Transcript Intro to xDSL Part 1

Introduction
to
DSL
Yaakov J. Stein
Chief Scientist
RAD Data Communications
Stein Intro DSL 1
PSTN
Stein Intro DSL 2
Original PSTN
UTP
UTP
Manual switching directly connected two local loops
Due to microphone technology, audio BW was 4 kHz
Stein Intro DSL 3
Analog switched PSTN
Invention of tube amplifier enabled long distance
Between central offices used FDM spaced at 4 kHz
(each cable carrying 1 group = 12 channels)
Developed into hierarchical network of automatic switches
(with supergroups, master groups, supermaster groups)
Stein Intro DSL 4
Data supported via
voice-grade modems
UTP
modem
modem
To send data, it is converted into 4 kHz audio (modem)
Data rate is determined by Shannon's capacity theorem
•there is a maximum data rate (bps) called the "capacity"
that can be reliably sent through the communications channel
•the capacity depends on the BW and SNR
In Shannon's days it worked out to about 25 kbps
today it is about 35 kbps (V.34 modem - 33.6 kbps)
Stein Intro DSL 5
Digital PSTN
CO SWITCH
“last mile”
TDM
analog
“last mile”
Subscriber Line
digital
PSTN
TDM
CO SWITCH
LP filter to 4 kHz at input to CO switch (before A/D)
Stein Intro DSL 6
Digital PSTN
Sample 4 kHz audio at 8 kHz (Nyquist)
Need 8 bits per sample = 64 kbps
Multiplexing 64 kbps channels leads to higher and higher rates
Only the subscriber line (local loop) remains analog
(too expensive to replace)
Can switch (cross connect) large number of channels
Noise and distortion could be eliminated due to
Shannon's theorems
1. Separation theorem
2. Source coding theorem
3. Channel capacity theorem
Stein Intro DSL 7
Voice-grade modems
still work over new PSTN
CO SWITCH
PSTN
UTP subscriber line
modem
CO SWITCH
modem
network/
ISP
Internet
But data rates do not increase !
Simulate analog channel so can achieve
Shannon rate < native 64 kbps rate
router
Stein Intro DSL 8
Where is the limitation ?
The digital network was developed incrementally
No forklift upgrades to telephones, subscriber lines, etc.
Evolutionary deployment meant that the new network
needed to simulate pre-existing analog network
So a 4 kHz analog channel is presented to subscriber
The 4 kHz limitation is enforced by LP filter
at input to CO switch (before 8 kHz sampling)
The actual subscriber line is not limited to 4 kHz
Is there a better way
to use the subscriber line for digital transmissions ?
Stein Intro DSL 9
UTP
Stein Intro DSL 10
What is UTP?
The achievable data rate is limited by physics of the subscriber line
The subscriber line is an Unshielded Twisted Pair of copper wires

Two plastic insulated copper wires

Two directions over single pair

Twisted to reduce crosstalk

Supplies DC power and audio signal

Physically, UTP is
– distributed resistances in series
– distributed inductances in series
– distributed capacitances in parallel
so the attenuation increases with frequency

Various other problems exist (splices, loading coils, etc.)
Stein Intro DSL 11
UTP characteristics

Resistance per unit distance

Capacitance per unit distance

Inductance per unit distance

Cross-admittance (assume pure reactive) per unit distance
X
R
L
G
C
Stein Intro DSL 12
UTP resistance
Influenced by gauge, copper purity, temperature
Resistance is per unit distance

