Introduction to xDSL Part III

1
IntroductiontoxDSL Part III

• Yaakov J. Stein
• Chief ScientistRAD Data Communications

2
Introduction to xDSL

• I Background
• history, theoretical limitations, applications
• II Modems
• line codes, duplexing, equalization,
• error correcting codes, trellis codes
• III xDSL - What is x?
• xI,A,S,V - specific DSL technologies
• competitive technologies

3
Quick recap

• Lecture 1
• How did we get to where we want to go?
• How far can we go?
• Lecture 2
• How can we get there?
• Lecture 3
• How can we get there?
• What do we do when we get there?

4
Quick Review

• DSL leaves concept of using 4KHz analog line
• Use UTP as general transmission line
• Rate limited by
• line loss
• thermal noise
• NEXT crosstalk
• FEXT crosstalk
• RF ingress (AM broadcast, ham, etc.)
• misc (splices, bridged taps, echo, filters, sync)

5
Introduction to xDSL III

• Applications
• Deployment topologies
• IDSL
• HDSL, HDSL2, SDSL
• ADSL, G.lite
• VDSL
• competitors (cable modems,wireless)
• HPNA

6
The Baby Bells had a problem ...

• 1993 cable TV companies started offering
• 10 Mbps Internet access
• Internet seen as potential future market
• RBOCs Plan HFC to every home by 1996!
• This didnt happen
• costs grew
• regulatory problems
• no standardization
• LECs had lower operating expenses
• What could be done?

7
Telco Alternatives

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

8
Analog (or V.90) modems
CO SWITCH
PSTN
UTP subscriber line
modem
CO SWITCH
modem
9
xDSL System Reference Model
CO SWITCH
PSTN
POTS-R
POTS-C
Analog modem
network/
UTP
ISP
POTS
POTS
PDN
SPLITTER
SPLITTER
DSLAM
xTU-R
router
WAN
xTU-C
x H, A, V, ...
10
VoDSL
CO SWITCH
PSTN
POTS-R
POTS-C
network/
UTP
ISP
POTS
POTS
PDN
SPLITTER
SPLITTER
xTU-R
DSLAM
router
xTU-C
WAN
11
Network Reference Model

• PDN (Premises Distribution Network) is ethernet
  or USB
• WAN is typically ATM or FDDI (even though FDDI is
  LAN protocol)
• Internet is TCP/IP
• HDSL connects to DACS and to CSU
• Many interconnect possibilities (may impact modem
  design)
• full STM, full ATM, full packet network,
  packet-ATM-packet, etc.
• Example, FR WAN, ATM over UTP, Ethenet PDN
• Modems should be cell pumps, not bit pumps
• (also need CIF protocol to tunnel ATM through
  Ethernet)

12
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

13
xDSL - Maximum Reach
14
Examples of Realistic Reach

• More realistical 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 2Km
• 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)

15
xDSL flavors

16
xDSL flavors

17
ITU G.99x standards

• G.991 HDSL (G.991.1 HDSL G.991.2 SHDSL)
• G.992 ADSL (G.992.1 full rate G.992.2 G.lite
  G.992.3,4,5 new)
• G.993 VDSL
• G.994 HANDSHAKE
• G.995 GENERAL (INFO)
• G.996 TEST
• G.997 PLOAM
• G.998 PNT (HPNA)

18
Some xDSL PSDs
PSD(dBm/Hz)
T1
IDSL
HDSL
HDSL2
ADSL

F(MHz)
19
Line Codes

• PAM
• IDSL, HDSL (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 (but QAM seems
  to be winning)

20
T1/E1

• DS1 rate
• 1 bit per symbol AMI
• Half duplex on each UTP
• Full duplex requires 2 UTP (4W)
• Simple DSP
• Linear equalization
• Needs conditioning
• Repeaters (every km)

21
IDSL

• Original DSL (1980s)
• 160 Kbps in 80 KHz BW
• resistance design reach (18Kft)
• popular in Europe, but not US
• 2 bit PAM called 2B1Q (2 Bits in 1 Quat)
• 10 3 (Gray code)
• 11 1
• 01 -1
• 00 -3
  
