Course 8 - 9

DSL type digital access techniques (Digital Subscriber Line)

o The term refers to the technologies and equipments used in a telephone

network to ensure the access to a high speed digital network on a twisted wire

line;

• there are two basic categories namely: SDSL – Symmetric DSL and

ADSL - Asymmetric DSL;

o SDSL ensures the same transfer rate in both directions, that is in:

• „upstream” → subscriber – exchange

• „downstream” → exchange – subscriber

• due to the attenuation and crosstalk these systems can work only at

medium frequencies;

• the symmetric DSL variants include: SDSL, SHDSL, MSDSL, HDSL,

HDSL-2, IDSL ;

• SDSL is ideal for LAN, bidirectional-video, web servers.

o ADSL ensures in „downstream” a large bandwidth channel, situated at high

frequencies and a more narrow band channel in „upstream” situated at low

frequencies;

o this division of the frequency bands has two reasons: the information quantity

transmitted in „downstream” is larger and it is reduced the crosstalk, which is

higher at high frequencies;

o ADSL variants include: ADSL, ADSL G.lite, RADSL and VDSL (can work

both in symmetric and asymmetric mode).

o The used transmission mode for different DSL techniques and the maximum

length of the subscriber loop are presented in tab. 1 and fig. 1; it is specified if

is possible the use of telephone service and the number of necessary pairs of

wire.

Fig. 1

Tab. 1

2

o for a condensed presentation of the main issues related to SDSL transmissions,

in fig. 2 are presented a HDSL system, the distant power supply and the

comparison with a simple bidirectional transmission of a primary PCM frame.

Fig. 2.1

Fig. 2.2

Fig. 2.3

o Types of SDSL techniques:

o HDSL (High data rate Digital Subscriber Line) – older version of symmetric

DSL created as an alternative to the T1 and E1 services.

� transmits a rate of 1,544Mbps on two pairs of twisted wire, on each pair a

784kbps rate is transmitted in full-duplex mode using the echo compensation

technique;

� allows the use of normal lines (0,5 mm – preconditioning is not necessary)

with a 12000ft (3700m) maximum length without the use of repeaters; does

not allows the standard telephone service on these lines.

o HDSL2 (Second generation HDSL) – ensures a rate of 1,5Mbps in both

directions on a single pair of twisted wire; does not allow the standard

telephone service on this line.

o IDSL (Integrated Services Digital Network (ISDN) DSL) – ensures

symmetrical rates up to 144kbps using existing telephone lines and ISDN

terminal (ISDN modem);

� differs from the ISDN by the fact that is continuously available; it is used for

WAN (Wide Area Network) type applications; does not allow the standard

telephone service.

o SDSL (Symmetric Digital Subscriber Line) – ensures high transfer rates on a

single twisted pair for T1 and E1 applications; the maximum transfer rate is

2,32Mbps; allows Ethernet interface between the SDSL modem and the user

equipment.

o SHDSL (Symmetric High bit rate Digital Subscriber Line) – allows a 20%

higher coverage than SDSL;

� allows the use of one or two pairs of twisted wire – for ex. 1,2Mbps transfer

rate can be transmitted at 20000ft (6100m) on two normal (0.4 mm) pairs.

o MSDSL (Multi-rate Symmetric Digital Subscriber Line) – allows the adaptive

change of the rate according to the type of the line;

� for ex. using the CAP modulation (Carrierless Amplitude & Phase

Modulation) are available 8 discrete rates between 64kbps/128kbps (29000ft –

8900m – 0,5mm) and 2Mbps (15000ft – 4600m).

o CAP (Carrierless Amplitude&Phase Modulation) – variant of the QAM

modulation (Quadrature Amplitude Modulation).

o QAM one of the best options for data transmissions – allows the use in the

same frequency band of two orthogonal sine carriers – a sine carrier and a

cosine carrier

• on each carrier it is possible to transmit different signals in the same

bandwidth

( ) ( ) ( )( ) ( ) ( ) ( )( ) ( ) ( ) ( ) ( ) ( )tsint'Qtcost'ItsinthtQtcosthtIts cccLPFcLPFQAM ω⋅+ω⋅=ω⋅∗+ω⋅∗= (1)

where hLPF(t) is the impulse response of low pass shaping filters.

