Lecture14 DSL Technology Digital Subscriber Line Ref: Vaidy-paper.pdf.

Lecture14

DSL Technology

Digital Subscriber LineRef: Vaidy-paper.pdf

Data Communication, lecture14 2

Different ways to connect

• Modem (POTS - Plain Old Telephone System)

• ISDN

• Wireless• Cable Modem (North America)

• xDSL


POTS Modem

V.34+

• Maximum Speed: 33.6 Kbps

• Expected Speed: 28.8 – 33.6 K

• Availability: Everywhere

• Symmetric


POTS Modem

V.90 (x2; K56Flex; V.92)

• Maximum Speed: 56 Kbps

• Expected Speed: 40 - 50 Kbps

• Availability: Almost everywhere

• Asymmetric

• Relative to distance


ISDN

BRI (Basic Rate Interface)

• 2B + C

• Two 64 Kbps “B” Channels

• One 16 Kbps “C” Channel

• Typically 128 - 144 Kbps

• Availability: Limited, Outdated

• Symmetric


ISDN

• North America: 23B + D (1536 Kbps)

• Europe: 30B + D (1984 Kbps)

• 64 Kbps “B” Channels

• One 64 Kbps “D” Channel

• Availability: Not for the general public

• Symmetric


Wireless

• Different wireless schemes proposed, planned and implemented throughout the world.

• Via satellite or ground antennas• Bandwidth: A few Kbps to many Mbps• Symmetrical or Asymmetrical• Deployment issues: Spectrum licensing,

interference, line of sight requirements, noise problems, bandwidth limitations…


Cable Modem

• Competes with DSL

• Cable TV is widespread in North America

• Satellite TV the norm for the rest of the world

• Based on NA’s Cable TV infrastructure (Coaxial)

• Bandwidth is shared among all users (like Ethernet)

• Maximum Speed: 30 - 50 Mbps

• Expected Speed: depends on number of users

• Asymmetric, uplink limited to 128Kbps by modem

• Everybody's download speed is greatly impacted if upload link is saturated


xDSL

Advantages• Speed (several Mbps)• Always connected• Uses and can even share POTS wiring

Disadvantages• Speed dependant on distance to center• Availability


xDSL

Varieties

• ADSL: Asymmetric DSL

• SDSL: Symmetric DSL

• VDSL: Very high speed DSL


ADSL

• Maximum speed depends on distance and contract– 8 Mbps for 2.7 Km– 6 Mbps for 3.7 Km– 2 Mbps for 4.9 Km– 1.5 Mbps for 5.7 Km

• Such is the case for the uplink (up to 800 Kbps)• Can share the regular phone line• May or may not require a splitter• Different Standards


SDSL

• Maximum speed– 2 Mbps for Europe– 1.5 Mbps for NA

• Up to 6.7 Km Range

• Can not share the regular phone line

• Different Standards


VDSL

• Only for short distances– 55 Mbps for 300 m– 27 Mbps for 500 m– 13 Mbps for 1500 m

• Such is the case for the uplink (up to 19 Mbps)• Can share the regular phone line• Different Standards• Fiber To The Neighborhood (FTTN) / Fiber To The

Curb (FTTC)


How it works

It all comes down to bandwidth!• For POTS it is 4 KHz• The installed wires can handle a lot more depending on

length and wire gauge• DSL isn’t the first to utilize the extra bandwidth, ISDN used

some of it too (generally < 0.1 MHz)• ADSL can use up to 1.5 MHz• Normal POTS voice uses the bottom 4 KHz, so a single

line can be shared


How it works

Carrierless Amplitude / Phase (CAP)

• 0 – 4 Khz: POTS• 25 – 160 Khz: Upstream• 0.24 – up to 1.5 Ghz: Downstream d d

d d d d


How it works

Discrete MultiTone (DMT)

• Divides the 1.5 Mhz available bandwidth to 250 x 6 KHz regions

• The lower 3 regions are reserved for POTS• Each channel is 4 KHz wide (2 KHz space between

them)• One way to think about it is that each channel is

assigned a virtual modem• Some of the lower channels are bidirectional


How it works

Discreet MultiTone (DMT)

• The official ANSI standard• More complex to implement• More flexibility on lines of differing quality


How it works

Line Sharing

• May or may mot require a splitter / low pass filter

• One of the key benefits of DSL• Only on some types of DSL


How it works

VDSL• Different standards (again!)• VDSL Alliance (Alcatel, Texas Instruments)

supports Discrete MultiTone (DMT)• VDSL Coalition (Lucent, Broadcom) supports

Carrierless Amplitude Phase (CAP) • Normal POTS voice uses the bottom 4 KHz, so

a single line can be shared


DSL Equipment

DSL Transceiver (Modem)

• At the customer side• Generaly have USB or

Ethernet connections• Some combine routers,

switches, etc.


