Broadband Bringing Home the Bits (2002) / Chapter Skim
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Appendix A: Broadband Technologies
Appendix A: Broadband Technologies
Pages 243-295
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Appendixes
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HYBRID FIBER COAX TECHNOLOGY Coaxial Cable The foundation upon hybrid fiber coax (HFC) broadband communications networks are based is coaxial cable (Figure A.1)
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. _ Two-Way Radio 216 - 470 MHz 79 Analog TV Channels ~ 5-40 MHz ~ ~ 54-550 MHz FIGURE A.2 Typical RF spectrum for analog cable television.
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Conceptually, coaxial cable provides cable operators with a private conduit through which RF signals are transported; in addition, the medium can support multiple signaling channels without regard to the baseband signals or modulation scheme that may be employed. This medium is generally immune to interfering influences that may exist in free space.
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Additionally, for a given design bandwidth, there were practical and theoretical limits to the number of amplifiers that could be cascaded. In order to maintain acceptable performance levels, it was necessary to limit the operational bandwidth of such cable systems to a few hundred megahertz, far below the potential of the cable alone.
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end. The cable industry quickly adopted this technology for a portion of its transmission plant, and continues to use it as a way to cost-effectively transform coaxial tree-and-branch systems into something much more powerful hybrid fiber coax (HFC)
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250 {6'~ I -_ Optical node HEAD ENE Optical cable FIGURE A.7 HFC networks allow smaller serving areas. Master head end APPENDIX A Typically 6 or fewer amplifiers in cascade an.
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The ability to assign and reassign spectrum to different uses is an important benefit of HFC architecture, because it allows for advances in digital services and technologies while continuing to support existing services. Thus, the architecture can simultaneously support many separate virtual networks.
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As nodes are divided and fiber is deployed closer to the customer, the total amount of usable bandwidth becomes greater; this makes it possible for every node division to more than double the available data capacity while reducing the number of users who share it.2 Similarly, breaking a 500-home node into four parts, each passing an average of 125 homes, increases the available reverse and forward capacities significantly more than fourfold and provides more than four times the bandwidth per user. Trials within the industry have made use of the spectrum from 900 MHz to 1 GHz (as compared with the traditional use of the 5- to 50-MHz region)
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The cable television industry will probably always carry some amount of NTSC signals, perhaps 20 or so RF channels; but it is anticipated that the number of these signals will decrease as most of them are incorporated into compressed digital formats. · Digital standard definition television (SDTV)
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10-Mbps data rates 3 channels (18 MHz) 100-Mbps data rates 20 channels (120 MHz)
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DIGITAL SUBSCRIBER LINE Introduction Digital subscriber line (DSL) service provides high-bit-rate digital service over ordinary phone lines, allowing from 100 kbps to tens of mega
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There are 500 million voiceband modems in existence today, most of which are used at speeds to 56 kbps to provide digital connection between various service providers and customers or to transfer data and facsimiles. Voiceband modems are limited in speed because the signals must traverse telephone company switches that allocate only 64 kbps maximum (of which 56 kbps are available)
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Generally, higher DSL data rates occur on shorter phone lines. As phone companies can afford the time and money to install fiber into more of their network, copper phone lines reduce in length.
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VDSL, the latest of the DSLs, can carry up to 60 Mbps on a single phone line and is in early trial and standardization phases. VDSL presumes some use of fiber to shorten phone-line lengths, consistent with eventual migration to fiber by phone companies.
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DSL Architectures There are almost 1 billion phone lines worldwide. The telephone lines are twisted pairs of copper wires, with the twisting invented by A.G.
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Such fiber is expensive, but the cost of labor, digging, and so on can be shared over a greater number of customers in the feeder segment, making certain upgrades economical. At the distribution point, splicing and connection to smaller cables containing fewer phone lines occurs, and those cables run through the "distribution plant" to pedestals or cabinets within a neighborhood where I Inside wire L: Central office equipment Main , distributing I frame ' I T 20,000 to 1 60,000 ..
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One can thus expect the achievable data rates to be the lowest for DSLs in the United States. Italy, Germany, and Sweden, for instance, are excellent candidates for higherspeed DSL service because a large fraction of their loops are within a kilometer or two of the central office.
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A DSL access multiplexer (DSLAM) resides at the telephone company side of the twisted pair.
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An alternate service provider must be given fair and equal access to the phone lines of the service provider's customers. Customer Premises Figure A.17 illustrates the customer premises end of a DSL connection.
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However, the DSL signals that traverse the much longer path from central office to customer need a high degree of sophistication to achieve the data rates desired in DSL. DSL Transmission Environment The DSL transmission environment is challenging, and should not be underestimated.
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Characterization of Twisted-Pair Telephone Lines Chapter 4 of Cioffi et al.7 details the calculation of the frequencyresponse of phone lines, which are often described by their "insertion loss." The insertion loss is measured in decibels (10 times the base-10logarithm) of the ratio of the power injected into a phone line at any given frequency to the power emanating at the end of the phone line at that same frequency.
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Werner.8) Figure A.l9 shows insertion loss characteristics of some shorter standardized loops,9 indicative of what might be used with VDSL over a yet wider bandwidth of 30 MHz.
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267 CO o x ~: > ~o cn cn o o .
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ADSL systems actually allow use of bandwidths up to 1,104 kHz, which would thus occur on lines that are shorter than those displayed in Figure A.l9, thus having less insertion loss at 1 MHz. Electromagnetically coupled noise occurs because the twisted pair is often bathed in radiation from a number of electronic sources.
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This wire is not twisted, and is much more susceptible to noise pickup; nonetheless, fortunately, this flat pair represents only a small segment of the total length of the phone line. Category 3 twisted pair, typically used by phone companies, has a few twists per inch.
