Changes are coming to Comcast's Xfinity Mobile service running over Verizon's network.

Comcast announced that, starting in February, it will move all of its mobile customers onto the "second generation" plans it first revealed this summer. The most notable element in Comcast's "second generation" plans involves the addition of more By-the-Gig pricing options.

However, Comcast's new plans also include an important new caveat: speeds will be slowed during periods of "network congestion."

"We manage the flow of data depending on network capacity similarly to how other major U.S. carriers do so. When the network is experiencing unusually high levels of demand, some customers temporarily may have slower data speeds," the operator wrote in explaining the new situation. "'High levels of demand' is most often based on where you are physically. For example, if you're in a crowded stadium, you may notice temporarily slower speeds."

Continued Comcast: "In practice, the vast majority of Xfinity Mobile customers probably will not notice a difference in the network's performance. If your data is slowed down, videos might buffer or pages may take a little longer to load. We don't make distinctions based on what you're using your device for, or the source of the traffic. The only factor that matters is how much network capacity is available."

The operator concluded that its Xfinity Mobile customers would be able to change the situation by purchasing services such as "HD Pass," which offers "higher-quality cellular service in times of network congestion."

Why this matters
Xfinity Mobile is a service that leans on Comcast's MVNO deal with Verizon. In its most recent quarter, Comcast added 204,000 lines to Xfinity Mobile, ending the period with a total of 1.79 million mobile lines.

The Wall Street analysts at research firm Cowen estimate that Comcast pays Verizon around $700 million a year for wholesale access to its network; the companies haven't disclosed the terms of their agreement.

But Comcast's MVNO agreement with Verizon is presumably also subject to Verizon's network-management techniques, and Verizon has made no secret of its own efforts to handle network congestion. "On certain plans, we may prioritize your 4G LTE data behind other traffic," Verizon writes on its own site. "If the cell site you are connected to begins experiencing high demand during the duration of your session, your 4G LTE data speeds may be slower than the other traffic's."

Part of the reason that wireless network operators like Verizon prioritize traffic is because they only have a finite amount of spectrum. If traffic on their network continues to increase, but they don't get any more spectrum, they are forced to manage traffic in ways that avid overloading their networks or cutting off service to some customers.

This is all part of the reason that Verizon has been urging the FCC -- the government agency charged with overseeing the nation's spectrum resources -- to release more spectrum for commercial uses, including 5G.

— Mike Dano, Editorial Director, 5G & Mobile Strategies, Light Reading | @mikeddano