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Forget that Loon's balloon burst, we just fired 700TB of laser broadband between two cities, says Alphabet

Up to 20Gbps link sustained over the Congo in comms experiment

Engineers at Alphabet's technology moonshot lab X say they used lasers to beam 700TB of internet traffic between two cities separated by the Congo River.

The capitals of the Republic of the Congo and the Democratic Republic of Congo, Brazzaville and Kinshasa, respectively, are only 4.8 km (about three miles) apart. The denizens of Kinshasa have to pay five times more than their neighbors in Brazzaville for broadband connectivity, though. That's apparently because the fiber backbone to Kinshasa has to route more than 400 km (250 miles) around the river – no one wanted to put the cable through it.

There's a shorter route for data to take between the cities. Instead of transmitting the information as light through networks of cables, it can be directly beamed over the river by laser.

In an effort dubbed Project Taara, X built two terminals, one in Brazzaville and another in Kinshasa, to transmit and receive data encoded in beams of laser light.

“In the same way traditional fibre uses light to carry data through cables in the ground, Taara’s wireless optical communication links use very narrow, invisible beams of light to deliver fiber-like speeds,” Baris Erkmen, Director of Engineering for Taara, explained today.

“To create a link, Taara’s terminals search for each other, detect the other’s beam of light, and lock-in like a handshake to create a high-bandwidth connection.”

About 700TB of data was exchanged over 20 days at speeds of up to 20 Gbps, with 99.9 per cent availability, with the help of Econet – the multinational telecoms giant, not the old Acorn networking system. The aim of the setup was to relay broadband internet traffic between the cities more as a test of the equipment than anything else.

A lot of effort went into tracking and pointing the light beam at a sensor a few kilometres away, and mitigating the effects of poor weather, interference from animals, and the like.

Diagram by X of Project Taara beaming broadband over a river

X's diagram of Project Taara beaming broadband between two cities

X has experimented with wireless optical communication techniques using lasers in various projects over the years, including from high-altitude balloons in the now-defunct unit Loon. In the past, it was more difficult to maintain bandwidth connections over large distances. Small disturbances in weather conditions, fog, or even birds flying across the laser beams was enough to interrupt internet service.

Erkmen said that the latest terminals installed in the Republic of the Congo and the Democratic Republic of Congo, however, have improved. They’re able to automatically adapt and can swivel their internal mirrors or direct their sensors to adjust to changing conditions in order maintain a direct line-of-sight connection with each other.

“Imagine pointing a light beam the width of a chopstick accurately enough to hit a 5-centimeter target (about the size of a US quarter) that's ten kilometers away; that’s how accurate the signal needs to be to be strong and reliable,” he said.

In an email to The Register, Erkmen told us: "Taara’s wavelength is in the near infrared part of the spectrum, near 1550 nm wavelength. This is a standard telecommunications wavelength for fiber and is also a wavelength at which atmospheric propagation has low loss."

"Over the last few years we’ve been developing a system that can withstand more challenging conditions like rain and that can auto-recover without operator intervention when conditions become better. Taara’s beams are precisely controlled and work a little bit like a telescope, relying on mirrors, lights, software, and hardware to move the beam to exactly where it needs to be.

Taara’s beams are precisely controlled and work a little bit like a telescope, relying on mirrors, lights, software, and hardware to move the beam

"We’ve been working on improving our pointing and tracking technologies with a combinations of hardware and software to better withstand normal environmental pressures like temperature fluctuations or rain and maintain error-free data transmission even in the presence of short interruptions (e.g., bird flying through the link or an unexpected disturbance to pointing).

"The terminals on the two ends of the link are continually monitoring the received power and performance as well as motion of the platform (e.g., if the tower is swaying in the wind or a truck is rumbling by) to make real time decisions on what is the optimal amount of power to transmit on the link. So when it rains the system can respond by compensating with more power."

X has, meanwhile, previously tested its wireless optical communications technology in India and Kenya. ®

Editor's note: This story was updated to include further information from Baris Erkmen and to clarify that X is a Google-stablemate within Alphabet.

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