0 x dsl

All Rights Reserved © Alcatel-Lucent 2007
7302 ISAM
xDSL
Alejandro Páez
alejandro.paez@alcatel-lucent.com

2 | xDSL
Table of content
Introduction
The DSL family
Standards
Restrictions
Modulation
Error detection and correction
ADSL flavors

Introduction
TOC

4 | xDSL
POTS modem communication
WWW
NB Access server
+ modem pool
modem
PSTN network
Modem to modem communication in POTS band through the PSTN network!
Frequencies within the voice band are transmitted through the switched
connection of a PSTN network
This voice band is used for voice or modem communication (e.g. fax, V.32, V.90,
...)

5 | xDSL
POTS vs non-POTS modem communication
Frequency (fHz)300Hz 3400Hz
Other frequencies used by DSL technologies:
ISDN > up to 80kHz
ADSL > up to 1,1MHz
Voice band
used by
POTS modems
(V.32, V.90, …)
Spectrum of local telephone line
DSL technologies use other frequencies outside the voice band to modulate
information on your local telephone line (UTP)
ISDN provides you with a 160 Kbps connection
ADSL gives you a high speed broadband connection on your local line

6 | xDSL
Solutions offered by ADSL
PROBLEM
Bitrate of analogue
modem limited to 56 kb/s
PSTN not suited for
high speed data
traffic
ADSL - modem
Redirection of data
traffic to specific
network (B-ISDN)
SOLUTION

7 | xDSL
ADSL
7300
ASAM
POTS,ISDN
ANT
Residential
unshielded twisted pair (UTP)
upstream : up to 800 kbps
←←←←←←←←←←←←
downstream : up to 8,1 Mpbs
→→→→→→→→→→→→→
ADSL : Digital Subscriber LineAsymmetrical
max 5,4 km

8 | xDSL
ADSL spectrum
Upstream Downstream
POTS
300Hz
3400Hz
1,1 MHz30kHz 125 kHz 164 kHz
ADSL
POTS
Copper
wire
±8Mbps
±800kbps
138 kHz
ADSL uses frequencies on the local line up to 1,1 MHz
These frequencies do NOT overlap with the POTS or ISDN band, allowing
simultaneous voice and data communication

9 | xDSL
POTS splitter (PS)
F
I
L
T
E
R
S
P
L
I
T
T
E
R
& UTP to LEX
The lower frequencies used by ADSL can disturb the audible spectrum and need
to be filtered out towards the telephone set
With on-hook / off-hook situations, the line impedance changes and this will
impact the ADSL modem communication (re-sync)

10 | xDSL
Voice / Data over DSL ?
Data
Standard ADSL ADSL with “derived” voice
VoDSL CPE
 
Data
Telephone Line
ADSL
POTS Lifeline
PS
POTS
Telephone Line
ADSL
ADSL CPE
PS
ADSL suitable for all types of communication (voice and data)
Evolution towards Full Digital Loop (FDL)
Elimination of POTS lifeline (and thus splitter)

11 | xDSL
ADSL overview
ISP
Corporates
NTATM
POTS
PSTN
LT
POTS
ADSL modem-modem communication
ATM PVC connection
End-to-end data connection
Service
providers Access providers End users
AS (BRAS)
PS PS
ADSL
modem pool
LT
voice
data

The DSL family
TOC

13 | xDSL
xDSL
ADSL
 first approach to deliver high bandwidth over existing UTP
 killer technology for residential internet access
SHDSL
 symmetrical high bandwidth service mostly for business users
VDSL
 very high speed bandwidth for BB entertainment (video)
 can work both in symmetrical & asymmetrical mode
FTTU
 fiber to the user = optical fiber all the way to the customer.
 conquering “the last mile”

14 | xDSL
SHDSL
Single-pair High speed DSL
 192 kbps … 2.312 Mbps bidirectional (Single pair UTP)
 384 kbps … 4.624 Mbps bidirectional (Double pair UTP)
 IMA support via SMLT board : combine 8 SHDSL lines links into one virtual link
(8 x 2,312 Mbps)
Limited in distance
 max 2,5 km loops
No POTS/ISDN service
 SHDSL uses the entire frequency range
 no splitters required
TC/PAM modulation technique

15 | xDSL
VDSL
Bitrates up to 55Mbps downstream on short loops (300m)
 maximum loop reach : 1500m
 ± 3000 carriers for DMT
Mostly needs a non-CO deployment
 via remote units (FTTN : fiber to the neighborhood)
Uses frequencies up to 12MHz
 zipper mechanism with FDD (frequency division duplexing)
 standardized bandplans (next slide)
Upstream Power Back Off
 to avoid FEXT
Disable carriers
 to avoid interference with HAM bands

