The Evolution of the U.S. Telecommunications Infrastructure Over the Next Decade

Transcription

1 The Evolution of the U.S. Telecommunications Infrastructure Over the Next Decade BROAD BANDWIDTH THROUGH DSL Technology Task Group (TTG-4) Reporter: McAdams Members: CIOFFI, TONG, TERRY, BLOOM As is well known, Digital Subscriber Line (DSL) service is delivered through modems 1 over standard copper, twisted-pair telephone wires. As of midyear 1999, the bandwidth usually supplied through ADSL (asymmetric digital subscriber line) is 1.5 megabits/sec from the central office (CO) to the customer and 640 kbps from the customer to the CO. Service at this speed is available within approximately 18,000 linear feet of a Telco's central office. ADSL, when used for high-speed connectivity for an individual customer premises, can simultaneously be used for POTS connectivity to the central office over the same copper wire pair. Most ILECs (Incumbent Local Exchange Carriers) and CLECs (Competitive Local Exchange Carriers) offer DSL service to customers, the latter through co-location with the former's central office facilities. DSL technology is available in several configurations symmetric and asymmetric, downstream bandwidth from 64 kbps to 52 Mbps, upstream bandwidth from 64 kbps to 6 Mbps, and with or without voice circuits. xdsl is usually employed to denote any product in the DSL category the products include ADSL (Asymmetric DSL), SDSL (Symmetric DSL), HDSL (High speed DSL), IDSL (ISDN DSL), and VDSL (Very high speed DSL). 1 A DSL modem is an all-digital device, as contrasted with the widely available and inexpensive analog dial-up modem. The analog modem translates the computer s digital signals into audio tones which can be carried on a switched telephone network. A digital modem establishes a permanent digital circuit between the home and the central office; over this circuit, any combination of services may be employed, up to the modem s bandwidth limit; external devices are required to convert analog signals (such as voice or video) to and from the DSL s digital signals. 1

2 Three very important characteristics of DSL technology are: (1) the user retains full POTS service, usually at no additional fee, and functionality is retained even in a power outage; (2) the user is provided with sole access to the DSL line; and (3) experts currently estimate that "DSL-lite," 2 asymmetric 1.5 megabit service, including a voice channel, in a robust and consumer-installable configuration, is available in COs serving approximately 70 percent of the US population. As yet, there are approximately 100,000 DSL circuits installed and operating in the US; nearly all of these are in business locations (business circuits tend to be on better-maintained and-documented wire and cable facilities); only a tiny proportion of these customers use residential DSL. The significance of (2) is that the full bandwidth of a DSL line is available to the user at all times. There is no contention among users for this line; it is not a "party line." These facts also have security implications, since it is unlikely that one will inadvertently receive a message intended for one's neighbor, and the only file sharing actively supported is by the customer devices attached to the modems. The circuit between the modems is reasonably secure from tampering and unauthorized entry. Most DSL technology is sold by manufacturers to LECs, and is not available for direct purchase by consumers or businesses. Many DSL installations today are capable of providing service at 6 megabits downstream with significant upstream bandwidth. In the not-too-distant future, it is expected that 6 megabit symmetric service will be readily available through DSL. This higher speed service 2 Also called G-Lite 2

3 is possible only over much shorter distances than is the 1.5 megabit service generally offered by ILECs and CLECs. The modem equipment currently provided for the 1.5 megabit service is capable of supporting data speeds of up to 8 megabits. Service at this speed can be delivered over the twisted pair, but as just noted, only for yet significantly shorter distances. The DSL Footprint If one were to create a diagram of a supplier's DSL service alternatives: 1.5, 6.0, and 8.0 Mb/s; one would see a series of concentric circles showing the areas within which each respective bandwidth would be available. For example, small circles around a LEC s central office, with circumference drawn of heavy black lines, might represent 8 megabit service; medium-size circles with medium black lines would represent 6 megabit service; and the largest circles with thin lines would represent 1.5 megabit service. The radii for these circles are on the order of 5,000, 11,000, and 18,000 cable-feet. In each instance, each respective service area can be extended to greater and greater distances from the CO by providing direct fiber connection from the CO to a remote node, called a feeder-distribution interface or "fiber distribution point," from which each respective service will then be available to more-distant customers. The remote node houses the "DSL optical network unit." Each such node then has the capability of serving customers at different speeds over circles of radius in inverse proportion to the increasing speed of the service delivered. This pattern of plant deployment exhibits other interesting characteristics. It is a pattern through which fiber is introduced into a region to permit access to customers at evergreater distances from the CO. It is a mechanism for extending the DSL, copper-based, high bandwidth service to more users located farther from the central office. Tactical Matters 3

