xDSL

1

xDSL xDSL Technology andTechnology andApplications:Applications:

Removing the Telephone Line BottleneckRemoving the Telephone Line Bottleneck

Krista S. JacobsenTexas Instruments

Broadband Access Group

[email protected]

© Texas Instruments 1999

2

Overview/Goals

u Introduction

u DSL network topology

u Transmission environment

u Line code alternatives

u ADSL

u Splitterless ADSL and G.lite

u VDSL

u Spectral compatibility

3

Acronyms

ADSL: Asymmetric digital subscriber lines

CO: Central office

DSL: Digital subscriber line(s)

HDSL: High-speed digital subscriber lines

ISDN: Integrated services digital network

ONU: Optical network unit

POTS: Plain old telephone service

SDSL: Symmetric digital subscriber lines

VDSL: Very high-speed digital subscriber lines

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Overview of xDSL Flavors

Technology Data rates Applications

ISDN 64+ kbps symmetric Data transmission

HDSL 1.054/2.044 Mbps symmetric 2-pair repeaterless T1/E1

HDSL-2 1.054/2.044 Mbps symmetric Single-pair T1/E1

SDSL Under discussion… Multiple HDSL, data transmission

ADSL Up to 8 Mbps downstreamUp to 800 kbps upstream

Internet, video, multimedia

VDSL Up to 52 Mbps downstreamUp to 26 Mbps upstream

Same as ADSL, but more of it

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Digital SubscriberDigital SubscriberLines (DSL)Lines (DSL)

Network TopologyNetwork Topology

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The Local Loop

Telephone lineCentral office (CO) Plain old telephone service(POTS)

Local loop characteristics:n The local loop has been designed solely to support voice traffic

n The 0-4 kHz bandwidth limitations of the local loop are a result of filters placedat the central office to help condition lines for voice traffic

n If the filters are removed, the local loop can support much higher data rates thanvoice band modems provide

n Achievable data rates depend on:

w Loop length (attenuation)

w Condition of line (bridged taps, loading coils, in-home wiring, etc.)

w Noise (crosstalk, impulse noise, radio-frequency ingress)

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Issues:• Bandwidth limited by the local loop (POTS/ISDN - 56 kbps/128 kbps)• Data traffic is congesting the voice network

• Voice network designed around 3-6 minute average call• Data connections typically last over 20 minutes

POTS or ISDN Internet

Voice NetworkCircuit-Switched

FrameRelay

T1/PRI

ISP

AccessConcentrator

Router

ATMCentral Office

Data Network Packet or Cell Switched

PSTN

Class5 Switch

Voice Band Topology

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Issues:• Removes local loop bottleneck

• Still have bandwidth limitations in the data network that must be addressed• Relieves congestion of voice network

• But analyst projections still show analog modems dominating until 2000+

PSTN

WANDSLAM or xDSL linecard in Class5 Switch

ATM

Internet

ISP

POTS/xDSL

Class5 Switch

Central OfficeATM

FrameRelay

FrameRelay

Voice NetworkCircuit-Switched

Data NetworkPacket or Cell

Switched

DSL Topology

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Transmission EnvironmentTransmission Environment

xDSL Transmission Environment

FACT:FACT:Telephone networks were deployed to transfer

analog voice signals, not high-speed digitalsignals.

RESULT:RESULT:Numerous impairments impede transmission in the

frequency band beyond the voice band.

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Environmental Variables

u Channel attenuation varies with:n length

n gauge

n frequency

n bridged taps

u Various noises also impede high-speed transmission:n crosstalk

n impulse noise

n radio-frequency ingress

u Channel and noise characteristics vary substantiallyfrom line to line

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Twisted-pair Channels

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Bridged Taps

u Unterminated stubs of twisted-pair cable connectedin shunt to main pairn Originally used to provide for future plant expansion

n No longer used

u Effect is notches/ripple in frequency spectrum

Central Office Customer

BridgedTaps

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Effects of Bridged Taps

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Crosstalk

Binder

xDSL linesT1 linesPOTS

• Copper twisted pairs are grouped in binders Up to 50 twisted pair per binder

• CrosstalkCaused by other pairs in the binder carrying xDSL, ISDN, T1, etc.Pairs in adjacent binders can also cause crosstalkCoupling (interference) increases with frequency

Tx Rx

Rx Tx

T1Line

xDSLLine

Near-end crosstalk (NEXT)Tx Rx

Tx Rx

T1Line

xDSLLine

Far-end crosstalk (FEXT)

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1 T1 NEXT24 HDSL NEXT

24 ISDN NEXT

0 Hz 500 kHz 1.0 MHz 1.5 MHz

Frequency

Pow

er D

ensi

ty (

dBm

/Hz)

