As 5G networks start to go live, mobile operators are working on one technology that's less sexy than connected cars or gigabit-speed video but could cause a lot of headaches if it doesn't work.

The issue is timing: Most carriers in North America rely on GPS to tell their cell sites what time it is. That works for timing and clock synchronization in 4G, but 5G networks have much more strict requirements, and getting out of sync could cause a site to go down.

The problem is one of the biggest worries for carriers implementing 5G transport networks, said Heavy Reading analyst Sterling Perrin, who led a panel on the topic at the Big 5G Event in Denver this week. In a Heavy Reading survey last month sponsored by four equipment vendors, 45% of respondents said network synchronization was of high importance.

Cell sites need to be on synchronized clocks to avoid interference among signals on the same frequencies. The T-1 lines that once served as backhaul for cell sites had built-in timing, but when carriers switched to Ethernet, that went away. Now, in North America, timing usually comes from GPS units on top of cell towers, using the atomic clocks of the GPS satellite network.

Several big changes coming with 5G are forcing carriers to rethink this approach, Perrin said. First, most North American 4G networks use frequency-division duplex (FDD) spectrum, but 5G networks are based on time-division duplex (TDD), which requires tighter synchronization. On top of that, the virtualized radio networks coming with 5G have their own strict timing requirements, with tolerances as low as sub-microsecond. And in the dense 5G small-cell networks coming to cities, GPS units would be expensive to deploy and prone to coverage problems in urban canyons.

It's not enough to rely on GPS anymore, said Kevin Boyle, a customer solutions sales manager for Ericsson North America. If a 5G site loses its timing, the carrier will have less than an hour to fix it before the site shuts itself down to prevent interference with neighboring cells, he said.

"GPS today has been great, but if you get somebody with a jammer, or you get a GPS unit failure, you're going to have a problem with actually keeping your cell sites up," Boyle said. GPS jamming devices are illegal but can be found for less than $100 online, he said.

To prevent this, he recommends operators build in a backup for GPS. Ethernet now has timing mechanisms that allow for timing via the transport network, and carriers can distribute clocks around their infrastructure to keep cell sites synchronized, Boyle said.

For synchronization in its fledgling 5G network, Verizon is using GPS units at cRAN (centralized RAN) hubs that serve multiple cell sites as well as border clocks within the network. After using these redundant systems to ensure synchronization works, it hopes to move to a solely network-based solution in the future, said Glenn Wellbrock, Verizon's director of optical transport network architecture, design and planning.

Why this matters
As operators introduce 5G elements into their networks, reliability will be essential for getting customers to trust 5G services. Consumers will want to see predictable performance before paying a 5G premium, and enterprises will demand it before committing to the advanced services that the technology was built for. How mobile operators carry out the transition to 5G in their infrastructure, including implementing systems such as cRAN and new approaches to timing and synchronization, will be critical to making the new mobile applications they envision into commercial realities.

— Stephen Lawson, special to Light Reading. Follow him on Twitter @sdlawsonmedia.