Comcast today announced that its gigabit cable Internet service will be available in Atlanta and Nashville early this year, with the next deployments coming to Chicago, Detroit, and Miami in the second half of 2016. Exact deployment dates haven't been revealed.

Today's announcement also said that Comcast's fiber-based 2Gbps service, which launched last year, is now available to 18 million homes as planned . That's a sizable chunk of Comcast's network, which passes 55 million homes and businesses in 39 states and Washington, DC. The 2Gbps fiber service launched first in Atlanta, spreading next to Chicago, Detroit, Miami, Nashville, and other metro areas.

While Comcast's fiber-to-the-home service—known as Gigabit Pro—delivers 2Gbps both downstream and upstream, it won't be available throughout Comcast's territory. Comcast says it does plan to bring gigabit cable to "virtually" its entire territory over the next two or three years. That's due to version 3.1 of DOCSIS, the Data Over Cable Service Interface Specification, which can bring gigabit download speeds without the need to deploy fiber to homes. New modems will be required.

Upload speeds on the gigabit cable service won't be as fast as download speeds, though specifics haven't been announced. Comcast also hasn't said what its gigabit cable service will cost, but expect it to be pricey: the 2Gbps fiber service's standard price is $300 a month with $1,000 in startup fees.

Comcast is not applying its monthly data caps and overage charges to the fiber service, but they could still apply to gigabit cable. A Comcast spokesperson told Ars that "we don't have product details to share today in terms of price or data plans" for DOCSIS 3.1 service.

Comcast has also tested DOCSIS 3.1 modems in customer homes in Pennsylvania and Northern California, though neither is on the list of first commercial deployments.

Comcast's first gigabit cable deployments will overlap a bit with Google Fiber and AT&T's fiber service. Google says it plans to bring fiber to Atlanta and Nashville and is considering a deployment in Chicago. AT&T's "gigapower" is already available in parts of Atlanta, Chicago, Nashville, and Miami, and Detroit is in AT&T's plans.