|Today’s networks simply were not built for multimedia and, in particular, for applications that involve video communications, multimedia collaboration, and/or interactive-rich media. Curiously, it is through the use of sophisticated computer applications and devices that we have been able to determine what the human information-processing model comprises: There is a very strong tendency for us to rely on the visual information stream for rapid absorption and longer retention. More than 50% of a human’s brain cells are devoted to processing visual information, and combined with the delights of sound, smell, and touchdespite our enormous dependence on the written wordwe’re very active in processing the cues from the physical world. By changing the cues, we can change the world. Digital-rich media, in every conceivable sort of formatincluding audio, animation, graphic, full-motion video, application, whiteboards, and communitieswill increasingly depend on multimedia. Video and multimedia applications require substantial bandwidth, as well as minimal latencies and losses. The table on the following page is a snapshot of the per-user bandwidth requirements for various services. Application Bandwidth Requirement E-mail and Web (not optimum support) 56Kbps Web as an always-on utility, crude hosted applications, 15-second video e-mail 500Kbps Hosted applications, reasonable videophone 5Mbps Massive multiplayer/multimedia communities 10Mbps Scalable NTSC/PAL-quality video 100Mbps Digital high-definition video-on-demand (uncompressed) 1Gbps Innovation applications (3D environments, holography, and so on) 10Gbps|
Digital video and digital audio also require minimal, predictable delays in packet transmission, which conventional shared-bandwidth, connectionless networks do not offer. (Chapter 3, “Establishing Communications Channels,” discusses connectionless networks in detail.) They also require tight controls over losses, and again, connectionless networks do not account for this. As more people simultaneously access files from a server, bandwidth becomes a significant issue. Correct timing, synchronization, and video picture quality are compromised if the bandwidth is not sufficient. As discussed in the following sections, two key issues relate to multimedia communications: the nature of digital video and the role of television. Digital Video One of the fascinating areas driving and motivating the need for broadband access is television. Although TV has a tremendous following throughout the worldmore than computing and even telecommunicationsit remained untouched by the digital revolution until recently. Despite major advances in computing, video, and communications technologies, TV has continued to rely on standards that are more than 55 years old. The biggest shortcoming with the existing TV standardsthat is, National Television Standards Committee (NTSC; used in North America and Japan), Phase Alternating Line (PAL; used throughout the majority of the world), and Systeme Electronique Couleur Avec Memoire (SECAM; used in France and French territories)is that they are analog systems, in which video signals degrade quickly under adverse conditions. Most of this signal degradation occurs along the path the picture travels from the studio to a TV. Digital TV (DTV) offers numerous advantages over the old analog TV signal, among which is the fact that it is nearly immune to interference and degradation. Another advantage of DTV is the ability to display a much better range of colors. The human eye can discriminate more than 16 million colors, and sophisticated computer monitors and DTVs can display those 16 million colors and more. DTV can transmit more data in the same amount of bandwidth, and it can also transmit more types of data. Combined with high-definition TV (HDTV) and digital sound, what this means to the end user is a better picture, better sound, and digital data. However, digital broadcasters are not restricted to just sending a high-definition picture; they can still broadcast a standard-definition picture over DTV, referred to as standard-definition TV (SDTV). But why would they want to do that? The answer is simple: In the same amount of bandwidth, they can deliver four standard-definition programs instead of only one high-definition program. But most importantly, digital technology is converting television from a mechanism that supports passive viewing to an interactive experiencean environment in which you choose when, where, and how you engage with the world at your disposal. Of course, you can still passively watch TV, but you can also customize the experience and make it your own. DTV is already offering us more choices, and it’s going to make our viewing experience even more interactive. People have only so much time and money to spend on electronic goods and services. In many parts of the world, the first thing people seem willing to spend their time and money on involves entertainment. Therefore, the television industry, as well as the content, entertainment, and application worlds, will be increasingly important to how the local loop develops and how this further demands the introduction of home area networking. Of course, TV and networks will deliver more than entertainment. They will deliver edutainment and infotainment, too, and the presentation of the information and knowledge you need will be in a format that is palatable and ensures assimilation and retention on a rapid and effective basis. Video and multimedia facilitate our ability to understand and retain information and therefore will become the basis of much information delivery. This will drive the need for more bandwidth not just to the home but also within the home, to network the growing variety of computing and entertainment systems. Very importantly, it will also drive the need to deliver programming and content on a mobile basisyet another argument for fixed-mobile convergence (FMC)and fuel the broadband wireless arena to new generations of wireless technology and spectrum utilization. What is required to carry a digitized stream to today’s TVs? In the North American system, a 6MHz NTSC channel requires approximately 