Addressing the bandwidth demands of IPTV

Along with the rapid increase in subscribers, there are two amplifying trends that will pose significant challenges and opportunities to network designers: video-on-demand (VoD), and the shift to high-definition TV (HDTV).

IPTV covers both live TV and stored video. VoD usage today in IPTV systems is a paltry seven percent, meaning that 93 percent of content is broadcast. As service providers continue to roll out time-shift functionality, also known as network digital video recording (DVR), this penetration for video-on-demand should increase from seven percent to 70 percent or higher. As users shift from watching broadcast TV to VoD, this shift will drive a tenfold increase in the bandwidth used required in the core network.

At the same time, users are shifting towards HDTV. These signals require four to eight times more bandwidth than standard definition to accommodate the increased resolution of the signal. High-definition penetration in 2006 was 15 percent (Consumer Electronics Association) and is forecast to grow to 60 percent over the next five years. This implies at least a fourfold increase in network bandwidth simply to accommodate this migration.

Taken together, these three trends (increasing IPTV penetration, VoD usage and a migration towards HDTV) mean that the network bandwidth used by IPTV will increase a staggering 1000 times, or 100,000 percent. This rapid growth presents network designers with a set of architecture and design challenges that must be addressed in order to allow cost-effective, high-quality video deployments.

IPTV vs. TV over the Internet " the same thing?
The terms IPTV and Television over the Internet are often used interchangeably but refer to two different technologies.

TV over the Internet, popularized by YouTube, is video clips delivered over the Internet to consumers who watch it on a PC. The display is usually a small window on a monitor, and the video may or may not play smoothly depending on the quality of the Internet connection. TV over the Internet is typically used to watch short video clips and is not positioned as a replacement for broadcast TV.

In contrast, IPTV is broadcast-quality TV, intended to compete directly with cable or satellite operators. IPTV is typically offered over DSL or a fibre-to-the-home (FTTH) network, and is offered by the carrier (usually the local phone company) who owns the network.

IPTV systems normally include an IP set-top box (IP-STB) that sits beside the TV and converts from IP to video signals. It is the combination of the IP-STB and managed network that provide the user with the same experience as possible through cable or satellite and allow the phone companies to compete for TV services.

IPTV Architectures
Only three network elements are required for a minimalist IPTV implementation: video encoders to convert from broadcast TV to IP packets, an access network to connect subscribers to their service provider and a set-top box to convert from IP to video at the subscriber premise. More typical networks include several additional systems: VoD servers, ad insertion systems, billing platforms and Digital Rights Management (DRM) systems. A typical IPTV system is shown in Figure 2.

There are two variants to deliver broadcast and VoD streams over the "last-mile" from the central office to the subscriber.

"Pure" IPTV systems deliver all channels, both broadcast and VoD, over IP through the network to the subscriber. This is the architecture used by DSL providers and some of the FTTH providers. In this architecture, the broadcast channels are sent as multicast streams from the video encoding units to the head-end DSLAMs, which pick out the specific channels being watched by each subscriber and send only those channels down the DSL link towards the subscriber. VoD streams are handled the same way, and flow down the DSL link beside the multicast broadcast channels.

"Hybrid" IPTV implements RF-encoded video for the broadcast channels and IP streams for VoD. Fibre deployments often use hybrid architectures, with one wavelength of light carrying the video signals and a separate wavelength used for the IP traffic. In hybrid architectures, an Optical Network Terminal [ONT] at the subscriber premise is needed to convert from fibre to TV and Ethernet.

Bandwidth Requirements
IPTV is practical only because of the staggering compression levels that are possible with modern video codecs, as video streams can be compressed more than 100:1 using the latest MPEG-4 (H.264) codec. Even with this level of compression, IPTV is easily the highest bandwidth service ever mass-deployed into the network. A single hour of IPTV requires five Gigabytes, the same as 1,000 hours of voice and more than the annual amount of email a consumer will receive and send.