Crosstalk Examples

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Radio-frequency Ingress

u Occurs when over-the-air signals couple into xDSLlinesn AM radio

n Amateur radio

18Frequency

Noi

se p

ower

leve

l

Radio-frequency Ingress

u AM interferers are localized in frequency, maintain constantpower with time

u HAM interferers are unpredictablen on/off keying

n hopping in frequency

n may be high or low power

u Strength of interferer depends on proximity of source to line

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Impulse Noise

u Caused by home appliances, lightning, power linedischarges, other unidentified sources

u Usually short-duration, high-power, fairly constantwith frequency

u Can be 10’s of millivolts, 100’s of microseconds

time

Vol

tage

frequency

PSD

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Frequency

Channel response RF ingress

CrosstalkBridged tap

Not an environment conducive to high-speed communication...Not an environment conducive to high-speed communication...

Composite Impairment Profile

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Line Code AlternativesLine Code Alternatives

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Two Linecode Classes

u Single-carrierSpreads information content during each symbol over entire channel

bandwidth

n Examples:w Carrierless Amplitude/Phase (CAP) Modulation

w Quadrature Amplitude Modulation (QAM)

u Multi-carrierDivides bandwidth into subchannels, allocates only some portion of

the entire information content to each subchannel during eachsymbol

n Examples:w Discrete Multi-Tone (DMT) Modulation

w Discrete Wavelet Multi-Tone (DWMT) Modulation

u Uses an inverse discrete Fourier transform (IDFT) to partitiontransmission bandwidth into subchannels

u Measures signal-to-noise ratio (SNR) of each subchannel and assignsdata accordinglyn Adapts to each line at start-up and automatically avoids severely degraded

regions of bandwidth

n Maximizes bit rate

n Adapts during steady-state to maintain bit rate and noise margin

Bits

per

sub

chan

nel

Frequency

Discrete Multi-Tone (DMT) Modulation

24

-30

-20

-10

0

10

20

30

40

50

SN

R (

dB

)S

NR

(d

B)

0 138 276 414 552 690 828 966 1104Frequency (kHz)Frequency (kHz)

0

2

4

6

8

10

12

0 138 276 414 552 690 828 966 1104

Nu

mb

er o

f B

its

Nu

mb

er o

f B

its

Frequency (kHz)Frequency (kHz)

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Bit Allocation Example

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Conceptual View of DMT

Power spectra of individual subcarriers

Composite DMT power spectrum

f

f

26

u CAP and QAM are similarn QAM generates in-phase (I) and quadrature (Q) signals using sine/cosine

mixer in analog domain

n CAP generates I and Q signals in the digital domain

u CAP/QAM symbols are short-duration, occupy entire availablebandwidthn Because the information is distributed evenly over the entire channel

bandwidth, effects of channel impairments and noise must be overcome bytransmit filters and equalizing receivers

Q

I

00

01

10

11Frequency

CAP/QAM Modulation

US DS

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DMT vs. CAP/QAM

Frequency

Bits

Atte

nuat

ion

Frequency

Bridged Tap

AM Interference

x-talk

• DMT dynamically adapts to the conditions of the line

• CAP/QAM require highly-flexible, potentially complex filters to enablethe receiver to dig out the signal

Atte

nuat

ion

Frequency

Bridged Tap

AM Interference

x-talk

Frequency

Bits

Bit distribution variesbased on channel andnoise characteristics

The bit distributioncannot be varied acrossthe frequency band

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Asymmetric Digital SubscriberAsymmetric Digital SubscriberLines (ADSL)Lines (ADSL)

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ADSL Basics

u Standard-compliant ADSL uses DMT

u Supports asymmetric data ratesn Distance and line condition dependent

w Upstream data rates: up to 800 kbps

w Downstream data rates: up to 8 Mbps

n 10:1 downstream-to-upstream ratiow Ideal for Internet traffic (TCP/IP)

w Ideal for MPEG movies

u Supports limited symmetric data ratesn Up to ~ 800 Kbps

u Co-exists with life-line POTS servicen Uses existing POTS infrastructure

8.0

Dat

a R

ate

(Mbp

s)

1.5

1812Distance (k ft)

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CO CO

POTSPOTS(or ISDN)(or ISDN)

SwitchSwitch

splitsplit

split

ATMATMoror

10BT/42210BT/422

8 Mbps

800 kbps ADSLADSL

video switchvideo switch

ADSLADSLAccessAccessMuxMux

(bridge)(bridge)

ADSLADSL

ADSLADSL

.

