DSL Advances

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). Unbundling first began in the United States as a consequence of the 1996 Federal Telecommunications Act. Current unbundling practice with 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. This standardization is known generally as spectrum management (see Chapter 10). However, there are many DSL types and bandwidths, and service providers that are often competitors , which complicate 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. For instance, the Tauzin-Dingell bill passed by the U.S. House and before the U.S. Senate at time of writing would (unintentionally) create a structure that allows the highest possible data rates and services technically, although its encouragement of competition and the consequent rate of DSL service installations is debated.

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 economically infeasible. The difficulty arises because there may be no spare fibers for the CLEC to access the optical network unit (ONU), 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 "colocation" when it is in the central office (CO). Space and facilitation of such CO colocation for unbundling of the dark copper is mandated by law in the United States. Presently, many ILECs are finding regulator acceptance of their control of all physical-layer signals that emanate from remote terminals (as long as they can prove they provide wholesale packet-level unbundled service fairly ), as opposed to emanating from the CO where the physical-layer unbundling is forced by law. This represents a change in architecture with respect to what many standards groups have presumed in SM 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, such as is now studied in DSM. Even without such coordination, there is much that DSM can offer, as we shall see in Section 11.3.

11.1.1 Cable Modem Architecture ”DSL's Competition

Figure 11.2 illustrates a general architecture common to cable-TV service providers. Of particular interest is that the cable system is operated by a single service provider, and the coaxial-cable 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 additional 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 (e.g., 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 United States has opened discussion on whether cable operators will be forced to provide other content. Even if not forced, many cable operators are investigating allowing competitive-service suppliers on the same cable, effectively implementing wholesale "unbundling." Also, as illustrated in Figure 11.2, 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 time-domain according to the same conventions as with the coax (just with more channels available for all services) ”this is similar to PONs (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.

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 DSL 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, whereas 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 in the cable system to coordinate data to different users. An aspect of having a MAC is that it moves the unbundling problem to a higher 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 DSM can be construed as a first step towards enabling such a MAC. This no-MAC observation provides a market-competition argument to support 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 11.1.2). 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.

11.1.2 DSL Evolution

An often-presumed DSL evolution appears in Figure 11.3(a) with remote-terminal-based DSL. Individual uncoordinated twisted pairs run to each 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 at the LT. Figure 11.3(a) illustrates how a second service provider would connect, with their own fiber from the central office to the DSLAM presumed if physical-layer unbundling were continued . ^[1] A third 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 the present form of physical-layer unbundling. Although a multiplicity of fibers connecting to DSLAMs colocated in a CO is perhaps a common expectation, the purpose of the use of fiber is to avoid many parallel wires/paths to a customer. Thus, clearly present physical-layer unbundling leads to a ludicrous technical evolution of multiple fibers to every customer; nonetheless existing DSL spectrum-management decisions (see [28]) address only this evolution path . Note this evolution is different that the cable system's evolution, which has one fiber shared among many customers, as discussed in the previous section.

^[1] If this separate fiber is not the case, then the ILEC controls a crucial link, which is then packet unbundling.

A clear alternative is to maintain one fiber as in Figure 11.3(b), but carry the different service providers' signals on that same fiber; that is, packet unbundling or wholesaling. 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 implement the modem. This allows coordination of the lines at the LT, which can lead to enormous gains in data rate as in Sections 10.3 “10.5, as well as a nearly arbitrary mixture of asymmetric and symmetric services. SM then becomes more of a multiplexing problem, than one of just fixed worst-case minimization of crosstalk between lines that may be operated by different service providers.

Figure 11.4 predicts the timeline of argued evolution of DSL from its present configuration to a likely packet unbundled future. The ILEC could actually be any service provider, and the twisted-pair network in the final step might actually be a private network. Early DSM could have the network maintenance center for DSL collecting performance/line information from the lines and possibly making recommendations to the lines/DSLAMs as to maximum binder-benefical data rates to attempt. Largely in early DSM, each DSL line operates autonomously without guidance from an SMC ”even in this case, considerable improvement is possible, as in Section 11.3. Clearly the DSL maintenance center can also provide information to the ILEC service personnel as to potential or identified problems, resulting in either manual or automatic repair/prevention. As DSM progresses and DSL evolves to fiber-fed remote terminals or line terminals, then highly advanced maintenance can be used remotely or placed directly at the common copper interface to customers in the LT. Signals with such later coordination could ultimately be cogenerated to avert crosstalking problems between lines that otherwise dramatically reduce data rates. The data rates listed are representative of what is possible at each stage of evolution with existing modem technologies, but with increasing management and coordination.

Ultimately coordinated drops lead to very high data rates, easily 100 Mbps symmetric service (effectively allowing 100BT connections that customers today often find in their internal office network) proliferating to all ends of the copper network. Such speed actually matches or exceeds the best envisioned by fiber networks that would require wide-scale replacement of facility that may not be practical (all the way to customer) over the same time-frame of evolution, and at a lot less cost than the all-fiber system.

11.1.3 The Essential Steps of DSM

Having no spectrum management is much like driving in a country with no traffic laws, no traffic signals, and no police. Static spectrum management is like driving with traffic laws, signals, and police control under an ultra -conservative dictatorship. Dynamic spectrum management is like having more mutually beneficial laws with traffic-controlled stoplights, car pool lanes, and drivers trained to at least somewhat respect their lanes and use their good common sense in avoiding collisions.

Figure 11.5 illustrates the essential steps of any DSM system. In autonomous DSM operation, the service provider (DSL operator) sets data rates for the DSL services they will provide. Each modem is configured to run in one of three modes:

RA ” rate adaptive (maximize data rate)

MA ” margin adaptive (use all available power at given fixed data rate)

FM ” fixed margin (use only power needed to guarantee high-quality service)

The last mode is most conducive to best overall use of the binder and is finding increasing use with enlightened service providers who want to see all services perform as best as is possible. The first two steps can lead to dramatic improvement in overall binder performance if the service provider sets the modes and data rates according to DSM-standard guidelines, and require absolutely no coordination among DSL lines or service providers. Such improvements are the subject of Section 11.3.

For yet greater performance as a function of individual situations, information about DSL lines can be collected, perhaps many times. Information acquired can be as simple as the DSL modem reporting it is operational at the supplied data rate (or can operate at some other data rate, lower or higher). Such information in the SMC can lead to setting yet better (revenue improving) rates and modes of the DSL modems, or perhaps suggest to the service provider other operations (like putting in a fiber, replacing DSL modems with better/ newer ones, etc.). With increasing sophistication of the DSM system, information acquired about the network can include loop characteristics (e.g., topology, channel and crosstalk description), transmission parameters (e.g., data rate, power, bandwidth, energy/bit allocation in frequency/time), and traffic information (e.g., statistics, user traffic characterization, real-time measurements). Ultimately, vectored generation of DSL modem signals can lead to the highest performance and (because of shared processing) the highest port densities , lowest power consumed, and lowest costs of the modems. Thus, DSM provides an enormous opportunity for service providers to improve DSL service and increase rates, enabling a migration to hundreds of millions of customers connected at high speeds, while simultaneously providing equipment vendors and component suppliers an opportunity to differentiate their products.