02/15/01 1 T1E1.4/ Contact: J. Cioffi, Dept of EE, Stanford U., Stanford, CA , F:

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1 02/15/01 1 T1E1.4/ Project: T1E1.4: Spectrum Management II Title: Unbundled DSL Evolution (088) Contact: J. Cioffi, Dept of EE, Stanford U., Stanford, CA cioffi@stanford.edu, , F: Date: Dist'n: February 19, 2001 T1E1.4 Abstract: Unbundling presumptions of DSL standards groups are reviewed and contrasted with their respective counterparts in cable broadband access. This comparison evaluates the ultimate DSL network architecture under those presumptions as fiber penetrates further into the existing loop plant. The results of the evaluation then raise questions as to the future emphasis and focus of advanced DSL spectrum management. NOTICE This contribution has been prepared to assist Standards Committee T1 - Telecommunications. This document is offered to the Committee as a basis for discussion and is not a binding on any of the companies listed as authors. The requirements are subject to change after further study. The authors specifically reserve the right to add to, amend, or withdraw the statements contained herein.

2 02/15/01 2 T1E1.4/ Unbundled DSL Evolution (088) J. Cioffi Department of Electrical Engineering Stanford University, Stanford, CA Phone: Fax: Abstract: Unbundling presumptions of DSL standards groups are reviewed and contrasted with their respective counterparts in cable broadband access. This comparison evaluates the ultimate DSL network architecture under those presumptions as fiber penetrates further into the existing loop plant. The results of the evaluation then raise questions as to the future emphasis and focus of advanced DSL spectrum management. 1. Introduction: Unbundling is the incumbent local exchange carrier s (ILEC s) lease of a telephone line or some part of its bandwidth to a competitive local exchange carrier (CLEC). Current unbundling practice with digital subscriber line (DSL) service usually allows the CLEC to place modulated signals directly on their leased physical copper-pair phone line, sometimes referred to as the lease of dark copper. Such unbundled signals may have services, and consequently spectra, that differ among the various service providers. The difference in spectra can magnify crosstalking incompatibilities caused by electromagnetic leakage between lines existing in close proximity within the same cable. ILECs and CLECs then try to ensure mutual spectral compatibility by standardizing the frequency bands that can be used by various DSL services. However, there are many DSL types and bandwidths, and service providers are often competitors, which complicates the process of such spectrum-management standardization. Further, the cooperation and connection between spectrum regulators and DSL standards groups is still in early evolution, so that regulators may allow practices different than those presumed in the process of spectrum-management standardization. In advanced DSL service the location of the line terminal (LT or central-office side ), as well as network termination (NT or customer premises side ), can vary. That is, not all LT modems are in the same physical location. Often the location may be an optical network unit or cabinet, where placement and attachment of CLEC equipment may be technically difficult if not altogether physically impossible. The difficulty arises because CLEC fiber access to the ONU may be restricted, and/or the ONU may not be large enough to accommodate a shelf/rack for each new CLEC. Placement of such CLEC equipment for dark copper is often called collocation, when it is in the central office. Space and facilitation of such central-office collocation for unbundling of the dark copper is mandated by law in the United States. Presently, the author is aware only of an FCC decision in September of 2000 that allows Southwestern Bell Corporation to provide what is essentially packet unbundling at the remote terminal, that is service bandwidth is leased at a layer-2 or 3 protocol level, not at the physical layer. This represents a change in architecture with respect to what many standards groups have presumed in spectrum studies. The control of all the physical layer signals by a single service provider allows potential coordination of the transmitted signals in ways that can be beneficial to the achievable data rates, reliability, and complexity of DSL service. This paper reviews cable competitors to advanced DSL service in terms of broadband access practice and service competition allowed through their architecture. Section 2 shows how these services evolve into unbundling at a packet level, not a physical-layer level. Section 3 then follows the DSL architecture to determine its possible unbundled evolution. This contribution concludes that it is near-inevitable that a DSL alternative to collocation at remote terminals (LTs) must evolve. The most likely alternative is seen to