24 gauge 0.15 W/kft

26 gauge 0.195 W/kft
Skin effect: Resistance increases with frequency
Theoretical result
R~f
1/2
In practice this is a good approximation
Stein Intro DSL 13
UTP capacitance
Capacitance depends on interconductor insulation
About 15.7 nF per kft
Only weakly dependent on gauge
Independent of frequency to high degree
Stein Intro DSL 14
UTP inductance
Higher for higher gauge
24 gauge 0.188 mH per kft
26 gauge 0.205 mH per kft
Constant below about 10 kHz
Drops slowly above
Stein Intro DSL 15
UTP admittance
Insulation good so no resistive admittance
Admittance due to capacitive and inductive coupling
Self-admittance can usually be neglected
Cross admittance causes cross-talk!
Stein Intro DSL 16
Propagation loss
Voltage decreases as travel along cable
Each new section of cable reduces voltage by a factor
1v
1/2 v
1/4 v
So the decrease is exponential
Va / Vb = e
-g x
= H(f,x)
where x is distance between points a and b
We can calculate g, and hence loss,
directly from RCLG model
Stein Intro DSL 17
Attenuation vs. frequency
24 AWG
26 AWG
Stein Intro DSL 18
Why twisted?
from Alexander Graham 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
Stein Intro DSL 19
Why twisted? - continued
So don't need shielding, at least for audio (low) frequencies
But at higher frequencies UTP has cross-talk
George Cambell was the first to model
(see BSTJ 14(4) Oct 1935)
a
b
Lbc
Cbc
Lad
Cbd
c
d
Cross-talk due to capacitive and/or inductive mismatch
|I2| = Q f V1 where
Q ~ (Cbc-Cbd) or Q~(Lbc-Lad)
Stein Intro DSL 20
Loading coils
Long loops have loading coils to prevent voice distortion
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
Stein Intro DSL 21
Bridge taps
There may also be bridged taps
Parallel run of unterminated UTP


unused piece left over from old installation
placed for subscriber flexibility
High frequency signals are reflected from the open end
A bridged tap can act like a notch filter!
Stein Intro DSL 22
Other problems
Splices
Subscriber lines are seldom single runs of cable
In the US, UTP usually comes in 500 ft lengths
So splices must be made every 500 ft
Average line has >20 splices
Splices are pressure connections that add to attenuation
Over time they corrode and may spark, become intermittent, etc.
Gauge changes
US binder groups typically start off at 26 AWG
Change to 24 AWG after 10 kft
In rural areas they may change to 19 AWG after that
Stein Intro DSL 23
Binder groups
UTP are not placed under/over ground individually
In central offices they are in cable bundles
with 100s of other UTP
In the outside plant they are in binder groups
with 25 or 50 pairs per group
We will see that these pairs interfere with each other
a phenomenon called cross-talk (XTALK)
Stein Intro DSL 24
CSA guidelines
1981 AT&T Carrier Service Area guidelines
advise as follows for new deployments
 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
Stein Intro DSL 25
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
Stein Intro DSL 26
Present US PSTN - continued
We will see, that for DSL - basically four cases

Resistance design > 18Kft loaded line - no DSL possible

Resistance design unloaded <18 Kft <1300 W - ADSL

CSA reach - HDSL

DA (distribution area) 3-5 kft - VDSL
Higher rate - lower reach
(because of attenuation and noise!)
Stein Intro DSL 27
xDSL
Stein Intro DSL 28
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)
Stein Intro DSL 29
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
Stein Intro DSL 30
xDSL System Reference Model
Analog
modem
CO SWITCH
PSTN
POTS-C
network/
ISP
POTS
SPLITTER
router
WAN
POTS-R
UTP
POTS
SPLITTER
DSLAM
xTU-C
PDN
xTU-R
x = H, A, V, ...
POTS
xDSL
frequency
DC 4 kHz
Stein Intro DSL 31
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 specified that splitter be separate from modem
No interface specification (but can buy splitter and modem from different vendors)
Splitter requires installation
 Costly technician visit is the major impediment to deployment
 ADSL has splitterless versions to facilitate residential deployment
Stein Intro DSL 32
Why is DSL better
than a voice-grade modem?
Analog telephony modems are limited to 4 KHz bandwidth
Shannon’s channel capacity theorem
gives the maximum transfer rate
N
S
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?
Stein Intro DSL 33
Maximum reach
To use Shannon's capacity theorem
we need to know how much noise there is
One type of noise that is always present
(above absolute zero temperature) is thermal noise
Maximum reach is the 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
We can compute the maximum reach from known UTP attenuation
Stein Intro DSL 34
xDSL - Maximum Reach
Stein Intro DSL 35
Other sources of noise
But real systems have other sources of noise,
and thus the SNR will be lower
and thus will have lower reach
There are three other commonly encountered types of noise

RF ingress

Near End Cross Talk (NEXT)

Far End Cross Talk (FEXT)
Stein Intro DSL 36
Sources of Interference
XMTR
RCVR
RCVR
XMTR
FEXT
NEXT
RCVR
XMTR
THERMAL
NOISE
XMTR
RCVR
RF INGRESS
Stein Intro DSL 37
Unger’s discovery
What happens with multiple sources of cross-talk?
Unger (Bellcore) : 1% worst case NEXT



