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.

The good news is the actual throttled speeds: we couldn't find it listed publicly anywhere, but a Comcast spokesperson told us that after 20GB, download speeds will be limited to 1.5Mbps, which is probably good enough for standard definition video streaming. With other carriers, throttled speeds are generally measured in kilobits per second.

(UPDATE: We've learned that upload speeds will be limited to 750kbps after unlimited data customers use 20GB in a month.)

Comcast is reselling Verizon Wireless mobile service thanks to a 2012 spectrum deal in which Verizon bought spectrum from cable companies. The 24/7 throttling after 20GB could limit the amount Comcast has to pay Verizon. Comcast also might not be able to fully monitor the Verizon network, which would make it hard to implement throttling only when there's network congestion.

Comcast mobile customers will be able to get faster speeds by connecting to Comcast Wi-Fi hotspots, of which there are 16 million around the country. Phones on the Comcast network will automatically connect to these hotspots when in range without the user having to select the Wi-Fi network or enter a password, Comcast said. But if you're over 20GB and not in range of Wi-Fi, you'll be throttled.

Comcast said the data limits are applied to cellular data only. Usage of Xfinity-branded Wi-Fi hotspots does not count against either home or mobile data caps.

There's no good way to purchase additional high-speed cellular data once you're past 20GB. You could switch to per-gigabyte pricing in the middle of a month, but that would be prohibitively expensive if you're using more than 20GB. This could eventually change, as Comcast said it is examining options for unlimited data users who go over 20GB.

$12 per GB, no zero-rating

Here are the other key points, based on the announcement materials, statements by Comcast executives during an investor call today, and information a Comcast spokesperson provided to Ars:

Each phone line comes with unlimited talking and texting.

There's no zero-rating. Comcast video will count against your data usage.

You need Comcast home Internet service in order to get Comcast mobile. While the unlimited plan is $65 a month for customers with basic Comcast service, the $45 unlimited price requires a subscription to Xfinity Premier Double or Triple Play, which starts at around $149 a month.

You can also purchase mobile service in 1GB increments, at $12 for each gigabyte of cellular data. A Comcast phone plan could therefore cost just $12 a month for 1GB of data and unlimited talking and texting.

You can mix and match in a family, so one person could have unlimited data while others pay by the gigabyte.

If you're using a lot of data one month, you can switch to the unlimited plan mid-month without any penalty in order to save money, and then switch back to per-gigabyte pricing the next month. You could also go the other way by switching from unlimited to per-gigabyte pricing if you're not using much data.

You can change plans in the Comcast mobile app. There's also 24/7 customer support via phone and text message.

There are no monthly "line access fees," so the full prices are as quoted above, plus the usual taxes and other fees.

Comcast will sell Apple, Samsung, and LG phones at retail price, either paid up front or spread over 24 months with a zero percent interest rate.

Comcast did not provide a specific availability date other than "Q2," meaning some time between now and the end of June.

You won't be able to bring your own unlocked phone to the Comcast mobile service at launch, but Comcast hopes to add that ability in the future.

It's about bundles, not nationwide market share

Though Comcast is relying on Verizon Wireless for cellular data, the cable company could end up building its own cell towers, as it may obtain some spectrum from the latest government auction. But Comcast really wants to lean on Wi-Fi. Most smartphone data usage is on Wi-Fi to begin with, and that proportion could increase with Comcast's strategy of automatically connecting customers to Xfinity Wi-Fi hotspots and imposing strict throttling on "unlimited" cellular data.

In the call with investors today, Comcast executives said they're not out to gain major nationwide market share. Instead, they want to increase the number of services existing Comcast customers buy and gain new customers who live in Comcast's footprint but haven't signed up for service. Comcast has nearly 25 million Internet customers in 39 states and Washington, DC.