Video streaming optimization in ADSL architecture

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(1)!. ". Universidade Federal de Pernambuco posgraduacao@cin.ufpe.br www.cin.ufpe.br/~posgraduacao. RECIFE, NOVEMBRO/2007.

(2) ESTE TRABALHO FOI APRESENTADO À PÓS-GRADUAÇÃO EM CIÊNCIA DA COMPUTAÇÃO DO CENTRO DE INFORMÁTICA DA UNIVERSIDADE FEDERAL DE PERNAMBUCO COMO REQUISITO PARCIAL PARA OBTENÇÃO DO GRAU DE MESTRE EM CIÊNCIA DA COMPUTAÇÃO.. ORIENTADOR: DR. DJAMEL SADOK CO-ORIENTADOR: DR. STENIO FERNANDES. RECIFE, NOVEMBRO/2007.

(3) Santos, Gustavo Gentil Barreto Video streaming optimization in ADSL architecture / Gustavo Gentil Barreto Santos. – Recife: O Autor, 2007. xvi, 102 folhas : il., fig., tab. Dissertação (mestrado) – Universidade Federal de Pernambuco. CIn. Ciênca da computação, 2007. Inclui bibliografia. 1. Transmissão de vídeo – Infraestrutura ADSL. 2. Qualidade de vídeo – Avaliação de desempenho – Linha digital de assinante (ADSL). I. Título.. 004.696. CDD (22.ed.). MEI2008-022.

(4) To my beloved wife Aline and my parents Anderson and Marilu. iii.

(5) Acknowledgments Thanks to God, because without him neither my existence nor this work would be possible. Thanks to Professors Djamel and Judith for all support given in both personal and professional difficulties. I would like to thank Professor Stenio for the patience and efforts in advising this dissertation. Thanks to everybody from GPRT (Networks and Telecommunications Research Group), who have helped and contributed in this work. Special thanks to Anderson, Augusto, Glauco, Josy, Rafael, Ramide and Rodrigo. I would also like to thank my wife Aline for understanding my absence during the period of this work and for keeping me always focused in my goals. Finally, I would like to thank my parents, Anderson and Marilu for making me continue with my dreams and for all support given since the beginning of this work.. iv.

(6) Summary Abbreviations and Acronyms. x. Abstract. xiii. Resumo. xv. 1. Introduction. 1. 1.1 Motivation............................................................................................................................................. 6 1.2 Objective ............................................................................................................................................... 6 1.3 Work Structure ..................................................................................................................................... 7 2. Theoretical Background. 9. 2.1 xDSL Technology ................................................................................................................................ 9 2.1.1 2.1.2 2.1.3. ADSL Fundamentals .................................................................................................................................11 Physical Layer Issues .................................................................................................................................15 Link Layer Issues .......................................................................................................................................22. 2.2 Video Content Issues ........................................................................................................................ 24 2.2.1 2.2.2 2.2.3 2.2.4 2.2.5 2.2.6. Digital Television and IPTV ....................................................................................................................24 MPEG Overview .......................................................................................................................................28 MPEG Video Structure ............................................................................................................................30 Profiles and Levels .....................................................................................................................................31 MPEG Algorithm ......................................................................................................................................32 Packetization ...............................................................................................................................................33. 2.3 Noise Impairments ............................................................................................................................ 35 2.3.1 2.3.2 2.3.3 2.3.4. Additive White Gaussian Noise – AWGN............................................................................................35 Crosstalk ......................................................................................................................................................36 Radio Noise ................................................................................................................................................37 Impulse Noise ............................................................................................................................................37. 2.4 Video Quality Assessment ................................................................................................................ 38 2.4.1 2.4.2. 3. Subjective Video Quality Metrics ............................................................................................................38 Objective Video Quality Metrics .............................................................................................................39. Related Work. 42. 3.1 Video Streaming Impairments ......................................................................................................... 42 3.2 Overcoming Video Streaming Impairments.................................................................................. 44 3.3 Evaluating Video Impairments ........................................................................................................ 49 4. Performance Evaluation 4.1 4.2 4.3 4.4 4.5 4.6 4.7. 51. Testbed Setup ..................................................................................................................................... 51 Analyzing ADSL Metrics and Parameters...................................................................................... 53 Layer 3 and Application metrics ...................................................................................................... 60 Analyzing Video Traffic .................................................................................................................... 63 Analyzing video traffic with concurrent traffic not using QoS................................................... 70 Noise Modeling .................................................................................................................................. 76 REIN Impact on Video Traffic ....................................................................................................... 79 v.

(7) 5. Video Streaming Optimization 5.1 5.2 5.3 5.4 5.5. 6. 83. SNR margin effectiveness against REIN ....................................................................................... 83 Interleaving Delay effectiveness against REIN ............................................................................. 85 INP effectiveness against REIN ..................................................................................................... 87 Analyzing video traffic with concurrent traffic using QoS .......................................................... 89 Advisable ADSL Line Configuration.............................................................................................. 93. Conclusions and Future Work. 95. 6.1 Contributions ..................................................................................................................................... 96 6.2 Future Work ....................................................................................................................................... 96 7. References. 98. vi.

(8) List of Figures Figure 1 Copper wire frequency usage by ADSL and POTS [67]. ...................................................... 3 Figure 2 End to end video delivery scheme [4] ..................................................................................... 6 Figure 3 An End-to-End transmission through PSNT infra-structure [56]..................................... 10 Figure 4 ADSL Access Network and its required equipments [56]. ................................................. 12 Figure 5 Spectrum use by CAP coding [56]. ......................................................................................... 17 Figure 6 Spectrum use by DMT coding [56]. ....................................................................................... 18 Figure 7 Different formats and scanning modes for HDTV and SDTV in pixels ......................... 25 Figure 8 Different Layers for the Digital Video Transmission Architecture ................................... 26 Figure 9 Video Transmission using multicast ....................................................................................... 28 Figure 10 Video Transmission using unicast......................................................................................... 28 Figure 11 Combination of YUV (YCbCr) Components ..................................................................... 29 Figure 12 Y Component (Y) .................................................................................................................... 29 Figure 13 U Component (Cb) ................................................................................................................. 29 Figure 14 V Component (Cr) .................................................................................................................. 29 Figure 15 MPEG Compression stages for both spatial and temporal redundancies removal [61]33 Figure 16 PES packet structure [32] ....................................................................................................... 34 Figure 17 MPEG TS packet structure [32] ............................................................................................ 35 Figure 18 NEXT and FEXT noise when coupled to the transmission [67] .................................... 36 Figure 19 Testbed Scheme for running the experiments .................................................................... 53 Figure 20 Downstream rates for different loop length ....................................................................... 55 Figure 21 Upstream rates for different loop length ............................................................................. 55 Figure 22 Downstream Interleaving Delay for different loop length ............................................... 56 Figure 23 Upstream Interleaving Delay for different loop length ..................................................... 56 Figure 24 Downstream SNR margin for different loop length.......................................................... 57 Figure 25 Upstream SNR margin for different loop length ............................................................... 57 Figure 26 Downstream INP value for different loop length.............................................................. 58 Figure 27 Upstream INP value for different loop length ................................................................... 58 Figure 28 Downstream Interleaving Depth for different loop length .............................................. 59 Figure 29 Upstream Interleaving Depth for different loop length .................................................... 59 Figure 30 Downstream FEC Bytes for different loop length ............................................................ 59 Figure 31 Upstream FEC Bytes for different loop length .................................................................. 59 Figure 32 Downstream Bits per Symbol for different loop length ................................................... 60 Figure 33 Upstream Bits per Symbol for different loop length ......................................................... 60 Figure 34 Synchronization of the transmitted frames with their correspondent from the original video .................................................................................................................................................... 62 Figure 35 Received video data rate at the multimedia client for different loop lengths ................. 66 Figure 36 Different videos average delay for different loop lengths................................................. 67 Figure 37 Different videos jitter for different loop lengths ................................................................ 68 Figure 38 Packet Loss in percents for different loop lengths ............................................................ 69 Figure 39 PSNR values for video with different bitrates and loop lengths...................................... 70 Figure 40 Received video rate at the multimedia client for background traffic occupying 50% of available ............................................................................................................................................... 72 Figure 41 Received background traffic rate at the multimedia client for the 50% background traffic .............................................................................................................................................................. 72 Figure 42 Received video rate at the multimedia client for the 100% background traffic............. 72 Figure 43 Received background traffic rate at the multimedia client for the 100% background traffic .............................................................................................................................................................. 72 Figure 44 Video traffic delay for the 50% background traffic scenario .......................................... 73 vii.