4 One vestige of regulated service offering in the DSL arena (to date, and likely into the next few years) is that DSL service is provided by some LECs to individual residential users, but not to businesses. Where this is done, it is clearly a tactical move on the part of these network suppliers. For example, a user establishing DSL service in both directions over two normal copper pair is able to achieve a symmetric throughput equivalent to a T-1 line. Yet each DSL line is priced to approximately $60 per month--only a fraction of the price of a T-1. Furthermore, a T-1 can be subdivided into 24 standard POTS lines. Therefore, a small business using such a facility could service 24 individual phone lines through 2 such DSL connections. 3 The same lines available as a "business service" are priced much higher through existing tariffs. DSL service is generally higher-cost for the LEC to supply for the residential market compared to business customers. The current trend is for CLECs to supply most of the business DSL services, with ILECs serving the spoils of the consumer market. (Some analysts have forecast that CLEC circuit growth volume will surpass that of the ILECs on an annual basis by 2001.) ILEC margins for business DSL are low on a per-circuit basis, but compensated by the gross revenue from multiple-circuit sales. Some ILECs have appeared to be offering residential DSL as a loss-leader or regulatory sop to prove the existence of a competitive market, one of the quid pro quo items for their entry into longdistance service. Cross-Talk: The Threat to DSL Laboratory and field tests suggest that when the proportion of copper wire in a given wire bundle reaches 40 percent, cross-talk is likely to become a major problem for users of the service. The estimate, supra, that as of 1999 DSL-lite can be made available to 70 percent of the US population takes this 40 percent danger-point proportion into account. That is, 3 The business would also have to purchase, install, and maintain multiplexers at each end of the DSL circuit. 4

5 current estimates are that DSL-lite can be deployed to 70 percent of US population before the proportion of wires in any significant number of bundles reaches 40 percent of the wires in those bundles. The potential crosstalk problem is also mitigated somewhat by the installation of DSL equipment at the end of a feeder fiber-run, rather than solely in the CO. And at 40% utilization, it is more than economical to run fiber from the CO to the fiber node. Existing standards have been established with cognizance of, and adaptation to, the known limitations of potential cross-talk. One factor that helps to explain these numbers is that over their history, the ILECs in general, and the companies of the former Bell System, in particular, have installed a great deal of "dark copper." That is, it was frequently the case that, with each new active line installed, one or more inactive lines had also been deployed. This practice was standardized at an average of 1.1 to 1.3 pairs per dwelling unit, and was economically justified because the cost of installing the additional lines after the initial construction was much greater than the cost of the initial over supply. The number of available (active) pairs is further reduced by weather damage, moves and changes, backhoe-damage, and additional unanticipated growth. Further, many households today have multiple phone lines to their home, only one of which might be a DSL line. This also reduces the proportion of DSLenabled wires in a bundle. Implications of Higher Bandwidth Services The picture changes radically as the bandwidth supplied to the user is increased. The likelihood of crosstalk in the presence of multiple users at even the 6 megabit rate is more than proportionally increased. This is one of the reasons why higher speed services have not been widely installed at the present time by many Telcos. US West is a clear exception. It is a leader in supplying high-speed DSL with more than five thousand current customers who are using 25 4 megabit service. 4 September 15, U S West is one of the few companies deploying the VDSL technology, which requires special line conditioning, but can provide compelling service at reasonable prices. 5

6 The likelihood of cross-talk is substantially reduced for and by installations attached to fiber distribution points. While the engineering design rules call for every POTS line to be directly copper-connected back to the central office, the volume of such copper becomes rather substantial and impractical as the area served by the CO increases. When a fiber connection-point is deployed, a single strand of fiber to-and-from the node replaces what traditionally would have been potentially several thousand return-copper lines attached to this remote, "mock-central office." There is an additional characteristic to the pattern implicit in the deployment of fiber distribution points. In these instances, fiber is used to deploy DSL connectivity "out of the neighborhood, and away from the central office". To the extent that central offices are geographically central to population clusters, fiber is being "deployed further away from the curb" -- at least for DSL-lite installations. The opposite is true for the higher and higher bandwidth services. For service deployments of increasing bandwidth, fiber to the remote distribution point must be introduced in greater and greater proximity to the central office, but the delivery medium remains copper. An intermediate step could well be fiber to the curb (or pole) with wireless from the pole to a small group of homes. For the truly high bandwidth services, this technology would be approaching an all-fiber solution. At bandwidths up to 25 Megabits and above, it would appear that direct fiber to the user could become the preferred distribution mechanism. However, the probability is small that ILECs would deploy such facilities until the probability of a given subscriber density (i.e., customers per length of plant) becomes much greater, more stable, and more predictable than seen today. The experience of US West suggests that DSL services in the range of 25 megabits might be widely implemented throughout the country in comparable technical and market conditions. US West has also just announced dial-up ADSL to be widely available for as little as $19.95/mo. 6

