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x DSL Introduction . Yaakov J. Stein Chief Scientist RAD Data Communications. PSTN wiring. subscriber line. subscriber line. Old (analog) PSTN. UTP. Voice-grade modems. modem. modem. New (digital) PSTN. CO SWITCH. “last mile”. TDM. analog. digital. PSTN. TDM.

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x dsl introduction
xDSLIntroduction

Yaakov J. Stein

Chief ScientistRAD Data Communications

new digital pstn
New (digital) PSTN

CO SWITCH

“last mile”

TDM

analog

digital

PSTN

TDM

“last mile”

CO SWITCH

voice grade modems over new pstn

network/

ISP

router

Voice-grade modems over new PSTN

CO SWITCH

PSTN

UTP subscriber line

modem

CO SWITCH

modem

Modem technology is basically unchanged

Communications speeds do not increase

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

n

a

V = (a+n) - (b+n)

b

why twisted continued
Why 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 V1where Q ~ (Cbc-Cbd) or Q~(Lbc-Lad)

loading coil
Loading 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

bridge taps
Bridge taps

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
Other problems

Subscriber lines are seldom single runs of cable

US 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

csa guidelines
CSA 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 pstn
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
alternatives for data services
Alternatives 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)

slide18
xDSL

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

xdsl system reference model

Analog modem

CO SWITCH

PSTN

POTS-C

POTS-R

network/

UTP

ISP

POTS

POTS

PDN

SPLITTER

SPLITTER

DSLAM

xTU-R

router

WAN

xTU-C

x = H, A, V, ...

xDSL System Reference Model
splitter
Splitter

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 modem

No 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 than a voice grade modem

N

S

Why is DSL better 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?

maximum reach
Maximum 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
sources of interference

THERMAL

NOISE

Sources of Interference

XMTR RCVR

RCVR XMTR

FEXT

NEXT

RCVR XMTR

XMTR RCVR

RF INGRESS

examples of realistic reach
Examples 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)
itu g 99x standards
ITU 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)
bonding
Bonding

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
t1 service
T1 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 conditioning
T1 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
slide36
HDSL

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

hdsl2
HDSL2

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)

SDSLSymmetricDSL (ETSI)

Now called

SHDSL Single pairHDSL (ITU)

adsl full rate
ADSL (full rate)

Asymmetric - high rate DS lower rate US

Originally designed forvideo on demand

Almost retired due to lack of interest

…but then came the Internet

Studies show DS:US should be about 10:1

full rate ADSL 512-640 kbps US, 6-8 Mbps DS G.lite 512 Kbps US, 1.5 Mbps DS

ADSLcould meanAll Data Subscribers Living

g lite
G.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)

adsl2
ADSL2

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

slide41
VDSL

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

vdsl2
VDSL2

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 cable modems
Competition - Cable modems

CABLE

MODEM

CMTS

CABLE

MODEM

OPTICAL

FIBER

NODE

CATV

HEADEND

fiber

coax

COAXIAL

AMPLIFIER

CABLE

MODEM

CABLE

MODEM

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)

the simplest modem dc
The 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)
the simplest modem dc1
The simplest modem - DC

What about RZ?

  • No long +1 runs, so DC decay not important
  • Still there is DC
  • Half width pulses means twice bandwidth!
the simplest modem dc2
The simplest modem - DC

T1 uses AMI (Alternate Mark Inversion)

  • Absolutely no DC!
  • No bandwidth increase!
the simplest modem dc3
The simplest modem - DC

Even better - use OOK (On Off Keying)

  • Absolutely no DC!
  • Based on sinusoid (“carrier”)
  • Can hear it (morse code)
slide51
PSK

Even better to use sinusoids with different phases!

BPSK

1 bit / symbol

or QPSK

2 bits / symbol

Bell 212 2W 1200 bps

V.22

slide52
QAM

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

star watching
Star watching

For QAM we can draw a diagram with

  • x and y as axes
  • A is the radius, f the angle

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

Each point represents 2 bits!

qam constellations
QAM constellations

16 QAMV.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!

8PSK

V.27

4W

4800bps

dmt continued
DMT - continued

frequency

time

xdsl line codes
xDSL 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

duplexing
Duplexing

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

adsl fdd duplexing

POTS

US

DS

8

32

256

ADSL FDD Duplexing

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

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

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