gunawan wibisono dept electrical engineering university of indonesia

Gunawan WibisonoGunawan Wibisono

Dept electrical engineeringDept electrical engineeringuniversity of indonesiauniversity of indonesia

Agenda

Cable Modem DSL

Broadband Network

9.4

CABLE TV FOR DATA TRANSFERCABLE TV FOR DATA TRANSFER

Cable companies are now competing with telephone Cable companies are now competing with telephone companies for the residential customer who wants companies for the residential customer who wants high-speed data transfer. In this section, we briefly high-speed data transfer. In this section, we briefly discuss this technology.discuss this technology.

BandwidthSharingCM and CMTSData Transmission Schemes: DOCSIS

Topics discussed in this section:Topics discussed in this section:

9.5

Division of coaxial cable band by CATV

9.6

Downstream data are modulated using the 64-QAM modulation technique.

The theoretical downstream data rateis 30 Mbps.

Upstream data are modulated using the QPSK modulation technique.

The theoretical upstream data rate is 12 Mbps.

CEENET #8 8.2002 7

Changes in the Cable Network Changes in the Cable Network

• The cable network was designed to deliver TV signals in one direction from the Head-End to the subscribers homes

• To provide TV services Cable Operators had to recreate a portion of the over-the-air radio frequency (RF) spectrum within a sealed coaxial cable line

• Operators had to upgrade the cable network so that signals could flow in both directions

CEENET #8 8.2002 8


• Cable Operators assign a spectrum of signal frequencies to the cable network

• One spectrum is used for the signals that move from the Head-End towards the cable subscriber

• Another spectrum of signal frequencies are used for the signals that move from the cable subscriber towards the Head-End

CEENET #8 8.2002 9


• By replacing existing one way amplifiers with two way amplifiers Cable Operators are able to separate the upstream and downstream signals and amplify each direction separately in the right frequency range

CEENET #8 8.2002 10


A Traditional Cable network

CEENET #8 8.2002 11


A Modern Cable network

CEENET #8 8.2002 12

What is a Cable Modem? What is a Cable Modem?

9.13

Figure 9.17 Cable modem (CM)

9.14

Figure 9.18 Cable modem transmission system (CMTS)

CEENET #8 8.2002 15

How Fast is a Cable Modem? How Fast is a Cable Modem?

• Cable modem speeds vary widely – Depends on the cable modem system

– Cable network architecture

– Traffic load.

• In the downstream direction (from the network to the computer), network speeds can be up to 27 Mbps– BUT, this is an aggregate amount of bandwidth that is

shared by users.

CEENET #8 8.2002 16


• Few computers will be capable of connecting at such high speeds or have exclusive access to the network– A more realistic number is 1 to 3 Mbps.

• In the upstream direction (from computer to network), speeds can be up to 10 Mbps. – However, most modem producers have selected a more

optimum speed between 500 Kbps and 2.5 Mbps

– AND, many cable operators limit the upstream bandwidth to 128 or 384kbs

CEENET #8 8.2002 17


• An asymmetric cable modem scheme is most common. The downstream channel has a much higher bandwidth allocation (faster data rate) than the upstream,

• primarily because Internet applications tend to be asymmetric in nature.

• Activities such as World Wide Web (http) navigating and newsgroups reading (nntp) send much more data down to the computer than to the network.

CEENET #8 8.2002 18


• Mouse clicks (URL requests) and e-mail messages are not bandwidth intensive in the upstream direction.

• Image files and streaming media (audio and video) are very bandwidth intensive in the downstream direction.

What is a Cable Modem &What is a Cable Modem & how does it work? how does it work?

A Cable Modem is a digital modem that uses a coaxial cable connection for the data transmission.

This data connection is received by a cable modem that decodes the signal into your PC.

http://www.cable-modems.org/tutorial/01.htm


MORE INFO...

http://www.whatis.com/coaxialc.htm



How fast is a Cable Modem?How fast is a Cable Modem?

Cable modems are up to 10-20Mbps downloads. Typical downloads are over 300Kbps, or close to 600Kbps, but the speed of the cable modem depends on a few things.

First it depends on how many users are on the system since the cable technology is a "shared" bandwidth. Too many users using too much throughput can drain this “shared” technology.

