adsl and flavors in a nutshell

8/6/2019 ADSL and Flavors in a Nutshell

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ADSL and flavors in a nutshell


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ADSL overview


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TOCGetting onto the internet the PSTN way

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 transmittedthrough the switched connection of a PSTN network

This voice band is used for voice or modemcommunication (e.g. fax, V.32, V.90, ...)


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4

TOCGetting onto the internet the DSL way

ISP

CorporatesNTATM

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


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TOCADSL

7300ASAM

POTS,ISDN

ANT

Residential

unshielded twisted pair (UTP)

upstream : up to 800

kbps

downstream : up to 8,1Mpbs

ADSL : Digital Subscriber LineAsymmetrical

max 5,4 km


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6

TOCSpectrum

UP DOWNPOTS

UP DOWN

ISDN

UP DOWNPOT

S

30kHz

1,1MHz

1,1MHz30kHz

138kHz

548kHz

G.dmt Annex A

G.dmt Annex B

G.lite

138kHz


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TOCPOTS splitter

FILTER

SPLITT

ER

& UTP to LEX

The lower frequencies used by ADSL can disturb theaudible spectrum and need to be filtered out towards the

telephone set

With on-hook / off-hook situations, the line impedancechanges and this will impact the ADSL modemcommunication (re-sync)


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TOCCrosstalk AoP & AoI

UP DOWNPOTS

1,1MHz30kHz

G.dmt Annex A

138kHz

UPDOWN

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 anAoP line overlap with the receiver frequencies of an AoIline.


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TOCDMT 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 QAMdetermined.

minimum : QAM-4 2 bits/symbol

maximum : QAM-16384 14 bits/symbol

Symbol period for each carrier : 250 s


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TOCDiscrete Multi Tone example

Ts (Symbol Time)

QAM-4 f1

QAM-16 f2

QAM-4 f3

= DMT

1 DMT Symbol


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TOCDMT vs. Line characteristics

7 29 38 255

4 30 125 165 1100

Frequencyinterference

frequency

attenuation

Bits / carrier

carrier

frequency (kHz)

ADSL filter

characteristics

Line characteristics


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TOCADSL 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 periodTs=17ms/69 = 246,377 sTs=17ms/68 = 250 s (symbol period for the data plane)


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TOCBitswapping explained

Bits/carrier

Carriers2

345

67

89

10

1112

13

14

1

Current max. bits/carrier

Current used bits/carrier


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TOCBit 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). Thisresults in extra bits that could be allocated on that carrier

During showtime (modem operation), the SNR is measured onall carriers at regular intervals (default 1 sec)

if the SNR on a certain carrier degrades resulting at a lower QAMthat can be used on that carrier, the bits of that carrier will bereallocated to other carriers where the maximum QAM is higherthan the actual used QAM.

the modems will try to spread out the reallocated bits overnumerous carriers.


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TOCReed-Solomon correction mode

Byte1

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 %


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TOCADSL & 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 RSword are framing overhead used for modem to modemcommunication (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


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TOCTrellis coding

Trellis coding is another error detection and correction

mechanism which is optional for ADSL.

Trellis principle

looking at the complete data, youre able to detect andcorrect errors, similar to detection and correction is spokenlanguage.

Example :

transmitted data the water is wet and cold

received data the water is llet and cold

by looking at the word let only, we can not decide that thesentence is wrong. by looking at the information before and after the word

(context), we can safely say that it should be wet instead oflet.


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TOCInterleaving

Messagevector

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!


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ADSL flavors

ADSL2, ADSL2+,

READSL2


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TOCADSL the next steps

With ADSL, unable to provide consistent performance

over longer distances. Several potential improvements defined in the last

years in areas as: 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.

So, after 3 years of field expierence with ADSL, the nextsteps are ADSL2, ADSL2+ and READSL


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TOCOverview 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


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ADSL2

G.992.3


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TOCADSL2 improvements

ADSL2 will improve the ADSL rate and reach on long lines.

ADSL2 is more robust in the presence of narrow bandinterference on long lines.

This is done via improvements on: Modulation efficiency Mandatory trellis coding

Enabling enhanced signal processing algorithm Reducing framing overhead Enabling achieving higher RS coding gain. Initialization state machine

Existing ITU G.992.1 & 2 ADSL standards remain in force.

New ADSL chipset should support ADSL2 and be backwardscompatibility with G.992.1 & 2


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TOCDifferences in ADSL and ADSL2 datarates

Standard mandatory and upperlimit downstream datarates.

