Part 2: To Gig-E or Not to Gig-E7/07/2002 8:00 PM Eastern
When we left off last time, we'd tapped into the meaning of this new thing called "gigabit Ethernet," or "Gig-E." This time, a look inside the thought processes of the industry's architecturally minded technologists, as they mull whether Gig-E's contributions to signal transportation make good sense.
Generally, Gig-E's proponents see it as an inexpensive way to outfit cable plant for the billowing bandwidth needs of on-demand TV, beyond films. Detractors say that Gig-E may be neither as cheap nor as fast as promised.
The answers lie partly in a cost comparison between digital-video storage and bandwidth, and partly in common sense about what works and doesn't work for specific cable systems. Thus, this week's translation will largely center on the philosophy of technology decisions.
To believe in the cost benefits of Gig-E is to believe in the inevitability of the many letters now preceding "-OD" (on-demand) in industry conversations. There's "V" and "SV," the old standbys, for "video" and "subscription video." But there's also "FOD" (free-on-demand), "EOD" (everything-on-demand), "GOD" (games-on-demand) and "E-I-E-I-OD" (kidding).
Offering on-demand service is suddenly about a lot more than movies. TV shows, sports events, short-form how-to clips, and any other digitized video material would also roost on those video-on-demand (VOD) servers, along with the movies. Consumers could watch what they wanted, when they wanted, with all the VCR-like features of VOD — fast forward, rewind, pause.
HUB OF THINGS
Today's VOD offerings — again, mostly films — are usually stored on servers in distribution hubs. Each hub manages the flow of signals to and from about 200,000 homes passed by cable service. The hub is also the aggregation point for the 500-home nodes you always hear about when people talk system architecture.
Say a system has five such hubs. Making more on-demand stuff available for customers would mean duplicating all of that material, five times. Doing so is expensive and unwieldy, Gig-E people say. Maybe it's cheaper to centralize the servers in one headend, and switch the video out over Gig-E to the hubs?
Here's where it makes sense to look a bit deeper. Let's assume a 50 percent penetration rate for digital-video service to that hub, which passes 200,000 homes (some MSOs go much higher). You're down to 100,000 homes, ready to watch on-demand TV and movies.
This is where the math changes. Early on-demand experiments with non-movie video show usage patterns that are radically different.
Most common VOD (i.e. movies-on-demand) models assume that at any given time, the network must be ready to support 10 percent of the people in that hub (1,000) to request the same video stream, at the same time. They call this the "peak simultaneous" usage rate.
Yet in TV on-demand, usage peaks could go much, much higher. Nobody knows for sure how much higher — it's too soon.
Let's go completely mad and say that at any one time, half of the people who could do on-demand TV viewing would do on-demand TV viewing. That would mean you'd need to store and prepare as many as 50,000 streams at one time — and that's just for one of the five hubs.
The other side of the model is to ascertain how much it would cost to centralize all the servers in the headend, put in a fast transportation method between the headend and the hubs — like Gig-E — and fold in a video-switching mechanism to use the available bandwidth more efficiently.
If you lived through the early 1990s chapters of cable's technology story — when MSOs and the telcos were poised to raid each other's core businesses — you're probably close to a gasp right now. Switch
the video? Isn't that industrial blasphemy?
If you didn't live through it, here's the recap: Back when cable and the telcos were starting to square off, some telcos picked switched digital video (SDV) technology to get TV signals into homes. For cable technologists, SDV was an easy target, scoffed at as "gold-plated," "frivolously expensive" and "unnecessary."
But then again, VOD was little more than convention glitz back then. Video wasn't yet digital. Hot sellers in set-tops the Scientific-Atlanta 8600x, and the General Instrument Corp. CFT-2200 — both analog. Gig-E was yet to become the grandchild of 10 megabit-per-second Ethernet, the fastest at that time.
There are no right or wrong answers yet to the question of Gig-E's applicability in lots-on-demand systems. In hubs where space is tight, it may make sense to put the servers elsewhere; other local conditions will skew the logic every which way.
But it's safe to say that Gig-E is worth consideration, and that switched video perhaps isn't the pariah it once was.