3 02/15/01 3 T1E1.4/ be packet unbundling 1, which is the resale of the digital bandwidth available on the twisted pairs, rather than the direct physical-layer lease of the line itself. Consequently, this possible evolutionary path deserves attention by standards groups in their development of advanced DSL standards. 2. Cable Architectures Figure 1 illustrates a general architecture common to cable service providers. Of particular interest is that the system is operated by a single service provider, and the bandwidth used is shared by all users. Cable modem technology basically uses time-division multiple access of the different users on a shared coaxial segment in a common up or down frequency band. The up band can be located below 40 MHz, but up bands may be appropriated from the existing TV channels for upstream transmission at higher frequencies. At least one downstream TV channel is also appropriated for the downstream cable modem, and shared again in the time domain. Cable systems today are operated by a single operator, and this operator controls all content (for example, which internet service(s) or voice service(s) may be offered, as well as what TV channels are offered). However, the FCC [2] in the USA has opened discussion on whether cable operators will be forced to provide other content. Even if not forced, many cable operators are electing to open their content to competitive service suppliers, effectively implementing unbundling. Also, as illustrated in Figure 1, as fiber moves into the HFC network, a single fiber will attach to many homes replacing the coax and providing higher bandwidth for all services. That fiber would consequently be shared in the timedomain according to the same conventions as with the coax (just with more channels available for all services) this is similar to PONs or passive optical networks, sometimes also studied as an alternative by telephone service providers for fiber migration to the customer. A single service provider, perhaps eventually restricted from controlling content, would control that fiber. Various mixes of time-, frequency-, or code-division access will not change this aspect of a single common carrier, with likely multiple services/contents provided on the system. Such a system is now, and will be in the future, a competitor to DSL. satellite Cable Head end Fiber split coax cablem odem Broadcast TV U D U 50 M Hz M Hz 5-40 M Hz Figure 1 Basic Cable modem architecture, HFC (hybrid-fiber coax). (All links shared among the common users to that link). 1 Sometimes also known as wholesaling and implemented in a few deployments in Canada for instance.

4 02/15/01 4 T1E1.4/ To emphasize and draw a comparison later with DSL, in the cable system, the issue of competition and unbundling is forced to a higher level, which we call here packet unbundling by the physical coordinated nature of the shared media. A single common carrier, the cable operator, maintains the physical layer and the consequent bits that flow over that layer. In DSL systems with current unbundling practice, the bits are managed independently by each service provider, and indeed the physical-layer signals may be different, so different that they cause harmful interference to each other. Spectrum management attempts to contain this harmful interference in DSL, while in cable, there is no such spectrum management because all signals provided by a single service provider are necessarily compatible. Because the physical medium is shared, a MAC (medium access control) is REQUIRED to coordinate data to different users. An aspect having a MAC is that it moves the unbundling problem up a layer in the protocol stack. The MAC doesn t care who s providing the incoming data it just routes it to the right customer. A similarity of the DSL and cable system is that the DSL crosstalking has the same electrical interference problem as the shared common media in cable, which is increasingly important at the higher frequencies used by DSLs on shorter lines from LTs, and causes a performance dependence between lines. However, DSL has yet no MAC to accommodate this problem, and cannot do so with line-layer unbundling, but could do so with packet-level unbundling. This no-mac observation also argues for the eventual DSL-regulatory migration to packet unbundling, which is inevitable anyway if multiple fibers to each home are to be avoided as DSL evolves (such arguments appear in Section 3). Again, as the cable system evolves to greater bandwidths and more fiber, a single fiber eventually reaches all customers and its bandwidth is shared among any common customers and content providers. 3. DSL Evolution DSL evolution appears in Figure 2(a) with remote-terminal-based DSL. Independent twisted pairs run to each customer, carrying upstream and downstream signals for that customer. The content of a pair is controlled by the service provider whose modem attaches to that pair in the line terminal (LT). If that LT modem is in a central office, several service providers may compete for the privilege to supply DSL service to that customer as mandated by law. However, the issue is yet formally undecided at the LT outside the central office (be it for ADSL, VDSL or any other DSL), although at least one ILEC in the United States (SBC) has permission to instead packet unbundle at a higher digital layer in the protocol stack. Figure 2(b) illustrates how a 2 nd service provider would connect, with their own fiber from the central office to the DSLAM presumed if physical-layer unbundling were continued, as has been presumed in spectrum management so far. (If this separate fiber is not the case, then the ILEC controls a crucial link, which is then forces packet unbundling). A 3 rd service provider would have their own fiber, and so on, resulting in many fibers to the LT. As this system evolves, the loop plant eventually has many fibers to each customer in FTTH to maintain unbundling. While a multiplicity of fibers connecting to DSLAMs co-located in a central office is perhaps a common expectation, the purpose of the use of fiber is to avoid many parallel wires/paths to a customer. Note this is different that the cable system, which has one fiber shared among many customers, as discussed in Section.

