AT&T counters cable threat with unlimited broadband, but for subscribers it comes at a cost.

AT&T will soon begin offering unlimited broadband data without any caps, an effort it is taking to keep customers from fleeing to cable after being penalized for using too much bandwidth. But the real issue at hand is the fact that many broadband subscribers don’t know what speeds they actually get, not just how much they use.

But customers have to shell out an additional $30 a month to get the unlimited data plan. A potential attraction is that a user can to switch unlimited home Internet data or add TV service at any time — even in the middle of their billing cycle.

While these plans seem compelling, the fact is most broadband subscribers don’t actually know what speed they actually get.

Parks Associates found that 43 percent of subscribers do not know their current broadband speed, while over one-third of consumers who switched service providers in the past 12 months did so in order to obtain a faster service at a comparable price.