160Mbps; a digitized PAL stream, used throughout Europe, requires about 190Mbps (the PAL system uses an 8MHz channel); and HDTV requires 1.5Gbps. Videoconferencing needs much less bandwidth than TV, but it still requires a substantial amount; the H.323 standard from the ITU allows videoconferencing to be carried at bandwidths ranging from 384Kbps to 1.5Mbps. Streaming video requirements vary, depending on the quality: Low quality requires 3Mbps, medium quality requires 5Mbps, and high quality requires 7Mbps. An important driver behind broadband access is content, and much of the content for which people are willing to pay is entertainment oriented. The television industry is now beginning to undergo the revolution that digital technology has caused in other communications-related industries, and it is now starting to capitalize on the potential new revenue-generating services that personal digital manipulation may allow. One example is digital video recording (DVR), also called personal video recording (PVR), in which television programs are digitally recorded onto a hard disk, letting viewers pause live TV, watch programs on their own schedule, and even skip commercials. With the introduction of DTV and the mandate by spectrum management agencies to phase out or decommission analog broadcasting, we will need a much greater amount of bandwidth in our homes to feed the new generations of televisions. In terms of information transfer, television has generally been associated with the concept of broadcast, terrestrial or satellite, or cable delivery of someone else’s programming on someone else’s timetable. Video is associated with the ability to record, edit, or view programming on demand, according to your own timetable and needs. Multimedia promises to expand the role of video-enabled communications, ultimately effecting a telecultural shift, with the introduction of interactive television. Before we begin our detailed discussion of video compression and DTV standards, it makes sense to provide a brief explanation of the key parameters that determine not only the viewing experience but the bandwidth required: Number of pixels on a screen A pixel (which is short for picture element) is one of the thousands of small rectangular dots that comprise television and computer screen images. Basically, the more pixels per screen, the greater, or better, the resolutionthat is, the more defined, detailed, and crisp the image appears. Frame rate The frame rate is a measure of how fluid or natural the motion onscreen appears. As a quick reference, motion pictures use 24 frames per second (fps), the North American NTSC television standard uses 30fps, and the European PAL standard uses 25fps. (Television standards are discussed later in this chapter.) Number of bits per pixel The number of bits per pixel is a measure of the color depth; the more bits per pixel, the more colors can be represented. Remember that the human eye can perceive more than 16 million colors. A digitally encoded image, using 24 bits per pixel, can display more than 16 million colors, providing a rich and natural experience. As we talk about compression techniques and digital television standards, you will notice that most of the standards define the number of pixels and frames per second that can be supported. Video Compression To make the most of bandwidth, it is necessary to apply compression to video. Full-motion digital video needs as much compression as possible in order to fit into the precious spectrum allocated to television and wireless communications, not to mention to fit on most standard storage devices. Moving Picture Experts Group (MPEG; www.chiariglione.org/mpeg) is a working group of the International Organization for Standardization (ISO; www.iso.ch) and the International Electrotechnical Commission (IEC; www.iec.ch) that is in charge of developing standards for coded representation of digital audio and video. It has created the MPEG compression algorithm, which reduces redundant information in images. One distinguishing characteristic of MPEG compression is that it is asymmetric: A lot of work occurs on the compression side, and very little occurs on the decompression side. It is offline versus real-time compression. Offline allows 80:1 or 400:1 compression ratios, so it takes 80 or 400 times longer to compress than to decompress. Currently, MPEG-2 generally involves a compression ratio of 55:1, which means it can take almost an hour to compress 1 minute of video. The advantage of this asymmetrical approach is that digital movies compressed using MPEG run faster and take up less space. There are several MPEG standards, in various stages of development and completion, and with different targeted uses. The following are some of the most common MPEG standards: MPEG-1 MPEG-1 is a standard for storage and retrieval of moving pictures and audio on storage media. MPEG-1 is the standard on which such products as Video CD and MP3 are based. MPEG-1 addresses VHS-quality images with a 1.5Mbps data rate. MPEG-1 can play back from a single-speed CD-ROM player (150Kbps or 1.2Mbps) at 352 x 240 (i.e., quarter-screen) resolution at 30fps. MPEG-2 MPEG-2 is the standard on which such products as DTV set-top boxes and DVD are based, and at this point, it is the compression scheme of choice. It addresses DTV- or computer-quality images. MPEG-2 carries compressed broadcast NTSC at a 2Mbps to 3Mbps data rate, broadcast PAL at 4Mbps to 6Mbps, broadcast HDTV at 10Mbps to 12Mbps, and professional HDTV at 32Mbps to 40Mbps. MPEG-2 supports both interlaced and progressive-scan video streams. (Interlaced and progressive-scan techniques are discussed later in this chapter.) MPEG-2 on DVD and Digital Video Broadcasting (DVB) offers resolutions of 720 x 480 and 1,280 x 720 at up to 30fps, with full CD-quality audio. On MPEG-2 over Advanced Television Systems Committee (ATSC), MPEG-2 also supports resolutions of 1,920 x 1,080 and frame or field rates of up to 60fps. MPEG-4 MPEG-4 is a standard for multimedia applications. MPEG-4, an evolution of MPEG-2, features audio, video, and systems layers and offers variable-bit-rate encoding for both narrowband