Broadband video applications are quite demanding, requiring high data rates, quality of service (QoS) features, and other capabilities that strain today's digital subscriber line (DSL) technologies.

Setting the bar for DSL performance at the level of video applications addresses two important issues. First, it accommodates a superset of capabilities that will support most other applications and avoids incremental DSL improvements which can drive up costs for operators. Second, home entertainment applications, many of which are video-orientated, will be a major driving force behind continued growth in broadband. With a projected 25 per cent compound annual growth rate for DSL through 2007, the stakes are huge for the entire DSL market.

The good news is that the newest versions of the DSL standards and a new delivery platform, Uni-DSL (UDSL), address the requirements for providing robust video services. Video applications enabled by UDSL include multi-channel digital video broadcast HDTV, streaming IP video, video on demand, and video conferencing.

Advancing standards

New DSL standards such as ADSL2+, VDSL, and VDSL2 (in development) are moving in the right direction when it comes to the higher data rates needed by video applications. Since DSL data rates are inversely proportional to loop length, the high data rates supported by these standards are becoming more practical as operators shorten their distribution loops (twisted copper wire pairs serving customers) by deploying equipment at remote terminals andcross-connect points.

The VDSL standard features higher data rates in both the downstream and upstream directions than ADSL2 or ADSL2+ because VDSL supports greater transmission bandwidth. However, the higher frequencies of VDSL provide an advantage only to about 4 kft. The VDSL2 standard, which is currently under development, will have a bandwidth in excess of 12 MHz. This extra bandwidth, useful only at short loop lengths, along with higher transmit power and other features will enable VDSL2 to outperform VDSL. (See Fig. 1 for a graphic comparison of the rate and range performance of the various DSL standards).

ADSL2+ and VDSL2 offer the highest data rates in the 5 - 6 kft region, which is a maximum loop length of interest when considering equipment deployment in telephone company cross-connect boxes. (Fig. 2 maps a network operator's equipment to the various DSL standards.) Downstream data rates >15 Mbps are achievable at these loop lengths. These data rates will accommodate multiple video streams, along with data and voice services, assuming MPEG4 video at ~6 Mbps for HDTV, 2 - 4 Mbps for SDTV, data service at 1.5 - 3 Mbps, and voice service at several hundred kbps.

Better DSL delivery

The features and capabilities of ADSL2+ and VDSL2 set the stage for a DSL delivery platform that is well suited to demanding video applications in homes and businesses. For example, these standards support a number of features such as multiple latency paths, basic dynamic spectrum management (DSM), and channel bonding. DSM techniques are used to improve utilisation of network capacity, and channel bonding, or the aggregation of multiple copper wire pairs into one DSL channel, is a cost-effective way of increasing DSL data rates to accommodate video streams.

A new delivery platform from Texas Instruments calledUni-DSL provides several advantages for operators seeking to deploy video and other new services. UDSL is designed to support aggregate (upstream and downstream) data rates up to 200 Mbps, and is a converged DSL delivery platform that takes advantage of ADSL2+ and VDSL2 technology to optimise the performance and service delivery in every local loop in the network.

At very short loop lengths (eg 300 - 500 feet), UDSL symmetric data rates of 100 Mbps are more than enough to enable simultaneous upstream and downstream transmission of multiple HDTV video streams, even using current MPEG2 compression at 19 Mbps per stream.

In addition, UDSL provides built-in features for bonding and the most advanced forms of DSM. As a converged delivery platform, UDSL will be compatible with all of the relevant discrete multi-tone (DMT)-based DSL standards, including ADSL, ADSL2, ADSL2+, VDSL, and eventually VDSL2 when it is completed.

Bonding extends the range at which video services can be provided. The overall data rate of a bonded DSL channel is approximately the sum (slightly less because of bonding overhead) of the DSL data rates on each pair.

For delivering video to the residence, two-pair bonding provides a way to approximately double the data rate that can be offered at a given distance.

For example, with two-pair bonded ADSL2+ or VDSL2,20 Mbps service can be provided to 9 - 10 kft, the range required for remote terminal deployment.

There are many aspects to DSM, but one of the most attractive is 'vectoring', which can be used as an extension to bonding. Using multiple bonded pairs to provide service to a single customer implies that the transmitters and receivers on each end have access to each other's signal streams. This allows the use of vectoring (also known as multiple-input,multiple-output, or MIMO) signal processing techniques to cancel crosstalk and further increase data rates. UDSL incorporates training and initialisation features, as well as physical layer architecture features, to facilitate vectoring.

Depending on the crosstalk environment and loop length, vectoring can produce substantial (eg 30 - 50 per cent) data rate gains relative to bonding. This allows operators to deliver more robust video services in terms of data rates, range, and immunity to interference.

Driving DSL growth

As the broadband market continues to evolve, two factors will drive the growth of DSL. First, the user experience in the home or business must be enhanced to spur new demand. This means new and exciting multimedia applications with robust video content. Second, operators must be able to cost-effectively and efficiently deploy new capabilities. Operators have invested much in their copper infrastructures.

Fortunately, new versions of the DSL standards and innovative DSL delivery systems offer a migration path for the installed infrastructure to the higher data rates and other features needed to cost-effectively support exciting new applications with significant video content.

William L Abbott is Manager, Systems Engineering, Peter S Chow is Chief Technology Officer, and James T Aslanis, is General Manager, DSL Technology Centre, Texas Instruments Inc, USA. www.ti.com/dsl