16 | xDSL
VDSL band plans
down up updown
down up updown
down up updown
Plan B (formerly plan 997) ETSI + ITU - compromisecompromise band plan
flexible plan Fx (ITU; under discussion in ETSI)
Plan A (formerly plan 998) ETSI + ANSI + ITU - optimized for asymmetryasymmetry
ADSL
0.138 3.0 5.1 7.05 12.0 MHz
0.138 3.75 5.2 8.5 12.0 MHz
0.138 3.752.5 Fx 12.0 MHz
0.138 1.1 MHz
example band plan for China - optimized for symmetrysymmetry
down up optional down
0.138 3.75 8.5 12.0 MHz

17 | xDSL
FTTU
Fiber To The User
 using PON (Passive Optical Network, passive star topology)
Extremely high bandwidth
 using Wavelength Division Multiplexing
 622 Mbps down, 155 Mbps up
Smaller-diameter, lighter-weight cables
Lack of crosstalk between parallel fibers
 no NEXT, FEXT
 Immunity to inductive interference
High-quality transmission
Low installation and operating costs

18 | xDSL
FTTU deployment scenario
OLT
LL Network
OTHER
POTS/ISDN
V5
VB5
G.703
ONU
ADSL ( < 6 KM )
< 8 Mbit/s
FTTEx
ONU
ADSL/VDSL ( < 1 KM )
< 26 Mbit/s
FTTCab
ONU
VDSL ( < 300 M )
< 52 Mbit/s
FTTC
ONT
< 622 Mbit/s down
< 155 Mbit/s up
FTTH/B
Central Office
XNT
XNT
XNT
ATM NETWORK

Standards
TOC

20 | xDSL
ANSI standards
ANSI T1.413 Issue 1 1995
 first ADSL specification out in 1995 was STM based and not clearly built.
ANSI T1.413 Issue 2 1998
 second ADSL specification which was mostly driven by Alcatel and ATM
based as is used today

21 | xDSL
ITU-T standards
ITU-T G.dmt or G992.1
 specification by ITU-T which is based on the ANSI T1.413 Issue 2 standard
plus an extra handshaking protocol.
 Annex A specifies operation above the POTS band
 Annex B specifies operation above the ISDN band
 Annex C specifies operation for the Japanese ISDN band
ITU-T G.lite or G992.2
 specification by ITU-T which is a “stripped down” version of the ANSI T1.413
Issue 2 standard plus an extra handshaking protocol. Based on
recommendations made by the UAWC workgroup (Microsoft, Compaq, Intel)
ITU-T G.hs or G994.1
 specifies the handshaking procedure for xDSL transceivers

22 | xDSL
Spectrum
UP DOWN
POTS
UP DOWN
ISDN
UP DOWN
POTS
30kHz
1,1MHz
1,1MHz30kHz
138kHz
548kHz
G.dmt Annex A
G.dmt Annex B
G.lite
138kHz

Restrictions
TOC

24 | xDSL
Data speed
Bits symbols bits
sec sec symbol
Question : how can we increase the data speed and respect the symbol rate
related constraint ? (Nyquist)
Answer : increase the number of bits per symbol via different modulation
techniques like QAM
 bit rate expresses in bits per second (bps)
 symbol rate expressed in symbols per second (baud)

25 | xDSL
Nyquist bandwidth constraint
Symbol period
For a given bandwidth (W in Hz), the maximum amount of symbols/second
(Rs in baud) is limited in order to avoid Inter Symbol Interference (ISI)
Time (sec)
Ts
Each symbol corresponds to a number of bits
You need to be able to distinguish one symbol from another
The symbol period is minimum the signal period of the lowest frequency.

26 | xDSL
Shannon-Hartley capacity theorem
W = bandwidth in Hz
SNR = Signal to Noise ratio in dB
G = Gainfactor achieved by error correction
Capacity [bps] 1/3 x W x SNR x G~~

27 | xDSL
Shannon-Hartley : Capacity vs. Distance
km
1 2 3 4 5 6
UTP Cable length
CapacityMb/s
25
20
15
10
5
Shannon Hartley capacity
ADSL
8,1 Mb/s
2 Mb/s
6 Mb/s

28 | xDSL
Attenuation vs. Frequency
10 KHz 100 KHz 1 MHz
0
20
40
60
80
Attenuation
(dB)
Frequency
(Hz)
1 km
2km
3km
4km
Cable cross section = 0,5mm²
POTS
band
ϕ x d
R =
Seff

29 | xDSL
Attenuation due to distance
Local exchange
UTP cable 0,5 mm2
4 km : Loss of 32dB at 150 kHz
Transmitted pulse Received pulse
5 km : Loss of 55dB at 150 kHz
ϕ x d
R =
Seff