• alternative line code
• 4B3T (4 Bits in 3 Ternary symbols)

3
1
-1
-3
22
HDSL

• Replace T1/E1 DS1 service
• Use 2B1Q line code, DFE
• Full duplex on each pair with echo cancellation
• Full CSA without conditioning/repeaters
• more complex DSP (250 MIPS)
• ANSI 2 pairs for T1 (each 784 Kbps)
• ETSI 1, 2, 3 or 4 pairs
• Most mature of DSL technologies

23
HDSL vs T1(AMI)
T1 HDSL
24
HDSL - continued

• HDSL is repeaterless T1/E1
• Major application - multiline POTS
• Reach is CSA (less than ADSL!)
• Can add doublers to extend range
• Other applications
• PBX extension
• digital local loop
• campus networks
• Internet

25
HDSL2, SDSL, SHDSL, OPTIS

• 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)
• SHDSL Single pair HDSL (ITU)
• This is the DS1 service that will last!

26
OPTIS Overlapping PAM Transmission with
Interlocking Spectra

• A solution that achieves these goals
• 16 level PAM with 517K baud rate
• very strong (512 state, gt5 dB) TCM
• 1D for low (216 msec) latency (speech)
• strong DFE
• tailored spectra (fits between HDSL and T1)
• partially overlapped (interlocking) spectra
• folding (around fb/2) enhances SNR!
• upstream bump for spectral compatibility

27
OPTIS - continued
28
OPTIS - continued
29
ADSL

• 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

30
Why asymmetry?

• NEXT is the worst interferer stops HDSL from
  achieving higher rates
• FEXT much less (attenuated by line)
• FDD eliminates NEXT
• All modems must transmit in the SAME direction
• A reversal would bring all ADSL modems down
• Upstream(US) at lower frequencies and power
  density
• Downstream (DS) at high frequencies and power

31
Why asymmetry? - continued
PSD (dBm/Hz)
US
DS
F(MHz)
32
Echo cancelled ADSL

• FDD gives sweet low frequencies to US only
• and the sharp filters enhance ISI
• By overlapping DS on US
• we can use low frequencies and so increase reach
• Power spectral density chart

33
ADSL - continued

• ADSL system design criterion BER 10-12 (1 error
  every 2 days at 6 Mbps)
• Raw modem can not attain this low a BER!
• For video on demand
• RS and interleaving can deliver (error bursts of
  500 msec)
• but add 17 msec delay
• For Internet
• TCP can deliver
• high raw delay problematic
• So standard defines TWO framers
• fast (noninterleaved ) and slow (interleaved)
  buffers

34
ADSL standard

• ITU (G.dmt) G.992.1, ANSI T1.413i2 standard
• First ADSL data implementations were CAP
• Standard is DMT
• DMT allows approaching water pouring capacity
• DMT is robust
• DMT requires more complex processing
• DMT may require more power

35
DMT

• Discrete Multitone is a form of FDM (Frequency
  Domain Multiplexing)
• Discrete Multitone is a form of MCM (MultiCarrier
  Modulation)
• It uses many different carriers, each modulated
  QAM
• Each tone is narrow
• low baud rate (long frame)
• channel characteristics are constant over tone
• Number of bits per tone chosen according to water
  pouring
• Put more bits where SNR is good

36
DMT - continued

• DMT is OFDM (Orthogonalized FDM)
• Carrier spacing is precisely baud rate
• Center of tone is precisely the zero of all other
  sincs
• ICI minimized
• ISI minimized by having a long interframe guard
  time
• DMT modem can be efficiently implemented using
  FFT
• DFT is mathematically equivalent to a bank of
  filters
• Filtering is equivalent to cyclic convolution
• So use cyclic prefix rather than guard time

37
DMT - continued

frequency
time
38
ADSL DMT

• Baud rate (and channel spacing) is 4.3125 KHz
• US uses tones 8 - 32 (below 30 KHz reserved)
• DS uses 256 tones (FDM from tone 33, EC from tone
  8)