(T1D1.3 185-244)
50 pair binders
22 gauge PIC
18 Kft
Found empirically that cross-talk only increases as N0.6
This is because extra interferers must be further away
Stein Intro DSL 38
NEXT
Only close points are important

Distant points are twice attenuated by line attenuation |H(f,x)|2
Unger dependence on number of interferers
Frequency dependence

Transfer function ~ I2Campbell / R ~ f 2 / f 1/2 = f 3/2

Power spectrum of transmission
Total NEXT interference (noise power)
KNEXT N0.6 f 3/2 PSD(f)
Stein Intro DSL 39
FEXT
Entire parallel distance important

Thus there will be a linear dependence on L
Unger dependence on number of interferers
Frequency dependence

Transfer function ~ I2Campbell ~ f 2

Power spectrum of transmission
Total FEXT interference (noise power)
KFEXT N0.6 L f2 |Hchannel(f)|2 PSD(f)
Stein Intro DSL 40
Example - Interference spectrum
Stein Intro DSL 41
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

10 Mbps
7 Kft

13 Mbps 4.5 Kft
1.4 km

26 Mbps
3 Kft
900 m

52 Mbps
1 Kft
300 m (SONET
(CSA 50% US loops)
2 km
STS-1 = 1/3 STM-1)
Stein Intro DSL 42
Bonding (inverse mux)
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)
ATM 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
Stein Intro DSL 43
Duplexing
Up to now we assumed that only one side transmits
Bidirectional (full duplex) transmission
requires some form of duplexing
For asymmetric applications we usually speak of
DS downstream and US upstream
Four methods are in common use:




Half duplex mode (4W mode) (as in E1/T1)
Echo cancellation mode (ECH)
Time Domain Duplexing (requires syncing all binder contents)
Frequency Domain Duplexing
POTS
US
DS
frequency
DC 4 kHz
Stein Intro DSL 44
Muxing, inverse muxing, duplexing
inverse
multiplexing
multiplexing
data streams
physical line
data stream
physical lines
Duplexing =
2 data streams in 2 directions on 1 physical line
Multiplexing =
N data streams in 1 direction on 1 physical line
Inverse multiplexing = 1 data stream in 1 direction on N physical lines
duplexing
Stein Intro DSL 45
(Adaptive) echo cancellation
Signal transmitted is known to transmitter
It is delayed, attenuated and distorted in the round-trip
Using adaptive DSP algorithms we can
 find the delay/attenuation/distortion
 subtract
modulator
4W to 2W
HYBRID
demodulator
Stein Intro DSL 46
xDSL types
and
history
Stein Intro DSL 47
DSL Flavors
DSL is often called xDSL
since there are many varieties (different x)
e.g. ADSL, HDSL, SHDSL, VDSL, IDSL, etc.
There were once many unconnected types
but now we divide them into three main families
The differentiation is by means of the application scenario



HDSL (symmetric, mainly business, data + telephony)
ADSL (asymmetric, mainly residential, Internet access)
VDSL (very high rate, but short distance)
Stein Intro DSL 48
PSD(dBm/Hz)
Some xDSL PSDs
T1
IDSL HDSL HDSL2
ADSL
F(MHz)
Stein Intro DSL 49
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.992.4 splitterless ADSL2
G.992.3 ADSL2
G.992.5 ADSL2+)

G.993 VDSL (G.993.1 VDSL

G.994 HANDSHAKE

G.995 GENERAL (INFO)

G.996 TEST

G.997 PLOAM

G.998 bonding (G.998.1 ATM
G.993.2 VDSL2)
G.998.2 Ethernet G.998.3 TDIM)
Stein Intro DSL 50
ITU xDSL layer model

Transport protocol (ATM, STM, PTM)

Transport Protocol Specific - Transmission Convergence (TPS-TC)

Physical Medium Specific - Transmission Convergence (PMS-TC)

Physical Medium Dependent (PMD)

