AT&T says it has replaced nearly half of the software in its vast operations with open-source software-defined networking (SDN) code.

Speaking to developers just before this year's CES conference kicked off on Tuesday, technology and operations veep John Donovan dropped that figure as evidence that the operator's SDN strategy is working.

Donovan said there are now "millions" of AT&T wireless subscribers connected to virtualized network services – for example, many will be relying on the so-called AT&T Integrated Cloud (AIC), which is based on OpenStack.

The US telco ended 2015 with AIC deployed to 74 of its locations around the world, and has more than 275 businesses using it, we're told. AT&T's internal tools and the customer-facing applications share the same code in the cloud.

OpenStack and SDN doesn't just mean the network as a whole is more resilient, since a failure in one zone doesn't cascade to others: Donovan said the configurability in AIC also makes it much easier to design an implementation or service to meet local regulations.

SDN also helps the company "contain security threats better," Donovan added.

While OpenStack is the most important of the SDN projects to AT&T's current requirements, he said the company is also contributing to OpenNFV, OpenDaylight, and ONOS.

Donovan's address to the developers starts at the 1hr 33min 40s mark of this webcast. ®