(9) Figure 45 Background traffic delay for the 50% background traffic scenario ................................ 73 Figure 46 Video traffic delay for the 100% background traffic scenario ........................................ 73 Figure 47 Background traffic delay for the 100% background traffic scenario ............................. 73 Figure 48 Video traffic jitter for the 50% background traffic scenario ........................................... 74 Figure 49 Background traffic jitter for the 50% background traffic scenario ................................ 74 Figure 50 Video traffic jitter for the 100% background traffic scenario ......................................... 74 Figure 51 Background traffic jitter for the 100% background traffic scenario ............................... 74 Figure 52 Video traffic packet losses for the 50% background traffic scenario............................. 75 Figure 53 Background traffic packet losses for the 50% background traffic scenario .................. 75 Figure 54 Video traffic packet losses for the 100% background traffic scenario .......................... 75 Figure 55 Background traffic packet losses for the 100% background traffic scenario ................ 75 Figure 56 PSNR values for the 50% background traffic scenario ..................................................... 76 Figure 57 PSNR values for the 100% background traffic scenario ................................................... 76 Figure 58 REIN Signal Composition ..................................................................................................... 77 Figure 59 1A Noise Profile Modeling .................................................................................................... 78 Figure 60 1B Noise Profile Modeling .................................................................................................... 78 Figure 61 2B Noise Profile Modeling..................................................................................................... 79 Figure 62 Packet loss for noise profiles with different frequencies .................................................. 82 Figure 63 Packet loss for noise profiles with different burst length ................................................. 82 Figure 64 PSNR for noise profiles with different frequencies ........................................................... 82 Figure 65 PSNR for noise profiles with different burst length .......................................................... 82 Figure 66 Packet loss for 1A noise profile and different SNR margin configuration .................... 84 Figure 67 Packet loss for 2A noise profile and different SNR margin configuration .................... 84 Figure 68 Video PSNR for 1A noise profile and different SNR margin configuration ................. 85 Figure 69 Video PSNR for 2A noise profile and different SNR margin configuration ................. 85 Figure 70 Average packet delay for Noise 1A ...................................................................................... 86 Figure 71 Average packet delay for Noise 2A. ..................................................................................... 86 Figure 72 Packet Loss for Noise 1A and different INP values ......................................................... 88 Figure 73 Packet Loss for Noise 2A and different INP values ......................................................... 88 Figure 74 INP impact on the downstream rate .................................................................................... 89 Figure 75 Received video rate at the multimedia client for the 50% background traffic ............... 91 Figure 76 Received background traffic rate at the multimedia client for the 50% background traffic .............................................................................................................................................................. 91 Figure 77 Video traffic delay at the multimedia client for the 50% background traffic ................. 91 Figure 78 Background traffic delay at the multimedia client for the 50% background traffic ...... 91 Figure 79 Video traffic jitter at the multimedia client for the 50% background traffic ................. 92 Figure 80 Background traffic jitter at the multimedia client for the 50% background traffic ....... 92 Figure 81 Received video rate at the multimedia client for the 50% background traffic ............... 93 Figure 82 Received background traffic rate at the multimedia client for the 50% background traffic .............................................................................................................................................................. 93. viii.

(10) List of Tables Table 1 Resistance of some wire gauges [67]. ....................................................................................... 16 Table 2 Related problems and parameters to the ADSL Line. .......................................................... 22 Table 3 Some profiles defined by the MPEG standard ...................................................................... 32 Table 4 Some levels defined by the MPEG standard .......................................................................... 32 Table 5 Experiment Setup Parameters................................................................................................... 54 Table 6 Synchronization algorithm and step by step example having the window size of 3 ......... 63 Table 7 Experiment Setup Parameters................................................................................................... 65 Table 8 Possible PSNR to MOS mapping [46]..................................................................................... 69 Table 9 Experiment Setup Parameters................................................................................................... 71 Table 10 REIN Noise Profiles. ............................................................................................................... 77 Table 11 REIN Noise Profiles................................................................................................................. 80 Table 12 Experiment Setup Parameters. ................................................................................................ 80 Table 13 Experiment Setup Parameters. ................................................................................................ 83 Table 14 Achieved downstream rate for different SNR margin values ............................................. 84 Table 15 Experiment Setup Parameters. ................................................................................................ 86 Table 16 Experiment Setup Parameters. ................................................................................................ 90. ix.

(11) Abbreviations and Acronyms CAP 3P Services AAC AAL ADSL ADSL2 ADSL2+ AM ANSI ATM ATSC ATU-C ATU-R AWG AWGN BER CATV CBR CD CLE CO CoS CPE CRC DCT DiBEG D-ITG DMT DSIS DSL DSLAM DTV DVB DVD EIA ES FDD FEC. Carrierless Amplitude and Phase Modulation Triple Play Services Advanced Audio Coding ATM Adaptation Layer Asymmetric Digital Subscriber Line Asymmetric Digital Subscriber Line - Version 2 Asymmetric Digital Subscriber Line - Version 2 Plus Amplitude Modulation American National Standards Institute Asynchronous Transfer Mode Advanced Television Systems Committee ADSL Termination Unit – Central ADSL Termination Unit – Remote American Wire Gauge Additive White Gaussian Noise Bit Error Ratio Cable TV Constant Bit Rate Compact Disc Cell Loss Error Central Office Class of Service Customer Premise Cyclic Redundancy Check Discrete Cosine Transform Digital Broadcasting Experts Group Distributed Internet Traffic Generator Discrete Multitone Double Stimulus Impairment Scale Digital Subscriber Line Digital Subscriber Line Access Multiplexer Digital Video Digital Video Broadcasting Digital Versatile Disc Electronic Industries Association Elementary Stream Frequency Division Duplex Forward Error Correction x.