The second factor to cable modem speed is a limit on the cable modem itself. Some cable providers will limit the upload or download speed on the cable modem, and this could affect your connection speed.

http://www.whatis.com/bandwidt.htm

How secure is a Cable Modem?How secure is a Cable Modem?

Cable connections are not 100% secure in any instance like many other connections on the Internet. Even though most cable providers block ports 137-139, cable modems are likely to be generated in any case where a user has file and print sharing turned on, or possibly other services like SMTP (Simple mail transfer protocol), Web Servers and Telnet services. A general rule is to keep passwords long and turn off any service that you don't absolutely need running. A firewall type application should be used to keep a network as secure as possible.

CEENET #8 8.2002 22

Real-world performanceReal-world performance

• The theoretical performance of a Cable Modem is based upon all other devices being able to work at the same speed and performance as the Cable Modem

• However, in a similar way that the actual usable bandwidth on a 10Mbps Ethernet connection reduces to a 4Mbps, so too will the performance of a Cable Modem connection be reduced

CEENET #8 8.2002 23


• The Cable network itself will suffer the same problems of Internet performance as any other Internet Service Provider (ISP)

• Although performance to services on the cable network itself can be amazingly fast, access to 'the outside world' will be slowed down by the performance of other connections on the way.

CEENET #8 8.2002 24


• As usage on your segment grows (as more customers are added) the bandwidth must be shared by more people– Adding more cable network segments is very expensive

for the cable operator

• If you connect to a remote Internet site that itself has a connection speed equivalent to a T1 connection (1.5Mbps), then that is as fast as the data can be served to you, no matter how fast your receiving equipment is

CEENET #8 8.2002 25

Who Makes Cable Modems? Who Makes Cable Modems?

• 3Com, Cisco Systems, Com21, General Instrument, Motorola, Nortel Networks, Phasecom, Samsung, Terayon, Toshiba, Zenith

• And many others

CEENET #8 8.2002 26

• It MOdulates and DEModulates signals• Much more complicated than their telephone

counterparts• Cable modems can be part modem, part tuner, part

encryption/decryption device, part bridge, part router, part network interface card, part SNMP agent, and part Ethernet hub

Cable Modem Technology Cable Modem Technology

CEENET #8 8.2002 27

• Typically, a cable modem sends and receives data in two slightly different fashions– In the downstream direction

• he digital data is modulated and then placed on a typical 6 MHz television channel, somewhere between 50 MHz and 750 MHz

• 64 QAM is the preferred downstream modulation technique, offering up to 27 Mbps per 6 MHz channel

• This signal can be placed in a 6 MHz channel adjacent to TV signals on either side without disturbing the cable television video signals.


CEENET #8 8.2002 28


– The upstream channel is more tricky• Typically, in a two-way activated cable network, the upstream (also

known as the reverse path) is transmitted between 5 and 42 MHz

• This tends to be a noisy environment, with RF interference and impulse noise. Additionally, interference is easily introduced in the home, due to loose connectors or poor cabling

• Since cable networks are tree and branch networks, all this noise gets added together as the signals travel upstream, combining and increasing

• Due to this problem, most manufacturers use QPSK or a similar modulation scheme in the upstream direction, because QPSK is more robust scheme than higher order modulation techniques in a noisy environment

• The drawback is that QPSK is "slower" than QAM.

CEENET #8 8.2002 29

Cable Modem Services Cable Modem Services

• The dominant service is high-speed Internet access– This enables the typical array of Internet services to be

delivered at speeds far faster than those offered by dial-up telephone modems

– Other services will include

– access to streaming audio and video servers, local content (community information and services)

– access to CD-ROM servers

– a wide variety of other service offerings. New service ideas are being developed daily.

CEENET #8 8.2002 30

• In North America, cable operators are packaging high-speed data services much like they do basic cable television service

• Typically charging $40 - $60 per month for an Internet service package – Includes software, unlimited Internet access,

specialized content and rental of a cable modem

Cost of Cable Modem ServiceCost of Cable Modem Service

CEENET #8 8.2002 31

Cost of Cable Modem ServiceCost of Cable Modem Service

• At the low end of this pricing scale, a very robust Internet service is available to consumers for about the cost of a dial-up account with a local Internet service provider and a second telephone line

• Even at $60 per month, cable is a far better value than ISDN.

