dsl technologies - massey university · • symmetric and asymmetric? – dsl technology supports...

DSL Technologies

Professor Richard Harris

Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 2

Objectives

• To be able to explain the different variants of DSL• To be able to identify the main features of DSL

technologies and describe their advantages and disadvantages

• To explain the marketplace for DSL• To describe the next generation of ADSL and its new

features


References

• “xDSL” – http://www.techguide.com• Networks 2000 Conference proceedings – Toronto,

Canada• ADSL & DSL Technologies, Walter Goralski, 2nd

edition (McGraw Hill)• DSL Forum Website


Presentation Outline

• Introduction to xDSL technology• Scenarios for use of xDSL in the marketplace• Explanation of the various types of xDSL


Introduction - 1

• Carriers such as Telecom New Zealand have extensive copper networks to support their telephony infrastructure.

• Although this infrastructure was designed to carry analogue voice in a narrow band of low frequencies up to 4 kHz, it is nowbeing successfully used to carry digital services occupying wider bands of frequencies.

• However, at higher frequencies the transmission degradation becomes progressively more difficult to deal with.


Introduction – 2

• A new technology has been developed that extends the life of copper based technology.

• The new technology is xDSL.• This technology encompasses several different

technologies and permits customers to access the network at high speed without the need for massive infrastructure replacement.

• Costs of xDSL equipment are quite reasonable.


Introduction – 3

• The market for xDSL equipment is expanding at a significant rate.

• Several areas are driving this development, viz:– Internet access for consumers and business people– Growing opportunities for telecommuting– Remote LAN access


Internet Access

• Demands for better and quicker service to the home is being driven by the need for Web access, computer gaming, video and music.


Telecommuting

• Working at home is becoming a reality for certain business sectors. High speed links are essential for such groups as they have the need for significant bandwidth.


Remote LAN Access

• Core competencies are grouped into more efficient and self directing units.

• Remote work locations consist of LANs connected over telecommunication services to corporate offices.


Network Scenario – 1


Network Scenario - 2


What is xDSL? - 1

• A set of similar technologies that provide high bandwidth over copper twisted pair local loop cable.

• It is used between a customer and the first node in the network.

• It is applicable to non-loaded local copper loops.– The majority of installed plant around the globe.


What is xDSL? - 2

• Modem-like technology– So-called because it has devices at either end of the subscriber’s line (like a

modem!)– Accepts a data stream (usually in digital format) and overlays it onto a high

speed analogue signal.– Three modulating techniques divide the signal frequency range into three

basic elements to carry POTS traffic as well as the upstream anddownstream high bandwidth signals.

– The 3 modulating schemes are:• 2B1Q• Carrier-less amplitude phase modulation (CAP)• Discrete multi-tone modulation (DMT)

POTSDownstream

Upstream


What is xDSL? - 3

• Symmetric and Asymmetric?– DSL technology supports both asymmetric and symmetric

bandwidth configurations.– Asymmetric configurations have higher speed in one

direction only. Eg. Web browsing– There are many different types of DSL.


The DSL Technology – 1

• To create multiple channels, ADSL modems divide the available bandwidth of a telephone line in one of two ways– Frequency Division Multiplexing (FDM) or – Echo Cancellation

• FDM assigns one band for upstream data and another band for downstream data. – The downstream path is then divided by time division

multiplexing into one or more high speed channels and one or more low speed channels.

– The upstream path is also multiplexed into corresponding low speed channels.



• Echo Cancellation assigns the upstream band to over-lap the downstream, and separates the two by means of local echo cancellation, a technique well know in V.32 and V.34 modems. With either technique, ADSL splits off a 4 kHz region for POTS at the DC end of the band.



• An ADSL modem organises the aggregate data stream created by multiplexing downstream channels, duplex channels, and maintenance channels together into blocks, and attaches an error correction code to each block.

• The receiver then corrects errors that occur during transmissionup to the limits implied by the code and the block length. The unit may, at the users option, also create super-blocks by interleaving data within sub-blocks; this allows the receiver to correct any combination of errors within a specific span of bits.

• This allows for effective transmission of both data and video signals alike.


What are the xDSL acronyms?