7 Currently, the equipment to support each level of bandwidth is being supplied by approximately 20 manufacturers. The modems in use for DSL are generally manufactured by Alcatel. This European manufacturer has seized a leading position as a supplier of "xdsl" technologies. Home Gateway Products An innovation likely in the next three years will be "home gateway" products. A host of such products already exists, some using the existing copper telephone wire inside the home, some using the house-power wiring, others using cable TV-type coaxial cable, some using Ethernet-type unshielded twisted pair, and still others using a range of radio solutions, both RF and infrared. At the core, however, the product is a router in the home that will interconnect multiple devices in the home. The router may connect to the phone-jack. A splitter might connect one group of products to the router, while simultaneously connecting TV and other devices within the home. Compaq Computer currently offers a product of this kind for its desktop computers connecting through an 8 Megabit LAN. Wireless Perceived as Increasing Threat to Profitable Deployment of DSL As perceived by the DSL community, 2-way, low-power wireless technologies are approximately 3 years behind development in DSL. Progress in the wireless arena is moving quite rapidly, however. Spatial reuse approaches appear to make possible higher and higher bandwidths, even in 2-way wireless. Wireless CDMA in portions of the PCS band also offer promise for low-cost, reliable circuits. Much like the DSL circuits, these mini-networks could be connected to a neighborhood fiber node for transit to the CO. Some cable companies have reviewed this strategy for connection to their fiber nodes. Glossary 7

8 ADSL AFN AML analog: ATM AWGN bandwidth: Baseband Baud BER broadband: BW/WL Byte CAP Asymmetric Digital Subscriber Line simultaneously transports high-bit-rate digital information towards the subscriber, lower rate data from the customer, and analog voice via one twisted-wire-pair. all-fiber network added main line (in telephony); Amplitude Modulated Link (in cable communications) Pertaining to signals in the form of continuously variable physical quantities. An analog channel can transmit information of any value between the limits defined by the channel. asynchronous transfer mode additive white Gaussian noise. A measure of the information-carrying capacity of a communication channel. The bandwidth corresponds to the difference between the lowest and highest frequency signal, which can be carried on a cable television system. Transmission using signal with carrier frequency around zero; The band of frequencies occupied by the signal in a carrier wire or radio transmission system before it modulates the carrier frequency to form the transmitted line or radio signal. A symbol period. Bit error rate. The probability that a bit is detected erroneously. A general term used to described wide bandwidth equipment or systems which can carry a large proportion of the electromagnetic spectrum. A broadband system able to deliver multiple channels and/or services to users or subscribers. Bandwidth per wavelength A group of eight bits Carrierless AM/PM modulation. Similar to QAM. It uses a single carrier, in contrast to DMT. 8

9 carrier: CATV: channels: CLEC coax cable: Crosstalk CSA DFT digital: diplex filter: DLC DMT DS0 DS3 An electromagnetic wave of which some characteristics is varied in order to convey information Community Antenna Television. A broadband communication system capable of delivering multiple channels of programming from a set of centralized antennas, generally by coaxial cable, to a community. A signal path of specified bandwidth for conveying information. In cable television the common bandwidth for an analog channel is 6 MHz. competitive local exchange carrier A type of cable used for broadband data and cable systems. Composed of a center conductor, insulating dielectric, conductive shield, and optional protective covering, this type of cable has excellent broadband frequency characteristics, noise immunity and physical durability. Synonymous with coax. The unintentional coupling of signals from different transmission lines. See NEXT and FEXT. Carrier serving area. Discrete Fourier Transform. A discrete form of Fourier transform that can be implemented efficiently using the fast Fourier transform (FFT). Descriptive of any process that uses discrete levels (usually 0 and 1) to represent characters or numbers. A device that provides signal branching on a frequency division basis. This circuit selects the frequency of desired channels, commonly used in a bidirection cable television system. digital loop carrier (also, derived local channel ) Discrete Multi-Tone. A modulation technique that transmits simultaneously several narrow band QAM signals. The number of bits in each carrier can be different depending on the condition of the transmission line. A 56 or 64 kbps full duplex service; a digitized voice channel. A 45 Mbps full duplex service. 9

