Broadband Bringing Home the Bits (2002) / Chapter Skim
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Pages 245-295
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From page 245... ...
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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From page 248... ...
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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Such devices had been in development for the digital market in an effort to achieve higher data-transmission speeds over optical fibers (in contrast to coaxial cable) , but further optimization was required for broadband applications.
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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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This makes the investment to upgrade to HFC a sustainable one for most cable companies. At least some cable operators plan to build as many as five separate (virtual)
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252 Fiber APPENDIX A HUB HFC plant 3 ~~ ~ it, = ~\ Mpeg video 1 ~ ~ ~ 1 I / f~ Switch data / I~U / 4, . ~ Traniport _ _ =~\—' ' ~~ ~, Or 'at ~~.~ /// / Analog I ~~ ;~%f~/ 'a 'I'm'' a' ~ "::::-:' ~u~aL=u 111 ~ head end FIGURE A.10 Capability to support multiple networks within HFC.
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and high-speed Internet access are examples of this type of service. The cable television (CATV)
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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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Several types of digital transmission on phone lines are shown for comparison. Generally, DSL today often really means ADSL, an asymmetric DSL service that can carry up to 8 Mbps downstream from a telephone company central office to a customer and up to 1.5 Mbps back upstream.
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In a short time, tens of millions of customers will be connected. ADSL service can now be ordered in nearly one-third of the United States, and telephone companies plan ubiquitous coverage in the near future.
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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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Arm .~= >| pOTS | Digital network modem ~ DSLAM Telephone company office FIGURE A.16 Telephone company central office and DSLAM.
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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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From page 266... ...
Notice the rippling of the insertion loss, corresponding to signal energy reflecting from the open-circuited extensions and returning later in time to the main line to add to the current signals there. At some frequencies, the reflected signals are 180 degrees out of phase and destroy the current signals, corresponding to the dips.
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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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From page 269... ...
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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From page 270... ...
When the insertion loss of the segment of wire between the coupling point in both directions is considered and the noise problem integrated over the total length of wire, basic physics leads to the standardized crosstalk coupling function for DSLs of PSDNExT(f ~ = (N/494 · 10 . f · PSDnear-en4,xmit~f )
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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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, control voltages to elevators (phone lines in apartment buildings often run through the elevator shafts) , and ringing of phones on lines sharing the same binder.
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mandates certain maximum levels of radiation in various frequency bands. In the case of DSL modems, the telephone line itself, while not inside the modem, does radiate, and so this type of radiation is typically limited by limiting the power spectral density of signals transmitted on phone lines.
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From page 274... ...
This particular technology will allow a number of solutions to the unbundling and crosstalk problems mentioned above; these solutions have yet to be implemented but are simulated here to allow an understanding of future research directions in this DSL area. As phone companies increasingly deploy fiber, telephone line lengths become shorter.
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From page 275... ...
Here, data rates on 1,000-ft loops, so-called pedestal drops, can exceed 250 Mbps with all possible and/or known improvements included. A last curve, "Ultra DSL," allows an increase of transmit power to 400 milliwatts, perceived as an analog limit for phone lines with VDSL parameters.
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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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The telco now needs not only to operate the tracks and trains but also to provide boxcars, tank cars, and transshipment between trains, ships, and planes. The carrier needs to address the following issues if it is to provide DSL services to its customers: · It must provide a data network connecting the ADSL terminated copper loops to the service providers desired by customers.
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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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can, under the terms of the Telecommunications Act of 1996, lease copper loops from the incumbent phone company (the incumbent local exchange carrier, or ILEC) and serve customers with ADSL.
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For both Internet and broadband services, wireless services have experienced a larger-than-expected gestation period owing to a combination of factors such as technology cost and performance problems, spectrum regulation barriers, and weak standards. However, wireless data and broadband Internet services seem poised for technical and market breakthroughs over the next 3 to 5 years, and should thus provide an important alternative for facilitation of broadband services in the United States and other parts of the world.
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, the focus for wireless systems will shift toward semimobile or mobile services, given that an increasing percentage of computing platforms will become inherently portable. It is noted that the wireless access network shown in Figure A.27 may be expected to interface with both the future public telephony network and the Internet, which are themselves experiencing some degree of convergence as they migrate toward broadband services.
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As shown in the figure, newer commercial or precommercial wireless technologies have reached the Mbps+ bitrate levels necessary for viable broadband services, either fixed or mobile. Figure A.29 shows the typical bit-rate and mobility regimes for various broadband wireless networks currently under consideration.
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283 in?
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radio modems based on advanced signal-processing techniques, such as equalization, spread spectrum, multicarrier modulation, spatial processing, and smart antennas. Examples include equalized QAM, used in several first-generation fixed wireless systems; equalized VSB in the U.S.
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· Spectrum regulation and management policies that facilitate rapid deployment of broadband services, while promoting efficient use. The pace of wireless network deployment is critically dependent on spectrum regulation policies, both international and domestic.
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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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For fixed wireless access, interface functions specific to the radio link are performed by the base station, which puts out standard IP and/or ATM data and control into the infrastructure network. For mobile scenarios, services (such as location management and handoff)
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advances as a function of time. In this view, advances in computing are much more rapid than are advances in access technologies.
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. With the evolution of flexible and scalable communications technology, one often has the option of considering input signals of higher bandwidth, as long as the compression is strong enough to delimit the output data rate to a specified number.
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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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From page 291... ...
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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From page 292... ...
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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In delay-sensitive communications, packet losses can be anticipated by redundancy in the packet generator. In sophisticated algorithms, such as embedded coding and multiple description coding, this redundancy is contained by unequal protection of subpackets, depending on models of perceptual importance of these subpackets, as in unequal bit error protection.
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From page 295... ...
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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