Introduction

The competition over which access technology is going to prevail in delivering voice, data and broadcast quality video and TV over the Internet to the home is fierce. On the one side of the ring are the Telcos, which control vast backbone networks, but have encountered obstacles in squeezing high bit rates over traditional last mile copper lines. On the other side of the ring are the cable companies, which want to use existing cable infrastructure to deliver high bandwidths to consumers. This competition reflects the convergence of broadcasting and networking into streaming media. Ultimately, it is the MSO (Multi Service Provider) that will come out on top. The success of MSOs depends on their ability to deliver video streaming, voice over IP and data services over broadband networks. This paper discusses how carriers can utilize DSL technology to turn traditional copper lines into broadband pipes for delivering rich media services to consumers' desktops.

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The Case for Broadband

To expand revenues from channels and justify pay-per-view services over the Internet, carriers have to be able to offer broadcast quality digital video. This means a compression rate and hence a delivery rate of at least 2 Mbps. Today, these bandwidths cannot be delivered over regular copper lines. Video, unlike voice and data, is a time-dependent, bandwidth thirsty media. Several factors are liable to affect smooth delivery:

Bottlenecks: Most networks are shared media, meaning that many users compete for available bandwidth. When a network becomes busy, all its traffic slows down. A few seconds delay in the delivery of a data packet is not crucial as long as the entire message comes through in one piece. With streaming, the opposite is true. Because video is time dependent, the broadcast will be distorted if the video transmission has to wait for other traffic to go through first.

Video File Size: Current networks cannot carry raw digital video which is still too large to be manipulated. An uncompressed 24 bit color video, with 640 x 480 resolution, at 30 frames per second, would require a bandwidth of over 200 Mbps. The goal of digital compression is to massively reduce the amount of data required to store and transmit a digital video file, while retaining its original quality. The following pages of this paper will discuss various formats for compressing digital video.

Bandwidth: Most home consumers today access the Internet at rates ranging from 28.8 Kbps to 56 Kbps over analog modems. The introduction of DSL access to the home means that home consumers can also benefit from the broadband Internet.

As the following diagram illustrates, the higher the quality of the video source, the greater bandwidth needed for transmission.
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Video Streaming over DSL

To overcome the obstacles mentioned above, Telcos and operators have begun the widespread deployment of DSL technology. Digital Subscriber Line (DSL) converts existing twisted-pair telephone lines into access paths for multimedia and high-speed data communications. The acronym DSL, refers to DSL modems and DSL lines. DSL modems are located on either end of a twisted-pair copper line.

ADSL (Asynchronous DSL) and VDSL (Very High Bit Rate DSL) are the two types of DSL most relevant for video and interactive multimedia delivery.

ADSL - Asynchronous DSL

ADSL is an ideal technology for broadband rich media streaming. ADSL provides sufficient bandwidth. Bi-directional ADSL lines provide a channel for management applications, such as billing, and interactivity. ADSL incorporates forward error correction that dramatically reduces errors caused by impulse noise. Most important: since ADSL modems are installed on existing copper lines, there is no need for massive re-wiring. Thus, the cost of initial deployment is financially feasible.

ADSL modems supply three separate frequency channels over the same line:

One channel carries telephone conversations.
The second carries a 16 to 640-kbit/s data signal upstream from a user to the Internet.
The third channel is a high-speed downstream connection running anywhere from fractional T1 (256 Kbps - 1.544 Mbps) to 8 Mbit/s.

Some ADSL modems offer a common configuration of 1.5 or 2.0 Mbps in one direction a 16 kbps duplex channel. Others provide rates of 6.1 Mbps and 64 kbps duplex. Data rates depend on a number of factors, including the length of the copper line, its wire gauge, the presence of bridged taps, and cross-coupled interference.

To create multiple channels, ADSL modems use two methods to divide the available bandwidth of a telephone line: Frequency Division Multiplexing (FDM) or Echo Cancellation.

FDM assigns one band for upstream data and another band for downstream data. The downstream path is divided by time division multiplexing into one or more high speed channels and one or more low speed channels. The upstream path is also multiplexed into corresponding low speed channels.
In Echo Cancellation, the upstream band overlaps the downstream band. The two are separated by means of local echo cancellation.

An ADSL modem prepares the data stream for transmission by multiplexing downstream channels, duplex channels, and maintenance channels together into blocks. An error correction code is attached to each block. The receiver then corrects errors that occur during transmission up to the limits implied by the code and the block length.

VDSL - Very High Bit rate DSL

VDSL is still in the specification stage. VDSL supports very high bit rates over short distances, for example, up to 52 Mbps over 300 m (984 ft). VDSL is an enabling technology for FTTN (Fiber to the Neighborhood) and FTTC (Fiber to the Curb) architectures. In these architectures, optic fiber delivers very high bandwidths to the curb. VDSL delivers the content from the curb to the living room. In terms of services, VDSL has the capacity to be a full service network, allowing Telcos and operators to offer multimedia, interactive TV, HDTV as well as broadband Internet, all on the same line. The following table shows VDSL supported bit rates at varying distances.

Downstream Bit rate	Upstream Bit rate	Distance
12.96 - 13.8 Mbps	1.6 Mbps	4500 ft or 1500 meters
25.92 - 27.6 Mbps	3.1 Mbps	3000 ft or 1000 meters
51.84 - 55.2 Mbps	6.4 Mbps	1000 ft or 300 meters

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DSL Enables Quality

Deploying DSL enables the use of high quality MPEG video. The various MPEG standards allow application developers to create customized trade-offs between playback rate, quality, bandwidth and cost.

MPEG-1, MPEG-2 and MPEG-4 offer different compression rates and schemes, allowing service providers to offer various video streaming services.

MPEG-1 provides very good quality at playback rates of up to 3 Mbps.
MPEG-2 provides a compression solution for applications that are not limited by bandwidths (3 to 15 Mbit/s). Such a broad compression range means that it is not necessary to sacrifice quality to compression rate limitations.
MPEG-4 is a set of compression/decompression formats and streaming technologies that address the need for distributing rich interactive media over narrow and broadband networks. Although MPEG-4 covers more or less the same encoding range as MPEG-1 and MPEG-2, its target applications are different. MPEG-4 defines interactivity, scalability and streaming of rich media. Thus content compressed according to the MPEG-4 standard can be streamed over the broad or narrowband Internet, used in Interactive TV applications or streamed to wireless appliances such as cellular phones and PDAs (Personal Digital Assistants).

The following diagram illustrates the correlation between input resolution, bandwidth, compression rate and video quality.

Infrastructure and System Configuration

The following diagrams shows how video can be delivered over ADSL to corporate and home consumers.

Video Streaming Applications

Below are some services that have the potential to increase channel revenue for service providers.
TV over IP
Interactive TV
Business TV
News on Demand
Distance Learning

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