.

ADSLADSL

internetinternet

ADSL Deployment

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Central Office

Downstream

ADSL Data Rates and Ranges

6 8 10 12 14 16 18 20 22 240

1

2

3

4

5

6

7

8

Mb

its/

sec

Distance (kft)

Downstream data rate vs. distance from CO

26-gauge

24-gauge

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ADSL Duplexing Alternatives

POTS

UPf

DOWN

ADSL

FDM (frequency division multiplexing)– Upstream and downstream channels are disjoint in frequency– Frequency separation of the two channels is easy to implement

Echo cancellation– Upstream and downstream channels overlap in frequency

• Echo canceler is used to separate channels– Better performance than FDM

• Downstream channel also uses higher-quality lower-frequency portions ofthe spectrum

f

POTS

DOWNUP

ADSL

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Splitterless Splitterless ADSL and G.ADSL and G.litelite

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POTS Splitter

u A splitter is a cross-over filter that separates voicebandsignals from passband DSL signals

u Splitters ease DSL implementation and allow coexistenceof POTS and high-rate DSL on the same line

Line

Remote Splitter

POTS

Central Office

LPF

HPF DSLModemPOTS

DSL

Splitter

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Why Eliminate the Splitter?

TRUCK-ROLL = $$

Central Office

• Original DSL Application: Video-on-demand• Constant bit rate service• Requires POTS splitter to satisfy service requirements over broadrange of conditions and service areas

• Latest DSL Application: Internet Access• Variable-bit rate service• Allows retransmission of corrupted data• Elimination of splitter reduces costs, but also reduces data rateslightly

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DSLZm

PhoneZ1 or Z2

Z0

CentralOffice

Line

ECHO

Splitterless DSL Challenge

ON-HOOK

n Impedance: Z1

n Echo: ~Linear

OFF-HOOK

n Impedance Z2

n Echo: Partially nonlinear

w nonlinear components in some phones

w inband AND out-of-band energy

Also have transient problems when transitioning from on-hookto off-hook state

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What is G.lite?

u Intended for consumer mass marketn Low cost/complexity

u Goal is easy installationn Minimize wiring changes

n Avoid POTS splitter (splitterless)

u POTS and G.lite operate simultaneously

u Provide maximum coverage of customersn Reach more important than data rate

u Ensure spectral compatibility with standardizedxDSL

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Full rate (G.dmt, T1413i2) U-ADSL (G.lite, UAWG)

Optimized for data rate Optimized for cost

8 Mbps/800Kbps 1.5Mbps/512 Kbps

256 tones 128 tones

15 bits per tone 8 bits per tone

Echo Canceled (EC) FDM with EC option

Full initialization

Not included

Fast retrain

Power management

Comparison of Full-rate and G.lite

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NID

ADSLModem

(ATU-R)

Existing or new wiring

HPFLPF

Splittered installation

Distributed Splitter installation

NID

ADSLModem

(ATU-R)

Existing premises wiring

LPFLPFLPF

HPF

NID

ADSLModem

(ATU-R)

Existing premises wiring HPF

Splitterless installation

G.lite Customer Installation Options

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Very High-Speed DigitalVery High-Speed DigitalSubscriber Lines (VDSL)Subscriber Lines (VDSL)

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“Classic” VDSL

u Intended for loops up to 4.5 kft (1.5 km) longn “Fiber-to-the-neighborhood”

n “Last mile”

u Uses a much wider bandwidth than ADSLn Appropriate bandwidth depends on loop length

n 10-12 MHz allows high bit rate transmission on a wide range ofloop lengths

n Very short loops (< 1 kft or 300 m) require even larger bandwidthto maximize performance

u VDSL may be deployed from the CO or local exchange(LEX) or from the ONU

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VDSL ONU Deployment

u Likely deployment in rural or less densely populated areasn Both VDSL size and power consumption must be minimized: the

ONU is small and its environment is uncontrolled

u ADSL lines may be in the same binder as VDSL linesemanating from the ONUn Spectral compatibility with ADSL an issue

CO/LEXCO/LEX......

Twisted-pair lines

ONUONUFiber

Large distance

Up to 4.5 kft (1.5 km)

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VDSL CO/LEX Deployment

u Likely deployment in densely populated areasn Minimizing VDSL size and power consumption is important but

not critical because the CO/LEX environment is controlled

u ADSL lines may also emanate from the CO/LEXn In this case VDSL never affects ADSL performance

..

..

..