(12) FEXT FR FTTC FTTH G-HDSL GOP HDSL HDTV HFC HSV IDCT IDSL IEC IEEE IGMP INP IP ISDB ISO ITU JCIC MBS MCR MIB MOS MPEG MPEG PS MPEG TS MPQM MSE NAB NCTA NEXT NR NS-2 NTP NTSC OFDM OID OSI P2P PAL PCR. Far-End Crosstalk Full Reference Fiber To The Curb Fiber To The Home G - Symmetric high-speed Digital Subscriber Line Group of Pictures High-bit-rate Digital Subscriber Line High Definition TV Hybrid Fiber-Coax Human Visual System Inverse Discrete Cosine Transform ISDN Digital Subscriber Line International Electrotechnical Commission Institute of Electrical and Electronic Engineers Internet Group Management Protocol Impulse Noise Protection Internet Protocol Integrated Services Digital Broadcasting International Organization for Standardization International Telecommunication Union Joint Committee on InterSociety Coordination Maximum Burst Size Minimum Cell Rate Management Information Base Mean Opinion Score Moving Picture Experts Group MPEG Program Stream MPEG Transport Stream Moving Pictures Quality Metric Mean Square Error National Association of Broadcasters National Cable Television Association Near-End Crosstalk Non Reference Network Simulator - Version 2 Network Time Protocol National Television Standards Committee Orthogonal Frequency Division Multiplexing Object ID Open Systems Interconnection Peer to Peer Phase Alternating Line Peak Cell Rate xi.

(13) PDU PES PIB POTS PSD PSNR PSTN PVC QAM QoE QoS REIN RF RLE RR RS RTP SCR SDSL SDTV SECAM SMPTE SNR SSCQE SSIM STB TCP TV UBR UDP UMTS VBR VC VDSL VLC VLC VOD VoIP VQEG VQM. Protocol Data Unit Packetized Elementary Stream General Purpose Interface Bus Plain Old Telephone Service Power Spectral Density Peak Signal-to-Noise Ratio Public Switched Telephone Network Permanent Virtual Circuit Quadrature Amplitude Modulation Quality of Experience Quality of Service Repetitive Electrical Impulse Noise Radio Frequency Run Length Coding Reduced Reference Reed-Solomon Real-Time Transport Protocol Sustainable Cell Rate Symmetric Digital Subscriber Line Standard Definition TV Séquentiel Couleur À Mémoire Society of Motion Picture and Television Engineers Signal-to-Noise Ratio Single Stimulus Continuous Quality Evaluation Structural Similarity Index Set-Top Box Transmission Control Protocol Television Unspecified Bit Rate User Datagram Protocol Universal Mobile Telecommunications System Variable Bit Rate Virtual Circuit Very high speed Digital Subscriber Line Variable Length Coding VideoLAN Client Video on Demand Voice over IP Video Quality Experts Group Video Quality Metric. xii.

(14) Abstract With the evolution of the broadband market and the decline of the traditional voice services, telecom service providers need to offer a larger combination of services to maintain competitiveness and gain market share. Video and audio content are now available like never before. With the seamless delivery of voice, video and data services, Triple Play Services (3P) are essential for operators and service providers to retain their customer base. The delivery of Internet Protocol TV (IPTV) using DSL connections is a technology that brings new business opportunities to service providers. DSL technologies such as ADSL2+, have data rates that make them possible to combine voice, video and data services over a single telephone line. Taking these technological enhancements into consideration, ADSL2+ has a theoretical bandwidth of 24 Mbps, which would be enough to deliver several SDTV (1.5~4 Mbps – depending on the codec used) and HDTV (8~12 Mbps – depending on the codec used) television channels to the residential user. However, factors such as noise and distance between user and service provider can make this scenario impractical. In addition, there are numerous factors involved in the delivery of IPTV across the core network and its transmission to the customer over ADSL2+ connections. In general, video service providers first code the video signal usually using MPEG-2 or MPEG-4. Video streaming applications require uninterrupted data transfer, but the smallest packet losses can result in great quality degradation. Packet loss can be caused in two different ways. By noise, which can be found in any transmission environment and can be a real undesired effect that may introduce more errors than expected, or by buffer overflow in different elements of the network.. Moreover,. delay and jitter can have crucial impact on the video delivery. This work tackles the Video Streaming impairments in different scenarios, such as noisy environments, concurrent background traffic and different distances from the customer and service provider. Variables related to the ADSL infrastructure, e.g. noise protection and QoS mechanisms, are observed and their adjustments are made in order to give improvements to the video quality. Parameters related to Layer 1, 2, 3 and application level are studied and analyzed in this work. In addition, different sort of noises, such as REIN, are added to simulate transmission problems that might occur in a real streaming scenario. The experiments show that for different scenarios where different kind of impairments are coupled to the line, there are many factors, such as INP, SNR Margin, Interleaving Delay, that can be adjusted aiming to optimize the video quality. At the end of xiii.

(15) this work, a desirable configuration for a real video transmission over ADSL systems has been advised in order to mitigate the existing problems.. Keywords: Video Streaming, ADSL, Video Quality, Network Measurements. xiv.

(16) Resumo Com a evolução do mercado de banda larga e a diminuição dos serviços de telefonia tradicionais, os provedores de serviço de telefonia têm a necessidade de oferecer uma grande gama de serviços para se manter competitivo e obter novas fatias do mercado. Novos serviços de áudio e vídeo podem ser oferecidos como nunca antes. Com a junção de serviços de voz, dados e vídeo, serviços Triple Play (3P) são essenciais para que as operadoras e provedores de serviço mantenham sua base seus usuários atuais. O oferecimento de serviços IPTV usando conexões DSL é uma tecnologia emergente que traz novas oportunidades de negócios para os provedores de serviços. Tecnologias de banda larga como o ADSL2+ possuem taxas de dados que tornam possível combinar serviços de voz, vídeo e dados utilizando uma única linha telefônica. Levando em consideração essas melhorias tecnológicas, o padrão ADSL2+ consegue obter taxas de 24 Mbps na teoria, o que seria o suficiente para oferecer alguns canais SDTV (1.5~4 Mbps – dependendo do codec utilizado) e HDTV (8~12 Mbps – dependendo do codec utilizado) para o usuário residencial. Entretanto outros fatores como ruídos e a distancia entre o usuário e o provedor de serviços pode tornar esse cenário impraticável. Na maioria das vezes o provedor de serviços de vídeo codifica o sinal utilizando o MPEG-2 ou MPEG-4. Aplicações de vídeo em rede requerem transferência de dados ininterrupta, pois a perda de poucos pacotes pode resultar em uma grande perda de qualidade do vídeo. Perda de pacotes pode ser causada de duas maneiras distintas. Por ruídos, que podem ser encontrados em qualquer ambiente de transmissão e podem gerar um efeito indesejável, introduzindo mais erros que o esperado durante a transmissão, ou por estouro do buffer em diferentes elementos da rede. Além do mais, em determinados cenários, a influência do delay e do jitter pode ser crucial na entrega do vídeo para o usuário. Este trabalho aborda os problemas presentes na transmissão de vídeo numa infra-estrutura ADSL, ilustrando diferentes cenários, tais como ambientes com ruído, com tráfego concorrente e diferentes distancias entre o usuário e a provedora de serviço. Variáveis relacionadas à infra-estrutura ADSL, como, por exemplo, proteção a ruído e mecanismos de QoS são observados e ajustes são realizados na intenção de oferecer ao usuário final uma melhoria na qualidade do vídeo. Parâmetros relacionados as Camada 1, 2 e 3 e a nível de aplicação são estudados e analisados. Além disso, diferentes tipos de ruídos na transmissão, tal como REIN, serão inseridos no intuito de simular xv.