• I”(t) şi Q”(t) signals obtained after the QAM demodulation using a local

carrier with fc+∆fc frequency and phase ϕ :

( ) ( ) ( )( ) ( ) ( ) ( ) ( ) ( )( ) ( )( ) ( )

( )( ) ( )[ ]

( )( ) ( )[ ] ( )

( )( )

( )( ) ( )thtsin

2

t'Qtcos

2

t'I

thtsintt2sin2

t'Qtcostt2cos

2

t'I

thttcostsint'Qtcost'Ithttcostst"I

LPFcc

LPFcccccc

LPFccccLPFccQAM

∗

ϕ+ω∆⋅−ϕ+ω∆⋅=

=∗

ϕ+ω∆−ϕ+ω∆+ω⋅+ϕ+ω∆+ϕ+ω∆+ω⋅=

=∗ϕ+ω∆+ω⋅ω⋅+ω⋅=∗ϕ+ω∆+ω⋅=

(2)

bit assignment on modulation

constellation

mapping

FTJ

FTJ

BPF

Q

I

X

Q

I bits

TX

sin(2πfct)

cos(2πfct) MODULATOR

⊕⊕⊕⊕

⊗⊗⊗⊗

⊗⊗⊗⊗ sQAM (t)

LPF

LPF

sQAM (t)

clock recovery

block +

decision block +

demapping block

FTJ

FTJ

BPF

Q”

I”

bits

sin(2πfct+∆fct+ϕ)

cos(2πfct+∆fct+ϕ) DEMODULATOR

⊗⊗⊗⊗

⊗⊗⊗⊗

carrier

recovery LPF

LPF

Fig. 3.1 QAM modulator

Fig. 3.2 QAM demodulator

( ) ( ) ( )( ) ( ) ( ) ( ) ( ) ( )( ) ( )( ) ( )

( )( ) ( )[ ]

( )( ) ( )[ ] ( )

( )( )

( )( ) ( )thtcos

2

t'Qtsin

2

t'I

thtt2costcos2

t'Qtsintt2sin

2

t'I

thttsintsint'Qtcost'Ithttsintst"Q

FTJcc

FTJcccccc

FTJccccFTJccQAM

∗

ϕ+ω∆⋅+ϕ+ω∆⋅=

=∗

ϕ+ω∆+ω−ϕ+ω∆⋅+ϕ+ω∆+ϕ+ω∆+ω⋅=

=∗ϕ+ω∆+ω⋅ω⋅+ω⋅=∗ϕ+ω∆+ω⋅=

(3)

o if the carrier recovery is perfect, meaning ∆ωc=0 and ϕ=0, then I”(t) and Q”(t)

demodulated signals are: ( ) ( ) ( ) ( ) ( ) ( )tht'Qt"Q;tht'It"I FTJFTJ ∗=∗= (4)

o the use of QAM type modulations in the subscriber loop implies the following

problems (see fig. 4 ):

� the bandwidth of the QAM signal is twice of the I(t) and Q(t) modulator

signals bandwidth.

� the spectrum of the QAM signal is centered on the carrier frequency.

� carrier recovery is necessary

o the use of a base-band transmission with high spectral efficiency is the best

idea.

o the CAP modulation ensures the transmission of two different base-band

signals in the same bandwidth.

f

|H c h a n n e l( f ) | ; |S Q A M ( f ) |

f c

B Q A M

B I= B Q

|H s u b s c r ib e r - lo o p ( f ) |

F ig . 4 S p e c t r u m o f Q A M m o d u la t e d

s ig n a l ; B P F a n d L P F c h a n n e l

t r a n s fe r c h a r a c t e r is t i c s

bit assignment on

modulation constellation

mapping

filtru emisie în

fază hef(t)

filtru emisie în cuadratură

heq(t)