Physical medium
Stein Intro DSL 51
More xDSL flavors
modem
speed
reach
main applications
IDSL
160 (144) Kbps
5.5 km
HDSL
2 Mbps (4-6W)
3.6-4.5 km
HDSL2
2 Mbps (2W)
3 km
POTS
replacement,
videoconferencing,
Internet access
T1/E1 replacement
PBX interconnect,
FR
same as HDSL
SHDSL
2.3 Mbps
3 km
same as HDSL
SHDSLbis
4.6 Mbps
3 km
same as HDSL
Stein Intro DSL 52
More xDSL flavors (cont.)
Not
DSL
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
VDSL2
200 Mbps
(aggregate)
up to 1.8 km
LAN interconnect,
HDTV,
combined services
“
cable modem
10-30Mbps DS
shared
50 km
residential Internet
HPNA
1, 10 Mbps
home wiring
residential
networking
Stein Intro DSL 53
T1 service (not DSL)
1963: Coax deployment of T1





2 groups in digital TDM
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 (but still not DSL)




Bring 1.544 Mbps to customer private lines
Use two UTP in half duplex mode
Requires expensive line conditioning
One T1 per binder group
Stein Intro DSL 54
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)
Stein Intro DSL 55
The first true xDSL!
1984,88: IDSL
 BRI access for ISDN
 4B3T (3 level PAM) or 2B1Q (4 level PAM) modulation
 Prevalent in Europe, never really caught on in US
 144 Kbps over CSA range
ITU-T G.961 describes IDSL
There are 4 appendices:
 Appendix I - 4B3T (AKA MMS43)
 Appendix II - 2B1Q
 Appendix III - AMI Time Compression Multiplex (TDD)
 Appendix IV - SU32 (3B2T + ECH)
Stein Intro DSL 56
HDSL - NA improved copy of IDSL
1991: HDSL
 Replaced T1/E1 service, but
– full CSA distance w/o line conditioning / repeaters
 AMI line code replaced with IDSL's 2B1Q line code
 Use 2 UTP pairs, but in ECH mode (DFE)
– For T1 784 kbps on each pair
– For E1, 1, 2, 3 and 4 pair modes (all ECH)
 Requires DSP for echo cancellation
 Mature DSL technology, now becoming obsolete
Stein Intro DSL 57
HDSL2
With the success of HDSL,
customers requested HDSL service that would :
 require only a single UTP HDSL
 attain at least full CSA reach
 be spectrally compatible w/ HDSL, T1, ADSL, etc.
The result, based on high order PAM, was called


HDSL2 (ANSI)
SDSL Symmetric DSL (ETSI)
and is now called

SHDSL Single pair HDSL (ITU)
Stein Intro DSL 58
SHDSL
Uses Trellis Coded 16-PAM with various shaping options
Does not co-exist with POTS service on UTP
Can uses regenerators for extended reach
single-pair operation
 192 kbps to 2.312 Mbps in steps of 8 kbps
 2.3 Mbps should be achieved for reaches up to 3.5 km
dual-pair operation (4-wire mode)
 384 kbps to 4.608 Mbps in steps of 16 kbps
 line rate is the same on both pairs
Latest standard (G.shdsl.bis - G.991.2 2003 version)
 bonding up to 4 pairs
 rates up to 5696 kbps
 optional 32-PAM (instead of 16-PAM)
 dynamic rate repartitioning
Stein Intro DSL 59
ADSL
Asymmetric - high rate DS, lower rate US
Originally designed for video on demand
New modulation type - Discrete MultiTone
FDD and ECH modes
Almost retired due to lack of interest
…but then came the Internet
Studies - DS:US for both applications can be about 10:1
Some say ADSL could mean
All Data Subscribers Living
Stein Intro DSL 60
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
Stein Intro DSL 61
ADSL Duplexing

US uses low DMT tones (e.g. 8 - 32)

If over POTS / ISDN lowest frequencies reserved

DS uses higher tones
– If FDD no overlap
– If ECH DS overlaps US
P
O
T
S
US
8
DS
32
G.992.1 FDD mode
256
* 4.3125 kHz
Stein Intro DSL 62
Why asymmetry? - continued
PSD (dBm/Hz)
US
DS
F(MHz)
Stein Intro DSL 63
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
Stein Intro DSL 64
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 the G.992.1 standard defines TWO framers
fast (noninterleaved ) and slow (interleaved) buffers
Stein Intro DSL 65
ADSL standard
ITU (G.dmt) G.992.1, ANSI T1.413i2 standard
DS - 6.144 Mbps (minimum)
US- 640 kbps
First ADSL data implementations were CAP (QAM)
ITU/ANSI/ETSI standards are DMT with spacing of 4.3125 kHz