(17) problemas existentes em um cenário real. Os resultados gerados pelos experimentos mostram que para diferentes tipos de problemas agregados a linha, existem muitos fatores, tais como INP, SNR, Interleaving Delay, que podem ser ajustados visando a melhoria da qualidade do vídeo. No final deste trabalho é apresentada uma configuração desejável para sistemas ADSL no processo de transmissão de vídeo visando minimizar os problemas existentes na linha.. Palavras chave: Transmissão de vídeo, ADSL, Qualidade de vídeo, Medições de rede. xvi.

(18) 1 Introduction “Beneath this mask there is more than flesh. Beneath this mask there is an idea, and ideas are bulletproof .”. Quote from the movie V for Vendetta. In recent years, many efforts have been made to stream multimedia content through the existing network infrastructure. In addition, a great deal of changes has appeared in the telecommunication industry. While the available bandwidth has been growing quickly, the operators and service providers realized that they could explore much more the actual Internet scenario. Furthermore, Triple Play Services (Voice, Video and Data) are being deployed more than ever before. Consequently, operators and service providers have initiated efforts to engineer their networks to provide commercial package services covering Voice over IP (VoIP), Standard Definition TV (SDTV) and/or High Definition TV (HDTV) in combination with the Internet accessibility. With this new growing market in mind, service providers find themselves having to change business strategies to continue their growth and supply costumers needs. Faced with fierce competition and regulatory uncertainty, service providers are looking at video delivery to increase revenues and deflect competition. However, profitable video delivery requires carriers to think carefully about how best to invest into their network infrastructure [1]. All of this coincides with increasing global deployment of broadband access technologies, many of which are being used to deliver IP-based services and applications to customers at increasingly faster speeds [14]. In the past, there have been attempts to bring faster access speed and to reduce the existing bottleneck to consumers, each with various degrees of success. Today, the most popular broadband access technologies today are cable modems and Digital Subscriber Lines (DSL), which are being deployed across the globe. Although each technology has its advantages and drawbacks, DSL seems to be promising because it is backed by virtually all phone companies, plus the technology has room to grow [56]. The DSL technology is widely used because it achieves broadband speeds provides a greater bandwidth comparing to the old dial-up connections that reached the maximum of 56Kbps, and utilizes the some ordinary copper wires medium as of the POTS (Plain Old Telephone Service). 1.

(19) While DSL technology offers dramatic speed improvements (up to 8+ Mbps downstream and 1+ Mbps upstream) compared to other network access methods, the real strength of DSL-based services lies in the opportunities driven by [67]: . Multimedia applications required by today's network users. . Performance and reliability. . Economics The basic idea of the DSL system is to use frequencies that are not used by the POTS. Taking. into account that POTS utilizes only frequencies up to 4 KHz and the wire twisted pair is capable to transmit data in higher frequencies than 4 KHz. The properly usage of those frequencies higher than 4 KHz is made by employing different modulation techniques such as DMT (Discrete Multitone) and Carrierless Amplitude and Phase Modulation (CAP). There are also many existing flavors of the DSL technology, but the one mostly used is the Asymmetric Digital Subscriber Line (ADSL). In the ADSL technology, the asymmetric term is used because the downstream bandwidth differs from the upstream one. The picture in Figure 1 depicts how the frequencies are used by the ADSL and the POTS together. Although ADSL offers great rate1 enhancements comparing to 56 Kbps obtained in earlier dial-up connections, the obtained rate is dependent on many factors present in the content delivery process, as ADSL utilizes the copper pair circuit. Some of these factors are wire gauge, distribution point joints, crosstalk, radio frequency interference, line attenuation and impulse noise [25]. As we have seen above, many new technologies have emerged to allow broadband access to home users. As an attempt to make it possible many compression techniques were proposed to guarantee that even applications with large data rates could be used with low bandwidth connections. Since video and other multimedia data are usually too large for raw transmission or storage, most video streams are compressed before transmission over a packet–oriented network [8] [7].. 1. Rate is the obtained data rate in the Physical Layer. 2.

(20) Figure 1 Copper wire frequency usage by ADSL and POTS [67].. There are two most commonly used compression schemes classes for data compression: lossy or lossless techniques. When a lossless compression technique is used, the original data can be reconstructed from the compressed data in its exact original form. In a simple manner, this is possible because the algorithm uses the intrinsic redundancy that is existent in the whole data. On the other hand, a lossy technique cannot obtain the exact original data from the compressed data. This technique is frequently used in cases that even with the loss in a particular case, the effect is not noticeable. These algorithms often have a compression level that measures how compressed the raw data becomes after the algorithm is applied. The compression level is sometimes called compression rate. Thus, there is a tradeoff between compression rate and quality of compressed data. When the compression rate increases then the quality of compressed data decreases and when the compression rate decreases then the quality of compressed data increases. It is worth emphasizing that a given compression algorithm is suitable to only a few specific types of application data. For instance, it is not certain that a good compression technique for audio will have the same results for video and vice versa. The main objective of video compression is to remove redundancy in the original source signal. Moreover, the objective is not restricted to reduce the space occupied by the original data, but also to guarantee a good quality of the compressed data, to minimize the compression complexity and to reduce the overall encoding/decoding delay. With all these requirements, choosing a compression scheme can be a tough task. Video compression is possible because of [59]: 3.

(21) . Spatial redundancy among neighboring pixels in a given picture frame. . Spectral redundancy among color images within frames. . Temporal redundancy among pixels or blocks of pixels in different frames. . Considerable irrelevant information from a perceptual viewpoint in the information contained in a video data.. A good compression scheme can exploit these four points to produce high compression rates and in the same time it maintains a good quality level. Considering the redundant information contained in the raw data, it is also known that human eyes are not so sensitive to some non redundant information contained in the raw data as well. So, taking this into consideration, it is possible to employ lossless encoding techniques (such as Huffman coding) and approximate the information which is not important for the sake of human perception of the quality. For example humans do not distinguish between two video sequences one of them encoded at 30 frames per second and the other at 60 frames per second. In addition we are more sensitive to luminance variation than chrominance information [59]. Many institutions including the International Telecommunication Union (ITU) and International Organization. for. Standardization/International Electrotechnical Commission. (ISO/IEC) published some standards for video/audio compression in terms of storage, internet streaming and Digital Video (DTV). The most important video codec standards for streaming video are H.261, H.262, H.263, H.264, MPEG-1, MPEG-2 and MPEG-4. Although these standards have different names, they propose the same algorithm, as H.264 which is an implementation of the MPEG-4 part 10. The Moving Picture Experts Group (MPEG) was established in January 1988 with the mandate to develop standards for coded representation of moving pictures and audio. It operates in the framework of Joint ISO/IEC Technical Committee (JTC 1) on Information Technology. Starting from its first meeting in May 1988 when 25 experts participated, MPEG has grown to an unusually large committee. Usually some 350 experts from around 200 companies and organizations, from about 20 countries take part in MPEG meetings [60]. The MPEG committee has already standardized MPEG-1, MPEG-2 and MPEG-4 (versions 1 and 2) and in their last phases of development they are working on MPEG-4 (version 3, 4 and 5), MPEG-7 and MPEG-21. Despite the name MPEG, they also define other components than only 4.