CEENET #8 8.2002 32

"Telco-Return" Modems"Telco-Return" Modems

• Not really a cable technology• Used more often with Direct Satellite video

systems• Satellite down link is used for fast downstream

transmission• A telephone modem handles upstream

communication over the public telephone network.

CEENET #8 8.2002 33

Support for Multiple PCsSupport for Multiple PCs

• A cable modem can provide Intenet access to multiple PCs, if they are connected via a local area network (LAN)

• Cable modems typically have an Ethernet output, so they can connect to the LAN with a standard Ethernet hub or router

• Each PC must have an assigned IP address– The cable ISP usually sells at a premium of $5-$10 a month per

PC

– NAT (Network Address Translation) can allow multiple PCs to "hide" behind a single IP Address

Cable Modems vs. ADSL

There is one major advantage that ADSL has over cable modems. Cable modems use a shared networking technology where all the cable modems share a single pipe to the Internet. This pipe speed will fluctuate depending on the number of subscribers on the network.

When ADSL is used, the pipe to the Internet is solely "yours", and is not shared along the way to a central office. This allows for a more consistent speed, and this speed does not typically fluctuate like cable modem networks.

MORE INFO... http://www.whatis.com/adsl.htm

http://www.whatis.com/adsl.htm

ADSL vs. cable modem

Pro:– Secure. “Point to point

connectivity” of ADSL ensures the security of the service. Cable, by contrast, is shared media and is not secure at all.

– Bigger coverage area.– Cheap. ADSL uses

existing twisted pair, hence is cheap in installation and also cheap in monthly payment.

Cons:– Bandwidth. ADSL has

about 1.1MHz BW due to loop limitations, while cable modem has about 745MHz BW.

– Bridge taps, DLCs, load coils can lead to problems.

– Mutual noise among different DSL lines, T1 lines.

Stein Intro DSL 36

Introduction Introduction

toto

DSLDSL

Introduction Introduction

toto

DSLDSL

Yaakov J. Stein

Chief ScientistRAD Data Communications

Stein Intro DSL 37

PSTNPSTN

Stein Intro DSL 38

Original PSTNOriginal PSTN

UTP

Manual switching directly connected two local loops

Due to microphone technology, audio BW was 4 kHz

UTP

Stein Intro DSL 39

Analog switched PSTNAnalog 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 40

Data supported viaData supported viavoice-grade modemsvoice-grade modems

UTP

modemmodem

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 kbpstoday it is about 35 kbps (V.34 modem - 33.6 kbps)

Stein Intro DSL 41

Digital PSTNDigital PSTN

“last mile”CO SWITCH

“last mile”Subscriber Line

PSTN

CO SWITCH

TDM

TDM

digitalanalog

LP filter to 4 kHz at input to CO switch (before A/D)

Stein Intro DSL 42

Digital PSTNDigital PSTNSample 4 kHz audio at 8 kHz (Nyquist) Need 8 bits per sample = 64 kbpsMultiplexing 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 theorems1. Separation theorem2. Source coding theorem3. Channel capacity theorem

Stein Intro DSL 43

Voice-grade modemsVoice-grade modemsstill work over new PSTNstill work over new PSTN

UTP subscriber line

CO SWITCH

network/ISP

router

modem

PSTN

modem

CO SWITCH

But data rates do not increase !

Simulate analog channel so can achieve Shannon rate < native 64 kbps rate

Internet

Stein Intro DSL 44

Where is the limitationWhere 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 45

UTPUTP

Stein Intro DSL 46

What is UTP?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 parallelso the attenuation increases with frequency

Various other problems exist (splices, loading coils, etc.)

Stein Intro DSL 47

UTP characteristicsUTP characteristics Resistance per unit distance

Capacitance per unit distance

Inductance per unit distance

Cross-admittance (assume pure reactive) per unit distance

R L

X

G C

Stein Intro DSL 48

UTP resistanceUTP resistance

Influenced by gauge, copper purity, temperature

Resistance is per unit distance

24 gauge 0.15 kft 26 gauge 0.195 kft

Skin effect: Resistance increases with frequency

Theoretical result R ~ f 1/2

In practice this is a good approximation

Stein Intro DSL 49

UTP capacitanceUTP 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 50

UTP inductanceUTP 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 51

UTP admittanceUTP 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 52

Propagation lossPropagation loss

Voltage decreases as travel along cable

Each new section of cable reduces voltage by a factor

So the decrease is exponential

Va / Vb = e - x = H(f,x)

where x is distance between points a and b

We can calculate and hence loss, directly from RCLG model

1v 1/2 v 1/4 v

Stein Intro DSL 53

Attenuation vs. frequencyAttenuation vs. frequency

0 2 4 6 8 10-90

-80

-70

-60

-50

-40

-30

-20

-10

024 and 26 AWG Cables

Freq [MHz]