DMT allows approaching water pouring capacity
DMT is robust
DMT requires more complex processing
DMT may require more power
Stein Intro DSL 66
Splitterless ADSL
Splitterless ADSL, UAWG, G.lite, G.992.2, G.992.4
Splitterless operation
 fast retrain
 power management to eliminate clipping
 initialization includes probing telephone sets for power level
 microfilters
 modems usually store environment parameters
G.992.2 - cost reduction features




G.992.1 compatible DMT compatible using only 128 tones
512 Kbps US / 1.5 Mbps DS (still >> V.34 or V.90 modems)
features removed for simplicity
simpler implementation (only 500 MIPS < 2000 MIPS for full rate)
Stein Intro DSL 67
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
 more flexible format (see next slide)
 stronger FEC
 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
Stein Intro DSL 68
More ADSL2 features
Dynamic training features

Bit Swapping (dynamic change of DMT bin bit/power allocations)

Seamless Rate Adaptation (dynamic change of overall rate)
Frame bearers

Multiple (up to 4) frame bearers (data flows)

Multiple latencies for different frame bearers (FEC/interleave lengths)

Dynamic rate repartitioning (between different latencies)
Stein Intro DSL 69
ADSL annexes (G.992.1/3)
Annex A ADSL over POTS
Annex B ADSL over 2B1Q/4B3T ISDN
Annex C ADSL over TDD ISDN
Annex D State diagrams (state machine for idle, (re)training, etc)
Annex E Splitters (POTS and ISDN)
Annex F North America - classification and performance
Annex G Europe - classification (interop options) and performance
Annex H Synchronized Symmetric DSL with TDD ISDN in binder
Stein Intro DSL 70
ADSL annexes (G.992.3)
Annex I
All digital ADSL (i.e. alone on UTP) with POTS in binder
Annex J All digital ADSL with ISDN in binder
Annex K Transmission Protocol Specific functions (STM, ATM, PTM)
Annex L Reach Extended ADSL2 over POTS
Annex M Extended US BW over POTS
Stein Intro DSL 71
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
Stein Intro DSL 72
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
1.6/13
3.2/26
6.4/52
6 Kft
4.5 Kft
3 Kft
1 Kft
(1.8 Km)
(1.4Km)
(900 m)
(300 m)
Stein Intro DSL 73
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 won’t block another user
ADSL compatibility off use spectrum down to 300 KHz
Stein Intro DSL 74
VDSL2
DMT line code (same 4.3125 kHz spacing as ADSL)
VDSL uses BW of 1.1 MHz - 12 MHz (spectrally compatible with ADSL)
VDSL2 can use 20 kHz - 30 MHz

new band-plans (up to 12 MHz, and 12-30 MHz)

increased DS transmit power
various algorithmic improvements
borrowed improvements from ADSL2
3 user data types - STM, ATM and PTM



Stein Intro DSL 75
VDSL2 band plans
North American bandplan
US0 (if present) starts between 4 kHz - 25 kHz
and ends between 138-276 kHz
Europe - six band plans (2 A and 4 B)
A (998) US0 from 25
DS1 from 138 or 276
US1 3750-5200 DS2 5200-8500
B (997) US0 from 25 or 120 or nonexistent
DS1 from 138 or 276
US1 3000-5100 DS2 5100-7050
Stein Intro DSL 76
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!
Stein Intro DSL 77
Shannon Theory
Stein Intro DSL 78
Shannon - Game plan
Claude Shannon (Bell Labs) 1948
Digital communications never worse than analog
and frequently better !
Basic idea:




Analog signals become contaminated by noise
Amplification doesn't help - noise is amplified too
Bits can not be degraded in a minor way - either 0 or 1
When bit flip - Error Correcting Codes can fix
Rigorous proof:



Source - channel separation theorem
Source encoding theorems
Channel capacity theorems
Stein Intro DSL 79
Shannon - Separation Theorem
Source channel separation theorem

Separate source coding from channel coding

No efficiency loss
info
source
encoder
bits
channel
encoder
channel
analog
signal
channel
decoder
bits
source
decoder
info
The following are NOT optimal !!!