39
DMT misc.

• bit handling ((de)framer, CRC, (de)scrambler, RS,
  (de)interleaver)
• tone handling (bit load, gain scaling, tone
  ordering, bit swapping)
• QAM modem (symbolizer, slicer)
• signal handling (cyclic prefix insertion/deletion,
  (I)FFT,
• interpolation,
  PAR reduction)
• synchronization (clock recovery)
• channel handling (probing and training, echo
  cancelling, FEQ, TEQ)

40
RADSL

• Rate Adaptive ADSL
• Not variable rate (not small fast variations)
• Increases percentage of useable lines
• Fine for Internet access
• but not for video on demand
• Standard ADSL supports 32Kbps steps
• RADSL provides management protocols

41
G.lite

• ITU (G.lite) G.992.2, 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)

42
New ADSLs

• ITU has continued development of G.dmt.bis,
  G.lite.bis
• Should become G.992.3, G.992.4, G.992.5
• ADSL2
• Longer reach with higher rate (1.5 Km _at_ 12 Mbps)
• 4D 16-TCM constellations, Stronger RS FEC
• Lower framing overhead (programmable 4-32Kbps
  overhead)
• Power cutback standby mode
• Algo improvements (e.g. real-time tone
  re-ordering, relocatable pilot tone)
• ADSL
• Uses more BW for higher bitrates for short
  reaches
• double BW (512 bins) - double speed (24 Mbps!)
• Annex J
• Symmetric 3 Mbps

43
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

44
VDSL - rate goals

• Symmetric rates
• 6.5 4.5Kft (1.4 Km)
• 13 3 Kft (900 m)
• 26 1 Kft (300 m)
• Asymmetric rates (US/DS)
• 0.8/ 6.5 6 Kft (1.8 Km)
• 1.6/13 4.5 Kft (1.4Km)
• 3.2/26 3 Kft (900 m)
• 6.4/52 1 Kft (300 m)

45
VDSL - Power issues

• Basic template is -60 dBm/Hz from 1.1MHz to 20
  MHz
• Notches reduce certain frequencies to -80 dBm/Hz
• Power boost on increase power to -50 dBm/Hz
• Power back-off reduces VTU-R power so that wont
  block another user
• ADSL compatibility off use spectrum down to 300
  KHz

46
VDSL - duplexing

• In Japan and campus applications can operate TDD
  (ping pong)
• SDMT Synchronous DMT
• (2 KHz frame can be heard in adjacent pairs or
  hearing aids)
• Rest of world PSTN only FDD is allowed
• Can divide US and DS into 2 areas (e.g. ADSL) or
  more
• Need guard frequencies because of clock
  master/slave problems
• Zipper - large number of interleaved frequency
  regions
• (even on a bin by bin basis)

47
VDSL line code wars

• VDSL Alliance VDSL Coalition
• DMT QAM
• MORE LESS
• robust to noise power
• capacity complex
• spectral compatibility
  expensive
• IPR A/D bits
• With no complexity constraints probably
  equivalent

48
T1E1.4 draft T1.424

• T1E1.4 has released a 3-part trial use draft
  standard
• Part 1 Common Specifications
• Part 2 Single Carrier Modulation
• Part 3 Multicarrier Modulation
• Objective tests have been specified (VDSL
  Olympics)
• Test definition may determine results
• SCM is NOT spectrally compatible with ADSL
• Present SCM implementations are more mature
• the tests should be of technology, not products
• MCM may be more robust in certain noise settings
• The trials should be finished by July-August 2003
• ITU and IEEE are waiting for the results

49
G.994.1 (G.hs)

• Handshaking
• Universal flexible method for initialization
• Includes
• tone negotiation for capability identification
• common mode identification
• exchange of nonstandard information
• line probing (line code dependent)
• Currently integral part of ADSL and G.lite
• Anticipated that future ITU DSL modems will
  support as well

50
G.997 (PLOAM)

• Physical Layer Operation Administration and
  Maintenance
• Includes
• physical layer management (SNMP based)
• configuration, fault and performance
  administration
• 4 management interfaces
• optional OAM channel
• far end management
• Currently integral part of G.992 (ADSL) family
• Anticipated that future ITU DSL modems will
  support as well

