Stein Intro xDSL 1.1 Introduction to x DSL Part I Yaakov J. Stein Chief Scientist RAD Data...

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


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


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?


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



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



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



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


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


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


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)


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



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



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


Problem III - where to put the wiresProblem III - where to put the wires Originally overhead with cross-bars NY nightmare


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


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


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)



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



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


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



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


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


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


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


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!)


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


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

]


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


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


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


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


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!


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


Other problemsOther problems

What does a loading coil do?

Flattens response in voice band

Attenuates strongly above voice frequencies


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


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


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!


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.


Sources of InterferenceSources of Interference

XMTR RCVR

RCVR XMTR FEXT

NEXT

RCVR XMTR

XMTR RCVR

RF INGRESS

THERMAL NOISE


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


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


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)


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)


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 …


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


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


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!


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 )


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


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


Water pouringWater pouring

How can we maximize the capacity?


Next time ...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

Stein Intro xDSL 1.1 Introduction to x DSL Part I Yaakov J. Stein Chief Scientist RAD Data...

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