30 | xDSL
Speed characteristics vs. distanceMbit/sKbit/s
km0
2
4
6
8
10
0 1 2 3 4 5 6
ADSL Downstream
km
0
200
400
600
800
1000
ADSL Upstream

31 | xDSL
Bridged taps
Main
Signal
Echo
Echo
Frequency (Hz)
Attenuation (dB) Increased
attenuation due to
Bridged Tap
1
3
2

32 | xDSL
Crosstalk
Tx
Rx
Tx
Rx
Near End Crosstalk
Far End Crosstalk
For ADSL there is no Near End Crosstalk only Far End Crosstalk!
Tx
Rx
Rx
TxRx
Tx RxTxRx

33 | xDSL
Crosstalk AoP & AoI
UP DOWN
POTS
1,1MHz30kHz
G.dmt Annex A
138kHz
UP DOWN
ISDN
1,1MHz138kHz
G.dmt Annex B
NEXT
When AoP (ADSL over POTS) and AoI (ADSL over ISDN) reside in the same binder
there is NEXT
Some frequencies of the downstream transmitter of an AoP line overlap with
the receiver frequencies of an AoI line.

Modulation
TOC

35 | xDSL
Quadrature Amplitude Modulation (QAM)
Transmitted data = Constellation
2
3
1
0
-1
-2
-3
0,5 1 1,5 2 2,5 3
1111 1001
0000
00110111 0101 0001
0110
1110
1101 1011
1100 1000 1010
0100 0010
1001 0000 1111
Symbol length (Ts)
Symbol is represented by a variation of amplitude & phase for a particular frequency
y = A . sin (2π f.t + ϕ)
4 bits/symbol
>> QAM-16
t
A
ϕ
A

36 | xDSL
QAM and Noise
Constellation
1111 1001
0000
00110111 0101 0001
0110
1110
1101 1011
1100 1000 1010
0100 0010
0
2
3
1
-1
-2
-3
0,5 1
1001
Parasite noise
Same frequency
Amplitude ≠
Phase ≠
The Shannon-Hartley theorem : Capacity bps= 1/3 x W x SNR x G
0
2
3
1
-1
-2
-3
0,5 1
1011
Transmit Receive

37 | xDSL
QAM vs. SNR
Bits/symbol
QAM Signal/Noise ratio (dB) for
BER<10-7
4 QAM-16 21,8
6 QAM-64 27,8
8 QAM-256 33,8
9 QAM-512 36,8
10 QAM-1.024 39,9
12 QAM-4.096 45,9
14 QAM-16.384 51,9
table can be used in 2 ways :
(a) what is minimum required SNR to modulate N bits on a carrier
(b) how many bits can be modulated given a SNR of Y dB

38 | xDSL
Discrete Multi Tone (DMT)
For ADSL, multiple carrier frequencies are modulated on the 1 ADSL line using
QAM.
These frequencies are equally spaced and for each carrier the SNR is measured
to determine the maximum achievable QAM.
The sum of all frequencies is put on the line
This concept is called Discrete Multi Tone (DMT)

39 | xDSL
Discrete Multi Tone example
Ts (Symbol Time)
QAM-4 f1
QAM-16 f2
QAM-4 f3
Σ = DMT
1 DMT Symbol

40 | xDSL
DMT and ADSL
The spectrum used for ADSL is divided into 255 carriers.
 each carrier is situated at n x 4,3125 kHz
For the upstream direction, carriers 7 to 29 are used
For the downstream direction, carriers 38 to 255 are used
On each carrier the SNR is measured and the QAM determined.
 minimum : QAM-4 2 bits/symbol
 maximum : QAM-16384 14 bits/symbol
Symbol period for each carrier : 250 µs

41 | xDSL
DMT vs. Line characteristics
7 29 38 255
4 30 125 165 1100
• • • • • •
Frequency
interference
frequency
attenuation
Bits / carrier
carrier
frequency (kHz)
ADSL filter
characteristics
Line characteristics

42 | xDSL
#bits per carrier
Bits/carrier
Carriers
2
3
4
5
6
7
8
9
10
11
12
13
14
Maximum value after SNR measurement per carrier at startup
Possible working value at startup
1

43 | xDSL
Bit swapping
After start-up we will use a lower QAM then possible on most of the carriers
 the measured SNR at startup determines the maximum possible QAM at start-
up
 Example : QAM-4096 corresponding with 12 bits per symbol ⇒ used QAM on
that carrier : QAM-1024 (10 bits per symbol). This results in extra bits that
could be allocated on that carrier
During showtime (modem operation), the SNR is measured on all carriers at
regular intervals (default 1 sec)
 if the SNR on a certain carrier degrades resulting at a lower QAM that can be
used on that carrier, the bits of that carrier will be reallocated to other
carriers where the maximum QAM is higher than the actual used QAM.
 the modems will try to spread out the reallocated bits over numerous
carriers.