The Future…

• In NA the competition is between Cable and DSL• Higher upstream speeds / symmetric configurations for

VDSL• VDSL Already requires at least “Fiber To The

Neighborhood” (FTTN) because of it’s bandwidth requirements

• Many phone companies are planning “Fiber To The Curb” (FTTC)

• The next leap could be “Fiber To The Home” (FTTH)• Not in the near future though!


telephone lines (twisted-pair channels) which were originally intended to carry speech signals (about 4 kHz bandwidth) are today used to carry several megabits of data per second.

This has been possible because of efficient use of high frequency regions which suffer from a great deal of line attenuation and noise.


Communication channels can be wireless or wire-line channels, or a combination of both. In any case they introduce linear and nonlinear distortions, random noise, and deterministic interference.

The transmission of information with high rate and reliability under such unfavorableconditions has been possible becauseof fundamental contributions from many disciplines such as information theory, signal processing, linear system theory, and mathematics.


The Noisy Channel


The transmitted signal power P is proportional to the mean square value of x(n).

Assume that x(n) is a wide sense stationary random process.

Then the power is the integral of the power spectrum.

Assumptions


That is: P =

We can decrease the error probability by transmitting more power. For fixed power,

the error probability increases with bit rate.

Note that the power spectrum of x(n) tells us how its power is distributed in frequency.


we can increase the achievable rate (for fixed error probability and transmitted power).

The idea is to “pour” more power in the regions where the channel gain is large and noise spectrum is small.


Ideal equalizer (zero-forcing equalizer).


The effective power spectrum of noise at receiver is equal to:


If C(z) has zeros close to the unit circle, then 1/C(z) has poles near the unit circle and the noise gain can be large.

In frequency regions where Sqq(j ω) is small, we should allocate more power.


Power Allocation and Water-Filling Strategy


• If we imagine a bowl with its bottom shaped like Sqq(j ω), then Sxx(j ω) is the height of water filling the bowl, with λ denoting the uniform water level everywhere.

• The choice of λ depends on the total available power P and Pe.


How do we shape the power spectrum Sxx(j ω) to satisfy the water-filling type of power allocation?

This is tricky because we do not have a great deal of freedom to shape things, especially that x(n) is user generated data!

• the different subband channels carry different parts of a single input stream (DMT).


Expander


Decimator


Interleaving


Example:M=3


It is clear that we can regard v(n) as a time-domain multiplexed or TDM version of the individual signals vk(n).

The components vk(n) are also called the polyphase components of v(n) with respect to M.


Deinterleaving


The Digital Transmultiplexer(a Filter bank)


It was originally intended to convert data between time division multiplexed (TDM) format and frequency division multiplexed (FDM) format.

• Fk(z) are called transmitting filters or interpolation filters.


The kth transmitting filter has output:


Discrete Multi-Tone Modulation (DMT)Parsing Stage


Parsing Stage

Here s(n) represents binary data to be transmitted over a channel.

This data is divided into nonoverlapping

b-bit blocks.

The b bits in each block are partitioned into M groups


• The collection of symbols:

{x0(n), x1(n), … , xM–1(n)}

referred to as the DMT symbol.

• The sample xk(n) is typically a PAM or a QAM symbol

• Notice that for a given constellation, the power can be increased or decreased by scaling the distance between the code-words.


• We therefore have the freedom to allocate different powers for different sub-band channels.

• In this way the classical water-filling rule can be approximated.


• The transmitting filters fk(n) create the M-fold higher rate signals uk(n) as before, which are then added to produce the composite signal x(n).

• In principle, the DMT idea is similar to sub-band coding, where a signal x(n) to be quantized is first decomposed into sub-ands.


A simple fact:


• Thus the transfer function Dkm(z) from

xm(.) to yk(.) is the decimated version of the product-filter Hk(z)C(z)Fm(z).

