There's a comment above that suggests that Comcast owns the physical layer and, as such, is allowed to limit CenturyLink's access on its infrastructure (though

There's a comment above that suggests that Comcast owns the physical layer and, as such, is allowed to limit CenturyLink's access on its infrastructure (though there is a contrasting reply to it). I've always wondered why CenturyLink's advertised speeds were so much slower.