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The factor of N represents the number of twisted pairs in the binder expected to carry crosstalking signals. This type of noise often dominates receiver noise when it exists.
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, (2) where d is the length of the line in feet and H is the insertion loss of the twisted pair.
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While smaller power is transmitted by ham radio operators, the ham antennae are distributed massively through residential environments, often being only 10 to lOOm away from a phone line. The level of interference is sometimes as large as -35 dBm/Hz, and typically on the order of -50 to -60 dBm/Hz, well above the levels of any other noise type.
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Unbundling and Standards Solutions The American National Standards Institute's TlEl.4 group has taken a lead role in discerning problems with crosstalk between various types of DSLs, standardized and nonstandard. The idea is that if all services comply with defined spectrum masks, coexistence of different service providers' equipment in the same cable of twisted pairs is possible without the transmission technique itself having to be standardized.
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The vertical axis plots data rate, while the horizontal axis plots line length in feet. As line length increases, all data rates decrease.
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FIGURE A.24 Current ADSL and projections. to be used on a twisted pair, which allows considerably higher data rates on shorter lines.
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276 50 Q 40 `:c 30 o APPENDIX A > Current ~ Cancel self-xtalk . E;~ ~ ~ ~ , Cancel all xtalk N;\ .
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This is the rate supported by the digital voice channels within and between the switches of the worldwide transmission network. Providing a DSL-based physical path on a copper loop will allow a user to transmit at high data rates over that loop.
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Use of DSL in very specialized environments such as college campuses, military bases, or condominium apartments. The Large Common Carrier A large local exchange telephone company may support more than 2 million telephone lines in a major metropolitan area.
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The service providers are also connected to the carrier's ATM network via similar high-speed trunks. The use of ATM allows for scalability of the service to support hundreds of thousands of users in a metropolitan area and a wide range of potential future ADSL services.
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The ADSL connections to the user support IP over HDLC directly on the ADSL physical layer.l4 The ADSL user appears as host directly on the service provider's IP network. WIRELESS Introduction Broadband wireless access is frequently mentioned as an important alternative to wired technologies, namely, to DSL, cable, and fiber.
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. It is expected that initial applications of broadband wireless access will start with fixed devices such as home PCs connecting to an ISP, with a gradual migration toward mobile applications as end-user devices become more and more portable.
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Physical Layer Broadband wireless networks require physical-layer bit rates that are orders of magnitude higher than those for current digital cellular or WLL systems, i.e., 10 to 100 Mbps versus the current 10 to 100 kbps. The higher bit rates must be achieved without introducing line-of-sight (LOS)
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283 in?
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, that is, 5 to 10 bps/Hz/cell versus the current 0.5 to 1 bps/Hz/cell. Clearly, if broadband wireless services are to reach significant penetration, cell sizes will have to be relatively small (~1- to 5km radius)
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· Cellular technology capable of scaling to small cells and multiple sectors necessary for effective coverage of areas with higher population density. Scaling of broadband wireless services to small cells is inevitable in areas with higher population density, where throughputs on the order of 100 Mbps to 1 Gbps/km2 must be achieved in order to serve even a modest fraction of the population.
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Radio Link Protocol Broadband wireless access requires a new type of radio link protocol (RLP) capable of reliably transporting both packets and media streams with specified QoS.
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This in turn permits wireless systems to operate in a higher C/I environment, thus increasing overall capacity of cellular networks. Infrastructure Network Broadband wireless access links are being designed as "plug-ins" to existing fixed network architectures based on IP and/or ATM.
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Ultimately, this technical direction will further accelerate fixed and wireless network convergence, which has been predicted for some time. MEDIA COMPRESSION Media signals include (digital)
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"Quality" refers to the quality of the signal after compression, measured in absolute terms or in terms of closeness to the original version of it. The "bit rate" is the data rate after compression.
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What is important, however, is to note that all compression algorithms are based on only two basic principles: removal of redundancy in the input signal, and the reduction of irrelevancy in it. "Redundancy" is usually characterized in a statistical fashion, while "irrelevancy" is best linked to a perceptual criterion.
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Interestingly, the geometric mean of this range is 300 kbps, a number typical of conservative ADSL and cable modem rates in the year 2000. The data rates in Figure A.30 are strict lower bounds in the sense that in most applications, the compressed information needs to be supplemented with ancillary data.
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Still continuous-tone images Addressable video on CD Videoconferencing Advanced TV Network transmission Bit-rate multiplexers, undersea cable Overload on undersea cable, data modem High overload rate for undersea cable Transmission at low delay Second-generation digital cellular Low-bit-rate videophone European digital cellular full-rate North American digital cellular-TDMA North American digital European digital cellular half-rate EVRC (1996) RCELP 8.5, 4, 0.8 kbps NA CDMA, 2nd generation IS-136 (1995)
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l ' .! 1 2 4 8 16 32 64 128 512 2 8 32 KILOBITS PER SECOND MEGABITS PER SECOND FIGURE A.30 Data rates in digital representations of signals.
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The overall effect of all of the above processes is that the data rate for digital communication is strictly higher than the data rates at the output of the signal compression stage. While there is no rigorous way of measuring the resulting overhead in data rate without regard to the application and the needs of it, it is useful to use the following guideline: Typical overall overheads are in the range of 10 to 100 percent, and the rates on the horizontal axis of the compression chart in Table A.4 need to be increased by factors as high as 2.0, especially in the case of unfriendly access methods such as wireless links that are power- and interference-limited and/or in networks that are operating in situations of overload.
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Research and Technology Outlook At this time, compression technologies are mature. Although it is difficult to define the fundamental limits in the game, typical data rates for specified levels of quality are generally known.
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