(22) video compression algorithms. These standards are composed of different parts which define issues related to video, audio, systems, and conformance tests, among others. Both MPEG-1 and MPEG-2 standards are similar in basic concepts. They both are based on motion compensated block-based transform coding techniques similar to those employed in H.263. However, MPEG-4 employs more sophisticated approaches. Even with the existence of MPEG-4, MPEG-2 still has a crucial contribution in many applications and telecommunication systems. MPEG-2 is intended to be generic in the sense that it serves a wide range of applications, bitrates, resolutions, qualities and services. Applications should cover, among other things, digital storage and television broadcasting. The choice of the compression algorithm depends on the available bandwidth or storage capacity and the features required by the application. MPEG-2 standard is a truly integrated audio-visual standard and is capable of compressing National Television Standards Committee (NTSC) or Phase Alternating Line (PAL) video into an average bit-rate of 3Mbps to 6 Mbps with a quality comparable to analog Cable TV (CATV). It is a widely used standard in today‟s visual multimedia applications [19]. In the new era of DTV, leaving the old PAL and NTSC behind, the Digital Video Broadcasting (DVB), Integrated Services Digital Broadcasting (ISDB) and Advanced Television Systems Committee (ATSC) makes use of MPEG-2 to send video signals to the customer‟s home. This is possible because MPEG-2 defines how applications should behave for a lot of different scenarios requirements and restrictions. Figure 2 shows an end to end design of multimedia transmission in many different formats such as DVB and Video on Demand (VoD). In addition, it shows that the end user is capable of receiving different services as voice, video and data, as we have mentioned previously.. 5.

(23) Figure 2 End to end video delivery scheme [4]. MPEG-2 video syntax defines three different types of pictures: I-frames (Intra Frames) are encoded without any temporal compression techniques. Lossy and lossless coding are employed on the current picture without reference to the adjacent frames. Inter frames (P or B) however, are encoded by taking into account motion prediction techniques (removing the temporal redundancy among frames) as well as those employed to encode I frames. Hence, the bit rate of I frames is very large compared to both P and B frames [59].. 1.1 Motivation Since the beginning of digital data transmission, video transmission is a challenging task due to data rates achieved when using old dial-up connections. With the enhancements made using new broadband technologies, such as ADSL and Cable Modems, now video streaming has become part of our lives. However, there are many other factors involved in the multimedia content delivery. Impairments existence in the transmission medium can degrade video quality badly because of packet losses. Although packet loss represent a great impairment in the video transmission, other metrics like delay and jitter are also very important because they can have a considerable impact in video quality.. 1.2 Objective The main objetive of this work is to propose an ADSL line configuration that brings enhancements to the video streaming process. This enhancement is obtained after a detailed analysis and investigation of the impact of the parameters related to the ADSL line, such as Impulse Noise Protecion, Signal to Noise Ratio (SNR) margin, Interleaving Delay among others. Moreover, we 6.

(24) present results for the equipments being used in our experiments and show whether they are in compliance with the standards implemented by their manufacturers. For more reliable results, an investigation was made to observe the impact of these metrics for different scenarios and diverse kind of videos. These scenarios includes impairments such as noises, concurrent background traffic being transmitted together with video traffic and the video traffic bahavior for different distances from the Central Office (CO) and the Customer Premise (CPE). With this investigation we are able to set parameters in the Layer 1 and Layer 2 that optimizes video traffic which is rolled out through an ADSL access network. At the end of this work we present a guideline for the ADSL line configuration that encloses different scenarios when video content is delivered to the end user‟s home.. 1.3 Work Structure This dissertation has been organized in the following way. Chapter 2 describes the xDSL technology and presents the advantages and disadvantages when this technology is deployed, such as noise protection techniques and the impact of different sort of noises over the system. In addition some details about video compression techniques and video quality assessment are given. Chapter 3 presents related works in the video streaming and ADSL area and several open research issues, as the lack of bandwidth, and jitter and delay problems when streaming video content. Moreover this chapter explains how researchers try to overcome these problems and what methodologies are used in order to support their approach. Chapter 4 shows details about how the testbed was set up and the methodology used in order to conduct the experiments. Moreover, it explains the structure of the equipments, third part tools used, and software/scripts developed to run the experiments and to extract the required information. In addition, it illustrates the first experiments, which familiarize readers to the ADSL metrics and parameters, and some results that demonstrate the existing problems when streaming video over a noisy environment and having concurrent background traffic. In the Chapter 5 the obtained insights, as the impact of long ranges from the CPE and CO on the video transmission, from the first experiments conducted in Chapter 4 are used, and based on the considerations made along the entire Chapter 4, we were able to make more focused experiments and utilize better parameters values so that the video streaming process can take benefit from the new line configuration. Here the new line configuration is a collection of parameters that 7.

(25) we are able to set within the ADSL line and equipments. The scenarios and considerations about the experiments are also shown in this chapter. Finally, conclusions, including a list of noticed contributions and some future work are shown in the Chapter 6.. 8.

(26) 2 Theoretical Background “If a man hasn't discovered something that he will die for, he isn't fit to live.”. Martin Luther King Jr.. This chapter has the goal of explaining the technologies involved in this work, allowing a better understanding of the objectives presented. Section 2.1 presents the xDSL technology, its flavor, its infra-structure and how the existing telephone copper wires were used to provide broadband access to the customer‟s home. In Section 2.2 some fundamentals about the video transmission scenario and new trends about this technology are presented. Impairments that might exist in data transmission, such as noises characteristics are presented in Section 2.3. Finally, Section 2.4 presents methods and different approaches in order to assess video quality.. 2.1 xDSL Technology When we go back in time, just to the 90‟s, we consider those 300 and later 2400 baud modems that we used to think that data transfer speed could not get any faster. Then, when encoding algorithms became more efficient and telephone lines became cleaner of electrical interference, V.90 standard emerged and 56 kbps of data could be transferred through the telephone copper line. When modem was in use, the applications were based mainly in text data transfer. Corporations were the majority of the users who had a need for employees or vendors to access and use a corporate database. For that purpose, modem had to deal with digital bits and convert into analog signals. Those analog waves were then transmitted through a Public Switched Telephone Network (PSTN), also known as Plain Old Telephone Service (POTS). These waves were received by another modem, which converted these signals into digital bits again so that the receiver computer could understand.. 9.

(27) Figure 3 An End-to-End transmission through PSNT infra-structure [56].. With new resources, in recent years what is called broadband connection has become reality to the home users. One possibility for this high-speed connection could be the use of technologies such as fiber, whether fiber to the curb (FTTC), fiber to the home (FTTH), or hybrid fiber-coax (HFC). A few years ago, telcos (Telephone Companies) became aware of the great possibilities when utilizing very high bandwidth of fiber to carry digital signals, though a new structure would be necessary to deliver data to the customer. This is where DSL (Digital Subscriber Line) takes place. DSL is a broadband connection invented by BellCore in the mid-1980s that utilizes existing telephone copper wire to deliver data through the last mile between network service providers and the users of those network services. In the beginning the goal of DSL was to deliver voice and video to the customer. Because the technology was not mature to efficiently achieve what had been invented for. DSL was forgotten for almost a decade. After this period phone companies realized that their voice network became overloaded with data. xDSL is used as a general term to identify a great variety of DSL standards. Some of the xDSL flavors are:  SDSL – It has a symmetric nature, which means that the same bandwidth is available both for upstream and downstream direction. It can achieve rates up to 2.3 Mbps and uses 2B1Q encoding. The typical distance from the CO and CPE using SDSL standard is approximately 4 km.  HDSL – Known as High-bit-rate Digital Subscriber Line, this standard made use of 2B1Q at higher speeds as an alternate way to provide T1 and E1 services, without repeaters. The technique consists of splitting the 1.544 Mbps service into two pairs (four wires), in which each pair ran at 788 Kbps. By splitting the service across two lines and increasing the bits per baud, the per-line speed and the need for frequency spectrum could be reduced to allow longer loop reach.  G.shdsl – It is considered to be the next generation of symmetric services. This standard encompasses all functions which are currently provided by the European SDSL standard and HDSL2. It uses copper wire pair circuit to deliver data to the customer. It may have different bitrates and it is able to negotiate the framing protocol. This multirate replacement for proprietary SDSL offers symmetric bandwidths between 192 Kbps to 2.3 Mbps, with a. 10.

