Stein Intro xDSL 1.1 x DSL Introduction Yaakov J. Stein Chief Scientist RAD Data Communications

Stein Intro xDSL 1.1

xxDSLDSL

Introduction Introduction

xxDSLDSL

Introduction Introduction

Yaakov J. Stein

Chief ScientistRAD Data Communications


PSTN wiringPSTN wiring


Old (analog) PSTNOld (analog) PSTN

subscriber line subscriber line


Voice-grade modemsVoice-grade modems

UTP

modemmodem


New (digital) PSTNNew (digital) PSTN

“last mile”CO SWITCH

“last mile”

PSTN

CO SWITCH

TDM

TDM

digitalanalog


Voice-grade modems Voice-grade modems over new PSTNover new PSTN

UTP subscriber line

CO SWITCH

network/ISP

router

modem

PSTN

modem

CO SWITCH

Modem technology is basically unchanged

Communications speeds do not increase


Unshielded Twisted PairUnshielded Twisted Pair


What is UTP?What is UTP?

Two plastic insulated copper wires

Two directions over single pair

Twisted to reduce crosstalk

Supplies DC power and audio signal

Due to physics attenuation increases with frequency


Why twisted?Why twisted?

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


Why twisted? - continuedWhy twisted? - 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


Loading coilLoading coil

What does a loading coil do?

Flattens response in voice band

Attenuates strongly above voice frequencies

loops longer than 18 Kft need loading coils

88 mH every 6kft starting 3kft


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!

Bridge tapsBridge taps


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 problemsOther problems


CSA guidelinesCSA guidelines

1981 AT&T 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% of US lines 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


xDSLxDSL


Alternatives for data servicesAlternatives for data services

Fiber, coax, HFC

COST: $10K-$20K / mile

TIME: months to install

T1/E1

COST: >$5K/mile for conditioning

TIME: weeks to install

DSL

COST: 0 (just equipment price)

TIME: 0 (just setup time)


xDSLxDSL

Need higher speed digital connection to subscribers

Not feasible to replace UTP in the last mile

Older 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


xDSLxDSL System Reference Model System Reference Model

POTSSPLITTER

UTP

CO SWITCH

DSLAM

xTU-C

network/ISP

router xTU-R

POTSSPLITTER

PSTN

PDN

POTS-RPOTS-C

WAN

x = H, A, V, ...

Analog modem


SplitterSplitter

Splitter separates POTS from DSL signals Must guarantee lifeline POTS services! Hence usually passive filter Must block impulse noise (e.g. ring) from phone into DSL

ADSLforum/T1E1.4 specify that splitter be separate from modemNo interface specification yet (can’t buy splitter and modem from different

vendors)

Splitter requires installation Costly technician visit is the major impediment to deployment G.lite is splitterless ADSL


Why is DSL better Why is DSL better than a voice-grade modem?than a voice-grade modem?

Analog telephony modems are limited to 4 KHz bandwidth

Shannon’s theorem tells us that the maximum transfer rate

for SNR >> 1

C = BW log2 ( SNR + 1 ) C(bits/Hz) = SNR(dB) / 3

So by using more BW we can get higher transfer rates!

But what is the BW of UTP?

S

N


Attenuation vs. frequencyAttenuation 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

]


Maximum reachMaximum reach

Length of cable for reliable communications

ASSUMING ONLY THERMAL NOISE

Bellcore study in residential areas (NJ) found -140 dBm / Hz white (i.e. independent of frequency)

is a good approximation

Real systems have other sources of noise,

and thus have lower reach (Shannon!)

We can compute the maximum reach from UTP attenuation


xDSL - Maximum ReachxDSL - Maximum Reach

0 10 20 30 40 50 600

1

2

3

4

5

6DSL MAXIMUM REACH

Rate[Mbps]

Re

ach

[Km

]


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]


Examples of Realistic ReachExamples of Realistic Reach

More realistic design goals (splices, some xtalk)