Atte

nua

tion

[dB

/Km

]

24 AWG

26 AWG

Stein Intro DSL 54

Why twisted?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

V = (a+n) - (b+n)

n

a

b

Stein Intro DSL 55

Why twisted? - continuedWhy 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)

Cross-talk due to capacitive and/or inductive mismatch

|I2| = Q f V1 where Q ~ (Cbc-Cbd) or Q~(Lbc-Lad)

a

d

c

b

C bc C bd

L bc L ad

Stein Intro DSL 56

Loading coilsLoading coilsLong loops have loading coils to prevent voice distortionWhat does a loading coil do?

Flattens response in voice bandAttenuates strongly above voice frequencies

loops longer than 18 kft need loading coils88 mH every 6kft starting 3kft

Stein Intro DSL 57

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!

Bridge tapsBridge taps

Stein Intro DSL 58

Splices

Subscriber lines are seldom single runs of cableIn 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 changesUS 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

Other problemsOther problems

Stein Intro DSL 59

Binder groupsBinder 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 othera phenomenon called cross-talk (XTALK)

Stein Intro DSL 60

CSA guidelinesCSA 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 61

Present US PSTNPresent 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 62

Present US PSTN - continuedPresent 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 - ADSL

CSA reach - HDSL

DA (distribution area) 3-5 kft - VDSL

Higher rate - lower reach

(because of attenuation and noise!)

ADSL — What is it?

ADSL — Asymmetric Digital Subscriber Line – High speed communications over twisted pair.– Concurrent with POTS (plain old telephone

service).– Secure way of Internet access.– Originally standardized in ANSI (American

National Standards Institute) T1.231-1993.– Currently standardized in ANSI T1.413-1998.– Growing really fast.

9.64

ADSL is an asymmetric communication technology designed for residential

users; it is not suitable for businesses.

The existing local loops can handle bandwidths up to 1.1 MHz.

ADSL is an adaptive technology. The system uses a data ratebased on the condition of

the local loop line.

Stein Intro DSL 65

Why asymmetry?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

Why ADSL?

Over the past 15 years, a thousand-fold transmission rate is realized. But it still does not meet today’s need.– Viewing a full-motion movie requires about 5Mbps.– Downloading Netscape requires 10 minutes.

ADSL:– 20 fold faster

9.67

Figure 9.10 Discrete multitone technique

9.68

Figure 9.11 Bandwidth division in ADSL

9.69

Figure 9.12 ADSL modem

9.70

Figure 9.13 DSLAM

9.71

Table 9.2 Summary of DSL technologies

Stein Intro DSL 72

ADSL DuplexingADSL 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

POTS

US DS

8 32 256

G.992.1 FDD mode

* 4.3125 kHz

Stein Intro DSL 73

Why asymmetry? - continuedWhy asymmetry? - continued

0 0.5 1 1.5 2 2.5 3 3.5-90

-80

-70

-60

-50

-40

-30US

DS

PSD (dBm/Hz)

F(MHz)

Stein Intro DSL 74

Echo cancelled ADSLEcho 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

0 0.5 1 1.5 2 2.5 3 3.5-90

-80

-70

-60

-50

-40

-30

Stein Intro DSL 75

ADSL - continuedADSL - 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 sec) 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 76

ADSL standardADSL standard

ITU (G.dmt) G.992.1, ANSI T1.413i2 standardDS - 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 77

... once more...