OSI layers

Separation of line code from ECC
Stein Intro DSL 80
Shannon - Channel Capacity
Every bandlimited noisy channel has a capacity
Below capacity errorless information reception
Above capacity errors
Shocking news to analog engineers
Previously thought:
only increasing power decreases error rate
But Shannon didn’t explain HOW!
Stein Intro DSL 81
Channel Capacity (continued)
Shannon’s channel capacity theorem:
If no noise (even if narrow BW):
Infinite information transferred instantaneously
Just send very precise level
If infinite bandwidth (even if high noise):
No limitation on how fast switch between bits
If both limitations:
C = BW log2 ( SNR + 1 )
Stein Intro DSL 82
Channel Capacity (continued)
The forgotten part:
All correlations introduce redundancy
Maximal information means nonredundant
The signal that attains channel capacity
looks like white noise filtered to the BW
Stein Intro DSL 83
Channel Capacity (continued)
That was for an ideal low-pass channel
What about a real channel (like DSL)?
Shannon says ...
Simply divide channel into subchannels and integrate
each bandpass channel
obeys regular Shannon law
S log2 (SNR(f) + 1) BW
 log2 (SNR(f) + 1) df
Only SNR(f) is important !
Stein Intro DSL 84
Water pouring (Gallager) theorem
Given total amount of energy, N(f) and A(F)
how can we maximize the capacity?
N(f) / A(f)
f
Stein Intro DSL 85
Line Codes
Stein Intro DSL 86
How do modems work?
The simplest attempt is to simply transmit 1 or 0 (volts?)
1
1
1
0
0
1
0
1
This is called NRZ (short serial cables, e.g. RS232)
Information rate = number of bits transmitted per second (bps)
Stein Intro DSL 87
The simplest modem - DC
So what about transmitting -1/+1?
1
1
1
0
0
1
0
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)
Stein Intro DSL 88
The simplest modem - DC
What about RZ?
1
1
1
0
0
1

No long +1 runs, so DC decay not important

Still there is DC
Half width pulses means twice bandwidth!

0
1
Stein Intro DSL 89
The simplest modem - DC
T1 uses AMI (Alternate Mark Inversion)
1


1
1
0
0
1
0
1
Absolutely no DC!
No bandwidth increase!
Stein Intro DSL 90
NRZ - Bandwidth
The PSD (Power Spectral Density) of NRZ is a sinc ( sinc(x) = sin(x)
)
x



The first zero is at the bit rate (uncertainty principle)
So channel bandwidth limits bit rate
DC depends on levels (may be zero or spike)
Stein Intro DSL 91
From NRZ to n-PAM
+1
NRZ
-1
1
1
1
0
0
1
0
+3
GRAY CODE
10 => +3
11 => +1
01 => -1
00 => -3
+1
4-PAM
(2B1Q)
-1
-3
11
10
01
01
00
11
01
GRAY CODE
8-PAM
111


001
010
011
010
000
110
Each level is called a symbol or baud
Bit rate = number of bits per symbol * baud rate
100 => +7
101 => +5
111 => +3
110 => +1
010 => -1
011 => -3
001 => -5
000 => -7
Stein Intro DSL 92
PAM - Bandwidth
BW (actually the entire PSD) doesn’t change with n !
BAUD RATE
So we should use many bits per symbol
But then noise becomes more important
(Shannon strikes again!)
Stein Intro DSL 93
The simplest modem - OOK
Even better - use OOK (On Off Keying)
1



1
1
0
0
1
0
1
Absolutely no DC!
Based on sinusoid (“carrier”)
Can hear it (morse code)
Stein Intro DSL 94
OOK - Bandwidth
PSD of -1/+1 NRZ is the same, except there is no DC component
If we use OOK the sinc is mixed up to the carrier frequency

(The spike helps in carrier recovery)
Stein Intro DSL 95
ASK
What about Amplitude Shift Keying - ASK ?
2 bits / symbol
11


10
01
01
00
11
01
Generalizes OOK like multilevel PAM did to NRZ
Not widely used since hard to differentiate between levels
Is FSK better?
Stein Intro DSL 96
FSK
FSK is based on orthogonality of sinusoids of different frequencies



Make decision only if there is energy at f1 but not at f2
Uncertainty theorem says this requires a long time
So FSK is robust but slow (Shannon strikes again!)
1
1
1
0
f1
0
1
0
1
f2
Stein Intro DSL 97
PSK
Even better to use sinusoids with different phases!
BPSK
1 bit / symbol
1
1
1
0
0
1
0
1
or QPSK
2 bits / symbol
Bell 212 2W 1200 bps
V.22
11
10
01
01
00
11
01
Stein Intro DSL 98
QAM
Finally, best to use different phases and amplitudes
2 bits per symbol
11

