Post on 27-Dec-2015
Stein Intro xDSL 1.1
IntroductionIntroduction
toto
xxDSL DSL
Part IPart I
IntroductionIntroduction
toto
xxDSL DSL
Part IPart I
Yaakov J. Stein
Chief ScientistRAD Data Communications
Stein Intro xDSL 1.2
Introduction to xDSLIntroduction 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
Stein Intro xDSL 1.3
What is DSL?What is DSL?
Drinking Straw LineA 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 don’t apply to much pressure If you use a lot of tricks
Why not buy a new fire hose?
Stein Intro xDSL 1.4
Timeline of UTP 1800-1876Timeline 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 Morse’s patent expires
Western Union connects US with single steel wires
1858 First subatlantic telegraph cable connects US with Europe
Stein Intro xDSL 1.5
Timeline of UTP 1876-1877Timeline of UTP 1876-1877
Feb 14 1876 Alexander Graham Bell’s 29’th 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
Stein Intro xDSL 1.6
Timeline of UTP 1877-1899Timeline 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
Stein Intro xDSL 1.7
Timeline of UTP 1900-1918Timeline 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
Stein Intro xDSL 1.8
The importance of Theodore VailThe importance of Theodore Vail
Theodore Who?Son of Alfred Vail (Morse’s coworker)Ex-head of US post officeFirst 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 R&DEstablished Bell Telephones IPR divisionExecuted merger with Western Union to form AT&T
Solved the four main problems
Stein Intro xDSL 1.9
Problem I - the metal to useProblem I - the metal to use
Galvanized iron inexpensive, good outdoors Steel stronger but didn’t 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
Stein Intro xDSL 1.10
Problem II - silencing the martiansProblem 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 didn’t know what that meant)
Stein Intro xDSL 1.11
Problem II - continuedProblem 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
Stein Intro xDSL 1.12
Problem II - continuedProblem II - continued
from 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
V = (a+n) - (b+n)
n
a
b
Stein Intro xDSL 1.13
Problem II - continuedProblem 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)
a
d
c
b
C bc C bd
L bc L ad
Stein Intro xDSL 1.14
Problem III - where to put the wiresProblem III - where to put the wires Originally overhead with cross-bars NY nightmare
Stein Intro xDSL 1.15
Problem III - continuedProblem 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
Stein Intro xDSL 1.16
Problem IV - the priceProblem 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
Stein Intro xDSL 1.17
Problem IV - continuedProblem 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)
Stein Intro xDSL 1.18
Problem IV - continuedProblem 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
Stein Intro xDSL 1.19
Problem IV - continuedProblem 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
Stein Intro xDSL 1.20
Line conditioningLine conditioning
In order for a subscriber’s 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
Stein Intro xDSL 1.21
Problem IV - continuedProblem 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
Stein Intro xDSL 1.22
Resistance design rulesResistance design rules
AT&T 1954 guidelines
maximum resistance 1300
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
Stein Intro xDSL 1.23
CSA guidelinesCSA 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
Stein Intro xDSL 1.24
Present US PSTNPresent 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
Stein Intro xDSL 1.25
Present US PSTN - continuedPresent US PSTN - continued
For DSL - basically four cases
Resistance design > 18Kft loaded line - no DSL possible
Resistance design unloaded <18 Kft <1300 ADSL
CSA reach HDSL
DA (distribution area) 3-5 kft VDSL
Higher rate - lower reach
(because of attenuation and noise!)
Stein Intro xDSL 1.26
DSL - another definitionDSL - another definition
Need high speed digital connection to subscribers
Too expensive to replace UTP in the last mile
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
Stein Intro xDSL 1.27
Line loss vs. frequencyLine loss vs. frequency
0 2 4 6 8 10-90
-80
-70
-60
-50
-40
-30
-20
-10
024 and 26 AWG Cables
Freq [MHz]
Atte
nua
tion
[dB
/Km
]
Stein Intro xDSL 1.28
UTP characteristicsUTP 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
Stein Intro xDSL 1.29
UTP resistanceUTP resistance
Influenced by gauge, copper purity, temperature
Resistance is per unit distance
24 gauge 0.15 Kft 26 gauge 0.195 Kft
Skin effect: Resistance increases with frequency
Theoretical result R ~ f 1/2
In practice this is a good approximation
Stein Intro xDSL 1.30
UTP capacitanceUTP capacitance
Capacitance depends on interconductor insulation
About 15.7 nF per Kft
Only weakly dependent on gauge
Independent of frequency to high degree
Stein Intro xDSL 1.31
UTP inductanceUTP 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
Stein Intro xDSL 1.32
UTP admittanceUTP 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!
Stein Intro xDSL 1.33
Propagation lossPropagation 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 - x = H(f,x)
where x is distance between points a and b
We can calculate and hence loss directly from RCLG
1v 1/2 v 1/4 v
Stein Intro xDSL 1.34
Other problemsOther problems
What does a loading coil do?
Flattens response in voice band
Attenuates strongly above voice frequencies
Stein Intro xDSL 1.35
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 - continuedOther problems - continued
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 changesBinders typically 26 AWG
Change to 24 after 10 Kft
In rural areas change to 19 AWG after that
Other problems - continuedOther problems - continued
Stein Intro xDSL 1.37
Is that all?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 didn’t find out about the noise.
We forgot about cross-talk!
and there are two kinds!
And there is RF ingress too!
Stein Intro xDSL 1.38
What noise is there?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.
Stein Intro xDSL 1.39
Sources of InterferenceSources of Interference
XMTR RCVR
RCVR XMTR FEXT
NEXT
RCVR XMTR
XMTR RCVR
RF INGRESS
THERMAL NOISE
Stein Intro xDSL 1.40
Interference for xDSLInterference for xDSL
0 0.5 1 1.5 2-200
-180
-160
-140
-120
-100
-80
-60
-40
-20
0ISDN NEXT, AM INGRESS, SELF FEXT
Freq [MHz]
Inte
rfe
renc
e [d
Bm
/Hz]
ISDN
DSL
AM BROADCASTRADIO
THERMAL NOISE
Stein Intro xDSL 1.41
Unger’s discoveryUnger’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
Stein Intro xDSL 1.42
NEXTNEXTOnly 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)
Stein Intro xDSL 1.43
FEXTFEXT
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)
Stein Intro xDSL 1.44
What do we do now?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 …
Stein Intro xDSL 1.45
Shannon - Game planShannon - 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
Stein Intro xDSL 1.46
Shannon - Shannon - SeparationSeparation TheoremTheorem
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
Stein Intro xDSL 1.47
Shannon - Channel CapacityShannon - Channel CapacityEvery 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 didn’t explain HOW!
Stein Intro xDSL 1.48
Channel Capacity (continued)Channel Capacity (continued)Shannon’s 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 )
Stein Intro xDSL 1.49
Channel Capacity (continued)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
Stein Intro xDSL 1.50
Channel Capacity (continued)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