Q

I

X

Q

I bits

MODULATOR

⊕⊕⊕⊕ sCAP(t) Q’

I’ in phase

filter hef(t)

in quadrature

filter heq(t) Fig. 5.1 CAP modulator

o Conditions for the transmission and reception filters transfer functions

( ) ( )

( ) ( ) ( ) ( ) 0if2

;0if2

HH

eqefeqef

eqef

<ωπ

−ωϕ=ωϕ>ωπ

+ωϕ=ωϕ

ω∀ω=ω

(5)

( ) ( ) 0thth eqef =∗ (6)

( ) ( ) ( ) ( )

( ) ( ) ( ) ( ) 0daca2

;0daca2

0daca2

;0daca2

efrqefrq

eqrfeqrf

<ωπ

−ωϕ=ωϕ>ωπ

+ωϕ=ωϕ

<ωπ

−ωϕ=ωϕ>ωπ

+ωϕ=ωϕ

(7)

o basic relations witch describe the modulation and demodulation ( ) ( ) ( ) ( ) ( )thtQthtIts eqefCAP ∗+∗= (8)

( ) ( ) ( ) ( ) ( ) ( ) ( )( ) ( ) ( ) ( ) ( )( ) ( ) ( ) ( )( )( ) ( ) ( )( )

( ) ( ) ( ) ( ) ( ) ( ) ( )( ) ( ) ( ) ( ) ( )( ) ( ) ( ) ( )( )( ) ( ) ( )( )ththtQ

ththtQththtIththtQthtIthtst"Q

ththtI

ththtQththtIththtQthtIthtst"I

rqeq

rqeqrqefrqeqefrqCAP

rfef

rfeqrfefrfeqefrfCAP

∗∗=

=∗∗+∗∗=∗∗+∗=∗=

∗∗=

=∗∗+∗∗=∗∗+∗=∗=

(9)

o the bandwidth of the CAP modulated signal is half of the QAM modulated

signal bandwidth.

o the spectrum is centered at low frequencies where the subscriber loop

characteristic has low attenuation distortions.

sCAP(t)

clock recovery

block+

decision block

filtru de recepţie în fază

hrf(t)

filtru de recepţie în

cuadratură

hrq(t)

Q”

I”

bits

DEMODULATOR

in phase

reception

filter hrf(t)

in

quadrature

reception

filter hrq(t) Fig. 5.2 CAP demodulator

o ADSL and VDSL techniques

o ADSL (Asymmetric Digital Subscriber Line) can ensure rates between 6 and 9

Mbps in „downstream” and up to 1Mbps in „upstream”;

� DSL was developed in the late 1980s, its original function being video

delivery over copper (twisted pairs).

� theoretically can deliver three VHS channels or one MPEG2 channel with

digital stereo sound;

� early ADSL systems were not capable ensure quality video services, and the

emphasis was put on high bit rate data communications – high speed internet

access;

� works on a single pair of twisted wire and allows the standard telephone

service;

o ADSL G.lite – is a simplified variant of ADSL for domestic users which can

deliver in „downstream” up to 1,5Mbps and up to 500kbps in „upstream”; the

connection to the telephone line is simpler.

o RADSL (Rate Adaptive Digital Subscriber Line) – allows an adaptation of the

transmission rate up to 7Mbps in „downstream” and up to 1Mbps in

„upstream”;

� The transceiver adapts automatically the bandwidth assigned to “upstream”

and “downstream” transmission in order to obtain the highest possible

effective rate (small files can be transmitted at a higher rate than large files);

� Allows both symmetrical and asymmetrical applications;

o VDSL (Very high bit rate Digital Subscriber Line) – transfer rates between

25Mbps and 50Mbps can be obtained over low distances se pot (ten meters,

maximum hundred meters), that is from the optic cable to the user;

� it can be configured also for symmetric transmissions;

� it is intended for university campuses, industrial parks where it is a small

distance to the fiber optic terminal;

� VDSL standardization activities started in 1995; in 1997 an association led

by British Telecom developed the first VDSL requirements specifications;

