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Translation Please

The quintessentially techie term “QAM,” for quadrature amplitude modulation, is back in the engineering limelight. This time QAM (pronounced “kwahm”) has 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.

One QAM equals 38.8 Mbps of downstream data. This equates to two to three HD streams, 10 to 12 “regular” digital video streams, or 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 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,” they really mean multipurpose, not being at the “edge” of the network, wherever that is.


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.

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