Comcast has begun selling its new gigabit cable service in parts of Atlanta, and the company is heavily pushing customers toward three-year contracts as it tries to fend off a challenge from Google Fiber.

Customers will be able to buy the Internet service for $70 a month and not face any data caps if they sign a three-year deal that has an early termination fee. Without a contract, customers would have to pay $139.95 a month and face a 300GB-per-month data cap.

Customers on the no-contract option can upgrade to unlimited data for an extra $35 a month. Thus, Comcast's gigabit Internet service with unlimited data costs $70 per month with a contract and about $175 without. (A DSLReports article described the data cap details today, and Comcast confirmed them to Ars.)

While there are no activation or installation fees for the gigabit cable service, the early termination fee for contract customers is $350 "but would drop monthly on a sliding scale for the duration of the three-year contract," Comcast said.

Comcast is using DOCSIS 3.1 technology to deliver gigabit download speeds over cable, but the upload speed is only 35Mbps, Comcast told Ars. By contrast, Comcast's fiber-to-the-home service offers 2Gbps in both directions but is also a lot more expensive.

Comcast's DOCSIS 3.1 deployment comes shortly after Google Fiber went live in apartments and condos in Atlanta. Comcast has also been distributing flyers urging customers not to "fall for the hype" of Google Fiber.

Google Fiber charges $70 per month for symmetrical gigabit Internet. A $100 installation fee is waived if customers sign a one-year commitment. Google is also offering Atlanta customers 100Mbps service for $50 a month.

Comcast has deployed residential fiber in a bunch of cities, but Atlanta is the first to get Comcast's gigabit cable. The company's announcement today described the Atlanta deployment as a trial in "a number of neighborhoods," with customers being encouraged to offer feedback about the service. With Comcast planning to deploy DOCSIS 3.1 throughout its US territory, the feedback "will be used to help ensure future market rollouts deliver the best possible customer experience," the company said.

Pricing could also be different when gigabit cable launches in other cities. "Once this advanced consumer trial is complete, Comcast plans to roll Gigabit service out at additional price points in other markets to gauge consumer interest in Gigabit speeds," Comcast said.