51
G.996.1 (G.test)

• Universal testing procedure for xDSL modems
• Finds margins in presence of
• POTS signaling
• impulse noise
• cross-talk from other services
• geographical position dependent test loops and
  wiring models
• Currently integral part of G.992 (ADSL) family
• Anticipated that future ITU DSL modems will
  support as well

52
G.bond

• ISDN defined BONDING of 2B channels to one
  128Kbps line
• G.991.2 (SHDSL) Annex E has physical layer
  bonding
• ITU G.bond objectives
• 1) be higher layer agnostic
• 2) be backward compatible with the present 2-wire
  G.shdsl Annex E solution
• 3) different rates on different pairs
• 4) be applicable to all DSL families, not just
  SHDSL
• 5) have low latency and overhead (to support TDM)
• 6) support dynamic addition and removal of pairs
• If succeeds no need for layer 2 aggregation
  protocols
• (ATM-IMA, MLFR, MLPPP, 802.3ad etc) with
• high overhead, high latency, same rates for each
  pair,
• no dynamic addition/deletion support, etc.

53
cVoDSL

• Standard VoDSL sends TDM over ATM layer
• Channelized VoDSL reserves N 64Kbps channels
  (N1..4)
• PRO Implemented on-chip (no GW), higher voice
  quality, lower delay
• CON Consumes BW even if not used

54
Competitorsandnon-DSLtechnologies
55
G.998 (G.pnt,HPNA)

• 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!

56
HPNA

• HPNA 1.0 (98Q3) has average data rate 1.0432 Mbps
  
• Line code is PPM (pulse position modulation)
• Each pulse is 4 cycles at 7.5 MHz (shaped)
• time between pulses 3.27 msec lt t lt 6.07 msec
• Can co-exist with full-rate ADSL and G.lite
• HPNA 2.0 (ITU G.pnt) 10 - 32 Mbps
• QAM line code
• HPNA 3.0 up to 100 Mbps
• Specification not yet finalized

57
Cable modems
CABLE MODEM
CMTS
CABLE MODEM
OPTICAL FIBER NODE
CATV HEADEND
COAXIAL AMPLIFIER
fiber
coax
CABLE MODEM
CABLE MODEM
58
Cable modems - continued

• Line Code (nonstandard, IEEE 802.14)
• QPSK/16 QAM US 1.5 Mbps (raw)
• 64/256 QAM DS 30 Mbps (raw)
• QPSK control channel
• FDD (US low frequencies, DS high frequencies)
• BW to CM is shared
• Performance degrades when too many users

59
Cable modems - continued

• DOCSIS - Data Over Cable System Interface
  Specification
• Evolving specification for high-speed
  data-over-cable systems
• DOCSIS 1.0 designed for transparent
  bi-directional IP traffic
• 3.2 MHz channel, 5.12 Mbps (QPSK)
• DOCSIS 1.1 enhancement
• 3.2 MHz channel, 10.24 Mbps (16-QAM)
• BW management features for QoS multimedia
  applications
• DOCSIS 2.0 improved modem
• 6.4 MHz channel, 30.72 Mbps (64-QAM /
  128-QAMTCM / S-CDMA)
• symmetric upstream and downstream,
• increased noise immunity
• Cable modems are not allowed to monitor each
  other
• so Ethernet (CSMA/CD) is not possible

60
MMDS

• Wireless cable services are only minor
  competition
• Services originated when telcos wanted to get
  into CATV
• Multichannel Multipoint Distribution System
  (Wireless CATV)
• 2.6 GHz (SHF) frequencies
• 54 Mbps DS (33 uncompressed video/data channels)
• Upstream traffic requires expensive subscriber
  transmitters
• Line of site range
• Technical problems weather, trees

61
LMDS

• Local Multipoint Distribution System (Cellular
  TV)
• 28 GHz frequency
• short-distance version of MMDS
• uses small cells
• small cell size requires many transmission
  antennas
• most suitable for business LAN extension

62
DBS

• Direct Broadcast Satellite
• Geosynchronous satellites already used for
  digital TV
• POTS return connection
• High powered transmitter return connection
• Significant propagation delay
• Low earth orbit (LEO) satellites
• Minimal delays
• Lower power uplink transmitters
• Too expensive for residential use