� the standardization activity was delayed due to debates related of which

modulation technique should be used, DMT or QAM. Different VDSL

technical associations supported different transmission technologies –

VDSL Alliance, a partnership between Alcatel, Texas Instruments and other

supported DMT technology, VDSL Coalition, led by Lucent and Broadcom,

supported QAM-CAP technology. Finally DMT technology won out, and in

G.993.1 standard (2003) (VDSL1), the main transmission technique is

DMT, the QAM technique being one alternative possibility;

o In the table 2 are presented some parameters concerning the ADSL and

VDSL techniques.

o ADSL access and main characteristics

Tab. 2

ADSL

FTTNode

FTTCurb

FTTHomee

FTTBuilding

local

exchange

distribution

frame;

concentrator

subscriber

copper

copper fiber

Fig. 6 ADSL access scenarios

Fig. 7 ADSL bit rates and coverage

o Bandwidth allocation of ADSL transmissions

Fig. 8 ADSL access architecture

computer

ADSL modem

telephone (POTS)

or

Dial-Up modem

splitter-R splitter-L

TF exchange

(POTS)

server/digital packet

switching equipment

DSL

HPF

LPF

POTS

DSL

HPF

LPF

POTS

central office

subscriber

loop

abonat

subscriber

Fig. 8.1 ADSL access architecture

RX

TX

PSD

POTS upstream downstream f

spectral distribution

seen by the exchange RX filter spectral distribution seen by the subscriber

POTS upstream downstream f

PSD

TX

RX

TX filter

Fig. 9.1 Bandwidth allocation of ADSL transmissions if the transmission directions

are separated using echo compensation

spectral distribution seen by the exchange

spectral distribution

seen by the subscriber

POTS upstream downstream f POTS upstream downstream f

PSD PSD

RX RX

TX TX

Fig. 9.2 Bandwidth allocation of ADSL transmissions if the transmission directions

are separated using frequency multiplexing

o Transmission parameters – distortions characteristic to ADSL transmissions

upstream

downstream

D/A

A/D

AMP

TX

FILT.

D/A

hybrid

hybrid

AMP

RX

FILT.

FILT.

ECOU

FILT.

ECOU

A/D

digital DSP components

converters, analog filters and amplifiers

downstream

upstream

+echo downstream

+echo upstream

+nonlinear

distortions

+nonlinear

distortions

Fig. 10.1 ADSL modem line interface – transmission direction separation using echo

compensation

D/A

AMP

TX

FILT.

TX signal

echo TX

+ wanted RX

wanted RX signal

converters, analog filters and amplifiers digital DSP

components external passive filter

with high dynamic

range

driver

FILT. RX internal+

external

hybrid

AMP

A/D

Fig. 10.2 ADSL modem line interface – transmission direction separation using

frequency multiplexing

|H(f)| (dB)

f (kHz)

0,4 mm diameter and 4,5km length cable

0 20 40 60 80 100 120 140

-30

-40

-50

-60

-70

-80

-90

-100

Fig. 11.1 Cable frequency transfer characteristic

o The modulation techniques used in ADSL/VDSL technologies are CAP and

DMT (Discrete Multi-Tone)

o The principles of DMT modulation are explained in the following figures:

Feeder cable

Distribution cable

Bridged taps

Slice

Central

office

Fig. 11.2 Typical cooper loop configuration

Fig. 11.3

o the sub-carriers are orthogonal; subcarrier separation=4,3125kHz=DMT

symbol period = sub-channel symbol period; 255 sub-carriers are used in the

case of ADSL systems; maximum number of bits / tone 15.

o The frequency bands occupied by the „downstream” and „upstream” channels

when frequency multiplexing is used for transmission direction separation – 25

tones for upstream; 215 tones for downstream.

o RADSL assigns adaptively the frequency bands (and tones) for “upstream”

and “downstream”.

o Adaptation of DMT modulation to the channel frequency transfer

characteristic

g(t)

g(t)

g(t)