• DSL – Digital Subscriber Line• ADSL – Asymmetric Digital Subscriber Line• HDSL – High-bit-rate Digital Subscriber Line• VDSL – Very High-bit-rate Digital Subscriber Line• RADSL – Rate Adaptive Digital Subscriber Line• SDSL – Symmetric Digital Subscriber Line


DSL – Digital Subscriber Line

• Just another name for ISDN-BRI (ISDN Basic Rate Interface) and it has 2 x 64kbps circuit switched channels and one 16kbps packet switching and signalling channel.

• Circuit carries voice and data in both directions at the same time.

POTSDownstream

Upstream


ADSL

• High bandwidth in only one direction.• High speed up to 6Mbps• Attractive for broadcast services, Web browsing, real

time and interactive video services, file downloads,….• Many rollouts of this technology around the world.


HDSL

• High bit rates in both directions using two copper loops.• Proven to be reliable and cost effective.• Can provide repeater-less T1/E1 services over the two twisted

pair loops.• Can transmit over 24-gauge unconditioned twisted pair loops for

a distance of 4.2km.• No repeaters needed and simplified labour and engineering

effort required to support the technology.


VDSL

• Provides very high bit rates – asymmetric with up to 52Mbps in one direction and 2Mbps in the other direction.

• Typical distance is about 1km which permits about 2Mbps.

• Used in Fibre to the Curb (FTTC) and FTTH


RADSL

• Simple extension to ADSL to allow a wide variety of data rates depending on the line’s transmission characteristics.

• Useful where line quality is not particularly good.


The Modulation Schemes - 1

•• 2B1Q2B1Q– Signal with 2 bits per baud arranged in a four level pulse

amplitude modulated scheme.– Transmits data at twice the frequency of the signal.

•• DMT (Discrete MultiDMT (Discrete Multi--tone Modulation)tone Modulation)– Divides the 3 channels into 256 sub-channels and the traffic

is overlaid on them.– Good management of interference capabilities.


The Modulation Schemes - 2

• Carrier-less Amplitude/Phase Modulation (CAP)– Proprietary modulation

technique– Offers 1.5Mbps downstream

and 64kbps upstream


Rates for technologies

• The following table summarises the rates, reaches and compatibility details:


Technology Comparisons


The Original ADSL Architecture

AccessNode

PremisesDistribution

Network

(LAN, CEBus, etc)

ATU-R

Telephone(s)PSTN

Switch

DigitalBroadcast

Broadband

Narrowband

Networkmanagement

PSTNServices

ATU-C

ATU-C

ATU-C

ATU-C

POTS-R

TE

TE

TE

TE

POTS-C

Vc Va U-C2 U-C U-R U-R2 T-SM T

B

Splitter

Loop


Interpreting the Interface Acronyms

V interface, access node side from ATU-C to access nodeVa

V interface, CO side, from access node to network serviceVc

U interface (Remote side from splitter to ATU-R)U-R2

U interface (Remote side)U-R

U interface (CO side from splitter to ATU-C)U-C2

U interface (CO side)U-C

May be internal to SM or ATUT

T-interface for service moduleT-SM

Interface between PSTN and splitter (Remote side)POTS-R

Interface between PSTN and splitter (CO side)POTS-C

DSL Access MultiplexerDSLAM

Auxiliary data input such as a set top boxB

ADSL Termination Unit (Remote side) ATU-R

ADSL Termination Unit (Central Office side) ATU-C


Current ADSL Architectures – 1


Current ADSL Architectures – 2

BroadbandNetwork

PhysicalInterface

SwitchingFunction

Narrow bandNetwork

PSTN

High-passFilter

Low-passFilter

CentralSplitter

High-passFilter

Low-passFilter

RemoteSplitter

ATU-C

U-C2

POTS orISDN

POTS orISDN

LocalLoop

Telephone,Modem, fax

Or ISDN

SwitchingFunction

PhysicalInterfaceATU-R

Customer Premises Equipment

U-R2

U-C U-R



Current ADSL Architectures – 3 - G.Lite

BroadbandNetwork

PhysicalInterface

SwitchingFunction

Narrow bandNetwork

PSTN

High-passFilter

Low-passFilter

CentralSplitter

Low-passFilter

ATU-C

U-C2

POTS orISDN

POTS orISDN

LocalLoop Telephone,

Modem, faxOr ISDN

SwitchingFunction

PhysicalInterfaceATU-R

Customer Premises Equipment

U-R2

U-C U-R


CPWiring

High-passFilter


What direction to take next ?