5 02/15/01 5 T1E1.4/ Network twisted pair, DSL Central Office fiber LT Fiber, FTTH Figure 2(a) Single service provider, LT-based (V)DSL. A clear alternative is to maintain one fiber as in Figure 2(a), but carry the different service providers signals on that same fiber i.e., packet unbundling. Technically, when one common fiber carrier carries all the signals, necessarily there is a demultiplexer in the LT for all the individual digital signals. If the common carrier must implement this demultiplexer, that carrier might also as well implement the modem this allows coordination of the lines at the LT, which can lead to enormous gains in data rate (see companion paper [3]) and arbitrary mixture of asymmetric and symmetric services. Spectrum management then becomes more of a multiplexing problem, than one of just minimization of crosstalk between lines that may be operated by different service providers. Network twisted pair, DSL Central Office SP #1 SP #2 fibers. multiple fibers in this area lead to multiple fibers to home SP #1 SP #2 LT Fiber, FTTH Figure 2(b) Multiple service provider, LT-based DSL evolution. In such a situation, if the common carrier also derived direct profit from providing content services, then that common carrier might exploit the situation to their competitive advantage with respect to the other service providers who do not own the LT. Such a situation might only be evaded to encourage competition by disallowing the common carrier to be in any content business. 2 That carrier would then derive profit on a fair basis by selling service as best it can to all comers. Each customer s needs could be met individually as they arise by either offering the demanded DSL service type (with consequent rates and symmetry levels) on the existing twisted pair, or by running a fiber to that customer if just not possible physically. 2 Indeed, various groups are considering presently such a division of Wireline Providers and Service Providers to ensure good competition in the future of broadband access. However, cable industry proponents vigorously oppose the same type of deregulation in the cable industry.

6 02/15/01 6 T1E1.4/ The ubiquitous single standardized interface (perhaps as simple as the RJ-11 jack today) would then be offered to all customers, but their service data rates would be customized as necessary. This approach might best be studied in spectrum management because there is a possibility that it will prevail, and the needs of DSL would be best served by considering such a possible evolutionary path. 4. Conclusion Spectrum management for LT-based services, particularly newer higher-speed asymmetric and higherspeed symmetric DSL services, depends on regulatory decisions not yet made. Presumption of only one particular type of unbundling at the LT is therefore unwise, because the consequent spectrum decisions may not correspond to or be close to the best that DSL can do if that presumed regulatory decision is not made. In contrast, spectrum management and advanced DSL descriptions still need to study and encompass the possibilities that regulatory policy may provide. A formal proposal for a study point on this type of situation is made in a companion contribution [1]. Some example improvements in DSL performance are provided in another companion contribution [2]. 3. References: [1] "Proposal for Study of Dynamic Spectrum Management for the Evolving Unbundling Architecture of DSL," J. Cioffi, J, T1E1.4 Contribution 2001-xxx, February 2001, Los Angeles, CA. [2] "Example benefits of Dynamic Spectrum Management," J. Cioffi, G. Ginis, W.Yu, and C. Zeng, J, T1E1.4 Contribution , February 2001, Los Angeles, CA.