(28) 30 percent longer loop reach than SDSL and improved spectral compatibility with other DSL variants within the network. G.shdsl is expected to be applicable worldwide.  IDSL – It is based in ISDN standard and uses 2B1Q encoding. Differently from ISDN that enables dynamically initiation/termination of the connection, the IDSL has a permanent connection. IDSL can achieve symmetric bandwidths up to 144 Kbps in both directions.  VDSL - VDSL (Very high speed Digital Subscriber Line) aims to deliver downstream data up to 52 Mbps. However, this high data rate decreases quickly when the distance increases. To achieve such high bitrates, VDSL uses bandwidth up to 12 MHz. VDSL is more complex than ADSL since there are several options for frequency band allocation and modulation techniques. While ADSL is designed only for asymmetric transmission only, the VDSL standard includes both asymmetric and symmetric profiles and it is targeted at both residential and business applications [13]. VDSL seems very promising in terms of offering services that deal with huge amount of data in the near future.  ADSL – ADSL (Asymmetric Digital Subscriber Line) is by far the most deployed xDSL flavor today. The reason is that ADSL is suitable for most today‟s consumer due to the available bandwidth both in upstream and downstream directions. ADSL provides highspeed data transmissions over the twisted copper wire, the so-called local loop that connects the customer home or office to their local telephone company. Taking into consideration the fact that the nature of Internet is generally asymmetric, since users send a briefly page request and receive an entire web page download, ADSL seems to fit this scenario because the different upstream and downstream data rates.. 2.1.1 ADSL Fundamentals The PSTN and supporting local access networks were designed following guidelines that limited transmissions to a 3,400 Hz analog voice channel. When observing that higher frequencies could be explored so that another type of data could be transmitted in these frequencies, ADSL systems begun to be deployed. ADSL is a broadband connection that uses the existing telephone line, which upstream and downstream data rates are different. ADSL provides high-speed data transmissions over the twisted copper wire, the so-called local loop that connects a customer home or office (Customer Premise – CPE) to their local telephone company (Central Office – CO).. 11.

(29) The typical telephone cabling is capable of supporting a greater range of frequencies, around 2MHz. With ADSL modems, the digital signal is not limited to 3,400 Hz of voice frequencies. ADSL modems enable up to 2MHz of bandwidth to be used for transmission of digital data and analogue voice signals on the same wire by separating the signals in the frequency domain, preventing them from interfering with each other. ADSL modems establish a connection from one end of a copper wire to the other end of that copper wire.. Figure 4 ADSL Access Network and its required equipments [56].. Any DSL system has a DSL modem at the subscriber end, this can be seen in Figure 4 as an ATU-R (ADSL Termination Unit – Remote, user side) and more commonly called as customer premises equipment (CPE). At the Central Office (CO), a corresponding DSL modem demodulates the signals modulated by the subscriber modem. The CO is equipped with a Digital Subscriber Line Access Multiplexer (DSLAM), which is a platform for ADSL. DSLAM provides high-speed data transmission over traditional twisted-pair wiring. In addition, DSLAM is able to concentrate the data traffic from multiple DSL loops onto the backbone network for connection to the rest of the network. In other words, the DSLAM consists of banks of ADSL Termination Unit – Central Office (ATU-Cs). Depending on the region that Central Office is serving, the DSLAM should have a corresponding ATU-C for each ADSL Termination Unit – Remote (ATU-R) at the subscriber end. The splitters are small and simple devices that separate ADSL data from voice data. In other words, the splitter identifies whether the signal is below 4 kHz or higher. This is obtained using a simple low-pass filter technology. Therefore, the use of this splitter technology allows ADSL using the upper frequency spectrum in the same pair of copper for both voice and DSL data. If a single pair of copper is used for both voice traffic and DSL data, a splitter is used at both CPE and CO end. 12.

(30) Over the last few years, ADSL has been the most popular technology for providing broadband access to residential customers. Improved versions of ADSL, called ADSL2 and ADSL2+, which provides higher bit rates have now been developed and are gradually being introduced in many networks [13]. The difference between these standards and their particularities are shown below:  ADSL [37] is a local loop based transmission system that integrates voice and data services with higher data rates on the downstream than the upstream. It can reach speeds of up to 10Mbps downstream and 1Mbps upstream. ADSL allows customers to use simultaneously the normal telephone service and the high-speed digital transmissions on an existing telephone line and is ideally suited to home and small office users that require fast download rates for video on demand, Internet access, remote LAN access, multimedia access and other specialized PC services.  ADSL2 [38] has improved performance and interoperability over ADSL. It provides support for new applications, services and deployment scenarios, and can achieve higher data rates of approximately 12Mbps downstream, depending on loop length and other factors. Among the changes are improvements in data rate and reach performance, rate adaptation, diagnostics, and the use of a stand-by mode. ADSL2 has been designed to improve downstream data rate, distance between CO and CPE of ADSL, that means range, and achieve better performance on long lines in the presence of narrowband interference. ADSL2 achieves downstream and upstream transmission data rates of 12 Mbps and 1 Mbps respectively, depending on loop length and other factors. ADSL2 executes this through adopting better modulation efficiency, reducing framing overhead, reaching higher coding gain, improving the initialization state machine and providing enhanced signal processing algorithms. ADSL2 provides better modulation efficiency through the use of fourdimensional, 16-state trellis-coded and 1-bit quadrature amplitude modulation (QAM) constellations, which provide higher data rates on long lines where the signal-to-noise ratio (SNR) is low. ADSL2 achieves higher coding gain from the Reed-Solomon (RS) code on long lines where data rates are lower. This is due to improvements in the ADSL2 framers that improve flexibility and programmability in the construction of the RS codewords. ADSL2 systems decrease framing overhead through their support for a frame with a programmable number of overhead bits. Unlike the first-generation ADSL standards in which the overhead bits per frame are fixed and consume 32 kbps of actual payload data, in 13.