⊕⊕⊕⊕

⊗⊗⊗⊗

⊗⊗⊗⊗

⊗⊗⊗⊗

tf2j 1eπ

X1

X2

XN/2

tf2j 2eπ

tf2j 2/Neπ

Q

I

Xi

⊕

⊗

⊗

⊗

nN

12j

eπ

X1

X2

XN/2

nN

22j

eπ

nN

2/N2j

eπ

digital

implemen-

tation

Fig. 12 Principles of DMT modulation

Power spectral density

Fig. 13 Tone and frequency allocation for ADSL transmissions frequency multiplexing

used for transmission directions separation

o DMT has superior performances comparatively to the CAP modulation - the

selective assignment of the number of bits/tone ensures an equalization of the

channel and it is possible to do a fine adaptation of the transmission rate to the

channel distortions

o VDSL frequency allocation plan

bits / sub-carrier

for ideal channel

line frequency transfer characteristic +

distortions

interference

bits / sub-carrier

for real channel

a) b) c)

Fig. 14 Adaptation of the DMT modulation to the channel transfer characteristic

binary rate

subscriber loop length

D M T

C A P

C AP

D M T

coverage

Fig. 15 Comparison between DMT and CAP in what concern the possibility to adapt the

transmission rate and the maximum length of the subscriber line

Fig. 16

o VDSL spectral mask and spectral compatibility with xDSL

Fig. 17

Fig. 18

o VDSL performances

Fig.

19.1

Fig.

19.2

o New DSL technologies

o ADSL2

o Extra facilities provided by the ADSL2 standard � higher transmission bit rate;

� larger coverage area;

� more flexible adaptation of the transfer rate to the channel characteristics;

� channel diagnostic facilities;

� stand-by functioning mode;

o ADSL2 ensures a bit rate up to 12Mbps in „downstream” and up to 1Mbps in „upstream”;

these performances are obtained by the increase of the modulation efficiency, the decrease

of framing overhead, by larger coding gain (ADSL uses a 4 dimensional trellis coded

modulation with 8 states – ADSL2 uses a 4 dimensional trellis coded modulation with 16

states), by improvement of the initialization process and by more complex signal

processing:

� there are used modulations with 1 bit per symbol – important for long

lines;

� reordering of the tones based on data provided by the receiver allows the

spreading of the interferences;

� variable overhead (fixed overhead in ADSL standard – 32kbps; variable in

ADSL2 standard, overhead between 4kbps and 32kbps);

� more flexible RS code;

o The initialization of the ADSL2 has a lot of improvements:

� capabilities to decrease the transmission power at both ends to reduce the

crosstalk;

� dynamic allocation of the pilot tones to avoid spectral zeros inserted by

the bridged taps and radio interferences;

� adaptive change of the initialization period for optimal training of

different components;

� adaptive choose of tones (subcarriers) used in to transmit the initialization

messages to avoid the spectral zeros inserted by bridged taps and radio

interferences;

� suppression of non used tones during the initialization period to allow the

measurement and suppression of radio interferences;

o fig. 20 shows a comparison between the bit rates are coverage area of the ADSL and

ADSL2 systems

Fig. 20 Comparison

between the bit rate and

coverage area of ADSL

and ADSL2 techniques

o in the case of ADSL2 modems are provided diagnostic and monitoring facilities; can be

measured the background noise, the loop attenuation, signal to noise ratio; the diagnostic

information are sent to the central office and are used for service quality monitoring.

o another significant improvement brought by ADSL2 technique is related to power

management – the ADSL modems from first generation operate continuously in the “full-

power” mode, the so called L0 mode (even when nothing is transmitted) – significant

consumption of energy which is a problem especially in the case of “cabinet based

ADSL” – there are problems related to power supply and heat dissipation; the ADSL2

technique brings two new power management techniques, namely: (see fig. 21):

� L2 “low-power mode”: dedicated to the ATU-C modem located in the CO (or

cabinet) – it is allowed the fast entering and exiting of the low-power mode based

on the data traffic on the connection – if the traffic is large the modem works

continuously in the full power mode (L0) to maximize the transmission speed; in

the moment when the traffic decreases the modem enters the L2 operation mode

and a reduced bit rate is used to transmit – the switching between the two

functioning modes is realized instantaneously and without affecting the error

probability or the service quality;