Broadband Network (Internet)

Wiring Distribution Frame) Customer

Premises Wiring

Stein Intro DSL 78

= Assymmetric Digital Subscriber Line

- inmodulation band (not baseband)

- ANSI standards (T1.413 of T1E1.4 group), ETSI (european requirement added to T1.413), ITU (groups of standards ITU-T G.991, 992, 995 etc. – they are downloadable from : ITU - publications – ITU-T)

ADSL

Specifications:• high bit rate transmission + telephone (and also analog) connection, or ISDN • max. downstream from 1,5 to 8 Mbps / max. upstream from 16 to 832 kbps (basic ADSL system) – various data speeds in dependence from user distance• freq.band up to 1,1 MHz, DMT modulation scheme (Discrete Multitone Transmission), max. 256 DMT channels, each is 4 kHz wide• for analog teleph.- lower 4 kHz, for ISDN up to 80 kHz (if there is ISDN transmission, the band for digital data is reduced)• reach - 5,5 km• frame transmission by means Cu- lines• Full / Lite versions

Stein Intro DSL 79

Analog teleph.signal

Frequency

Fig. 1 ADSL spectrum with various variants [2]

ADSL variant

number of subchannels

from to speed number of subchannels

speedfrom to

only data

Tab.1 Comparison of ADSL variants

ISDN-BRA

Stein Intro DSL 80

POTS Upstream Downstream

Fig. ADSL spectrum in frequency multiplex

Frequency

POTS Upstream Downstream

Frequency

Fig. ADSL spectrum with echo compensation

Stein Intro DSL 81

ADSL and ISDN

Upstream Downstream

Frequency

Frequency

Frequency

Basic Access (4B3T link code)

Basic Access (2B1Q link code)

Stein Intro DSL 82

ADSL

Stein Intro DSL 83

btw., relation between bandwidth and data speed:

Shannon-Hartley theorema for information capacity of channel with both digital signal with mean power S and additive Gauss noise with mean power N. Bandwidth of channel is B [Hz].

N

SBC 1log2 [bps] ... channel information capacity

B ... bandwidth [Hz]

S ...power of signal in the given band B [V2 or W]

N...power of noise in the given band B [V2 or W]

S/N . .. signal–to-noise ratio [-]

(we know already SNR[dB] =10 log (S/N) )

Stein Intro DSL 84

symmetrical pair

LF

HF

Fig.2 Typical termination of ADSL line on the user side

Fig.3 ADSL line configuration with splitters

user

Provider

Data network

user line

ATU-C = ADSL transceiver unit at the central office, ATU-R .....at the Remote home or business

Stein Intro DSL 85

Splitter

Stein Intro DSL 86

xDSLxDSL

Stein Intro DSL 87

Alternatives for data servicesAlternatives 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 88

xDSLxDSL

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 89

xDSLxDSL System Reference Model System Reference Model

POTSSPLITTER

UTP

CO SWITCH

DSLAM

xTU-C

network/ISP

router xTU-R

POTSSPLITTER

PSTN

PDN

POTS-RPOTS-C

WAN

x = H, A, V, ...

Analog modem

frequencyDC 4 kHz

POTS xDSL

Stein Intro DSL 90

SplitterSplitter

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 modemNo 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 91

Why is DSL better Why is DSL better than a voice-grade modem?than a voice-grade modem?

Analog telephony modems are limited to 4 KHz bandwidth

Shannon’s channel capacity theorem gives the maximum transfer rate

C = BW log2 ( SNR + 1 )

So by using more BW we can get higher transfer rates!

But what is the BW of UTP?

S

N

for SNR >> 1

C(bits/Hz) SNR(dB) / 3

Stein Intro DSL 92

Maximum reachMaximum reachTo 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 93

xDSL - Maximum ReachxDSL - Maximum Reach

0 10 20 30 40 50 600

1

2

3

4

5

6DSL MAXIMUM REACH

Rate[Mbps]

Re

ach

[Km

]

Stein Intro DSL 94

Other sources of noiseOther 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 95

Sources of InterferenceSources of Interference

XMTR RCVR

RCVR XMTR FEXT

NEXT

RCVR XMTR

XMTR RCVR

RF INGRESS

THERMAL NOISE

Stein Intro DSL 96

Unger’s discoveryUnger’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

Channel Modeling(characteristic impedance, propagation constant, channel attenuation)

)()(

)()()(

fsCfG

fsLfRsZ

)(686.8)(10ln

20),(log20),( 10 fdfdfdHfdLdB

)()()(),( fdjfdsd eeesdH

Noise

There are three main types of noise that affect

DSL system performance:

NEXT (Near End Crosstalk) FEXT (Far End Crosstalk) Impulse Noise

NEXT

When a transceiver sends a signal and a nearby transceiver at the same end “hears” the signal, it’s NEXT.