(31) the ADSL2 standard the overhead bits per frame can be programmed from 4 to 32 kbps. In first-generation ADSL systems, on long lines where the data rate is low (e.g. 128 kbps), a fixed 32 kbps (or 25% of the total data rate) is allocated as overhead information. In ADSL2 systems, the overhead data rate can be reduced to 4 kbps, which provides an additional 28 kbps for payload data. ADSL2 transceivers are responsible for the measurement of line noise, loop attenuation, and signal-to noise ratio (SNR) on both ends of the line. These measurements are collected using a special diagnostic testing mode even when line quality is too poor to actually complete the ADSL connection. Additionally, ADSL2 includes real-time performance monitoring capabilities which informs line quality and noise conditions at both ends of the line. This information is interpreted by software and then used by the service provider to monitor the quality of the ADSL connection which prevents future service failures. It can also be used to determine which data rate services a customer can subscribe for.  ADSL2+ [40] doubles the bandwidth used for downstream data transmission of the ADSL2 by increasing the downstream rate to around 20Mbps on phone lines as long as 1.5km. The ADSL2+ standard doubles the maximum frequency spectrum used for downstream data transmission from 1.1MHz to 2.2MHz. In other words, ADSL2+ extends downstream bandwidth to 2.2 MHz (512 subcarriers) for all operation modes (POTS/ISDN/All Digital Mode). ADSL2+ solutions interoperate with ADSL and ADSL2 chipsets. The members of the ADSL2 standards family specify a downstream frequency band up to 552 KHz and 1.1 MHz. ADSL2+ specifies a downstream frequency up to 2.2 MHz. The result is a significant increase in downstream data rates in shorter phone lines. ADSL2+ upstream data rate is 1 Mbps but it depends on loop conditions. ADSL2+ can also be used to reduce crosstalk. ADSL2+ provides the capacity to use only tones between 1.1 MHz and 2.2 MHz. Crosstalk of ADSL services from the remote terminal in the lines from the central office can significantly impair data rates in the line from the CO. ADSL2+ can correct this problem using frequencies under 1.1 MHz from the central office to the remote terminal, and frequencies between 1.1 MHz and 2.2 MHz from the remote terminal to the customer premise (a type of FDD scheme). This eliminates a large amount of crosstalk between the services and maintains data rates in the line from the central office. ADSL2+ enables service providers to change their networks to support advanced services such as video in a flexible way, with a singular solution for both short-loop and long-loop applications. It includes all the feature and performance benefits from ADSL2 while maintaining the capability to 14.

(32) interoperate with legacy equipment and allow the system a gradual transition to provide advanced services. One should notice that the ADSL technology is still being improved and better rates have been achieved. This occurs because different modulation techniques have been developed and more spectrum bandwidth is now utilized. However, in order to exploit what ADSL has to offer, there are a lot of factors involved in different layers that this technology covers. Therefore, all these factors have to be investigated and analyzed.. 2.1.2 Physical Layer Issues When considering the POTS structure that ADSL takes advantage of, there are a lot of related issues about the adaptation of an old service running within the copper wires to a more sophisticated service as data transfer using higher frequencies. In this section we are going to present some of these issues and how they can affect the ADSL technology deployment.. Existing Copper Wire Structure Loop length has an important impact on ADSL deployments. For short loops scenarios great data rates are achieved while longer loops, which means greater ranges, will make the CO and CPE equipments synchronize at lower rates, thus advanced services (video, real-time data, etc.) may not be feasible depending on how far the customer is from the CO. Synchronization between CO and CPE equipments means an agreement of their capabilities during data transmission. In the old telephone systems, what we call bridge taps were frequently used to connect and disconnect portions of a loop cabling. Bridged taps are unused sections of twisted pair connected at some point along another pair. Their presence causes additional loop loss for frequencies of the quarter wavelength of the extension length. This would be caused by shared telephone copper wires, extension of the cable beyond the drop of the customer premises and any repairs made by just splicing the broke wired pairs. These bridge taps impact on data transfer in higher frequencies such as ADSL does. Therefore they must be removed Another existing element in the PSTN infra-structure is the loading coils. Loading coils are typically 88 millihenry (mH) chokes placed were used to reduce attenuation when transmitting voice signal in long loops to the customer premise. The loading coils technique was created to increase the telephony transmission quality, offering lower loss in the POTS band and great attenuation at frequencies above those used by POTS. This technique consists in placing a series of physical inductors at equally spaced intervals. The problem of using load coils is that it works for low 15.

(33) frequencies only and when ADSL is deployed in a loading coil environment, they act as a low-pass filter. Therefore, in order to successfully deliver ADSL services though the POTS infra-structure, the loading coils had to be removed, thus causing extra costs. Attenuation is the dissipation of a transmitted signal power as it travels through the copper wire line. The high-frequency signals transmitted through DSL loops attenuate power faster than the lower-frequency signals of the POTS band. One way to minimize attenuation is to use lower resistance wire. This is achieved by modification of the wire gauge. Resistance of some wire gauges can be seen in Table 5. Table 1 Resistance of some wire gauges [67].. AWG2 19 22 24 26. Diameter (mm) 0.9 0.63 0.5 0.4. Loop Resistance (Ω/Km) (20º C) 55.4 110.9 175.2 281.4. ADSL Modulation Techniques In order to offer high speed data transfer and makes a better utilization of what the copper pair has to offer, ADSL uses line coding techniques such as DMT (Discrete MultiTone) and CAP (Carrierless Amplitude and Phase Modulation). CAP modulation was used in the early deployment of ADSL systems and it is a variation of the QAM (Quadrature Amplitude Modulation) which was developed by AT&T. Moreover, it divides the entire spectrum in three different bands. The first ranges from 0 to 4 KHz and is allocated for the POTS. The second ranges from 25 KHz to 160 KHz and is allocated for upstream data transmission. The third ranges from 240 KHz to 1.5 MHz and is allocated for downstream transmission of the data. Figure 5 depicts how CAP acts when splitting the frequency spectrum for POTS, for upstream and downstream directions.. AWG (American Wire Gauge) is an indicator of the wire size. The heavier the gauge, the lower the AWG number and the lower the impedance. 2. 16.

(34) Figure 5 Spectrum use by CAP coding [56].. Although CAP was used many years when ADSL systems were being deployed, a more sophisticated coding technique was introduced later as DMT. Though CAP is not standardized, DMT was initially standardized as ANSI (American National Standards Institute) T1.413 and was then forwarded to ITU as G.992.1. Differently from CAP, DMT also known as OFDM (Orthogonal Frequency Division Multiplexing) divides the available spectrum in 256 (ADSL/ADSL2) or 512 (ADSL2+) small channels of 4.3125 KHz each. Each channel is able to carry 60 Kbps of data. DMT defines two data paths: fast and interleaved. Fast offers low latency, while the purpose of interleaving is to avoid consecutive errors delivered to the Reed-Solomon (RS) forward error correction (FEC) algorithm at the receiving end of the circuit. RS is much more effective on single errors or errors that are not consecutive [56]. Looking at Figure 6, the 4.3125 KHz small channels can be seen for upstream and downstream bandwidth utilization. These 4.3125 KHz channels are each bin inside the upstream and downstream frequency division.. 17.

