How MLB’s and T-Mobile’s ABS Challenge System Corrects Strike Calls in Seconds

Baseball can be a game of physics and statistics, so I was surprised during a recent major league game in Seattle when a technical measurement sent the crowd into a tizzy.

In the May 1 matchup, Kansas City Royals pitcher Cole Ragans threw a fastball to hitter Leo Rivas of the Seattle Mariners toward the bottom-left corner of the strike zone. The umpire called a strike, putting Rivas out — until Rivas tapped his helmet twice.

That gesture activated an Automatic Ball-Strike, or ABS, challenge to review the pitch, which must be triggered within seconds of the ball being caught. 

All eyes turned to the big screen to watch an animation of the pitch. It showed, empirically, that the ball did nick the corner of the strike zone, confirming that the umpire had made the correct call, much to the groaning disappointment of the crowd.

ABS is the product of a joint collaboration between Major League Baseball and T-Mobile. The technology got called up to the majors this season after more than 7,000 games of testing in the minor leagues. The system is now used in all 29 MLB stadiums in the US (the Toronto Blue Jays’ ballfield hasn’t deployed ABS because T-Mobile is not licensed to operate in Canada).

Speaking at T-Mobile Park in Seattle before that early May game, John Stanton, owner and chairman of the Seattle Mariners (and founder of VoiceStream Wireless, which became T-Mobile), explained that the challenge system was developed after a new TV technology changed how people experienced the game.

Unlike fans in the stadium relying on umpire calls behind the plate, viewers at home could plainly see whether pitches were balls or strikes, thanks to a visible box for the strike zone on television. 

“It became apparent when there was a bad call. And the umpires were embarrassed by it, and we were in a situation where it undermined some of the credibility of baseball,” Stanton said. “It was important to find a way to fix that.”

How the ABS system calls strikes and balls

Behind the scenes — or rather, surrounding the park — are 12 Hawk-Eye cameras pointed at the pitching mound and home plate to track pitcher handoffs and ball movement. 

They communicate over a private T-Mobile 5G network within the park using Ericsson Dot radios, easily-overlooked finned discs the size of dinner plates mounted in various locations, including each team’s dugout and the press box above home plate. The network uses T-Mobile’s licensed N41 (2.5 GHz) spectrum to minimize interference.

Also key to the network is low latency, which averages around 2.3 milliseconds, according to a LinkedIn post by T-Mobile’s John Saw, president of technology and chief technology officer.

The ABS system analyzes the pitch, builds an animation of where the ball landed in the strike zone and projects it onto the big screen for the stadium’s fans to see. T-Mobile’s internal goal is a 17-second turnaround from when the player taps their head. Stanton said that, in practice, it happens within 15.4 seconds on average.

Although one could argue that the animation of the pitch crossing the plate and appearing in a zoomed-in representation of the strike zone isn’t technically necessary, it’s surely more compelling for the audience than a simple binary decision over whether it’s a strike. The lead-up to the call can feel as dramatic for the crowd as waiting to see if an outfielder will catch a high fly ball.

“When there’s a call made, the fans, the umpires and the players all find out at exactly the same time,” said Stanton, creating “that hold‑your‑breath dynamic that’s particularly special.”

MLB maintains a video record of all of the ABS challenges at the MLB Film Room, but fans can head to the league’s official ABS website to dig deeper into how the tech is affecting teams and players this year. Front and center are stats showing that, currently, 53% of calls have been overturned by an ABS challenge at this point in the season. 

While that ostensibly suggests that umpires are missing calls, players are only requesting reviews of the most uncertain situations — and anyone who’s seen an ABS result knows how these pitches often barely nick the edge of the strike zone. 

Is instant data ruining the spirit of the game?

What about baseball purists who see this as an encroachment of technology on a game that relies on the eyes and judgments of players and skilled umpires? I would have expected fans who think ABS is dampening the heart of the game to push back harder against even more Moneyball-style embrace of data over tradition.

T-Mobile’s Amy Azzi, vice president of sponsorships for sports, entertainment and hospitality, said that sentiment was their biggest point of nervousness. But MLB shared statistics with T-Mobile showing that 91% of respondents said ABS improved the game overall, and 76% said it improved the game experience. 

“This stadium just lights up when a call is overturned, or it becomes a rally moment for the crowd,” Azzi said.

Each team gets two ABS challenges during a game. If a challenge is overturned, the team that made it loses one of its challenges. If it’s upheld, the team keeps the challenge and can use it again later. So if a team challenges two calls it thinks were strikes, and the ABS system confirms they were strikes, the team can still make further challenges until they lose them.

ABS is also adding another strategic element to the game. Jerry Dipito, president of baseball operations for the Mariners, said that the team is learning that patience matters. 

“We always value patience and thoughtfulness among our players,” Dipito said, noting that’s even more important today with this technological option. “We just need to flip the switch at the right time on an ABS challenge to affect the outcome of a game.”

Even with precision ABS technology, it still doesn’t remove the human factor. Dipito gave an example of a game against the Minnesota Twins, who he said have been aggressive in using ABS challenges. They’d run out of their challenges in the sixth inning, and when it got to the ninth, Seattle threw two pitches that would have been identified as balls if a challenge were available.

However, Mariners catcher Cal Raleigh framed the pitch as a strike, something he’s historically been very good at, Dipito noted. 

“[Raleigh] effectively flipped the game in our favor because he made it look like a strike,” he said.

Although the Mariners eventually lost the early May game 7-to-6, it was still exciting to watch in person, with five home runs, four ABS calls and a home-team comeback that almost prevailed. At the end of the night, despite all the data and statistics, it’s still the experience that matters.