44 | xDSL
Bitswapping explained
Bits/carrier
Carriers
2
3
4
5
6
7
8
9
10
11
12
13
14
1
Sudden frequency interference decreases
SNR on a number of carriers
Current max. bits/carrier
Current used bits/carrier

45 | xDSL
Bitswapping explained (2)
Bits/carrier
Carriers
2
3
4
5
6
7
8
9
10
11
12
13
14
1
A lower SNR also lowers our max QAM
(the number of bits on those carriers)
Affected frequencies

46 | xDSL
Bits/carrier
Carriers
2
3
4
5
6
7
8
9
10
11
12
13
14
1

47 | xDSL
Bits/carrier
Carriers
2
3
4
5
6
7
8
9
10
11
12
13
14
1
Noise margin is spread over the full spectrum

48 | xDSL
Communicating modems
DOWN
modulation
UP
demodulation
Unshielded Twisted Pair
DOWN
demodulation
UP
modulation
ADNT DMT FRAME
(ATU-R)
2 analogue signals (UP,DOWN) travelling in opposite directions over the UTP
The ATU-R locks to the pilot carrier in the downstream direction (PLL : Phase
Locked Loop)
The better the lock, the better the overall SNR !

49 | xDSL
ADSL superframe
DS 3DS 2DS 1 DS 4 DS 68DS 67. . . . . SS 69
SUPERFRAME
17 ms
DMT Symbol
DMT symbol
 a DMT symbol is the sum of all symbols on each individual carrier
Data Symbol (DS)
 a data symbol is used to transmit payload information
Synchronization Symbol (SS)
 a synchronization symbol is transmitted after 68 data symbols to assure
synchronization and to detect possible loss of frame
ADSL symbol period
 Ts=17ms/69 = 246,377 µs
 Ts=17ms/68 = 250 µs (symbol period for the data plane)

TOC

51 | xDSL
Error detection and correction principle
DISTANCE = 2 t + 1
Correction Mode Detection Mode
1 bit error 2 bit error 3 bit error 1 bit error2 bit error
= Valid data
= Invalid data
• 2 bit error correction (t=2)
• 3 bit error detection (d=3)
• More than 3 bit error results
in wrong correction
•5 bit error detection (d=5)

52 | xDSL
The choice of correction mode or detection mode is a consensus
 will you choose the ability to detect AND correct errors with the possibility
to introduce more errors
 OR will you choose the ability to detect a higher number of errors with no
possibility to correct them but at least you don’t introduce more errors
Some coding mechanisms will switch from correction mode to detection mode as
soon as errors are detected (e.g. ATM)
 this is done as errors are mostly of a bursty nature and never come alone.

53 | xDSL
Reed-Solomon correction mode
Byte
1
2
3
4
239
k byte
message
vector
n byte code
vector
254
255
240
n - k
check
bytes
Code RS(255,239)
Distance : n-k+1
d= 255-239+1
d=17
Correction: (d-1)/2
c=(17-1)/2
c = 8
With 16 check bytes, the RS code
can correct up to 8 erroneous bytes
per code vector
Error correction overhead = 16/255 = 6.3 %

54 | xDSL
Reed Solomon
Message vector Ctrl
Received data
Transmitted data
Distance = 15-11+1= 5 Correction = (5-1)/2= 2
More then 2
lost bytes
Burst of errors
Data to be transmitted
Lost data

55 | xDSL
Interleaving
Message
vector
Ctrl
Data to be transmitted
Transmitted Data
Bloc 0 Bloc 1 Bloc 2
Received Data
CtrlCorrection CtrlCorrection CtrlCorrection CtrlCorrection CtrlCorrection
Bloc 3 Bloc 4
Bloc 0 Bloc 1 Bloc 2 Bloc 3
Burst errors
6 lost bytes
1 Byte error
per bloc!

56 | xDSL
Reed Solomon ATM burst
ATM cell: 53 Bytes
RS Decoder
+ Buffer
Burst of 4
ATM cells
ATM
functions
Reed Solomon
overhead
1B
Reed Solomon words coming from lower levels
Traffic
shaper
RS has the characteristic of generating bursty ATM traffic
 the result might be that a policing function discards ATM cells
Therefore we need to shape the ATM traffic
 ATM cells will be transmitted at line rate

57 | xDSL
Delay and Interleaving depths
FAST =
NO
INTERLEAVING !