and broadband delivery in a single file. It also uses an object-based compression method, rather than MPEG-2’s frame-based compression. MPEG-4 enables objectssuch as two-dimensional or three-dimensional video objects, text, graphics, and soundto be manipulated and made interactive through Web-like hyperlinks and/or multimedia triggers. The best feature of MPEG-4 is that the RealNetworks players, Microsoft Windows Media Player, and Apple QuickTime all support MPEG-4. MPEG-4 is intended to expand the scope of audio/visual content to include simultaneous use of both stored and real-time components, plus distribution from and to multiple endpoints, and also to enable the reuse of both content and processes. MPEG-4 Advanced Video Compression (AVC) MPEG-4 AVC, also called Part 10 or ITU H.264, is a digital video codec standard noted for achieving very high data compression. It is the result of a collaborative partnership effort between the ITU Video Coding Experts Group (VCEG) and the ISO/IEC MPEG known as the Joint Video Team (JVT). AVC contains a number of new features that allow it to compress video much more effectively than older standards and to provide more flexibility for application to a wide variety of network environments. H.264 can often perform radically better than MPEG-2 video compression, typically achieving the same quality at half the bit rate or less. It is planned to be included as a mandatory player feature in an enormous variety of implementations and standards. MPEG-7 MPEG-7 is a multimedia content description standard for information searching. Thus, it is not a standard that deals with the actual encoding of moving pictures and audio, like MPEG-1, MPEG-2, and MPEG-4. It uses XML to store metadata and can be attached to timecodes in order to tag particular events, or, for example, to synchronize lyrics to a song. MPEG-21 Today, many elements are involved in building an infrastructure for the delivery and consumption of multimedia content. However, there is no big picture to describe how these elements relate to each other. MPEG-21 was created to provide a framework for the all-electronic creation, production, delivery, and trade of content. Within the framework, we can use the other MPEG standards, where appropriate. The basic architectural concept in MPEG-21 is the digital item. Digital items are structured digital objects, including a standard representation and identification, as well as metadata. Basically, a digital item is a combination of resources (e.g., videos, audio tracks, images), metadata (such as MPEG-7 descriptors), and structure (describing the relationship between resources). MPEG-1, MPEG-2, and MPEG-4 are primarily concerned with the coding of audio/visual content, whereas MPEG-7 is concerned with providing descriptions of multimedia content, and MPEG-21 enables content to be created, produced, delivered, and traded entirely electronically. Faster compression techniques using fractal geometry and artificial intelligence are being developed and could theoretically achieve compression ratios of 2,500:1. Implemented in silicon, this would enable full-screen, NTSC-quality video that could be deliverable not only over a LAN but also over the traditional PSTN as well as wireless networks. Until better compression schemes are developed, we have standardized on MPEG-2, which takes advantage of how the eye perceives color variations and motion. Inside each frame, an MPEG-2 encoder records just enough detail to make it look like nothing is missing. The encoder also compares adjacent frames and records only the sections of the picture that have moved or changed. If only a small section of the picture changes, the MPEG-2 encoder changes only that area and leaves the rest of the picture unchanged. On the next frame in the video, only that section of the picture is changed. MPEG-2 does have some problems, but it is a good compression scheme, and it is already an industry standard for digital video for DVDs and some satellite television services. One problem with MPEG-2 is that it is a lossy compression method. This means that a higher compression rate results in a poorer picture. There’s some loss in picture quality between a digital video camera and what you see on your TV. However, MPEG-2 quality is still a lot better than the average NTSC or PAL image. By applying MPEG-2 encoding to NTSC, we can reduce the bandwidth required. Another important video compression technique is Windows Media 9 (WM9). The WM9 series codec standard, implemented by Microsoft as Windows Media Video (WMV) 9 Advanced Profile, is based on the VC-1 video codec specification currently being standardized by the Society of Motion Picture and Television Engineers (SMPTE; www.smpte.org) and provides for high-quality video for streaming and downloading. By making use of improved techniques, VC-1 decodes high-definition video twice as fast as the H.264 standard while offering two to three times better compression than MPEG-2. WM9 is supported by a wide variety of players and devices. It supports a wide range of bit rates, including high-definition at one-half to one-third the bit rate of MPEG-2, as well as low-bit-rate Internet video delivered over a dialup modem. (More detailed information is available at www.microsoft.com/windows/windowsmedia/9series/codecs/video.aspx.) Even if we achieve the faster data rates that MPEG-2 offers, how many of us have 20Mbps pipes coming into our homes? A 1.5Mbps connection over DSL or cable modem cannot come close to carrying a 20Mbps DTV signal. Therefore, broadband access alternatives will shift over time. We will need more fiber, we will need that fiber closer to the home, and we will need much more sophisticated compression techniques that can allow us to make use of the even more limited wireless spectrum to carry information. We will also need to move forward with introducing new generations of wireless technologies geared toward the support of multimedia capacitiesa combination of intelligent spectrum use and highly effective compressionwith support for the requisite variable QoS environment and strong security features. We will also need better compression techniques.