A simplified NEXT model for N disturbers:

2

3

13

6.0

10134.1

1)

49( fN

NEXTN

Stein Intro DSL 100

NEXTNEXTOnly 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)

FEXT

When a transceiver sends a signal and a transceiver at the far end “hears” the signal, FEXT occurs.

A simplified FEXT model for N disturbers:

226.0 )()49

( fHdfkN

FEXTN

Stein Intro DSL 102

FEXTFEXT

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)

Impulse Noise

Impulse noises are large surges of noise with short duration. The sources of impulse noises are not well understood yet. It is a very devastating noise if not handled well.

A concatenated code, using a 2-dimensional 8-state trellis code and a 4-error-correcting Reed-Solomon code with an interleaving depth of 18 symbols, was found to be suitable for eliminating impulse noise.

FDM (Frequency Division Multiplexing) ECH (Echo Canceller with Hybrid)

Multiple Access

Two main contenders: DMT — Discrete MultiTone

– A multi-carrier system using Discrete Fourier Transforms to create and demodulate individual carriers.

CAP — Carrierless Amplitude and Phase– A version of suppressed carrier QAM.

Line Code

DMT

Existing ANSI and ETSI standards Consists of up to 256 sub-channels, (also

called tones or bins), of 4.3125KHz– upstream use 25-163KHz (bins 6 to 38)– downstream use 142KHz-1.1MHz (bins 33 to 255)– bins 16 (69KHz) and 64 (276KHz) are pilot tones.

Outperforms CAP in field trials More expensive and complex

DMT Line Code

Observations

Three Channels: POTS channel

– POTS channel is split off from the digital modem by filters, thus guaranteeing uninterrupted POTS.

High speed downstream channel– Its data rate depends on length of the copper line,

its wire gauge, presence of bridged taps, cross talk, etc.

Medium speed upstream channel

DMT Features

Discretely divides the available frequencies into 256 sub-channels or tones.

Incoming data is broken down into a variety of bits and distributed to a specific combination of sub-channels.

To rise above noise, more data resides in the lower frequencies and less in the upper frequencies.

DMT Transmission Parameters

Downstream– symbol rate: 4KHz– FFT size: 512– Cyclic prefix: 32– Sampling rate:

2.208MHz– Transmit power:

20dBm– Highpass filter:

62.5kHz

Upstream– Symbol rate: 4kHz– FFT size: 64– Cyclic prefix: 4– Sampling rate:

276kHz– Transmit

Power:7dBm– Lowpass filter:

43.875kHz

DMT Block Diagram

PSD of DMTPSD is useful for finding received signal power, thus useful for analyzing NEXT and FEXT noises.Upstream and downstream PSD models are:

838

8

8

6

2

, )1020()10104.1

(1

1)sin(2

f

fff

fT

ZT

VPSD DSADSL

2

22

, )()sin(2

fHf

fT

ZT

VPSD USUSADSL

Stein Intro xDSL 3.113

DMTDMT

Discrete Multitone is a form of FDM (Frequency Domain

Multiplexing)

Discrete Multitone is a form of MCM (MultiCarrier Modulation)

It uses many different carriers, each modulated QAM

Each tone is narrow low baud rate (long frame) channel characteristics are constant over tone

Number of bits per tone chosen according to water pouring

Put more bits where SNR is good


DMT - continuedDMT - continued

DMT is OFDM (Orthogonalized FDM) Carrier spacing is precisely baud rate Center of tone is precisely the zero of all other sincs ICI minimized

ISI minimized by having a long interframe guard time

DMT modem can be efficiently implemented using FFT DFT is mathematically equivalent to a bank of filters

Filtering is equivalent to cyclic convolution

So use cyclic prefix rather than guard time


DMT - continuedDMT - continued

time

frequency


ADSL DMTADSL DMT

Baud rate (and channel spacing) is 4.3125 KHz

US uses tones 8 - 32 (below 30 KHz reserved)

DS uses 256 tones (FDM from tone 33, EC from tone 8)

POTS

US DS

8 32 256


DMT misc.DMT misc.

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 118

Splitterless ADSLSplitterless 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)

Frame Structure

Frame Structure (cont.)

A super frame is defined for every 68 IFFT/FFT

operations.The super frame has a time duration

of 68/4k=17ms for baud rate of 4kHz.