1.5 Mbps 18 Kft 5.5 km (80% US loops)

2 Mbps 16 Kft 5 km

6 Mbps 12 Kft 3.5 km (CSA 50% US loops)

10 Mbps 7 Kft 2 km

13 Mbps 4.5 Kft 1.4 km

26 Mbps 3 Kft 900 m

52 Mbps 1 Kft 300 m (SONET STS-1 = 1/3 STM-1)


xDSL flavorsxDSL flavors

modem speed reach main applications

IDSL 160 (144) Kbps 5.5 km POTSreplacement,videoconferencing,Internet access

HDSL 2 Mbps (4-6W) 3.6-4.5 km T1/E1 replacementPBX interconnect,FR

HDSL2 2 Mbps (2W) 3 km same as HDSL

SDSL <= 2 Mbps 3 km same as HDSL


xDSL flavorsxDSL flavors

modem speed reach main applications

ADSL 6 Mbps DS 640 Kbps US

3.5-5.5 km residential Internet, video-on-demand

ADSL2 8 Mbps DS 800 Kbps US

> ADSL Internet access, VoIP

ADSL2+ 16 Mbps DS 800 Kbps US

< 2 km “

VDSL <= 52 Mbps 300m - 1 km LAN interconnect, HDTV, combined services

VDSL2 200 Mbps (aggregate)

up to 1.8 km “

cable modem 10-30Mbps DS shared

50 km residential Internet

HPNA 1, 10 Mbps home wiring residential networking


ITU G.99x standardsITU G.99x standards

G.991 HDSL (G.991.1 HDSL G.991.2 SHDSL)

G.992 ADSL (G.992.1 ADSL G.992.2 splitterless ADSL G.992.3 ADSL2 G.992.4 splitterless ADSL2 G.992.5 ADSL2+) G.993 VDSL (G.993.1 VDSL G.993.2 VDSL2)

G.994 HANDSHAKE

G.995 GENERAL (INFO)

G.996 TEST

G.997 PLOAM G.998 bonding (G.998.1 ATM G.998.2 Ethernet G.998.3 TDIM)


BondingBonding

If we need more BW than attainable by Shannon bounds

we can use more than one UTP pair (although XT may reduce)

this is called bonding or inverse multiplexing

There are many ways of using multiple pairs: ATM - extension of IMA (may be different rates per pair)

cells marked with SID and sent on any pair Ethernet - based on 802.3(EFM) frames are fragmented, marked with SN, and sent on many pairs Time division inverse mux Dynamic Spectral Management (Cioffi) Ethernet link aggregation


xDSL typesxDSL types


T1 serviceT1 service

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


T1 line conditioningT1 line 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


The first xDSL!The first xDSL!

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


HDSLHDSL

Replace T1/E1 DS1 service

Use 2B1Q line code, DFE

Full duplex on each pair with echo cancellation

CSA reach w/o conditioning/repeaters

more complex DSP

ANSI: 2 pairs for T1 (each 784 Kbps)

ETSI: 1, 2, 3 or 4 pairs

Most mature of DSL technologies


HDSL2HDSL2

Customers request HDSL service that is single UTP HDSL at least full CSA reach spectrally compatible w/

HDSL, T1, ADSL, etc.

Variously called

HDSL2 (ANSI)

SDSL Symmetric DSL (ETSI)

Now called

SHDSL Single pair HDSL (ITU)


ADSL (full rate)ADSL (full rate)

Asymmetric - high rate DS lower rate US

Originally designed for video on demand

Almost retired due to lack of interest

…but then came the Internet

Studies show DS:US should be about 10:1full rate ADSL 512-640 kbps US, 6-8 Mbps DS G.lite 512 Kbps US, 1.5 Mbps DS

ADSL could mean All Data Subscribers Living


G.liteG.lite

Splitterless ADSL, UAWG

ADSL compatible DMT compatible using only 128 tones

512 Kbps US / 1.5 Mbps DS

Still much faster than V.34 or V.90 modems

No splitter required!

Certain features removed for simplicity

simpler implementation (only 500 MIPS < 2000 MIPS for full rate)


ADSL2ADSL2

ADSL uses BW from 20 kHz to 1.1 MHz

ADSL2 Increases rate/reach of ADSL by using 20 kHz - 4.4 MHz

Also numerous efficiency improvements better modulation reduced framing overhead stronger ECC reduced power mode misc. algorithmic improvements

for given rate, reach improved by 200 m

3 user data types - STM, ATM and packet (Ethernet)

ADSL2+ dramatically increased rate at short distances


VDSLVDSL

Optical network expanding (getting closer to subscriber)

Optical Network Unit ONU at curb or basement cabinet

FTTC (curb), FTTB (building)

These scenarios usually dictates low power

Rates can be very high since required reach is minimal!

