7.2 Applications and Their Interfaces
| Applications for VDSL are manifold , with the consequent relative impacts yet indeterminate. This section begins by some degree of analysis of different data-rate combinations and applications for VDSL in terms of the presently evolving views of broadband access. Generally, this section argues that speeds for DSL access will steadily increase from the current installed asymmetric 100s of kilobits to 1 Mbps-plus to 10s of Mbps of symmetric transmission speed as fiber reaches closer to the DSL customer. The application types of audio, video, and data as usual will guide vision of the requisite speeds of future broadband DSL access here, followed by an example progression of DSL applications that could, for instance, motivate the speed combinations illustrated by the VDSL transmission methods discussed in Sections 7.3 and 7.4. 7.2.1 VDSL Applications and Speed TypesThis section will individually consider the three application types of audio, video, and data/computing. Each area shows considerable need from a very realistic viewpoint for the increasing DSL speeds of VDSL. In combination, the different application types further suggest that within a reasonable time period, DSL will be choking for yet greater bandwidth, a need to which the DSL industry will need to respond. Audio"Audio" to telecommunications engineers often means a voice signal, although clearly today high-fidelity audio and music are of increasing interest. Good quality voice signals require 64 kbps (although some networks reduce that via compression to 32 kbps or 16 kbps). Small businesses and high-end residences are using increasing numbers of phone channels, multiplexed on a single line. The recent success of the ADSL area known as VoDSL (voice-over DSL) suggests that 16 voice channels on a single DSL link (1 Mbps) is highly desirable ”indeed symmetric 1.5 Mbps service (24-channel T1) has long been a staple of new telco business. Clearly transmission of this or higher symmetric rates allows more economic service of a mutiline business. One could argue that symmetric data rates from 1 to 150 Mbps are clearly of immediate interest, but are limited in their use to places where those speeds can be implemented. Clearly, extension over phone lines to a greater number of termination points is of interest. However, new audio applications also abound. The success of Napster and other similar applications admits the desirability of downloading CD-quality audio to a variety of home storage devices for immediate or delayed playback. The best high-quality compression methods today can achieve this quality with no less than 64 kbps. Thus, Table 7.2 suggests some transfer speeds for perhaps a teenager using DSL to download new music, both an audio single and an entire album. Clearly, 56 kbps is unacceptable, but even current ADSL speeds of today (1.5 Mbps) lead to annoying wait times. Speeds of 6 Mbps ( asymmetrically ) do lead to reasonable transfer times. DataAnyone familiar with e-mail attachments (often documents) or downloads of documents or software applications from the Web is aware that many files today can easily be 2 MB in size. That size has increased accordingly to Moore's law (even faster in the storage industry) by a factor of 2 every 2 years, meaning that in 2 “5 years , file sizes of 10 MB might be common (and in fact can exist today, albeit rare in everyday use). Ideally, one would like to purchase a computer (perhaps a laptop) and connect it to a phone line and then have file transfer at the highest speeds that the connection of the computer DSL modem to the network DSL modem would allow. Some parts of the industry have already found that current DSL speeds are not yet sufficient for best e-mail/surfing file transfer. Such file transfers may be in either direction, especially for someone working at home who has extracted a file, altered it, and then wants to restore it to the corporate server with changes, suggesting symmetric transmission may also become desirable. Table 7.3 illustrates file transfer speeds for a 10 MB file. Table 7.2. Music Transfer Times with Various DSL Speeds
Table 7.3. 10 MB Data-File Transfer Times with Various DSL Speeds
Video (and Image)A single photo image from a digital camera today can easily consist of 1 MB, whereas an entire 50-photo album would take then 50 MB. Table 7.4 lists DSL speeds and times for transferring such a photo or album. Again bidirectional transfer could be common, lending to increasing demand for symmetric DSL services. For video, real-time video takes from 1.3 to 4 Mbps for a single good quality channel. HDTV (high-definition TV) requires 20 Mbps, but perhaps a more realistic DSL application would be the transfer of a DVD movie over the Internet to a residential display device, for subsequent viewing (perhaps with stops and starts while the viewers take leave of the viewing room for personal needs). A DVD two- hour movie requires about 2 GB to store, leading to the transfer times in Table 7.5. Future HDTV movies could occupy as much as 130 GB. The tables clearly indicate a desirability for faster DSL speeds, even when real-time video (TV) is ignored. Distributed ComputingAs a final example, view the Internet as an example of distributed computing. The parallel bus transfer speed of a 1.5 GHz Pentium 4 processor is 48 Gbps, symmetric. Clearly in the limit, anything less slows the processor capability in a fully distributed environment. Thus, higher and higher broadband access speeds are motivated by the basic applications of which we know today. DSL broadband access speeds will become (if not already) the bandwidth limiter, and thus VDSL seeks to address this limitation. For this reason, VDSL standards have outline a series of increasing speeds and symmetries as objectives of VDSL, as summarized in Table 7.6. Table 7.4. Single Photo and Album Transfer Times with Various DSL Speeds
Table 7.5. DVD Movie Transfer Times with Various DSL Speeds
