The Evolution of the U.S. Telecommunications Infrastructure Over the Next Decade BROAD BANDWIDTH THROUGH DSL TTG4: McAdams, Cioffi, Bloom, Hargadon Digital Subscriber Line (DSL) telecommunication 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) was 1.5 Mbps from the central office (CO) to the customer and 640 kbps from the customer to the CO. Service at this speed is available as long as the length of the copper wire does not exceed approximately 18,000 linear feet. ADSL, when used for high-speed connectivity for an individual customer's premises, can simultaneously be used for POTS (plain old telephone service) connectivity to the CO over the same copper 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 CO facilities. A general depiction of the DSL architecture is shown in Figure 1. 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. The term xdsl is usually employed to denote any of the technologies in the DSL category this includes 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 dialup modem. The analog modem translates the computer s digital signals into voice-band audio tones which can be carried on a switched telephone network. By contrast, a digital modem establishes a permanent digital circuit between the home and the central office; over this circuit, any combination of communication services may be employed, as long as they do not exceed the modem s bandwidth limit; devices external to the DSL circuit are required to convert analog signals (such as voice or video) to and from the DSL s digital signals. LaymanDSL.doc 1
Figure 1: Generalized DSL Architecture 2 The precise data rate which can be delivered by any DSL circuit depends on the length of the copper telephone wire between the customer and a network terminating unit (NTU). The NTU may be located at the local telephone company s CO or at a remote electronics site. (In Figure 1, the DSLAM acts as the NTU.) The copper telephone wire is usually the standard, copper, twisted pair telephone lines, 26 AWG at termination, without special conditioning or electronics. Three very important characteristics of DSL technology are: 1. For most DSL services, the user may retain full POTS connectivity, usually at no additional fee, and, as with POTS generally, functionality is retained even in a power outage; this is particularly the case with the DSL.Lite 3 technology, which has been designed as a residential consumer service; 4 2 3 4 Pacific Bell, 1997 Also called G-Lite The original DSL.Lite specification called for implementation at the residence without a splitter (bandpass filter) between the voice and data circuits. This would have been easy to install, but would have removed a voice channel and given little back in return. Although DSL.Lite is not as popular in 1999 among the LECs as other DSL solutions, the DSL.Lite development work did yield microfilter technology which has proven useful to other DSL approaches in minimizing certain types of noise problems at the customer s location. LaymanDSL.doc 2
2. The user is provided with sole access to the DSL circuit; and 3. Experts estimate that some version of asymmetric 1.5 Mbps service, including a voice channel, in a robust and consumer-installable configuration, could be installed in COs serving approximately 70 percent of the US population as of the year 2000. 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, other things being equal, one will inadvertently receive a message intended for one's neighbor, and the only file sharing actively supported is by the devices attached to the modems at each end. The circuit between the modems is a modulated carrier, and thus subject to unauthorized interception, as are all modulated signals; however, the modulation and signaling patterns are reasonably complex and of moderate amplitude, and the wire itself is usually buried, in conduit, or high on utility poles, rendering the circuit reasonably secure from tampering and unauthorized entry. Additional security precautions may be taken, if desired, at the NTU, the customer premises equipment (e.g., by the use of a firewall), and/or the DSLAM at the CO. Most DSL is sold by manufacturers to LECs, and is not generally available as a technology for direct purchase by consumers or businesses. Because the DSL service usually employs copper pairs which are pre-existing in the LEC s Distribution Network and on the customer s premises, existing Operations Support Systems (OSS) are usually adequate to enable the telecommunications service provider to set up the circuit and activate the account in a matter of a few minutes to an hour. Many of the existing DSL installations are capable of providing service at 6 Mbps downstream with significant upstream bandwidth. The modem equipment currently provided for the 1.5 Mbps service is capable of supporting data speeds up to 8 Mbps. By the year 2010, it is expected that 6 Mbps symmetric service will be readily available through DSL. This higherspeed service is possible today only over much shorter distances than the 1.5 Mbps service generally offered by ILECs and CLECs. As noted above, the furthest practical distance from customer to NTU using existing technology is about 18,000 feet of wire. The potentially available asymmetric downstream capacity is inversely related to the distance; e.g., 1.5 Mbps LaymanDSL.doc 3