15 Mbps8 MbpsADSL2 (G.992.3)

8 Mbps (15Mbps for optional S=1/2)6.144 MbpsADSL (G.992.1)

Standard architecture upperlimit downstream datarate

Mandatory downstreamdatarate

Recommendation

Standard mandatory and upperlimit upstream datarates.

1,5 Mbps800 KbpsADSL2 (G.992.3)

1.5 Mbps640 KbpsADSL (G.992.1)

Standard architecture upperlimit upstream datarate

Mandatoryupstream datarate

Recommendation


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TOCImprovements

Better modulation efficiency by mandatory trellis-coding. Was optional for ADSL (G.992.1).

The 1-bit QAM constellation provide higher data rateson long lines where the SNR is low.

In ADSL (G.992.1) the overhead bits per frame consumemin. 32Kbps of the payload data. By a low data rate of128Kbps this is 25% overhead. In ADSL2 the overhead bits can be programmed from 4 to

32Kbps. This provides an additional 28Kbps for payload

data. Improved performance by allowing data modulation on

the pilot tone.


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TOCPower Management (Power consumption)

Current ADSL operates always in full-power mode, evenwhen no user data is transmited.

ADSL2 brings in two power management modes, whichreduce the overall power mode, while maintaining theADSL always on functionality and reduce the overall

power consumption. L0 = full power mode, used during high data traffic.

L2 low-power mode: is based on the internet traffic overthe ADSL connection. For example when there is onlybackground traffic to keep sessions alive.

L3 low-power mode: is a sleep mode when the user is noton-line. When user returns on-line, ADSL transceiver use aFAST STARTUP (duration 3 sec), to reinitialise and enter intoshowtime.


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TOCPower Management diagram

Normal operations

keep alive

Sleep


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TOCIMA Bonding for higher data rates

With bonding multiple phone lines together, data ratesto homes and businesses can be significantly increased.

ADSL2 uses as bonding mechansime, the IMA (inversemultiplexing for ATM) standard.

Through IMA, ADSL2 chipset can bind two or morecopper pairs in an ADSL link, which results in higherdownstream data rates.

ADSL2

ADSL 1

ADSL x

ATM

IMA

ATM


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TOCADSL2: Fast Start-up

Reduction of initialization time from 10 sec (ADSL) to 3sec.

Allow ATUs to quickly enter Showtime:

From a L3 power management state In case of error during Showtime

Data Rate fine tuning in Showtime.

Following a Fast Start-up, Seamless Rate Adaptation (SRA)is used, to optimise the ATU settings. This because thefast startup makes estimations during the short trainingphase which will be most of the times not optimal.


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TOCAll 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 POTSe.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


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ADSL2+

G.992.5


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TOCADSL2+ doubles the frequency spectrum


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TOCADSL2+ 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

remoteADSL2+

ADSL2+ADSLdistance


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TOCADSL2+ doubles the max. data rate

O i f d t d t t (ADSL2+


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TOCOverview of downstream data rates (ADSL2+added)

Recommendation Mandatorydownstream datarate

Standard architecture upper limitdownstream 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


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TOCADSL2+ used to improve spectral compatibility


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Reach Extended ADSL2 (READSL2)

G.992.3 Annex L


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TOCReach Extended ADSL2 concept

A new ITU project studies Long Reach DSL (LDSL)

Targets operation on long loops (e.g. up to 18 kft 26 AWG)

Introduction of Reach Extended ADSL2 (READSL2)

Not to much expected in long reach anyway (Shannons limit!!)

New ADSL2 PSD mask with reduced crosstalk to existing services.

Leads to a small reach increase on the longest loop of about 0,5 kftrelative to ADSL2, if SHDSL is a dominating upstream killer.

However, in self-crosstalk the length increases up to 2kft.

Defined by ITU-T in Annex L of G.992.3 (2003)

For a DS data rate of 500 Kbps, READSL2 results in an increaseof coverage area of about 18%.

Longer reach achieved by using a higher power level (PSD) but in asmaller band so that the total PSD remains the same as for ADSL2


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TOCReach Improvement by READSL2Performance ADSL and READSL

0

500

1000

1500

2000

2500

14 15 16 17 18

Kfeet 26 AWG loop

bitrate

(k

bps)

ADSL US

READSL US

READSL DS

ADSL DS

=4,3km =5,2km =5,5km

adsl and flavors in a nutshell

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