CAP

Initial ADSL implementations were done using CAP

1996 - 90% of world-wide ADSL implementation based on CAP

Variant of QAM - widely understood Not yet incorporated in ANSI standards

T1.413 or ETSI Supported by GlobeSpan Technologies

CAP Transmission Parameters

Downstream– Constellation size: 64– Baud rate: 266.67KHz– Throughput: 1.6 Mbps– Sampling

rate:1.0667MHz– Transmit power: 12dBm– Signal spectrum:

170 ~ 410KHz

Upstream– Constellation size: 16– Baud rate: 6KHz– Throughput: 24Kbps– Transmit power:-

4.8dBm– Signal spectrum:

96 ~ 102KHz

Stein Intro DSL 123

Example - Interference spectrumExample - Interference spectrum

0 0.5 1 1.5 2-200

-180

-160

-140

-120

-100

-80

-60

-40

-20

0ISDN NEXT, AM INGRESS, SELF FEXT

Freq [MHz]

Inte

rfe

renc

e [d

Bm

/Hz]

Stein Intro DSL 124

Examples of Realistic ReachExamples 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 (CSA 50% US loops)

10 Mbps 7 Kft 2 km

13 Mbps 4.5 Kft 1.4 km

26 Mbps 3 Kft 900 m

52 Mbps 1 Kft 300 m (SONET STS-1 = 1/3 STM-1)

Stein Intro DSL 125

ISDN

ADSL

FTTx, VDSL2,ADSL2plus

EnhancedCopper

Hybrid Fibre/Copper

Pure Fibre

ADSL Speed ComparisonADSL Speed Comparison

Voice bandModem

FTTH

Stein Intro DSL 126

ADSL RangeADSL RangeoIn general, the maximum range for DSL without a repeater

is 5.5 km oAs distance decreases toward the telephone company

office, the data rate increases

For larger distances, you may be able to have DSL if your phone company has extended the local loop with optical fiber cable

Data Rate Wire gauge Wire size Distance

1.5 or 2 Mbps 24 AWG 0.5 mm 5.5 km

1.5 or 2 Mbps 26 AWG 0.4 mm 4.6 km

6.1 Mbps 24 AWG 0.5 mm 3.7 km

1.5 or 2 Mbps 26 AWG 0.4 mm 2.7

Stein Intro DSL 127

ADSL Speed FactorsADSL Speed Factors

The distance from the local exchange

The type and thickness of wires used

The number and type of joins in the wire

The proximity of the wire to other wires carrying ADSL, ISDN and other non-voice signals

The proximity of the wires to radio transmitters.

Stein Intro DSL 128

ADSL network componentsADSL network components

The ADSL modem at the customer premises(ATU-R)

The modem of the central office (ATU-C)

DSL access multiplexer (DSLAM)

Broadband Access Server (BAS)

Splitter - an electronic low pass filter that separates the analogue voice or ISDN signal from ADSL data frequencies DSLAM .

Stein Intro DSL 129

Bonding (inverse mux)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 130

DuplexingDuplexingUp to now we assumed that only one side transmits

Bidirectional (full duplex) transmission requires some form of duplexing

For asymmetric applications we usually speak ofDS 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

frequencyDC 4 kHz

POTS US DS

Stein Intro DSL 131

Muxing, inverse muxing, duplexingMuxing, inverse muxing, duplexing

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

data streams physical line

multiplexing

data stream physical lines

inverse multiplexing

duplexing

Stein Intro DSL 132

(Adaptive) echo cancellation(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

demodulator

4W to 2W

HYBRID

Stein Intro DSL 133

xDSL types xDSL types and and

historyhistory

Stein Intro DSL 134

DSL FlavorsDSL Flavors

DSL is often called xDSLsince there are many varieties (different x)

e.g. ADSL, HDSL, SHDSL, VDSL, IDSL, etc.