Twisted-pair lines

CO/LEXCO/LEX

Up to 4.5 kft (1.5 km)

ù Must be robust to common VDSL impairmentsBridged taps, crosstalk, radio-frequency (RF) ingress, impulse noise

ù Must support both symmetric and asymmetric bit rates and avariety of asymmetric ratios

Symmetric: 26 Mbps, 13 Mbps, 8 Mbps, etc.

Asymmetric: 52/6.4 Mbps, 26/3.2 Mbps, 12/2 Mbps, 6/2 Mbps, etc.

Ratios: 8:1, 6:1, 4:1, 3:1, 2:1

ù Must be spectrally compatible with ADSL and other services

ù Low power consumption (1.5 W/line) and small size required forONU deployment

ù Low cost also desirable

VDSL System Requirements

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VDSL Egress Control

u Inverse of ingress is egress

u Emissions from VDSL lines into amateur radio bands willoccur if VDSL PSD is too high in those bands

u The VDSL PSD must be limited to -80 dBm/Hz within theamateur radio bands to ensure interference is inaudible

Frequency-division duplexing (FDD)Frequency-division duplexing (FDD)Inflexible/Expensive:Inflexible/Expensive:

w in general, bandwidths of upstream and downstream channelsmust be determined in advance

w to accommodate different downstream:upstream bit rate ratios,channel bandwidths must be programmable, which leads toincreases in system complexity

UPUP DOWNDOWN

POTS/ISDNPOTS/ISDN

ff

Duplexing Alternatives for VDSL

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Duplexing Alternatives for VDSL

Echo-cancellation (EC)Echo-cancellation (EC)Impractical:Impractical:

w self-NEXT increases with frequency, so echo-cancellation isonly practical over small, low-frequency bandwidths

w support of symmetric bit rate ratios is infeasible becauserequired overlapping bandwidth is too high

UPUP DOWNDOWN

POTS/ISDNPOTS/ISDN

ff

u A single frequency band is used to support both upstream anddownstream transmission

u Modems can either transmit or receive at any time, but not bothsimultaneously

u Modems in a binder are synchronized to a common clock

u A superframe structure is used to coordinate when the VTU-Os andVTU-Rs transmit

uu Synchronized DMT (SDMT) = DMT modems operating in a TDDSynchronized DMT (SDMT) = DMT modems operating in a TDDfashionfashion

POTS/ISDNPOTS/ISDN

ffDOWN/UPDOWN/UP

Time-division Duplexing (TDD)

u A superframe is a set of consecutive symbols, each of which isclassified as downstream, upstream, or quiet

u By varying the number of upstream and downstream symbols in thesuperframe, a wide range of data rate ratios can be supported

n Example: 20-symbol superframew 9-Q-9-Q superframe supports symmetric transmission

w 16-Q-2-Q superframe supports 8:1 transmission

w 12-Q-6-Q superframe supports 2:1 transmission

u Ideally, all lines in a binder support the same data rate ratio

Quiet intervals

Downstream

“Ping-pong”“Ping-pong”

Upstream

TDD Superframes

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Synchronization Requirements

u Superframes from different modems must be synchronized to avoid NEXTbetween lines

u Synchronization can be achieved by:n Providing a common clock at the ONU/CO (i.e., 8 kHz network clock)

n Allowing one VTU-O to source the master clock for all lines

n Using GPS technology to derive a common clock

DS US

US US

US

DSDS

DS

DS US USDS

DS US USDS

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Spectral CompatibilitySpectral Compatibility

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Bandwidth Utilization

100 MHz10 MHz1 MHz100 kHz10 kHz

POTS ADSL

VDSLISDN

HDSL

100 MHz10 MHz1 MHz100 kHz10 kHz

POTS Pre xDSL

Post xDSL

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In this configuration, VDSL can interfere with ADSL

If band below 1.104 MHz supports upstream VDSL, then ADSLdownstream performance is degraded by VDSL NEXT

If band below 1.104 MHz supports downstream VDSL, then ADSLdownstream performance is degraded by VDSL FEXT

SDMT can prevent interference to ADSL band by turning off subchannelsSDMT can prevent interference to ADSL band by turning off subchannelsbelow 1.104 MHzbelow 1.104 MHz

In situations when interference from VDSL to ADSL is not of concern, thisIn situations when interference from VDSL to ADSL is not of concern, thisband can be enabled to maximize performanceband can be enabled to maximize performance

CustomerPremise

(CP)

Spectral Compatibility with ADSL

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Summary

u Reviewed xDSL network topologies and associated issues

u Described transmission environment and channelimpairmentsn xDSL environment is harsh

u Examined line code alternativesn DMT

n CAP/QAM

u Described xDSLs:n ADSL

n Splitterless

n G.lite

n VDSL

Documents

xDSL