Proposed standard has multiple rates and reaches


VDSL2VDSL2

VDSL uses BW of 1.1 MHz - 12 MHz (spectrally compatible with ADSL)

VDSL2 uses 20 KHz - 30 MHz

new band-plans increased DS transmit power various algorithmic improvements borrowed improvements from ADSL2 3 user data types - STM, ATM and packet (pure Ethernet)


HPNA (G.PNT)HPNA (G.PNT)

Studies show that about 50% of US homes have a PC

30% have Internet access, 20% have more than one PC!

Average consumer has trouble with cabling

HomePNA de facto industry standard for home networking Computers, peripherals interconnect (and connect to Internet?)

using internal phone wiring (user side of splitter) Does not interrupt lifeline POTS services Does not require costly or messy LAN wiring of the home Presently 1 Mbps, soon 10 Mbps, eventually 100 Mbps!


Competition -Competition - Cable modems Cable modems

CATV

HEADEND

OPTICAL

FIBER

NODE

COAXIAL

AMPLIFIER

CABLE

MODEM

CABLE

MODEM

CABLE

MODEM

CABLE

MODEM

fiber coax

CMTS


Modem TheoryModem Theory


How do modems work?How do modems work?

The simplest attempt is to simply transmit 1 or 0 (volts?)

This is called NRZ (short serial cables, e.g. RS232)

Information rate = number of bits transmitted per second (bps)

1 1 1 00 1 10


The simplest modem - DCThe simplest modem - DC

So what about transmitting -1/+1?

This is better, but not perfect! DC isn’t exactly zero Still can have a long run of +1 OR -1 that will decay Even without decay, long runs ruin timing recovery (see below)

1 1 1 00 1 10



What about RZ?

No long +1 runs, so DC decay not important Still there is DC Half width pulses means twice bandwidth!

1 1 1 00 1 10



T1 uses AMI (Alternate Mark Inversion)

Absolutely no DC! No bandwidth increase!

1 1 1 00 1 10



Even better - use OOK (On Off Keying)

Absolutely no DC! Based on sinusoid (“carrier”) Can hear it (morse code)

1 1 1 00 1 10


PSKPSK

Even better to use sinusoids with different phases!

BPSK 1 bit / symbol

or QPSK

2 bits / symbol

Bell 212 2W 1200 bps

V.22

1 1 1 0 0 1 0 1

11 10 01 01 00 11 01


QAMQAM

Finally, best to use different phases and amplitudes

2 bits per symbol

V.22bis 2W full duplex 2400 bps used 16 QAM (4 bits/symbol)

This is getting confusing

11 10 01 01 00 11 01


Star watching Star watching

For QAM we can draw a diagram with x and y as axes A is the radius, the angle

For example, QPSK can be drawn (rotations are time shifts)

Each point represents 2 bits!


QAM constellations QAM constellations

16 QAM V.29 (4W 9600 bps)

V.22bis 2400 bps Codex 9600 (V.29) 2W

first non-Bell modem (Carterphone decision)

Adaptive equalizer

Reduced PAR constellation

Today - 9600 fax!

8PSKV.27

4W

4800bps


QAM constellations QAM constellations (cont)(cont)


DMT - continuedDMT - continued

time

frequency


xDSL Line CodesxDSL Line Codes

PAM IDSL (2B1Q) HDSL2 (with TCM and optionally OPTIS) SDSL

QAM/CAP proprietary HDSL/ADSL/VDSL

DMT ADSL G.lite

VDSL line code war is still raging


DuplexingDuplexing

How do we send information in BOTH directions? Earliest modems used two UTP, one for each direction (4W) Next generation used 1/2 bandwidth for each direction (FDD) Alternative is to use 1/2 the time (TDD)

More advanced DSP uses adaptive echo canceling

m odulator

dem odulator

4W to 2W

H YBR ID

U TPLEC


ADSL FDD DuplexingADSL FDD Duplexing

US uses tones 8 - 32 (below 30 KHz reserved)

DS uses 256 tones (FDM from tone 33, EC from tone 8)

POTS

US DS

8 32 256

Documents

Stein Intro xDSL 1.1 x DSL Introduction Yaakov J. Stein Chief Scientist RAD Data Communications