xDSL Introduction

1
xDSLIntroduction

• Yaakov J. Stein
• Chief ScientistRAD Data Communications

2
PSTN wiring
3
Old (analog) PSTN
4
Voice-grade modems
modem
modem
5
New (digital) PSTN
last mile
CO SWITCH
TDM
digital
analog
PSTN
TDM
last mile
CO SWITCH
6
Voice-grade modems over new PSTN
CO SWITCH
PSTN
UTP subscriber line
modem
CO SWITCH
modem
Modem technology is basically unchanged Communicat
ions speeds do not increase
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Unshielded Twisted Pair
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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

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Why twisted?

• 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

n
a
V (an) - (bn)
b
10
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 V1 where Q (Cbc-Cbd) or
  Q(Lbc-Lad)

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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

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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!

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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 gt20 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

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CSA guidelines

• 1981 ATT 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

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Present US PSTN

• UTP only in the last mile (subscriber line)
• 70 unloaded lt 18Kft
• 15 loaded gt 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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xDSL
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Alternatives for data services

• Fiber, coax, HFC
• COST 10K-20K / mile
• TIME months to install
• T1/E1
• COST gt5K/mile for conditioning
• TIME weeks to install
• DSL
• COST _at_ 0 (just equipment price)
• TIME _at_ 0 (just setup time)

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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 dont assume anything
• Use whatever the physics of the UTP allows

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xDSL System Reference Model
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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 (cant buy
  splitter and modem from different vendors)
• Splitter requires installation
• Costly technician visit is the major impediment
  to deployment
• G.lite is splitterless ADSL

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Why is DSL better than a
voice-grade modem?

• Analog telephony modems are limited to 4 KHz
  bandwidth
• Shannons theorem tells us that the maximum
  transfer rate
• 
  for SNR gtgt 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?

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Attenuation vs. frequency
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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

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xDSL - Maximum Reach
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Sources of Interference

• XMTR RCVR
• RCVR XMTR
• FEXT
• NEXT
• RCVR XMTR
• XMTR RCVR
• RF INGRESS

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Interference for xDSL
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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)

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xDSL flavors

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xDSL flavors

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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)

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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

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xDSL types
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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

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T1 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

35
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

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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

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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)
• SDSL Symmetric DSL (ETSI)
• Now called
• SHDSL Single pair HDSL (ITU)

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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 DSUS should be about 101
• full 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

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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 lt 2000 MIPS
  for full rate)

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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

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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

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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)

43
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!

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Competition - Cable modems
CABLE MODEM
CMTS
CABLE MODEM
OPTICAL FIBER NODE
CATV HEADEND
COAXIAL AMPLIFIER
fiber
coax
CABLE MODEM
CABLE MODEM
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Modem Theory
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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)

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The simplest modem - DC

• So what about transmitting -1/1?
• This is better, but not perfect!
• DC isnt 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)

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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!

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The simplest modem - DC

• T1 uses AMI (Alternate Mark Inversion)
• Absolutely no DC!
• No bandwidth increase!

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The simplest modem - DC

• Even better - use OOK (On Off Keying)
• Absolutely no DC!
• Based on sinusoid (carrier)
• Can hear it (morse code)

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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

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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

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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!

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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!
• 8PSK
• V.27
• 4W
• 4800bps

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QAM constellations (cont)
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DMT - continued

frequency
time
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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

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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

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ADSL FDD Duplexing

• US uses tones 8 - 32 (below 30 KHz reserved)
• DS uses 256 tones (FDM from tone 33, EC from tone
  8)