Real world: Bridged taps, Crosstalk & Narrowband

Interferers

Ease of CPE installation

Enabling applications:voice, games and video

The green line:Power savings

Adaptation to time varying line conditions

Enabling implementation

technologies

ADSL Anywhere: RU deployment

Monitoring and troubleresolution tools

All Digital Mode

Egress Friendliness

Multi-vendor Interoperability


ADSL2 - Modest Performance Improvements

• Now specifies operation up to 8 Mb/s downstream and 800 kb/s upstream

• Downstream bit-rate performance increased by 128-196 kb/s on most loops

• Upstream bit-rate performance increased by 32-64 kb/s on most loops

• Loop-reach increased by 0.5 to 1 kft for equivalent bit-rate on most loops

• Much stricter rate/reach requirements: per DSL Forum TR-48


ADSL2 Performance Enhancements

• Reduced framing overhead• 1-bit signal constellation• Receiver allocated pilot tone

(RFI & bridged tap)• Receiver directed rate

negotiation• Mandatory trellis code and

R=16 RS coding

• Tone re-ordering to mitigate RFI

• Tone disable to enable RFI cancellation

• Adjustable initialization interval to aid start-up on difficult lines

• More robust bit-swap• Data may be modulated on

pilot tone


ADSL2 - Expanded Line Diagnostics

• Special, robust, diagnostics initialization• Built-in double-ended line testing with standard

messages to retrieve far-end results– Line attenuation– Background line noise vs. frequency– Signal-to-noise vs. frequency


ADSL2 - New L2 Power Management State(permits statistical power savings)

• L0: full normal operation• L3: full off• L2: very low power state for use when there is little or

no traffic– Allows keep-alive traffic– Return to L0 within 0.5 ms– Wideband energy transmitted during L2

• Improved specifications for power cut-back


ADSL2 - Improved Framing

• Seamless rate adaptation permits ADSL to better track changes in line conditions

• Dynamic rate repartitioning for CVoDSL (channelized voice over DSL)

• Support for up to 4 bearers, each with it own latency setting

• Specification of IMA to bond multiple pairs (Inverse multiplexed ATM)


Planning Implications

• Implications for planners– Increased traffic demand– Higher capacity access networks


DSLAM Introduction

• The Digital Subscriber Access Multiplexer (DSLAM) is an important component of a DSL network.

• DSLAMs are becoming more powerful and have greater feature sets as vendors introduce more sophisticated product sets.

• A DSLAM interfaces with the ATU-C and is essentially the “access node” referred to in earlier slides.

• DSLAMs have their own architectures and may support other technologies apart from DSL. (Not just ADSL either, xDSL technologies are also supported.)


DSLAM Architectures

DSLAM

IP Router

ATMSwitch

OtherBroadband

Devices

ATMServer

OtherBroadband

Services PSTNSwitch

ATU-R

HDSL

SDSL

RemoteAccess

MultiplexerVarious

Various

Set-top box

PC/phone

PC/phone

PC with NIC cardExchange


DSLAM View of the Network

AccessNetwork

ServiceNetwork

DSLAM ATU-R

ATU-R

ATU-R

ATU-C

ATU-C

ATU-C

Premises networkor equipment

ServiceNetwork

ServiceNetwork



Network AccessProvider

Network serviceproviders:ATM, ISP,

Frame relay etc

Service user:Residential, SOHO


Vendor View of Network (Alcatel) – 1


Conclusions

• Significant opportunity to extend lifetime of copper cables.

• Many planning issues.


Conclusions

• Clearly, the main challenges are to determine appropriate methods for ensuring QoS is transferred from the PSTN to the new converged network in a manner that is transparent to the user.

• Many problems to be overcome before the true dream of converged networks is realised but this is being worked on around the world and systems are being rolled out very rapidly.

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