intro xDSL 1.22
Resistance design rules
AT&T 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
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Stein Intro xDSL 1.23
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
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Stein Intro xDSL 1.24
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
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Stein Intro xDSL 1.25
Present US PSTN - continued
For DSL - basically four cases
Resistance design > 18Kft loaded line - no DSL possible
Resistance design unloaded
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Stein Intro xDSL 1.26
DSL - another definition
Need high speed digital connection to subscribers
Too expensive to replace UTP in the last mile
Voice grade modems assume
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Stein Intro xDSL 1.27
Line loss vs. frequency
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UTP characteristics Resistance per unit distance
Capacitance per unit distance
Inductance per unit distance
Cross-admittance (assume pure reactive) per unit distance
R L
X
G C
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Stein Intro xDSL 1.29
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
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Stein Intro xDSL 1.30
UTP capacitance
Capacitance depends on interconductor insulation
About 15.7 nF per Kft
Only weakly dependent on gauge
Independent of frequency to high degree
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Stein Intro xDSL 1.31
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
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Stein Intro xDSL 1.32
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!
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Stein Intro xDSL 1.33
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
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Stein Intro xDSL 1.34
Other problems
What does a loading coil do?
Flattens response in voice band
Attenuates strongly above voice frequencies
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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!
Other problems - continued
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Stein Intro xDSL 1.36
Subscriber lines are seldom single runs of cableUS UTP usually comes in 500 ft lengths
Splices must be made
Average line has >20 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
Other problems - continued
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Stein Intro xDSL 1.37
Is that all?
We know the signal lossas 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!
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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.
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Stein Intro xDSL 1.39
Sources of Interference
XMTR RCVR
RCVR XMTRFEXT
NEXT
RCVR XMTR
XMTR RCVR
RF INGRESS
THERMAL
NOISE
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Stein Intro xDSL 1.40
Interference for xDSL
ISDN
DSL
AM
BROADCAST
RADIO
THERMAL
NOISE
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Stein Intro xDSL 1.41
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
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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 ~ f2 / f1/2
= f3/2
Power spectrum of transmission
Total NEXT interference (noise power)
KNEXT N0.6 f3/2 PSD(f)
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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)
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Stein Intro xDSL 1.44
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
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Stein Intro xDSL 1.45
Shannon - Game plan
Claude Shannon (Bell Labs) 1948
No loss in going to digital communications
All information canbe converted to bits
Source channel separation theorem
Source encoding theorems
Channel capacity theorems
All information shouldbe converted to bits
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Shannon - SeparationTheorem
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
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Shannon - Channel Capacity
Every bandlimited noisy channel has a capacity
Below capacity errorless information reception
Above capacity errors
Shocking news to analog engineersPreviously 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 )
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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 channelWhat about a realchannel (like DSL)?
Shannon says ...
Simply divide channel into subchannels and integrate
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Water pouring
How can we maximize the capacity?
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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 rangefor a given information rate
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