Short-speeds have been tending toward 100 Mbps symmetric, whereas 10 Mbps symmetric on one or more coordinated lines is also of newer interest. Clearly, not all of the speeds can be achieved at all the ranges in all environments, but Table 7.6 gives an idea of the types of applications, audio, data, and video above that might be enabled and over what lengths, basically consistent with Figures 7.2(a) and 7.2(b). As an example, the most ambitious of ADSL plans today use fiber to bring all line lengths to less than 4 km, allowing a maximum ADSL speed of about 6 Mbps downstream. Thus, a possible progression of VDSL speeds as line lengths get shorter might follow Figure 7.8's progression of bandwidth use [18]: Figure 7.8. Possible VDSL rate progression and bandwidth use. The use of bandwidth from DC to 20 MHz would allow successively, 4 km ADSL service (up to 6 Mbps) at 4km, to be followed by an increase of the upstream data rate to an equivalent 6 “8 Mbps by using spectrum from 1 “5 MHz judiciously to create equal upstream and downstream data rates of 6 “8 Mbps over 2 km or perhaps also to allow some increase in the downstream data rate. The band from 5.3 MHz to 10 MHz would be used to increase downstream data rates to 26 Mbps at a range of about 1 km, and the band above 10 MHz subsequently used to restore symmetry at 26 Mbps or higher on line lengths below 300 m where that upper bandwidth is useful. In Figure 7.8, the U/D areas mean that the band could be assigned to either direction. One can readily note that trade-offs in symmetry level, range, and data rate will easily produce other possibilities. The reality of VDSL application is that the exact spectrum use and data rates are still not carefully evaluated. In an attempt to define spectrum usage, a group of phone companies known as the Full Service Access Network (FSAN) tried to specify a reduced set of data rates and associated spectrum use, but the outcome was controversial with independent frequency plans in Europe and the United States as in Figures 7.4(a) and (b) depending on the belief in real-time TV as a DSL application (or lack thereof) and a third programmable international plan (Figure 7.4[c]) that may have been the only sanguine output from the study because it encompassed the reality that the application base was not yet well-enough understood to standardize. It appears at time of writing that efforts in EFM (see Section 7.5) and also new understandings of spectrum compatibility (see Chapter 11) may merit adjustment again to the bandplans. The U.S. group standardized on 22 Mbps down and 3 Mbps upstream to be supported over a target range of 1 km, and then specified an incompatible 6/6 and 13/13 Mbps target objective at the same and shorter ranges ”some Europeans preferred a more symmetrical set of data rates. The basic problem is that the exact driving economics and applications of VDSL were not yet known at the time of specification, thus the third programmable alternative that allows some degree of ability to change as necessary (see Section 7.3 on digital duplexing ). Table 7.6. VDSL Speeds and Ranges
Regulators in all countries of the world have only just begin to investigate the spectrum use in future VDSL. Indeed it may be that the packet-level unbundling in Chapter 11 becomes the regulator choice (as indeed has happened already for one major operator in the United States [19]), opening the issue of spectrum use in VDSL once again. The overall conclusion here is that the only sanguine solution may be one that offers considerable flexibility to adjust to market and regulatory requirements as broadband DSL access develops. Section 7.3 and Chapter 11 further investigate such flexibility. 7.2.2 Common VDSL Reference ConfigurationsIn an attempt to keep the two standardized transmission methods discussed in Section 7.1.4 somewhat in common application compliance, a common reference model was adopted and illustrated in Figure 7.9. The interfaces and functionality associated with the two interfaces are common to both VDSL transmission methods. The PMS-TC (physical medium-specific transmission convergence) and PMD (physical-medium dependent) interfaces are specified for each transmission method, while the spectra of the U interfaces and the splitter are again specified in common for the two transmission methods. Like ADSL, VDSL also specifies two paths, a slow or interleaved path and a fast or non-interleaved path as in Figures 7.10 and 7.11. The former undergoes interleaving as well as forward-error correction to allow for maximum impulse-noise protection while the latter allows for minimum delay (no more than 1 ms in VDSL). The application-specific reference is the DSLAM device that basically makes use of a subset of the functionality for a given PMS-TC interface, converting from the g interface. The g interface can be an ATM or STM interface, for instance at speeds well above those of an individual DSL modem and perhaps is the interface to the DSLAM itself. The application specific layer then extracts the pertinent bits (presumably set by the normal ATM method for setting permanent or virtual channels) for each of the fast and slow data paths through the VDSL modem, formats those bits into a known and reversible format within each stream, and then forwards fast and slow bits to the PMS-TC interface. Theoretically, these two bit streams could be visualized as the same for the two line codes; in reality, the DSLAM manufacturer and application-module manufacturer would know which of the two line codes it is using and then make the appropriate translations. Thus, even though both might interface to a common fiber interface (for instance OC-12 or Gig Ethernet), the ensuing DSLAMs will be quite different depending on the line code used. Figure 7.9. VDSL reference model from [15]. Figure 7.10. VTU-O reference model. Figure 7.11. VTU-R reference model. Indeed the greatest challenge of VDSL, presuming a working modem, may be the high-speed extraction and identification of the individual application signals, likely sent through an ATM switching system. Later chapters deal with that process, but this section notes a few characteristics of interest for transmission. One can envision the application devices as multiplexing and demultiplexing the applications signals discussed in Section 7.2.1, of which there may be many simultaneously present in both the fast and slow buffers, and formatting them for/from the modem itself. The high-bandwidth data channel created by VDSL may allow for numerous applications to simultaneously flow. 7.2.3 OperationsThe VDSL standard Part I [15] has an elaborate list of operational and maintenance capabilities. As with previous DSLs, the main parameters of interest are the state of the modems, the likelihood or presence of errors on the link, and the synchronization of network functions. VDSL allows passage of the 8 kHz Network Timing Reference. | |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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