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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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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Present US PSTN - continued

For DSL - basically four cases

Resistance design > 18Kft loaded line - no DSL possible

Resistance design unloaded

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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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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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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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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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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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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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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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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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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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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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Sources of Interference

XMTR RCVR

RCVR XMTRFEXT

NEXT

RCVR XMTR

XMTR RCVR

RF INGRESS

THERMAL

NOISE

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Interference for xDSL

ISDN

DSL

AM

BROADCAST

RADIO

THERMAL

NOISE

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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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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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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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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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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

DSL Intro-1

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