Comcast today announced pricing for its forthcoming smartphone data plans and said the Xfinity Mobile service will be available as an add-on for Comcast home Internet customers before the end of June. There will be some good deals, including $12-per-gigabyte pricing for light Internet users, but “unlimited” customers who use more than 20GB a month will end up being throttled to about 1.5Mbps.

The mobile plans will be available only for Comcast home Internet customers.

The “unlimited” plan will cost either $45 or $65 a month, depending on which Comcast cable bundle you have at home, but cellular speeds are throttled for the remainder of the month after you pass 20GB. Mobile carriers that throttle unlimited plans usually do so only in times and places of network congestion, but there is no such caveat with Comcast: once you pass 20GB of cellular data, your cellular speeds will be slower until the monthly billing period is over.