As part of its new traffic-shaping system, Comcast occasionally slows down Web visits during periods of high congestion. For customers who use companies like Vonage or Skype for voice-over-IP telephone service, the slowdowns could result in "choppy" phone calls. But Comcast does not intend to degrade its own digital telephone service, even in times of high congestion.

Last month, the FCC demanded that Comcast justify its "disparate treatment of its own VoIP service as compared to that offered by other competitors on its network."

Comcast responded by arguing that its own phone service is routed through a "managed IP network," while competitors like Vonage or Skype are routed over the public Web.

"Significantly, CDV (Comcast Digital Voice) customers do not need to subscribe to Comcast HSI (high-speed Internet) service," the company wrote in a letter to Dana Shaffer, chief of the FCC's wireline competition bureau, and Matthew Berry, FCC general counsel. "Many companies offer IP-enabled services over their networks, including voice and video services that are distinct from their high-speed Internet access service."

But net neutrality advocates aren't convinced. "Comcast still has some legal and technical questions to answer," said Ben Scott, policy director of Free Press. "For example, the filing does not state whether the use of Comcast Digital Voice contributes to congestion and triggers throttling. We'll continue to monitor this issue closely."

Last year, the FCC sanctioned Comcast for violating net neutrality principles by impeding peer-to-peer traffic. The broadband company acknowledged that it slowed such traffic to manage congestion, but while under investigation by the FCC, it promised to deploy protocol-agnostic techniques going forward.

Comcast's new system places users in two categories--"priority best effort" or the potentially slower "best effort." Subscribers start off in "priority best effort," but can be downgraded to "best effort" if they have used a high amount of bandwidth in a 15-minute period at a time of peak traffic. When those users decrease bandwidth use for a 15-minute period, they are returned to "priority best effort" status. Users who have been downgraded might notice choppiness in VoIP calls, but not calls placed through its own Internet telephone services.

This news story is important. I recommend that everyone read it.