10
01
01
00
11
01
V.22bis 2W full duplex 2400 bps used 16 QAM (4 bits/symbol)
This is getting confusing
Stein Intro DSL 99
The secret math behind it all
The instantaneous representation



x(t) = A(t) cos ( 2 p fc t + f(t) )
A(t) is the instantaneous amplitude
f(t) is the instantaneous phase
This obviously includes ASK and PSK as special cases



Actually all bandwidth limited signals can be written this way
Analog AM, FM and PM
FSK changes the derivative of f(t)
The way we defined them A(t) and f(t) are not unique

The canonical pair (Hilbert transform)
Stein Intro DSL 100
The secret math - continued
How can we find the amplitude and phase?
The Hilbert transform is a 90 degree phase shifter
H cos(f(t) ) = sin(f(t) )
Hence

x(t) = A(t) cos ( 2 p fc t + f(t) )

y(t) = H x(t) = A(t) sin ( 2 p fc t + f(t) )

A(t) =

f(t) = arctan( y(t)
x2(t) + y2(t)
x(t)
)
Stein Intro DSL 101
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!
Stein Intro DSL 102
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
Received symbols are not points
due to noise and Inter Symbol Interference
(ISI removed by equalizer)
Stein Intro DSL 103
QAM constellations (cont)
1664 points
Stein Intro DSL 104
Multicarrier Modulation









NRZ, RZ, etc. have NO carrier
PSK, QAM have ONE carrier
MCM has MANY carriers
Each is essentially an independent, standalone modem
Achieve maximum capacity by direct water pouring!
PROBLEM
Basic FDM requires has Inter Channel Interference
To reduce effect require guard frequencies
Squanders good bandwidth
Stein Intro DSL 105
OFDM

Subsignals are orthogonal if spaced precisely by the baud rate

Sinc function has zero at center of nearby modem
This implies that the signals are orthogonal - no ICI
No guard frequencies are needed
Don’t need N independent modems
– efficient digital implementation by FFT algorithm



Stein Intro DSL 106
DMT





Measure SNR(f) during initialization
Water pour QAM signals according to SNR(f)
Each individual signal narrowband --- no ISI
Symbol duration > channel impulse response time --- no ISI
No equalizer required
Stein Intro DSL 107
DMT - continued
frequency
time
Stein Intro DSL 108
Summary of xDSL Line Codes
PAM
 IDSL (2B1Q)
 HDSL
 SHDSL/HDSL2 (with TCM and optionally OPTIS)
 SDSL
QAM/CAP

proprietary HDSL/ADSL/VDSL
DMT




ADSL
ADSL2, ADSL2+
G.lite
VDSL2
Stein Intro DSL 109
Misc. Topics
in
DSL Modem Theory
Stein Intro DSL 110
Bit scrambling
We can get rid of long runs that cause DC at the bit level
out
in



D
D
...
D
D
...
D
Bits randomized for better spectral properties
Self synchronizing
Original bits can be recovered by descrambler
in
D
D
...
D
D
...
D
out
Still not perfect! (one to one transformation)
Stein Intro DSL 111
Timing

Proper timing
1
1
1
0
0
1
0
1
Provided by separated transmission

… uses BW or another UTP
Improper timing

causes extra or missed bits, and bit errors
Stein Intro DSL 112
Timing (baudrate) recovery
How do we recover timing (baud rate) for an NRZ signal?

For clean NRZ - find the GCF of observed time intervals

For noisy signals need to filter

PLL
b=T/t
t = a t + (1-a) T/b
How can we recover the timing for a PSK signal?



The amplitude is NOT really constant (energy cut-off)
Contains a component at baud rate
Sharp filter and appropriate delay
Similarly for QAM
BUT as constellation gets rounder
recovery gets harder
Stein Intro DSL 113
Carrier recovery
Need carrier recovery for PSK / QAM signals
How can we recover the carrier of a PSK signal?
X(t) = A(t) cos ( 2 p fc t ) where A(t) = +/- 1
So X2(t) = cos2 ( 2 p fc t )
For QPSK X4(t) eliminates the data and emphasizes the carrier!
Old saying
“square for baud, to the fourth for carrier”
Stein Intro DSL 114
Constellation rotation recovery
How can we recover the rotation of the constellation?
Simply change phase for best match to the expected constellation!
How do we get rid of 90 degree ambiguity?
We can’t! We have to live with it!
And the easiest way is to use differential coding!
DPSK
00
10
11 1
01
NPSK Gray code
000 100 110 010 011 111 101 001 000
QAM put the bits on the transitions!
Stein Intro DSL 115
ISI - BW reduction
Stein Intro DSL 116
QAM ISI
The symbols overlap and interfere
Constellations become clouds
Only previous symbol
Moderate ISI
Severe ISI
Stein Intro DSL 117
Equalizers
ISI is caused by the channel acting like a low-pass filter