(35) Figure 6 Spectrum use by DMT coding [56].. Error Correction Techniques FEC and interleaving technique have an important role in the ADSL systems. Since FEC is more effective against single error, an interleaving can be applied in order to make a better use of the FEC algorithm. ADSL employs Forward Error Correction (FEC) as a way to overcome transmission errors caused by line impairments. ADSL supports FEC in both upstream and downstream directions and the FEC coding algorithm adopted by the recommendation [37] [38] [40] is the Reed-Solomon coding. FEC is the technique that mathematically corrects data which somehow was corrupted during transmission (without any retransmission being used). The role of FEC is to deal with transmission errors caused by noise and other impairments on the wire line. FEC bytes are added to the user data stream to produce a means to calculate the presence of corrupted data and it also generates a corrected frame [56]. The best advantage of not retransmitting data is that for real time applications retransmission is sometimes not tolerated. In addition, it uses the available bandwidth to repeatedly send the same information, so the user perceives very slow throughput. FEC results in greater effective throughput of user data because valuable bandwidth is not being used to retransmit wrong data. For a message of K bytes, the RS coding append R redundancy bytes to form a RS codeword of N = K + R bytes. The R parameter defines the strength of the coding against line disturbance. The RS decoder at the receiver can recover R / 2 erred bytes per codeword. When more than R / 2 bytes errors occur in the same codeword, the receiver is unable to recover the original message, what is indicated by the CRC verification. Possible values of R are 0 (no protection), 2, 4, 8, 12 and 16 bytes per codeword with maximum size of 255 bytes. The greater the value of R is, the more protected the coding is against noise. However more bandwidth is used with non-user data (protection data). There is a trade-off between the rate loss generated by corrupted 18.

(36) frames and by the amount of redundancy bytes. Redundancy bytes must be kept as little as possible while still permitting the original message to be recovered. As a matter of name convention, when the corrupted data is corrected due to the use of FEC, they are reported as corrected errors. However, when these errors cannot be recovered or corrected, they are reported as uncorrected errors. Although FEC seems to be an excellent technique to correct corrupted data, its benefits does not come without a cost. For data being corrected, what we call FEC bytes have to be inserted in the user data. It is important to note that when FEC is being used, the user data has to share the available bandwidth with the redundant data, leading to a bandwidth loss. As a very basic and general rule is that the more FEC bytes used the more effective is the error correction. But in errorfree transmission paths, an unnecessary number of FEC bytes serve only to displace user data in the available bandwidth. Knowing how FEC acts and how it can lead to a misuse of the available bandwidth, its use must have some attention. Interleaving is the process of scrambling user data in a very precise sequence [56]. The interleaving process scrambles RS codewords in such way consecutive errors are spread over many codewords so RS decoding can be more successful. It is used to minimize the burst effect caused by the impulse noise and acts by spreading codewords over a longer time and giving more successful probability to the FEC technique. RS (Reed Solomon) FEC is much more effective on single errors or errors that are more spaced in time (not consecutive). So, when noise events occur during transmission, they can be a short and brief impulse or they can be great and lengthy. In cases that short period noises happens FEC can act in such a manner that correction can be made through the entire affected data, though when lengthy noises occur interleaving acts like a scrambler and mix the consecutive erred data with the corrected data, leaving the erred bits separated. Consecutive errors can be generated by bursty impulsive noise and affect long sequences of bits that may exceed the correction capacity of one RS codeword (8 × R / 2 bits) but less likely when more than one codeword is affected. For instance, for 2 interleaved codewords, the length of the maximum noise burst that can be corrected raises to 2 × 8 × R / 2 bits. Therefore, the erred bits are no longer consecutive and the RS FEC process is much more effective. The interleave depth indicates how many FEC frames are scrambled together, and assume values as 1, 2, 4, 8, 19, 32, and 64, but the Amendment 1 of ADSL2+ [39] extended it to 96, 128, 160, 192, 224, 256, 288, 320, 352, 384, 416, 448, 480 and 511. A greater value of interleave depth improves the noisy immunity but it can turn delay sensitive applications impracticable. 19.

(37) While providing greater protection against long consecutive erred bit sequences, the interleaving process has the drawback of increasing the transmission latency. The FEC frames are held in the interleaving buffer until the interleaving depth is reached at the transmitter. After that, the interleaved frames are sent through the line to the receiver, where they are buffered again to be de-interleaved. Greater values of D do not cause rate loss but impose larger delay to users and applications. Impulse Noise Protection (INP) is the maximum number of consecutive DMT symbols that can be recovered from an impulsive noise burst. The INP limits the size of the recoverable noise burst so that symbols affected by bursts lager than “INP” DMT symbols cannot be reconstructed properly. The impulse noise immunity is determined by FEC and interleaving parameters. The RS coding provides a recover capability of R / 2 erred bytes by data frame, where R is the number of redundant bytes. Applying interleaving depth D, the overall byte protection is D × R / 2 bytes. With DMT symbols of size L bits, the impulse noise protection obtained is given by. INP  D. R 1 2 L/8. where INP is the calculated impulse noise protection, D is the interleaving depth, R is the number of redundancy bytes and L is the size of the DMT symbols in bits.. Trellis Coding Trellis coding may be used by ADSL to avoid noise interference when transmitting data. The usage of trellis is an optional feature that ADSL provides. However, Trellis coding decreases bandwidth and increases the complexity of the decoder needed to recover the data. The decoder is more complex because it needs to take into account every subsequent symbol that would be affected by the transmitted data before it makes a decision. For example, for a simple 16 point QAM on the raw data, if Trellis coding is not used then 4 bits per symbol will be used to transmit the data. Using Trellis coding will take two out of every four raw bits, and convolutionally encodes them to 3. The symbol takes now 5 bits, and a 32 point QAM is needed to send the data. The raw error rate when decoding a 32 point QAM is much higher when compared when decoding a 16 point QAM. If a point decoded from the 32 point QAM is wrong, the likely correct choice should be one of the adjacent ones. The Trellis coding is able to spread symbol information in others symbols, this mean that, if the decoder is not able to decide whether the received symbol really fits in the QAM constellation, 20.

(38) knowing the previous received symbols will make the decoder able to do this task. This is done by not only considering single symbols in a symbol by symbol detection as it is done in uncoded modulation, but by taking into account whole sequences of received data. This is possible by only allowing certain sequences of signal points, while others are excluded. In order to get the freedom to exclude certain sequences without loss in data rate or increase in transmission bandwidth, the transmit constellation has to be increased.. Bit Swapping and SNR Bit Swapping is a mandatory ADSL feature that reallocates bit loading among the allowed sub carriers according to the SNR, however, after a bit swapping reconfiguration, the total data rate is unchanged [37]. Bit swap only works if the noise amplitude does not increase too rapidly, and the Power Spectral Density (PSD) of the noise is narrow enough such that not too many sub carriers are simultaneously affected by the noise. Despite this, the method provides good protection against narrow-band noise; against impulsive noise types bit swap provides little protection [38]. In practical terms, bit loading have a serious limitation: requires extensive synchronization, and the protocol implemented for this purpose is quite slow. The SNR margin represents the amount of increased received noise when compared to the noise power that the system can tolerate and still meet the target BER (Bit Error Ratio) of 10 -7. The default configuration of an ADSL transceiver allows a SNR margin of 6 dB, in other words, the noise can increase by 6 dB, without causing BER to increase above 10-7 [38]. One possibility is to configure the transceivers to a high SNR margin to deal with sudden transients, but this approach has the drawback that the bit rate decreases with higher SNR margin as the modem needs to fall back to lower rate modulation schemes in an attempt to maintain the same BER. If the noise margin falls below this minimum level, the modem should attempt to increase its power output, if that is not possible the modem will attempt to re-initialize or shutdown [38]. It would be interesting to investigate how power control is beneficial for capacity optimization. Although it may be intuitive that for a single modem increasing its power may help it to recover from a low SNR, such benefits may be limited when a number of channels and wires apply the same policy. As discussed in this section, the ADSL technology and its structure have many factors from its physical organization to its parameters defined in the standards. Table 2 exposes these problems/parameters and shows the advantages and disadvantages of them.. 21.