Book Excerpt: Next-Generation Network Services — continued

VDSL and VDSL2
Other prominent variants of DSL technology are known as Very High Data Rate DSL (VDSL) and Very High Data Rate DSL 2 (VDSL2). VDSL was standardized as ITU-T G.993.1 in 2004, and VDSL2 was standardized as ITU-T G.933.2 in 2005.

Both VDSL and VDSL2 attempt to push the limit of data transmission over 24-gauge copper wire pairs with both asymmetric and symmetric DSL data versions. Many view the VDSL technologies as the next step in providing a complete home communications and entertainment package. By supporting entertainment video, VDSL can offer competing service to cable TV. Some providers such as Qwest currently offer VDSL service in selected areas in the United States, and VDSL is very popular in South Korea, Japan, and China.

VDSL benefits from recent advances in digital signal processor technology to provide an incredible amount of xDSL bandwidth—speeds up to about 52 Mbps are possible with VDSL and up to 100 Mbps with VDSL2 (even in the symmetric version) on very short copper loops of about 250 to 500 feet. Compare that with a maximum speed of 6 to 8 Mbps for ADSL or 25 Mbps for ADSL2+, and it's clear that the move from current ADSL technology to VDSL could be as significant as the migration from a 56 K data modem to any type of xDSL.

In simple terms, VDSL technology operates over the twisted pair of copper wires in a phone line in much the same way that ADSL does, with a range of speeds depending on actual line length. Nonetheless, there are a couple of important distinctions for VDSL. The maximum downstream rate is 52 Mbps over lines up to 1000 feet (304.8 meters) in length. Downstream speeds as low as 13 Mbps, over lengths beyond 4000 feet (1219 meters), are also common. Upstream rates in early models are asymmetric, just like ADSL, at speeds from 1.5 to 2.3 Mbps. VDSL2 pushes speeds to 100 Mbps while further limiting copper wire distance.

The VDSL technologies' amazing performance comes at a price: VDSL can only operate over the copper line for a short distance, up to a maximum of about 4500 feet (1372 meters) and perhaps 500 feet or less for VDSL2 at maximum rate. So a strategy for getting VDSL closer to the subscriber is in order. Both VDSL downstream and upstream data channels will be separated in frequency from bands used for basic telephone service and ISDN, enabling service providers to overlay VDSL on existing services. At present, the two high-speed channels are also separated in frequency between themselves.

VDSL and VDSL2 achieve extra bandwidth capacity by using different frequency ranges within the copper loops. The frequency range of about 2 MHz to 12 MHz is used for VDSL so as not to overlap the ADSL frequency windows. VDSL2 uses frequencies even higher than 12 MHz, perhaps up to as much as 30 MHz in order to access more bandwidth capacity. These higher-frequency ranges are possible because these VDSL standards place limitations on the length of the copper loop—the shorter the loop, the less that high frequencies are attenuated. VDSL offerings will largely target no more than a few hundred to a few thousand feet of copper, with providers pushing their optical network backbones and distribution systems with ONUs closer to businesses and residences.

Also, the VDSL standards include support for either of two line-coding mechanisms—DMT and Quadrature Amplitude Modulation (QAM). Line coding is used to encode as many bits of customer data into symbols or time periods that travel over the DSL path. The more bits per symbol, the higher the bandwidth capacity. Covered earlier, DMT uses a very large number of chip-based transceivers, each to create channels or "tones" working in parallel. QAM, which uses a combination of phase shift keying and amplitude modulation, uses a smaller number of these transceivers, each working in a particular band of the frequency range. QAM is another method of encoding multiple bits in a symbol or time period. QAM chips come in various bit constellations, such as 16-QAM, 32-QAM, and so on.

The real key to the viability of VDSL is that the service providers are replacing many of their main binder cable feeds with optical fiber cable, effectively reducing the overall length of the copper facility from the provider's CO to the subscriber. In fact, many service providers are planning fiber to the curb (FTTC), which means that they will replace all existing copper lines right up to the point where your phone line branches off at your house or business. At the very least, most companies expect to implement fiber to the neighborhood (FTTN). Instead of installing optical fiber cable along each street, FTTN has fiber going to the main provider point of presence or an optical network unit (ONU) for a particular neighborhood.

Placing a VDSL transceiver in your home and a DSLAM with VDSL modem cards in the nearest DLC cabinet or ONU overcomes the speed and distance limitation. The DSLAM takes care of the analog-digital-analog conversion problem that disables ADSL over optical fiber lines. It also converts the data aggregated by the DSLAM transceivers into pulses of light that can be transmitted over the optical fiber system to the CO, where the data is routed through a BRAS to the appropriate network to reach its final destination. When data is sent back to the subscriber, the DSLAM converts the signal from the optical fiber cable and sends it to the VDSL remote transceiver at the subscriber location. Figure 8-8 shows a conceptual diagram of the devices in a VDSL network.

Figure 8-8 Devices in a VDSL Network

Source: Cisco Systems, Inc.

Early versions of VDSL use FDM to separate downstream from upstream channels and both of them from basic telephone service and ISDN. Echo cancellation is typically required for later-generation systems featuring symmetric data rates. A rather substantial distance, in frequency separation, is maintained between the lowest data channel and basic telephone service to enable very simple and cost-effective basic telephone service splitters. Normal practice would locate the downstream channel above the upstream channel.

VDSL downstream data rates derive from submultiples of the SONET and Synchronous Digital Hierarchy (SDH) canonical speed of 155.52 Mbps, namely 51.84 Mbps, 25.92 Mbps, and 12.96 Mbps. That's because the industry wants to efficiently pack DSL data into upstream SONET/SDH infrastructure, the wireline provider's primary optical distribution backbone. It's more simplistic to refer to these data rates as 13, 26, and 52 Mbps. Each rate has a corresponding target distance range, as shown in Table 8-4.

Table 8-4 VDSL Asymmetric Speed Range per Distance (Typical)

Source: Cisco Systems, Inc.

Table 8-5 shows VDSL symmetric rates for the corresponding target distance.

Table 8-5 VDSL Symmetric Speed Range per Distance (Typical)

Source: Cisco Systems, Inc.

Table 8-6 shows the target VDSL2 asymmetric and symmetric rates as known during the 2005 standardization effort.

Table 8-6 VDSL2 Asymmetric and Symmetric Data Rates (Typical)

Figure 8-9 compares VDSL, VDSL2, and ADSL transfer rates. There are about a half dozen DSL data encapsulations that are commonly deployed among service providers. There are so many because there are so many different engineers with different ideas on network design. Although each type of DSL encapsulation improves on the other along some vector of scalability, security, or performance, a typical provider would seldom use more than two or, at the most, three of these architectures on an ongoing basis. It is not the focus of this book to describe the various DSL encapsulations or designs.

See the "Recommended Reading" section at the end of this chapter for books that provide a deeper look at DSL network architectures, design, and implementation.

Figure 8-9 Comparison of Transfer Rates: ADSL and VDSL

Source: Cisco Systems, Inc.

— End

Reproduced from the book Next-Generation Network Services. Copyright 2006, Cisco Systems, Inc.. Reproduced by permission of Pearson Education, Inc., 800 East 96th Street, Indianapolis, IN 46240.

Visit www.ciscopress.com for a detailed description and to learn how to purchase this title.