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With recent advances in technology, carriers are now offered a number of new access technologies for delivering advanced services over their network. By adding IPTV, "triple-play" bundled packages offer service providers a whole new revenue stream--but also a whole new set of challenges. The first and most important question that needs to be addressed is, "What access technology should I use to deliver video?" Carriers looking to maximize their existing copper infrastructure have several choices, including ADSL2+, recently standardized ITU G.998.1 Bonded ADSL2+, and VDSL2 to deliver multi-channel video directly to the customer premises.

To begin, carriers need to understand the bandwidth requirements for different forms of video. To transmit broadcast-quality MPEG-2 standard-definition (SD) video requires a data rate of around 3 to 4 Mb/s for each channel. MPEG-2 high-definition (HD) television requires between 15 and 18 Mb/s. Let’s assume for that a minimum service level for high-speed data should be 1.0 Mb/s dedicated, each SD video channel requires 3.5 Mb/s, and each HD channel requires 15 Mb/s, both with MPEG-2 (re-encoded data). Of course, these rates continue to fall with improvements in video compression technology such as MPEG-4 and ITU-T H.264, which reduce this requirement to only 6 to 9 Mb/s for HD video.

Last year brought the first large scale deployments of ITU standard compliant ADSL2 and ADSL2+ compliant equipment, culminating several years of work across the DSL industry. The new ADSL2+ standard, which specifies a frequency up to 2.2 MHz, result in a significant increase in downstream data rate. The new standard also has optional annexes (Figure 1) to meet the service provider specific deployment requirements.

Figure 1: ADSL Standard Annexes

Standard-compliant ADSL2+ chipsets have a maximum achievable data rate of approximately 24.5 Mb/s. ADSL2+ data rates offer more than three-quarters of potential subscribers at least one channel of standard-definition television, about two-thirds can receive two channels, and a half qualify for three channels. (Figure 2)

In January 2005, the ITU-T Study Group 15 passed the G.998.1 (G.bond-ATM) standard. G.bond-ATM enables ADSL equipment to electronically “bond” together ADSL2 or ADSL2+ links using multiple copper phone lines, which can then be used to dramatically increase the bandwidth provisioned to subscribers via a single ATM data stream. Loop bonding technology provides the telecom industry with a revolutionary technique for combining multiple ADSL2+ connections together into a single, aggregated connection, even if they have different data rate capacities. These ultra-high data rates support advanced, bandwidth-hungry services such as broadcast video and video on demand. Even multiple streams of high-definition television can be supported using Bonded ADSL2+.

As shown in Figure 1, just two ADSL2+ lines can be bonded to provide residential customers with more than 33 Mb/s on lines shorter than 5 kilofeet. On longer phone lines, two ADSL2+ lines can be bonded to provide 8 Mb/s out to 12 kilofeet; enough to provide two channels of standard resolution video. As with all DSL connections, achievable reach will also depend on the quality and gauge of the phone line, as well as possible disturbers that can impede data rates. The performance in Figure 1 was based on 26 AWG using 12 self NEXT/FEXT disturbers.

Figure 2: ADSL2+ vs. Bonded ADSL2+ Data Rate Comparison

The Bonded ADSL2+ standard has several important features, including:

• Disparate data rates (up to a ratio of 4:1) among its pairs. This way, if some copper lines have lower capacity than others, it is not necessary to reduce the data rates on the other lines.
• Ports on the ADSL2 line card to be bonded may be done so randomly. That is, any combination of ports may be bonded, and they do not require "hardwired" preassignment.
• May be used over any DSL physical layer. As an example, bonded ADSL2 Annex M links could be used to double the upstream bandwidth.

Because ADSL and ADSL2/2+ chipsets currently support an ATM interface, ADSL2+ bonding can be easily supported by existing off-the-shelf central office and customer premises chipsets which leverages years of work invested in achieving interoperability based on a singular standard, as well as improvements in chipset power and spatial density. Bonding functionality does require the addition of some small multiplexing technology in order to divide data from a single ATM stream across different phone lines and then aggregate the data at the receiver in the correct order.

Bonded ADSL2+ promises to deliver the increased bandwidth service providers need to move to the next level of profitable service offerings over broadband. Multiple video streams and HD video expands the telecom service portfolio beyond traditional POTS voice and high-speed Internet access to include these exciting new competitive offerings. The demand for these offerings already exists, and with the finalization of the American National Standard for ATM-based multi-pair bonding standard, service providers will open the door to a new host of opportunities.

Figure 3: Video Delivery Options for DSL subscribers (Mean Loop Length: 10Kft)

VDSL has held great promise for delivering video services since the late 1990s. Unfortunately, the industry was not able to agree on a physical layer line code, and service providers were hesitant to deploy non-standard equipment. Some pre-standard VDSL products were deployed as end-to-end architectures, primarily in Korea and Japan and to a limited degree in regional North America deployments. In 2004, the industry finally came together to end the long-standing debate and ratified the ITU VDSL1 standard (G.993.1), which is based on the line code DMT but contains an optional QAM annex.

Last year the ITU began work on the next -generation VDSL standard designed to support new services such as IP TV. VDSL2 is in progress and expected to be ratified this year. It is designed to increase both rate and reach over the copper network and supports three primary modes or bandplans.

The first mode, sometimes referred to as Long Reach VDSL2 (LR-VDS2), supports European and North American service provider network plans to deploy remote DSLAMs deeper into the network. LR-VDSL2 specifies spectrum allocation of 8.8MHz. The target data rate for this bandplan is 20Mb/s at 5 kilofeet. This would allow for three to four channels of SD video or one channel of HD video. LR-VDSL2 will enable service provides to reach a majority of their customers with triple-play services.

The second VDSL2 mode provides symmetric data rates up to 100 Mb/s over very short loops by increasing the frequency spectrum up to 30 MHz. Obviously, many channels of HD video could be delivered with this technology. As expected, the date rate decline is very steep over distance making this VDSL2 mode ideally suited multiple dwelling unit applications.

The third mode of operation similar to the ITU VDSL1 DMT standard, supporting a spectrum of 12MHz. This mode can realize downstream data rates greater than 50Mp/s at 1 kilofoot, making it best suited for fiber-to-the-premises deployments. Under this mode, three channels of HD video would be available to subscribers.

It is anticipated that the new VDSL2 standard will include many of the features and functionality contained in the approved ITU ADSL2+ standard. As an example, support for Diagnostics Mode, which enables service providers to easily provision and monitor DSL services, will likely be required. The VDSL2 standard is expected to be consented at the next ITU standards meeting scheduled for May.

As standard-compliant equipment is in early deployment stage for both ADSL2 and ADSL2+, and the VDSL2 standard has not reached consent, it is still to early to tell which technology will capture the largest deployment figures for video. However, all three are promising and each serves a unique purpose in the network, so a combination of the technologies is likely to appear around the globe. One thing is for certain: carriers are flocking to deploy video as fast as possible to compete in the entertainment market and expand their service offerings, which not only helps to reduce churn, but also tends to up revenues as customers add new subscriptions to their service. As carriers look to deploy video-capable technology, they need to consider the future potential of their network and ensure that the technology they choose is scalable, interoperable, and standard-compliant. In doing so, they can begin to roll out video offerings to a waiting list of customers eager to embrace the new service.

Peter LeBlanc is Vice President of Marketing for Aware Inc.

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