58 | xDSL
Trellis coding
Trellis coding is another error detection and correction mechanism which is
optional for ADSL.
Trellis principle
 looking at the complete data, you’re able to detect and correct errors,
similar to detection and correction is spoken language.
 Example :
 transmitted data the water is wet and cold
 received datathe water is llet and cold
 by looking at the word “let” only, we can not decide that the sentence is wrong.
 by looking at the information before and after the word (context), we can safely say
that it should be “wet” instead of “let”.

59 | xDSL
ADSL & Reed Solomon
DS 3DS 2DS 1 DS 4 DS 68DS 67. . . . . SS 69
SUPERFRAME
17 ms
DMT Symbol
Assume Trellis coding is NOT used !
1 data symbol corresponds to a 255 RS word. Some bytes in the RS word are
framing overhead used for modem to modem communication (EOC, AOC, IB,
CRC)
If RS is not used, our data still runs through the RS decoder.
The maximum downstream ADSL speed for our data :
 with RS (255-16-1)*8bits/byte*4000 symb/sec = 7,616 Mbps
 without RS (255-1)*8bits/byte*4000 symb/sec = 8,128 Mbps

60 | xDSL
Coding gain
Bits/symbol QAM uncoded Trellis RS Trellis + RS
4 QAM-16 21,5 16 17,5 12,5
6 QAM-64 27,5 22 23,5 18,5
SNR for BER = 1E-7
From the table QAM vs. SNR, we have seen that to attain a BER of 10-7
for a
specific QAM you need a certain SNR.
 if the SNR is lower than this value, the BER will be too high.
 by introducing error detection and correction you lower the BER because a
number of the introduced errors will be corrected.
The mechanism introduces a coding gain resulting in an actual lower SNR that is
needed to achieve a certain constellation.
 Trellis introduces a coding gain of approximately 5,5dB
 RS introduces a coding gain of approximately 4dB
 Trellis & RS together introduce a gain of approximately 9dB

61 | xDSL
ADSL rates
Trellis
overhead
Attainable
line rate
Reed Solomon
overhead
ATM data
Framing overhead
1 up to 6 Bytes
Max. 255 Bytes
1/2bit per carrier
+ 4 bits
1B
ATM attainable rate
 maximum possible ATM rate
ATM used rate
 currently used ATM rate
Used line rate
 actual ADSL line rate
Attainable line rate
 maximum attainable line rate based on SNR measurements

ADSL Flavors
ADSL2 ADSL2+ READSL2

63 | xDSL
Table of contents
Overview new standards
ADSL2 Improvements
ADSL2+
READSL2 (Reach Extended ADSL2)
Multi-DSL

Overview new standards
Challenges for ADSL today

65 | xDSL
ADSL today
Reuse of existing copper wire
High Speed Data (Internet) Access
Separate network for data
Data and voice can be used simultaneously
Maximum Distance
Discrete MultiTone (DMT) modulation
Error detection/correction
Physical point-to-point connection

66 | xDSL
The Access Evolution Challenge
Tier 1: 1.5 Mb/s
Tier3: 5.5 Mb/s
Tier 4: 7.5 Mb/s
Tier 5: 10 Mb/s
Green Zone Grey Zone Red Zone
Tier 2: 3.5 Mb/s
ADSL coverage fromCO
Bandwidth
Services
Coverage
CO
The Access Challenge: Coverage and Bandwidth

67 | xDSL
Increasing bandwidth needs
15 MbpsTier 6
10 MbpsTier 5
7.5 MbpsTier 4
5.5 MbpsTier 3
3.5 MbpsTier 2
1.5 MbpsTier 1
Downstream BWTier
Upstream requirements could vary from 0.5 Mbps to
1.5 Mbps for symmetrical HSI consumer services
+
++
Sample grouping of services into BWTiers
Communication/Entertainment to N
devices
Video Broadcast
Interactive video (VOD)
Gaming (PC & consoles)
High Speed Internet
Increased appetite for symmetrical BW
Voice
Baseband voice
Residential
35-45
Increased
ARPU
>100
Communication/Entertainment to N
devices
Video broadcast, Interactive video
Access services
Corporate leased line, access, business
access
Voice
Baseband voice, VoDSL
Business

68 | xDSL
Coverage
VDSL
ADSL2(plus)
ADSL
ADSL2
READSL2

69 | xDSL
Higher coverage needs: extending the reach
6 Mb/s
Remote
DSLAM
Remote
DSLAM
Remote
DSLAMCentral
Office

70 | xDSL
DSL standards evolution to meet the challenges
ADSLADSL
ADSL2plus
Downstream bandwidth
boost up to 24 Mb/s
Reach Extended DSL (READSL2)
Loop reach increase of 600-900m up
to 6 km(192 Kb/s DS – 96 Kb/s US)
(0.4 mm loop)
Next generation ADSL= ADSL2
•10% improved performance on
longer loops
•Improved OAM
VDSLVDSL
Different bandplans
•Plan 997 : compromise
bandplan for symmetric
and asymmetric traffic
•Plan 998 : optimized for
asymmetry
•Bandwidths available up
to 55 Mb/s on short loops

