DSL Technologies. Objectives. References

Transcription

1 DSL Technologies Professor Richard Harris 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 Advanced Telecommunications Slide 2 References xdsl s 2000 Conference proceedings Toronto, Canada ADSL & DSL Technologies, Walter Goralski, 2 nd edition (McGraw Hill) DSL Forum Website Semester Advanced Telecommunications Slide 3 1

2 Presentation Outline Introduction to xdsl technology Scenarios for use of xdsl in the marketplace Explanation of the various types of xdsl Semester Advanced Telecommunications Slide 4 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 now being 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 Advanced Telecommunications Slide 5 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 Advanced Telecommunications Slide 6 2

3 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 Advanced Telecommunications Slide 7 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 Advanced Telecommunications Slide 8 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 Advanced Telecommunications Slide 9 3

4 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 Advanced Telecommunications Slide 10 Scenario 1 Semester Advanced Telecommunications Slide 11 Scenario - 2 Semester Advanced Telecommunications Slide 12 4

5 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 Advanced Telecommunications Slide 13 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 and downstream high bandwidth signals. The 3 modulating schemes are: 2B1Q Carrier-less amplitude phase modulation (CAP) Discrete multi-tone modulation (DMT) POTS Upstream Downstream Semester Advanced Telecommunications Slide 14 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 Advanced Telecommunications Slide 15 5

6 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 Advanced Telecommunications Slide 16 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 Advanced Telecommunications Slide 17 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 transmission up 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 Advanced Telecommunications Slide 18 6

7 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 Advanced Telecommunications Slide 19 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. POTS Upstream Downstream Semester Advanced Telecommunications Slide 20 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 Advanced Telecommunications Slide 21 7

8 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 Advanced Telecommunications Slide 22 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 Advanced Telecommunications Slide 23 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 Advanced Telecommunications Slide 24 8

9 The Modulation Schemes - 1 2B1Q 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 Multi-tone tone Modulation) Divides the 3 channels into 256 sub-channels and the traffic is overlaid on them. Good management of interference capabilities. Semester Advanced Telecommunications Slide 25 The Modulation Schemes - 2 Carrier-less Amplitude/Phase Modulation (CAP) Proprietary modulation technique Offers 1.5Mbps downstream and 64kbps upstream Semester Advanced Telecommunications Slide 26 Rates for technologies The following table summarises the rates, reaches and compatibility details: Semester Advanced Telecommunications Slide 27 9

10 Technology Comparisons Semester Advanced Telecommunications Slide 28 The Original ADSL Architecture V c V a U-C 2 U-C U-R U-R 2 T-SM T Digital Broadcast Broadband Narrowband management Access Node Splitter Loop ATU-R Premises Distribution (LAN, CEBus, etc) B TE TE TE TE PSTN Services PSTN Switch POTS-C POTS-R Telephone(s) Semester Advanced Telecommunications Slide 29 Interpreting the Interface Acronyms ADSL Termination Unit (Central Office side) ATU-R ADSL Termination Unit (Remote side) B Auxiliary data input such as a set top box DSLAM DSL Access Multiplexer POTS-C Interface between PSTN and splitter (CO side) POTS-R Interface between PSTN and splitter (Remote side) T-SM T-interface for service module T May be internal to SM or ATU U-C U interface (CO side) U-C 2 U interface (CO side from splitter to ) U-R U interface (Remote side) U-R 2 U interface (Remote side from splitter to ATU-R) V c V interface, CO side, from access node to network service V a V interface, access node side from to access node Semester Advanced Telecommunications Slide 30 10

11 Current ADSL Architectures 1 Semester Advanced Telecommunications Slide 31 Current ADSL Architectures 2 Customer Premises Equipment Broadband Physical Interface Switching Function ATU-R Switching Function Physical Interface U-C 2 High-pass Filter Local Loop U-R 2 High-pass Filter Premises Distribution Narrow band PSTN POTS or ISDN Low-pass Filter Central Splitter U-C U-R Low-pass Filter Remote Splitter POTS or ISDN Telephone, Modem, fax Or ISDN Semester Advanced Telecommunications Slide 32 Current ADSL Architectures 3 - G.Lite Customer Premises Equipment Broadband Physical Interface Switching Function ATU-R Switching Function Physical Interface U-C 2 High-pass Filter Premises Distribution High-pass Filter Local Loop Telephone, CP Low-pass Modem, fax Narrow band Wiring Filter POTS or Or ISDN Low-pass ISDN POTS or Filter U-C U-R PSTN ISDN Central Splitter U-R 2 Semester Advanced Telecommunications Slide 33 11

12 What direction to take next? Multi-vendor Interoperability Adaptation to time varying line conditions Monitoring and trouble resolution tools Enabling applications: voice, games and video Enabling implementation technologies Real world: Bridged taps, Crosstalk & Narrowband Interferers The green line: Power savings All Digital Mode ADSL Anywhere: RU deployment Egress Friendliness Ease of CPE installation Semester Advanced Telecommunications Slide 34 ADSL2 - Modest Performance Improvements Now specifies operation up to 8 Mb/s downstream and 800 kb/s upstream Downstream bit-rate performance increased by kb/s on most loops Upstream bit-rate performance increased by 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 Advanced Telecommunications Slide 35 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 Advanced Telecommunications Slide 36 12

13 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 Advanced Telecommunications Slide 37 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 Advanced Telecommunications Slide 38 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 Advanced Telecommunications Slide 39 13

14 Planning Implications Implications for planners Increased traffic demand Higher capacity access networks Semester Advanced Telecommunications Slide 40 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 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 Advanced Telecommunications Slide 41 DSLAM Architectures Exchange PC with NIC card IP Router ATU-R Set-top box PC/phone ATM Server ATM Switch DSLAM HDSL SDSL Various PC/phone Other Broadband Services Other Broadband Devices PSTN Switch Remote Access Multiplexer Various Semester Advanced Telecommunications Slide 42 14

15 DSLAM View of the Service DSLAM ATU-R Premises network or equipment Service Access ATU-R Premises network or equipment Service Access Provider ATU-R Premises network or equipment service providers: ATM, ISP, Frame relay etc Service user: Residential, SOHO Semester Advanced Telecommunications Slide 43 Vendor View of (Alcatel) 1 Semester Advanced Telecommunications Slide 44 Vendor View of (Alcatel) 2 Semester Advanced Telecommunications Slide 45 15

16 Vendor View of (Alcatel) 3 Semester Advanced Telecommunications Slide 46 Conclusions Significant opportunity to extend lifetime of copper cables. Many planning issues. Semester Advanced Telecommunications Slide 47 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. Semester Advanced Telecommunications Slide 48 16