Comcast's gigabit cable service is now for sale in four US markets, with 11 more to be connected by early 2017.

Detroit, Michigan, was hooked up yesterday, joining Atlanta, Chicago, and Nashville as Comcast's existing gigabit cable cities. Comcast also said that ten new cities will get gigabit cable in early 2017. The company previously said that Miami, Florida, will have it before the end of this year.

The newly announced cities are Denver, Colorado; Indianapolis, Indiana; Jacksonville, Florida; Kansas City, Missouri; Knoxville, Tennessee; Portland, Oregon; San Francisco Bay Area, California; San Jose, California; Salt Lake City, Utah; and Seattle, Washington. In all, that's 15 cities that either have Comcast's gigabit cable now or will by early 2017.

Comcast has two gigabit Internet services, one that uses cable and another that uses fiber only. The one we've talked about so far in this article uses existing cable lines to deliver 1Gbps download speeds and 35Mbps upload speeds with version 3.1 of DOCSIS (Data Over Cable Service Interface Specification). Comcast should be able to boost upload speeds significantly with advances in DOCSIS 3.1 technology, but it hasn't announced any specific plans to do so yet.

While the upgrades are beginning in major metro areas, Comcast has said it plans to install DOCSIS 3.1 throughout its 39-state US territory by 2018. The upgrade requires new equipment in customers' homes and at the Cable Modem Termination Systems (CMTS) in Comcast facilities, but it can be scaled out more quickly than fiber because it doesn't require new cable lines.

Not everyone in each market will be eligible to purchase gigabit cable immediately upon the early 2017 rollouts; Comcast told Ars that availability will depend on the pace of DOCSIS 3.1 upgrades in each market. In Michigan, Comcast said the gigabit speeds are "available to the entire city of Detroit." Comcast said it plans to bring gigabit cable "into surrounding metro areas and across Michigan in 2017."

Comcast's other ultra-fast service is called Gigabit Pro and offers 2Gbps downloads and uploads over a fiber-to-the-home network. Comcast is not planning to install fiber throughout its entire territory, but it is already selling Gigabit Pro in parts of about 25 markets.

Despite the much slower upload speeds, many customers will prefer the 1Gbps service because of the price difference. Comcast has been selling Gigabit Pro for $300 a month with $1,000 in startup fees. The gigabit cable service has generally cost $140 a month without a contract or $70 a month when customers sign a three-year contract. Signing the contract also provides unlimited data, which normally costs $50 a month extra. (Signing up for that $70 deal has proven to be difficult in Chicago, however, with some Comcast reps not aware that it exists.)

Comcast hasn't imposed data caps throughout its territory yet, but it has greatly expanded the number of cities facing data caps as of yesterday.