10 DSL DSLAM DWDM E1 EMI Equalizer F/T FDD FEC FEXT FFT frequency: FTTB FTTC FTTD FTTH gigahertz (GHz): Digital Subscriber Line. digital subscriber line access multiplexer dense wavelength-division multiplexing European Mbps symmetrical transmission digital carrier system; conceptually analogous to the US T-1 channel (1.544 Mbps); carries 32 voice channels electromagnetic interference A filter that minimizes channel distortion. fibers per trunk Frequency division duplex. Two way transmission via two separate frequency bands. Forward Error Correction. Far-End Crosstalk. Unintended coupling of signal from far end. fast Fourier transform The number of complete alterations of a sound or radio wave in a second, measured in Hertz. One Hertz equals one cycle per second. fiber to the bridger (cable television networks); fiber to the building (telephony/telecommunications networks) fiber to the curb fiber to the desktop Fiber to the home One billion (10^9) cycles per second HDSL High-bit-rate Digital Subscriber Line. Permits (in the US) or (in Europe) Mbps symmetric transmission over two or three wire pairs. HDTV: headend: High Definition Television The control center of a cable television system, where incoming signals are amplified, converted, processed and combined into a common cable along with any origination cablecasting, for transmission to subscribers. 10

11 System usually includes antennas, preamplifiers, frequency converters, demodulators, modulators, processors and other related equipment. HFC: IDSL ILEC Impulse Noise kbps Mbps Hybrid-Fiber-Coax network. A network consisting of a combination fiber optic and coax distribution systems. ISDN digital subscriber line incumbent local exchange carrier Noise with short duration. kilobits (thousands of bits) per second. Megabits (millions of bits) per second megabits per second (Mbp/s): A unit of measure equal to 8 bits equal a byte, 8 million bits equal a megabyte. megahertz (MHz): NEXT nodes: NTU PEG Channels PON PON PSTN QAM QoS RADSL RFI SDSL SDTV: One million (10^6) cycles per second Near-end crosstalk. Unintended coupling of signal from near end. For networks, a branching or exchange point. The transistion point from an optical to electrical distribution network. network termination unit public, education, and government access channels in a cable system passive optical network passive optical network Public Switched Telephone Network. Quadrature Amplitude Modulation. A passband modulation technique in which each transmitted symbol is represented by its amplitude and phase. quality of service rate-adaptive DSL radio frequency interference symmetric digital subscriber line Standard Definition Television 11

12 signal-processing: SLC SNR SONET A process of converting incoming signals to an intermediate frequency for filtering, level control, and other processing, then reconverts to a desired output frequency. subscriber loop carrier Signal to noise ratio. synchronous optical network T-1 a digital circuit with Mbps symmetric capacity, usually provisioned using two or three wire pairs T-1c VDSL WL/F xdsl XTLK a channelized T-1 circuit divided into 24 voice channels (DSOs) Very-high bit rate Digital Subscriber Line. VDSL systems transmit at 12 to 52 Mbps. wavelengths per fiber A generic term for all DSL technologies. Crosstalk 12

13 REFERENCES Recent Books W. Chen, DSL: Simulation Techniques and Standards Development for Digtal Subscriber Line Systems, Macmillan Technology Series, Howard Hecht, John Freeman, Marlis Humphrey, DSL: ADSL, RADSL, SDSL, HDSL, and VDSL, McGraw-Hill, T. Starr, J.M. Cioffi and P. J. Silverman, Understanding Digital Subscriber Line Technology, Prentice Hall, Tutorial J. Cioffi, symmetric Digital Subscriber Lines, Chapter 34 in Communications Handbook, J.D. Gibson Ed., CRC Press, J.J. Werner, The HDSL Environment, IEEE J. Select. Areas Comm., vol. 9, no. 8, Czajkowski, High-speed copper access: A tutorial overview, Elec. And Comm. Eng. J., vol. 11, no. 3, 1999, pp Web Resources: ADSL Forum: Find out what is going on in the industry. Download technical reports. Everythingdsl.com: Find more glossary and references here. 13