Gearing for VOD: Architecture, Sizing, QAM


As the year of video-on-demand opens, it seems timely to respond to a mailbox full of inquiries about the "hows" of VOD technology, architecture sizing and related modulation implications.

If cable providers do what they've said they'll do this year-lead interactive services with a fairly hefty push on VOD rollouts-then VOD will become the industry's first major foray into session-based services. Watch for Time Warner Cable to be the most active, igniting the technology in as many as half of its 38 regions by the end of 2001.

AT&T Broadband, Comcast Corp., Cox Communications Inc., Charter Communications Inc. and the rest of the pack plan to be similarly aggressive with '01 VOD launches.

Getting ready for VOD starts with understanding how much you'll need in terms of server storage space, headend equipment and bandwidth capacity.

If you believe VOD simply requires servers and receptive set-tops, think again. There is more.

At the least, to properly offer VOD you'll need: video file servers; channel upconverters (to take the raw output of the servers and put it on a tuneable channel); quadrature amplitude modulation (QAM, pronounced "kwahm," a commonly used digital-modulation technique in cable); security methods; and set-tops capable of handling rentable VOD content. On the back end, you need hooks into your billing software to collect rental fees.

An architectural rule of thumb for VOD modeling is to start with fully saturated homes passed by cable services, on a per-node basis.

Say you've already built your system to run fiber out to nodes serving 500 customers, and that, ultimately, you expect 80 percent of those homes to take basic-cable services. That yields a starting point of 400 customers.

Next, assume half of those homes will ultimately take a digital box. You're up to 200 VOD-capable homes, over time. (If you start with your current digital penetration number-20 percent to 30 percent-you run the risk of undersizing the network's future VOD needs, which will cost you later.)

Calculating how many simultaneous VOD streams you'll need comes next. Currently, the thinking is to assume that at any given time, one in 10 people (10 percent) opt to watch movies at the same exact time. Using that math, you'll need to organize equipment and bandwidth to serve 20 concurrent VOD video streams.

(At this point, it should be noted that straight math doesn't always correlate to actual human action. Think of your neighborhood. Maybe you rent one or two videos each weekend. But chances are everyone in your neighborhood doesn't rent flicks with the same regularity. In this writer's house, for example, about 2 films are rented every two or so months, then returned well past the due date. That creates a bit of domestic debate over who's next to go to Hollywood Video, because each subsequent trip necessitates stopping at the ATM to pick up enough cash to pay the late fees.)

We're up to bandwidth sizing for 20 VOD streams. This is where digital modulation comes into play. Modulation, simply put, is the process of imprinting information onto a communications carrier so it can get from one place (the headend server) to another (the VOD-capable set-top).

Cable currently uses a digital-modulation technique known as 256-QAM, which equates to about 38 Mbps of useable bandwidth per 6 MHz channel.

Handily, each digital-video film that's been compressed with MPEG-2 (Moving Picture Expert Group) uses roughly 3.8 Mbps of bandwidth. Divide 256-QAM's capacity (38 Mbps) by MPEG-2's data rate (3.8 Mbps), and you get 10. This means you can stuff about 10 films into a single 6 MHz cable channel. If you need 20 simultaneous VOD streams, you need two 6 MHz channels-assuming a peak 10 percent level of simultaneous usage by VOD customers.

The process of placing digitized content into the HFC system for receipt by digital boxes varies, predictably, from manufacturer to manufacturer. Simply put, here's what one needs to do to get a VOD movie to a home:

Digitize and compress it (MPEG-2); Store it (the work of companies like Concurrent Computer Corp., Diva Systems Corp., nCUBE Corp. and SeaChange International) Inc.; Upconvert the output of the server to a specific channel location; Wrap it with conditional access and encryption safeguards against theft; Multiplex (smoosh) it onto a carrier using 256-QAM; And send it to the digital set-top box.

This can happen in varying order. Both of the industry's major suppliers-Motorola Broadband Communications Sector and Scientific-Atlanta Inc.-ultimately execute the same functions, but do so in different order.

As VOD becomes reality, instead of models and tests the name of the game will be scale-the ability to augment the model with more serving capacity, and more 6 MHz channels dedicated to VOD (thus, more QAMs). Until then, the models and equipment lists cited here should yield at least an intellectual start to the process.

Stumped by jargon? Dazed at cable's technology acronym soup? Send your requests to, with "Translation Please" in the header.