71 | xDSL
ADSL restrictions
Unable to provide consistent performance over longer distances.
Several potential improvements defined in the last years:
 Data rate versus loop reach performance
 Loop diagnostics
 Deployment from remote cabinets
 Spectrum control
 Power control
 Robustness against loop impairments and RFI, operations and
maintenance.
After 3 years of field expierence with ADSL, the next steps are ADSL2, ADSL2+
and READSL2

72 | xDSL
Overview of the new standards
• G.dmt = G.992.1 = ADSL
• G.dmt.bis = G.992.3 = ADSL2
– Main improvements:
– performance: raising the bar;
– loop diagnostics tools;
– improved initialization & fast start-up ;
– power management;
• G.adslplus = G.992.5 = ADSL2+
– ADSL2+ is defined as delta to ADSL2
– Downstream bandwidth increase
(frequency spectrum up until 2.2 MHz)
– At least 16 Mbit/s should be supported (up to 24 Mbit/s)
• READSL =Annex L G.992.3
– Reach Extended ADSL2
– Targets 192 kbit/s DS – 96 kbit/s US on 6km 0.4mm
loops
G.dmt = G.992.1
= current ADSL
G.dmt.bis = G.992.3
= second generation ADSL2
G.adslplus = ∆ on G.992.3
= ADSL2+
ITU-T
READSL= G.992.3 annex L
= Reach Extended DSL

73 | xDSL
ADSL standards overview

74 | xDSL
Some examples of ‘new’ annexes: Annex I, J and M
3kHz 138kHz
13.3 dBm
upstream
UP
1,1MHz
19.9 dBm
downstream
DOWN G.992.3
Annex I
276kHz
13.4 dBm
upstream
UP
1,1MHz
19.3 dBm
downstream
DOWN G.992.3
Annex J
254kHz3kHz
276kHz
UP
1.1 MHz
DOWN G.992.3
Annex M
POTS

ADSL2 IMPROVEMENTS
G.992.3

76 | xDSL
Improvements in area’s
Real world: Bridged taps, Crosstalk
& Narrowband Interferers
Ease of CPE installation
Enabling applications:
voice, games and video
The green line:
Power savings
Adaptation to time
varying line conditions
Enabling implementation
technologies
ADSL Anywhere:
RU deployment
Monitoring and trouble
resolution tools
All Digital Mode
Egress Friendliness
Multi-vendor
Interoperability
After 3 years of field experience …

77 | xDSL
ADSL2 mayor improvements
Performance: raising the bar
 Improved modulation and coding gain
Power management
Improved initialization
Fast start-up
Loop diagnostics
On-Line configuration
All digital mode ADSL
Framing
Improved application support
Home installation

78 | xDSL
Differences in ADSL and ADSL2 datarates
Standard mandatory and upperlimit downstream datarates.
Recommendation Mandatory downstream datarate
Standard architecture upper limit downstream
datarate
ADSL (G.992.1) 6.144 Mbps 8 Mbps (15Mbps for optional S=1/2)
ADSL2 (G.992.3) 8 Mbps 15 Mbps
 Standard mandatory and upperlimit upstream datarates.
Recommendation Mandatory upstream datarate Standard architecture upper limit upstream datarate
ADSL (G.992.1) 640 Kbps 1.5 Mbps
ADSL2 (G.992.3) 800 Kbps 1.5 Mbps

79 | xDSL
Performance difference between ADSL and ADSL2
data rate increase of about 50-80 kbps reach increase of 250 Ft 26 AWG loop
3000
0
500
1000
1500
2000
2500
14 15 16 17 18
26 AWG loop (Kfeet)
bitrate(kbps)
ADSL downstream
ADSL2 downstream
ADSL upstream
ADSL2 upstream
Noise condition IC2:
At CO side:
12 Self Xtlk, 12 G992.1
ADSL
and -140dBm/Hz white
noise
At CPE side:
6 Self Xtlk, 6 G992.1 ADSL
and -140dBm/Hz white
noise

80 | xDSL
Performance : raising the bar
Better modulation efficiency by mandatory trellis-coding. Was optional for ADSL
(G.992.1).
The 1-bit QAM constellation provide higher data rates on long lines where the SNR is low.
Receiver determined tone reordening is a better defence against AM radio interference.
 Spread out non stationary noise due to AM RFI to get better coding gain from Viterbi
decoder
Improved performance by allowing data modulation on the pilot tone.
Mandatory support of
 DSL Forum TR-048 for North-America (annex A)
 ETSI TS 101 388 V1.3.1 for Europe (annex A&B)