• If m ≠ k then the symbol yk(n) is affected

by xm(i) resulting in interband interference.

• Similarly if Dkk(z) is not a constant then yk(n) is affected by xk(i), i ≠ n. (Intraband interference).


Perfect Recovery (PR) DMT Systems

• If interband and intraband interferences

are eliminated, the DMT system is said to

be free from intersymbol interference (ISI).

• In this case, we have the perfect symbol recovery or PR property.


Biorthogonal DMT

• we have perfect symbol recovery if and only if the transmitting and receiving filters satisfy the biorthogonality property defined as:



• This means that the impulse response gkm(n) of the product filter Gkm(z)= Hk(z)Fm(z) has the Nyquist(M) or zero-crossing property:

gkm(Mn) = 0 for k ≠ m

and

gkk(Mn) = δ(n)


Example: M=3


Channel Noise

• The only remaining distortion is due to the

channel noise. The received symbol can be written as:

yk(n) = xk(n) + qk(n)

• where qk(n) is the channel noise filtered through Hk(z)/C(z) and decimated.



Optimization of DMT

Note that:

• The variance of the symbol xk(n) represents its average power Pk.

• xk(n) comes from a bk-bit constellation with equal probability for all codewords.

• We assume that the noise qk(n) is Gaussian with variance square-root of σqk.


• The most important point to note is that the power P can be minimized by carefully controlling the variances of the noise components qk(n) at the detector inputs.

• The only freedom we have in order to control the power of noise is the choice of the filters Hk(z).

• But we have to control these filters under the constraint that {Hk, Fm} is biorthogonal.


• Since the scaled system { αkHk, Fm/αm}

is also biorthogonal it appears that the noise variances can be made arbitrarily small by making αk small.

• The problem is that the transmitting filters Fm(z)/αm will have correspondingly larger energy which means an increase in the power actually fed into the channel.


Orthonormal DMT systems

• We can regard the subchannel signal uk(n) as belonging to a subspace spanned by the basis functions:

• Note that:


• The composite signal x(n) which enters the channel is therefore a linear combination of the basis functions from all the channels.

• We say that a set of M filters {Fk(z)} is orthonormal if these basis functions are orthogonal to each other, and each of them is normalized to have unit energy.


• For perfect symbol recovery the transmiting and receiving filters in any orthonormal filter bank are related by:

which is called time reversed-conjugation.


Example:


Here f0(n) is chosen as a rectangular pulse of length M and:

• This is called the DFT filter bank because it can be implemented with a DFT matrix and an inverse DFT (IDFT) matrix


Optimal Orthonormal DMT Systems


ADSL services• Is used for data transmission on twisted pair

channels ( Telephone Lines)




• Nominally 1.5 Mbps (range is 500 kbps to 12 Mbps) downstream.

• Roughly about 1/3 to 1/10 of this rate as upstream

• ADSL is world-wide standardized in ITU Standard G.992.1 and uses DMT.


• The symbol rate is Ts = 250µs. • The downstream modulation uses 256 subchannels.• Each subchannel is 4.3125 kHz wide.• In each subchannel the sampling rate is 1/T =2.208

MHz.• The cyclic prefix is 40 samples. So, each time-domain

symbol contains then 512+40 or 552 samples. • Hermetian symmetry is used to create a real signal for

transmission over the band from 0 to 1.104 MHz.• Typically, the first 2-3 tones near DC and DC are not

used to prevent interference into voiceband telephony (POTS= plain old telephone service),


• Upstream transmission uses 32 tones to frequency 138 kHz.

• Tone 256 is also not used. Tone• 64 (276 kHz) is reserved for pilot signal (known point

in 4 QAM sent continuously) that is used to recover the symbol and sampling rates.

• The sampling rate upstream is 276 kHz and the cyclic prefix is 5 samples for a block size of 69 samples.

• Upstream is then exactly 1/8 downstream.• Downstream tones may or may not share the

upstream band.




• A maximum power of 20.5 dB is permitted in downstream, and 14.5 dB upstream.

• The maximum number of bits permitted to be loaded on to any single tone is bn=15.

Date post:	28-Mar-2015
Category:	Documents
Upload:	stephanie-meech
View:	222 times
Download:	1 times

Lecture14 DSL Technology Digital Subscriber Line Ref: Vaidy-paper.pdf.

Documents