� L3 “low-power mode”: dedicated both to modems ATU-C and ATU-R – the

modem enters a “sleep mode” in the moment when no traffic is detected on the

connection for a longer time interval; transition to normal operation requires

approximately 3s

o another important facility of the ADSL2 technique is the so called “seamless rate

adaptation” (SRA), meaning a continuous adaptation of the bit rate – it is possible to

modify continuously the bit rate during the transmission, without any interruption of the

service or modification of the bit error probability (the standard ADSL systems allows the

modification of the bit rate only at the beginning of the transmission) – changes in the

channel state are detected and the bit rate is adapted to the new conditions – it is based on

the separation of modulation and framing processing – the bit rate is modified without

affecting the framing (synchronization) – the bit clock synchronization is maintained in

this process – it is used the so called OLR procedure – “online reconfiguration” – shortly

this procedure acts in the following way:

� the receiver monitors the SNR on the channel and decide if it is necessary a

change of the transmission rate on the channel to compensate the channel

modifications;

Fig. 21 Power management

capabilities characteristic to

ADSL2 technique

� the receiver transmits a message to the transmitter to initiate the bit rate changing

procedure. This message includes all the data necessary for the transmission of the

new bit rate and specifies the number of bits per symbol and the power on each

tone (subcarrier);

� the transmitter sends a „Sync Flag” signal which is used as a marker and identifies

exactly the moment of the bit rate change.

� the marker is detected by the receiver and both ends of the communication change

the bit rate transparently to the provided service;

o The increase of the bit rate can be achieved by coupling several telephone lines in a single

ADSL connection (bonded ADSL) – it is an important facility of the ADSL2 technique; it

is necessary the insertion of a multiplexing/demultiplexing layer which allows the

distribution of a larger flow on different physical connections – bonded ADSL2+ was

introduced in 2005.

o ADSL2 offers the possibility to split the bandwidth in channels with different

characteristics for different applications. For ex. ADSL2 allows simultaneous support for

voice applications – higher allowed bit error probability but smaller delay and data

applications – smaller allowed bit error probability but larger delay. This capacity for

channelization provides support for CVoDSL – Channelized Voice over DSL –

transparent transmission method of TDM voice channels with DSL technique. 64kbps bit

rate channels are reserved for transmission of DS0 to CO or to a multiplexer – see fig. 22.

o Additional benefits offered by the ADSL2 technique:

� Improved interoperability – additions to the initialization procedures offer a better

interoperability between equipments with chips from different suppliers;

� Fast initialization (“startup”) – the initialization time can be reduced from 10s

(standard ADSL) to 3s (ADSL2);

� All-digital mode – optional mode in which the standard telephone band is used for data

transmissions – the bit rate can be increased with 256kbps;

� Support for packet based services – it is provisioned a special convergence level which

allows packet based services (for ex. Ethernet) to be transported over ADSL2;

o ADSL2+

� It is an extension of the ADSL2 standard; ADSL2 specifies a downstream band of

1.1MHz or 552kHz (ADSL G.lite.bis), and ADSL2+ specifies a band of 2.2MHz –

significant increase of the downstream bit rate for lines shorter than approximately

5000 feet; the upstream bit rate is around 1Mbps, according to the transmission channel

parameters (length of the line); see fig. 23 for frequency bands allocation for ADSL2+;

Fig. 22 CVoDSL technique for

transmission of TDM voice channels

using the DSL technique

in fig. 24 is presented a comparison between the bit rates ensured by ADSL2 and

ADSL2+ techniques according to the length of the line.

� ADSL2+ can be used to decrease the crosstalk by using only the frequency band

located between 1.1MHz and 2.2MHz – ADSL2+ could be an option for “cabinet

based DSL” (shorter lines), and ADSL2 for “central office based DSL”; see fig. 25

relatively to the problem of crosstalk.