There were once many unconnected typesbut 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 135

Some xDSL PSDsSome xDSL PSDs

0 0.5 1 1.5 2 2.5 3 3.5-90

-80

-70

-60

-50

-40

-30

F(MHz)

PSD(dBm/Hz)

IDSLT1

HDSL HDSL2

ADSL

Stein Intro DSL 136

ITU G.99x standardsITU 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.992.3 ADSL2 G.992.4 splitterless ADSL2 G.992.5 ADSL2+) G.993 VDSL (G.993.1 VDSL G.993.2 VDSL2)

G.994 HANDSHAKE

G.995 GENERAL (INFO)

G.996 TEST

G.997 PLOAM G.998 bonding (G.998.1 ATM G.998.2 Ethernet G.998.3 TDIM)

Stein Intro DSL 137

ITU xDSL layer modelITU 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 138

More xDSL flavorsMore xDSL flavors

modem speed reach main applications

IDSL 160 (144) Kbps 5.5 km POTS replacement, videoconferencing, Internet access

HDSL 2 Mbps (4-6W) 3.6-4.5 km T1/E1 replacement PBX interconnect, FR

HDSL2 2 Mbps (2W) 3 km same as HDSL

SHDSL 2.3 Mbps 3 km same as HDSL

SHDSLbis 4.6 Mbps 3 km same as HDSL

Stein Intro DSL 139

More xDSL flavors (cont.)More xDSL flavors (cont.)

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 LAN interconnect, HDTV, combined services

VDSL2 200 Mbps (aggregate)

up to 1.8 km “

cable modem 10-30Mbps DS shared

50 km residential Internet

HPNA 1, 10 Mbps home wiring residential networking

Not DSL

Stein Intro DSL 140

HDSL2HDSL2

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 141

SHDSLSHDSL

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 142

ADSLADSL

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 143

Fig.6 ADSL 2+ system

2,2 MHz

1,1 MHz

Up to 18,000 feet (5.5 km)

Up to 25 Mbps down

Up to 1 Mbps Upstream

ADSL2+

Stein Intro DSL 144

ADSL2

- ITU-T G. 992.3, .4

- 2nd generation of ADSL standard

- downstream - up to 12 Mbps

- DMT modulation

- bandwidth - up to 2,2 MHz

- but: shorter reach (only from 1,5 to 2 km) !

- CVoDSL

ADSL2 + (fig.6)

- ITU-T G. 992.5

- downstream - up 24 Mbps

- bandwidth - up to 2,2 MHz (512 subchannels DMT, each 4kHz wide, up to 2,2 MHz)

- full data speed only in reach of max. 1,5 km from DSLAM (!)

Stein Intro DSL 145

• RE-ADSL = Reach Extended ADSL

-ITU-T G.992.3 – Annex L (it is annex to ADSL2 standard)

- optimalized DMT channels with the goal of larger length (manipulation with PSD of some channels their higher throughput

- dedicated to long lines (not for short ) – up to 5,5 km with the same date speed as in ADSL2

• RADSL = Rate Adaptive DSL- it is in development- both symetrical and assym.transmissiontransmission speed is adaptive (it depends on transmission conditions and distance)down 1-12 Mbps / up 128kbps-1Mbps DMT or CAP (and QAM) are supposed- for applications without synchronization requirements (IP services, ATM, Frame Relay)

• Bonded ADSL

- combines (bonds) 2 or more (up to 32) Cu-pairs for higher or extreme data speeds (for big and reach companies)

Stein Intro DSL 146

ADSL2ADSL2

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 147

More ADSL2 featuresMore 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 148

VDSLVDSL

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 149

VDSL - rate goalsVDSL - 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 6 Kft (1.8 Km)

1.6/13 4.5 Kft (1.4Km)

3.2/26 3 Kft (900 m)

6.4/52 1 Kft (300 m)

Stein Intro DSL 150

VDSL - Power issuesVDSL - 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


VDSL - duplexingVDSL - duplexing

In Japan and campus applications can operate TDD (ping pong)

SDMT Synchronous DMT (2 KHz frame can be heard in adjacent pairs or hearing aids)

Rest of world PSTN only FDD is allowed

Can divide US and DS into 2 areas (e.g. ADSL) or more

Need guard frequencies because of clock master/slave problems

Zipper - large number of interleaved frequency regions

(even on a bin by bin basis)


VDSL line code warsVDSL line code wars

VDSL Alliance VDSL Coalition

DMT QAM

MORE LESS

robust to noise power

capacity complex

spectral compatibility expensive

IPR A/D bits

With no complexity constraints probably equivalent

Stein Intro DSL 153

VDSL2VDSL2

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 154

VDSL2 band plansVDSL2 band plansNorth 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

gunawan wibisono dept electrical engineering university of indonesia

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