What’s an Edge QAM, Again?
by Leslie Ellis // November 27 2007
The quintessentially techie “QAM,” for quadrature amplitude modulation, is back in the engineering limelight, this time with a new prefix: Edge.
When you hear people talking “edge QAMs,” chances are high that you’ll hear the term “switched digital video” within a few sentences. That’s because digital video switches created the need for edge QAMs.
It goes like this: All digital cable services carried over cable plant use QAM modulation. It’s the conveyor belt. It’s what moves video, voice and data services from headends to homes.
(Brush-up basics: People tend to pronounce it as a word — “kwahm,” as in, rhymes with “Guam.” As for physical location, it’s a headend/hub thing: An unassuming, rack-mounted metal box, enclosing a series of slide-in cards. Carrying capacity goes like this: One QAM equals 38.8 Mbps of downstream data, or two to three HD streams, or 10 to 12 regular digital video streams. It is the equivalent of one analog channel.)
Up until recently, cable providers purchased QAMs on a service-specific basis. The QAM assigned to move that Web page into your cable modem, for instance, couldn’t also be used to move those digital video channels into your high definition TV.
Likewise, a QAM dedicated to video on demand (VOD) couldn’t also be used for switched digital video, even though their architectural constructs are identical.
Enter the edge QAM. It’s built to carry both VOD and switched digital video streams. Ultimately, the edge QAM will move three categories of service (those two plus the Internet Protocol side of the house, meaning data and voice.)
When people say “edge QAM,” then, they mean multi-purpose. They aren’t talking about where it is, physically — at the “edge” of the network, wherever that is. (For directions, see “A Pocket Map to the Edge of the Network,” in the 03/15/05 edition.)
In the big picture of cable technology development, the edge QAM is fairly unobserved — but bubbling with supplier activity. By my count, seven companies are building the gear, up from two a few years ago. Among the providers: Arris, Big Band Networks, Harmonic Inc., Motorola, RGB Spectrum, Scientific-Atlanta/Cisco, and at least one skunk-works outfit.
Cable operators look for three things when evaluating edge QAM innovation: Price, density, and, for lack of a better term, “open-ness.”
Price is predictable. Right now, QAMs run in the $250 range. The goal, however ambitious, is to lop off the zero.
One rack-mounted unit typically contains cards that operate between eight and 24 QAMs. That’s the “density.” One pointed area of innovation, then, is upping the density. Higher density, lower price.
And then there’s the “open-ness” piece. Switching is a big move. All major cable operators are or will be doing it as a way to preserve bandwidth. None of them want to get painted into a corner with a single, monolithic supplier, seeking to control the economics of what happens beyond the switch. They’ve seen that movie before, and they don’t like the ending.
An edge QAM is considered a network resource. It talks to at least two other devices: The “session manager,” which sets up the linkage between you and the VOD server (or the switch), when you choose to watch something, and the “resource manager,” which figures out which QAMs are supposed to be moving what stuff to where.
An industrial movement is well underway to open up the conversations (the “protocols”) between those linkages, so that operators can buy things modularly. A switch from one guy, a session manager from another, a resource manager from a third, and edge QAMs from whoever is the most dense, open, and affordable.
Mix and match is the name of the game.
From an overall cost perspective, edge QAMs matter because they currently represent something like 70% of the capital spend for deploying switched digital video, according to the MSO-side technologists who track such matters.
By opening things up and making them more modular, operators reason, the actual QAM costs come down, which brings that percentage down, which makes switching more affordable, which make Wall Street happy. Lather, rinse, repeat.
This column originally appeared in the Platforms section of Multichannel News.