� Comparisons between the ADSL, ADSL2 and ADSL2+ downstream bit rates for

situations with and without crosstalk are presented in fig. 26.1 and 26.2.

Fig. 23 Frequency bands

allocation for ADSL2 and

ADSL2+

Fig. 24 Downstream bit rates

provided by ADSL2 and

ADSL2+ techniques according

to the length of the line

Fig. 25 Decrease of the crosstalk

by appropriate allocation of the

ADSL2+ downstream frequency

bands

o VDSL2

o Is the newest and most advanced DSL technology; was designed to support a wide

category of services like voice, video, data, high definition TV (HDTV), interactive

gamming.

o It is specified by the ITU-T G.993.2 standard and it is an enhancement of the G.993.1

(VDSL1) standard. It is a complex standard with numerous profiles and frequency

bands adapted to various bit rate and reach requirements.

o Allows symmetrical and asymmetrical full duplex transmissions up to bit rates of

200Mbps on twisted pairs using a bandwidth of 30MHz. The maximum bit rate of

250Mbps at source deteriorates quickly to 100Mbps at 0.5km and 50Mbps at 1km, but

Fig. 26.1 Comparison between the downstream bit rates ensured by ADSL, ADSL2 and

ADSL2+ techniques in white noise conditions for different loop lengths

Fig. 26.2 Comparison between the downstream bit rates ensured by ADSL, ADSL2 and

ADSL2+ techniques in crosstalk conditions for different loop lengths

after that the maximum bit rate degrades slowly and at a 1.6 km distance from the

source the performances are identical with those of ADSL2+.

o The VDSL2 standard includes many of the features and functionalities contained in the

ADSL2+ standard. It is about advanced diagnostic facilities, advanced management,

ability to maximize the bit rate and the efficiency of bandwidth use – VDSL2 is an

ideal technology for video services.

o VDSL2 allows an ADSL like long reach, meaning 1 – 4 Mpbs at de 4, 5 km distance,

in which case we have an ADSL2+ type functioning mode. VDSL2 type systems,

unlike VDSL1 type systems, are not limited to short loops.

o The transmission profiles defined for the VDSL2 techniques are presented in fig. 27.

These profiles characterize the allocated bandwidth, the number of tones (subcarriers),

used especially in downstream, the tone separation, the transmit power and the bit rate,

used especially in downstream.

o Allocation of the frequency bands is realized according to the plans presented in fig. 28.

Fig. 27 VDSL2 transmission profiles

Fig. 28 Frequency band allocation plans for VDSL1 and VDSL2 techniques

o VDSL2 features and performances

o The typical reach of VDSL1 systems is around 3kft (3000 feet); one of the VDSL2

systems basic characteristic is the larger reach, around 9kft – some characteristics of

ADSL systems are included in VDSL2.

o VDSL2 is uses completely different initialization procedures of those used by VDSL1

(channel measurement, and modem training); the VDSL2 system allows the use of the

US0 frequency band (used by ADSL systems) on long channels.

o The VDSL2 system provides an improved protection against impulse noise, one of the

most important distortions which affect DSL transmissions. The VDSL2 system allows

the correction of error packets generated by impulses with length between 250µs and

3,75ms.

o VDSL2 ensures support for services based on packet transmissions (for ex. Ethernet or

IP). Evolved management facilities allow the transmission on the same physical layer

of packets with different priorities and lengths. VDSL2 ensures also support for

transmission on the same physical channel of services with completely different

requirements (bit error probability, delay, protection against impulse noise).

o VDSL2 ensures evolved channel diagnostic and fault identification.

o VDSL2 ensures improved compatibility between equipments with different chip-set

and compatibility with ADSL and ADSL2 techniques.

o In fig. 29 it is presented a comparison between the performances (bit rate / reach) of

VDSL2, VDSL1, ADSL2+ techniques.

♦ The VDSL2 technique must provide full duplex connections with 100Mbps bit rate

at 350m distance and with 30Mbps at 1.2 – 1.5km distance.

Fig. 29 Comparison between performances of VDSL2, VDSL1, ADSL2+ techniques