Can correct by filtering with inverse filter
modulator
channel
filter
equalizer
demodulator
This is called a linear equalizer
 Can use compromise (ideal low-pass) equalizer
plus an adaptive equalizer
 Usually assume the channel is all-pole
so the equalizer is all-zero (FIR)
How do we find the equalizer coefficients?
Stein Intro DSL 118
Training equalizers
Basically a system identification problem



Initialize during training using known data
(can be reduced to solving linear algebraic equations)
Update using decision directed technique (e.g. LMS algorithm)
once decisions are reliable
Sometimes can also use blind equalization
e
e = e (ai)
Stein Intro DSL 119
Equalizers - continued
Noise enhancement
modulator
channel
filter
noise
equalizer
demodulator
This is a basic consequence of using a linear filter
But we want to get as close to the band edges as possible
There are two different ways to fix this problem!
Stein Intro DSL 120
Equalizers - DFE
ISI is previous symbols interfering with subsequent ones
Once we know a symbol (decision directed) we can use it
to directly subtract the ISI!
linear
equalizer
slicer
out
feedback
filter
Slicer is non-linear and so breaks the noise enchancement problem
But, there is an error propogation problem!
Stein Intro DSL 121
Equalizers - Tomlinson precoding

Tomlinson equalizes before the noise is added
noise
Tomlinson
modulator


precoder
channel
demodulator
filter
Needs nonlinear modulo operation
Needs results of channel probe or DFE coefficients
to be forwarded
Stein Intro DSL 122
More on QAM constellations
What is important in a constellation?




The number of points
The minimum distance between points
N
dmin
The average squared distance from the center E = <r2>
The maximum distance from the center
R
Usually




Maximum E and R are given
bits/symbol = log2 N
PAR = R/r
Perr is determined mainly by dmin
Stein Intro DSL 123
QAM constellations - slicers
How do we use the constellation plot?



Received point classified to nearest constellation point
Each point has associated bits (well that’s a lie, but hold on)
Sum of errors is the PDSNR
Stein Intro DSL 124
Multidimensional constellations




PAM and PSK constellations are 1D
QAM constellations are 2D (use two parameters of signal)
By combining A and f of two time instants ...
we can create a 4D constellation
From N times we can make 2N dimensional constellation!
Why would we want to?
There is more room in higher dimensions!
1D 2 nearest neighbors
2D 4 nearest neighbors
How do I draw this?
ND 2N nearest neighbors!
Stein Intro DSL 125
Trellis coding
Modems still make mistakes
 Traditionally these were corrected by ECCs (e.g. Reed Solomon)
 This separation is not optimal
 Proof: incorrect hard decisions - not obvious where to correct
soft decisions - correct symbols with largest error
How can we efficiently integrate demodulation and ECC?
 This was a hard problem since very few people were expert
in ECCs and signal processing
The key is set partitioning
Stein Intro DSL 126
Set Partitioning - 8PAM
Final step
First step
Original
4
2

Subset 0 Subset 1
00
01
10
11
Stein Intro DSL 127
Set Partitioning - 8PSK
Stein Intro DSL 128
Trellis coding - continued

If we knew which subset was transmitted,
the decision would be easy
So we transmit the subset and the point in the subset
But we can’t afford to make a mistake as to the subset
So we “protect” the subset identifier bits with an ECC

To decode use the Viterbi algorithm (example for 4 states - 2 subsets)



Stein Intro DSL 129
OPTIS Overlapping PAM Transmission with Interlocking Spectra
An single pair HDSL replacement
that is spectrally compatible with HDSL and T1
16 level PAM with 517K baud rate
very strong (512 state, >5 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
Stein Intro DSL 130
OPTIS - continued
Stein Intro DSL 131
OPTIS - continued
Stein Intro DSL 132
DMT processing
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)
Stein Intro DSL 133