downstream at 18,000 feet, 26 Mbps at 3,000 feet, or 52 Mbps at 1,000 feet. 5 (Although the performance curve is continuous, the available data rates are generally not, because equipment is available from the vendor community only at specific data rates.) A majority of the customers with access directly from the CO will probably be 7,000 cable-feet or more away from the NTU and thus will have a maximum of 8 Mbps downstream capacity. A very high percentage of American businesses are within 10,000 cable-feet of a CO, and thus may have access to DSL circuits with bandwidth of 8 Mbps or greater. In order to extend the availability of DSL service to customers, many LECs have begun to build DSL circuits using fiber Feeder Network and digital loop carrier circuits from the CO, and placing Figure 2: Typical application of digital loop fiber to support DSL at long distances from the CO RESIDENCE Phone Distance could be up to 25,000 feet, though optimally (for DSL) less than about 5,000 feet Fiber cable from CO to the O/E CENTRAL OFFICE Main Distributing Frame Splitter Network Termination Fiber Mux and O/E Fiber Mux and O/E DSLAM Modem ISP/CLEC Network PC Metallic cable from O/E to the home; Fiber Node and NTU probably located to serve one or a few neighborhoods; this will minimize the length of the metallic cable and make it possible to maximize the DSL bandwidth Distance could be several miles, though typically less than 30,000 feet 5 These distances are approximate, and will vary due to actual field conditions, presence of bridge tap, splices, age of the plant, construction quality, local weather, inter alia. LaymanDSL.doc 4
the fiber node and NTU at locations that are close enough to the customer s premises to permit higher-bandwidth DSL circuits over the last few hundred or few thousand feet of copper wire. Figure 2 shows one approach to such an installation. Cross-Talk: A Threat to DSL The potential cross-talk problem that exists for DSL has not been significant for ADSL. The picture changes radically as the bandwidth supplied to the user is increased. The likelihood of cross-talk in the presence of multiple users at even the 6 Mbps rate is more than proportionally greater. This is one of the reasons why higher speed services had not been widely installed by many LECs by the end of 1999. As noted above, for service deployments of increasing bandwidth, fiber to the remote distribution point (i.e., the NTU) may be introduced into circuits which are increasingly-close to the CO. The effect is to increase the bandwidth available to the customer without having to rebuild the plant that enters the customer s premises; that is, the entry facility remains narrowgauge copper wire. The likelihood of cross-talk is substantially reduced for and by installations attached to such fiber distribution points. When a fiber connection-point is deployed, a single strand of fiber each way, to-and-from the node replaces what potentially would have been several thousand return-copper lines attached to this remote node. Implications of Higher Bandwidth Services Some urban areas in America, have much shorter local loop lengths than the US average, and thus the capacity to deliver VDSL (within 3,000 feet) directly from the CO. This is also true for newer communities in the US with both suburban new-builds and urban redevelopment areas. These latter tend to be built either close to a major CO or with new wire and cable technology, either of which will make it easier for customers to receive DSL service in excess of 1.5 Mbps. Overseas, countries and companies have different development patterns. For example, Deutsche Telekom utilizes technology from Orckit and Fujitsu to provide VDSL to its customers who reside within 3,000 feet of a central office; this represents approximately 85 percent of DT s customer base. LaymanDSL.doc 5
In the US, the experience of US West suggests that DSL services in the range of 25 Mbps 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. DSL implementations work best when they push fiber deep into the architecture (i.e., toward the customer) with much higher per-end-user capacity and many fewer technical problems with noise and cross-talk. It can be expected that by 2005 and beyond, fiber will be at the neighborhood level for all DSL technologies; it will certainly be within 3,000 feet, and probably less for many subscribers. Later in the decade, we expect to see substantial fiber penetration into the outside plant (cf., Figure 2, supra) in order to keep copper-wire based implementations within DSL s theoretical limits; i.e., up to 26 Mbps per line, symmetric, and within 1,000 feet. Recent Books REFERENCES Chen, W., DSL: Simulation Techniques and Standards Development for Digital Subscriber Line Systems, Macmillan Technology Series, 1999. Hecht, Howard, John Freeman, and Marlis Humphrey, DSL: ADSL, RADSL, SDSL, HDSL, and VDSL, McGraw-Hill, 1999. Starr, T., John M. Cioffi, and P. J. Silverman, Understanding Digital Subscriber Line Technology, Prentice Hall, 1999. Tutorials Cioffi, John M., Symmetric Digital Subscriber Lines, Chapter 34 in Communications Handbook, J. D. Gibson, Ed., CRC Press, 1997. Werner, J. J., The HDSL Environment, IEEE J. Select. Areas Comm., vol. 9, no. 8, 1991. Czajkowski, High-speed copper access: A tutorial overview, Elec. And Comm. Eng. J., vol. 11, no. 3, 1999, pp. 125-148 Web Resources: ADSL Forum: http://www.adsl.com. Find out what is going on in the industry. Download technical reports. Everythingdsl.com: http://www.everythingdsl.com Find more glossary and references here. TeleChoice: http://www.telechoice.com Leading industry analyst on DSL service and technology LaymanDSL.doc 6