81 | xDSL
Reduced framing overhead/Optimal RS coding gain
In ADSL (G.992.1) the overhead bits per frame are fixed and consume 32Kbps of the
payload data. By a low data rate of 128Kbps (on long lines), this is 25% overhead.
In ADSL2 the overhead bits can be programmed from 4 to 64Kbps. This provides an
additional 28Kbps for payload data.
Through improved flexibility and programmability in the construction of the RS
codewords, ADSL2 achieves higher gain from the RS code when the data rates are low
on long lines.
Support of up to 4 frame bearers and 4 latency paths.

82 | xDSL
Improved Initialization
Receiver gives feedback to the CO.
Pilot tone is allocated by receiver
Arbitrary allocation (by receiver) of carriers used for initialization messages.
Tone blackout (disabling tones) to enable RFI cancellation schemes.
ATU-R chooses configuration
 taken into account the constraints given by the CO (improved rate adaptivity
concept).
Receiver and transmitter determine duration of init signals
Power cutback possible at both ends

83 | xDSL
Power Management (Power consumption)
ADSL: always in full-power mode
ADSL2: three power modes:
 L0 full power mode: showtime (high data traffic)
 L2 low-power mode: keep alive mode (background traffic)
 L3 low-power mode: sleep mode (user is off-line)

84 | xDSL
Power Management diagram
Normal operation
“keep alive”
Sleep

85 | xDSL
Loop Diagnostics Tools to determine field problems
ADSL
 trouble-shooting during and after installation and performance
monitoring was a challenging obstacle in the service deployment.
To tackle problem, ADSL2 transceivers are enhanced with extensive diagnostic
capabilities:
 Collecting measurements at the beginning of showtime.
 This mode is comparable with the existing Alcatel Golden ATU functionality.
 In-service (showtime) monitoring capabilities
 providing information on line quality and noise conditions at both ends of the
line: signal attenuation, SNR margin, attainable net data rate, SNR in function
of frequency… Most of these capabilities are comparable with the existing
Alcatel Golden ATU functionality.

86 | xDSL
Loop Diagnostics Tools to determine field problems
 Special non-showtime diagnostic-testing mode
 Collecting measurements
 even when line quality is too poor to actually get into showtime. This mode can be entered
under operator control.
The following test parameters can be measured and passed to the near end management‑
entity:
• Channel Characteristics Function H(f) per subcarrier (CCF-ps);
• Quiet Line Noise PSD QLN(f) per subcarrier (QLN-ps);
• Signal-to-Noise Ratio SNR(f) per subcarrier (SNR-ps);
• Line Attenuation (LATN);
• Signal Attenuation (SATN);
• Signal-to-Noise Margin (SNRM);
• Attainable Net Data Rate (ATTNDR);
• Near End Actual Aggregate Transmit Power (ACTATP);‑
• Far-end Actual Aggregate Transmit Power (ACTATP);

87 | xDSL
On-Line Reconfiguration (OLR)
Reconfiguration without interruption of service and without errors.
 “Bit Swapping” (BS): data rates are kept
 “Dynamic Rate Repartitioning” (DRR):
data rates are repartitioned among different latency paths
 “Seamless Rate Adaptation” (SRA):
total data rate is changed

88 | xDSL
Bit swapping explained (1)
Bits/carrier
Carriers
2
3
4
5
6
7
8
9
10
11
12
13
14
1
A lower SNR also lowers our max QAM
(the number of bits on those carriers)
Affected frequencies

89 | xDSL
Bit swapping explained (2)
Bits/carrier
Carriers
2
3
4
5
6
7
8
9
10
11
12
13
14
1

90 | xDSL
Seamless Rate Adaptation (SRA) - details
ADSL: re-initialization.
ADSL2: adapting the data rate in real time.
ADSL ADSL2
Modem reset
Initialization of 10 sec
Reduced rate
forever
time time
Kbps Kbps
SRA
SRA
Radio
interference
Radio
interference
Radio
interference
Reduced
rate
Back to
original rate
Radio
interference

91 | xDSL
Fast Start-up
Reduce the initialization time from 10 s to 3 s.
Allow ATU’s to quickly enter Showtime:
 From a L3 power management state
 In case of error during Showtime
Data Rate fine tuning in Showtime.
 Seamless Rate Adaptation (SRA) should be implemented

92 | xDSL
All Digital Mode ADSL (no underlying service)
All Digital Loop: extend the upstream bandwidth.
 ADSL2 Annex I:
Upstream tones 1-31 instead of 6-31 for ADSL over POTS
e.g. 100 kbps extra upstream
 ADSL2 Annex J:
Upstream tones 1-63 instead of 28-63 for ADSL over ISDN
e.g. 750 kbps extra upstream
UP DOWN
UP DOWN
POTS/
ISDN

93 | xDSL
Channelization and Voice over DSL
Split the bandwidth into different channels with different link characteristics
for different applications. ADSL2 simultaneous support:
 Voice application with low latency but higher BER.
 Data application with high latency but low BER.
Channelized Voice-over-DSL (CVoDSL): transport of voice directly over DSL.
 No transport of voice into higher layer protocols such as ATM or IP but
directly onto DSL (DMT symbols).

