DOCSIS 3.1 isn't exactly spreading like wildfire yet, but it is officially in commercial deployment now, and additional field trials have proven that the technology works in a variety of plant environments for serving up gigabit speeds.

Jorge Salinger, vice president of access architecture for Comcast Corp. (Nasdaq: CMCSA, CMCSK), shed some light on his company's early D3.1 rollouts in a paper presented at this week's SCTE Cable-Tec Expo in Philadelphia. According to Salinger, Comcast has "hundreds of customers" using D3.1-powered gigabit services in Nashville, Atlanta and Chicago so far. Salinger emphasized the importance of spectrum allocation, equipment configuration testing and technician training for any operator considering a DOCSIS 3.1 future.

For Comcast's part, Salinger pointed out that the company is using spectrum for D3.1 that it reclaimed from its migration to digital video from analog years ago. He added that Comcast is in the process of moving all of its HD video delivery to MPEG-4, which will free up further capacity for the rollout of D3.1-enabled gigabit services.

So far, Comcast is only deploying DOCSIS 3.1 in its downstream channels. But the modems it's putting in customer homes support D3.1 in the upstream as well, giving Comcast the ability to switch on that functionality for higher upstream speeds when ready.

One pleasant surprise Comcast discovered in its early work with DOCSIS 3.1 is that existing D3.0 modems can be used to monitor D3.1 signals. The legacy modems certainly weren't built with that function in mind, but they're nonetheless able to show how D3.1 signals are propagating through the network even if they can't interpret what those signals mean. This is useful for helping operators figure out how to configure their D3.1 networks for optimal performance.

Speaking alongside Salinger, Jason Miller, technical marketing engineer at Midcontinent Communications (Midco) , was also able to tout successes in D3.1 testing. Midco has run several D3.1 field trials, beginning with a test in Fargo, North Dakota where the company was already operating on a 1GHz plant. In a technical paper submitted to the SCTE, Miller goes into further detail on how different QAM modulation profiles impact network performance, and where operators should be aware of differences in error reporting with DOCSIS 3.1 equipment compared to traditional DOCSIS 3.0 modems.

Miller also cautioned at the Cable-Tec Expo event that the availability of D3.1 modems for field trials is still extremely limited. All told, he estimated that Midco had fewer than 20 of the next-generation modems in lab tests and field trials across Fargo and Sioux Falls, S.D. (See Gigabites: Google's Back on the Pole.)