AT&T Communications CEO John Donovan is retiring and leaving his position at the operator effective Oct. 1. Donovan joined the company as CTO in 2008, and has been leading the communications business for the last two years.

His departure follows an especially busy summer where Donovan played a key role in landing major deals with Microsoft and IBM to advance AT&T’s push into the cloud. He’s also been a central figure in the operator’s efforts around 5G, SDN, and virtualization.

“No other non-CEO has changed the company as much as he did. He put the foundation for SDN and got 5G going,” said Roger Entner, founder and lead analyst at Recon Analytics. “John is a visionary and how he implemented it. Now it’s time for someone else to execute and bring it over the finish line.”

Donovan has shepherded big changes at AT&T, including plans to gain software-defined control of 75% of its network operations by 2020; a 5G network that will be nationwide within 12 months; and migrating most non-network workloads to the public cloud by 2024.

The multi-year agreement with Microsoft, which is reportedly valued at more than $2 billion, will bring Office 365 to AT&T’s workforce and consolidate AT&T’s data center infrastructure and operations on Microsoft Azure.

“It’s been my honor to lead AT&T Communications during a period of unprecedented innovation and investment in new technology that is revolutionizing how people connect with their worlds,” Donovan said in a prepared statement. “All that we’ve accomplished is a credit to the talented women and men of AT&T, and their passion for serving our customers.”

AT&T Chairman and CEO Randall Stephenson described Donovan as a “terrific leader and a tech visionary who helped drive AT&T’s leadership in connecting customers, from our 5G, fiber and FirstNet build outs, to new products and platforms, to setting the global standard for software-defined networks.”

The general timing of Donovan’s retirement is not a surprise, according to Entner, but the announcement indicates that Donovan made the decision on his own. “You can see that he is leaving on his terms because they haven’t named a successor. Usually when they spring it on you they have a successor in mind,” Entner said.

As for who might replace Donovan, Entner believes five current AT&T executives are in the running: Lori Lee, CEO of AT&T Latin America and global marketing officer; Jeff McElfresh, president of AT&T Technology and Operations; Thaddeus Orroyo, CEO of AT&T Business; Rasesh Patel, senior executive vice president at AT&T Communications; and David Christopher, president of AT&T Mobility and Entertainment.

AT&T says it will announce a replacement for Donovan soon.