94 | xDSL
Channelized Voice over DSL
“Normal” VoDSL

95 | xDSL
Bandwidth efficiency comparison
VeDSL = CVoDSL

96 | xDSL
ADSL2: frame bearers and latency paths
A specific frame bearer is connected to only one latency path
 Frame bearer could be any data: ATM, voice, …
 Every latency path could have different characteristics
SCRAMBLER
RS
FEC
INTERLEAVER
MULTIPLEXER
RS Coding Info
Interleaving depth
Latency path #0
Framebearers
LATENCYPATH
MULTIPLEXER
PM
Latency path #p
#0
#n
…
…
n= 0 .. 3
P= 0 .. 3

97 | xDSL
IMA bonding for higher data rates
Higher data rates by bonding two or more copper pairs together
Bonding mechanism: IMA (inverse multiplexing for ATM).
ADSL2
ADSL 1
ADSL x
…
…
ATMIMA
ATM

98 | xDSL
Inverse Multiplexing over ATM (IMA)
IMA : ATM stream is transported on a number of lower-rate physical links
... ...
IMA Group
Physical
links
1
2
3
1
2
3
IMA control protocol
cells (ICP)
Single ATM cell stream
from ATM layer Original ATM cell stream
to ATM layer
TX RXIMA virtual link

99 | xDSL
Downstream rate for bonding 2 lines

ADSL2+
G.992.5

101 | xDSL
ADSL2+ doubles the frequency spectrum

102 | xDSL
ADSL2+ characteristics
ADSL2+ : downstream frequencies up to 2.2 MHz (512 carriers)
Increased downstream data rates on shorter lines (in Mbps):
Improved spectral compatibility between CO and remote cabinet
3.55.95.53.0 km
1.03.03.04.0 km
01.01.05.0 km
7.2106.22.0 km
10.0137.41.0 km
12.014.580.5 km
remote
ADSL2+
ADSL2+
ADSLdistance

103 | xDSL
ADSL2+ doubles the max. data rate

104 | xDSL
Overview of downstream data rates (ADSL2+ added)
Recommendation
Mandatory
downstream datarate
Standard architecture upper limit
downstream datarate
ADSL (G.992.1) 6.144 Mbps 8 Mbps (15Mbps for optional S=1/2)
ADSL2 (G.992.3) 8 Mbps 15 Mbps
ADSL2+ (G.992.5) 16 Mbps 24,5 Mbps

105 | xDSL
ADSL2+ used to improve spectral compatibility

Reach Extended ADSL2 (READSL2)
G.992.3 Annex L

107 | xDSL
Reach Extended ADSL2 concept
On long loops (e.g. up to 18 kft 26 AWG)
Increase in reach of 1 to 2 kft (300-600m) (26 AWG, 0.4mm loop)
New ADSL2 PSD mask with reduced crosstalk to existing services.
 Leads to a small reach increase on the longest loop of about 0,5 kft
relative to ADSL2, if SHDSL is a dominating upstream killer.
 In self-crosstalk the length increases up to 2kft - 600m.
Longer reach achieved by using a higher power level (PSD)
in a smaller band (same or even less total Tx power)

108 | xDSL
READSL2 PSD masks
Mandatory non-overlapped
downstreamPSDmask
UpstreamPSDmask
number 1

109 | xDSL
Reach Improvement by READSL2
Performance ADSL and READSL
0
500
1000
1500
2000
2500
14 15 16 17 18
Kfeet 26 AWGloop
bitrate(kbps)
ADSL US
READSL US
READSL DS
ADSL DS
=±4,3km =±5,2km =±5,5km
12 self crosstalkers with –140dBm/Hz white noise at CO side;
6 self crosstalkers with –140dBm/Hz white noise at CPE side.

110 | xDSL
Reach for ADSL vs. READSL2
Service ADSL READSL2
800 Kbps DS / 96 Kbps US 16 Kft (4,9 km) 18 Kft (5.5 km)
400 Kbps DS / 96 Kbps US 17 Kft (5,2 km) > 18 Kft (5.5 km)

111 | xDSL
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