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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
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 3
References
• “xDSL” – http://www.techguide.com• Networks 2000 Conference proceedings – Toronto,
Canada• ADSL & DSL Technologies, Walter Goralski, 2nd
edition (McGraw Hill)• DSL Forum Website
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 4
Presentation Outline
• Introduction to xDSL technology• Scenarios for use of xDSL in the marketplace• Explanation of the various types of xDSL
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 5
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.
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 6
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.
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 7
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
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 8
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.
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 9
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.
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 10
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.
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 11
Network Scenario – 1
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 12
Network Scenario - 2
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 13
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.
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 14
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
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 15
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.
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 16
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.
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 17
The DSL Technology – 2
• 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.
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 18
The DSL Technology – 3
• 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.
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 19
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
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 20
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
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 21
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.
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 22
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.
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 23
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
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 24
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.
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 25
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.
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 26
The Modulation Schemes - 2
• Carrier-less Amplitude/Phase Modulation (CAP)– Proprietary modulation
technique– Offers 1.5Mbps downstream
and 64kbps upstream
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 27
Rates for technologies
• The following table summarises the rates, reaches and compatibility details:
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 28
Technology Comparisons
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 29
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
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 30
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
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 31
Current ADSL Architectures – 1
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 32
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
PremisesDistribution
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 33
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
PremisesDistribution
CPWiring
High-passFilter
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 34
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
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 35
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
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 36
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
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 37
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
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 38
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
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 39
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)
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 40
Planning Implications
• Implications for planners– Increased traffic demand– Higher capacity access networks
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 41
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.)
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 42
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
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 43
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
Premises networkor equipment
Premises networkor equipment
Network AccessProvider
Network serviceproviders:ATM, ISP,
Frame relay etc
Service user:Residential, SOHO
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 44
Vendor View of Network (Alcatel) – 1
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 45
Vendor View of Network (Alcatel) – 2
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 46
Vendor View of Network (Alcatel) – 3
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 47
Conclusions
• Significant opportunity to extend lifetime of copper cables.
• Many planning issues.
Semester 2 - 2005 Advanced Telecommunications 143.466 Slide 48
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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