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McKay Brothers, a firm that sells high capacity links to trading firms, is connecting the financial districts of New York City and Chicago with a new network that aims to execute the fastest trades in the country. This new network, however, won’t be built on private fiber lines. Instead, McKay is taking to the airwaves, building a chain of tower-mounted microwave radios between the Big Apple and the Windy City.

Why is microwave faster? Electromagnetic waves travel at different speeds depending on the materials they traverse, said Gary Croke, director of product marketing for Aviat Networks(s avnw), the vendor McKay selected to supply its Eclipse Packet Node radios. Optical fiber has a much higher refraction index than plain old air, thus slowing down light waves moving through it, Croke said in an e-mail interview. By sending microwaves over the air, McKay is able to cut down the round-journey of a packet by 4 milliseconds, Croke claimed.

That may not sound like much, but according to McKay and Aviat, those 4 millionths thousandths of a second can make all the difference in the world to a high-frequency trader. Executing a trade even the tiniest fraction of a second earlier than a competitor can lock in the smallest change in pricing, which, when applied over massive volumes of shares or commodities, can translate into millions dollars. Aviat estimated said that a single millisecond advantage could equate to an additional $100 million a year to large high-frequency trading firm.

McKay isn’t revealing the exact route or number of towers of radios used in the network, which is scheduled for completion this summer, though it is sure to use as few links as possible. The more times a transmission passes from radio waves to electronic components the more it’s slowed down. Fiber transmissions have the same limitations, passing through routers and repeaters many times along their journey. McKay wants to ensure its not only using the fastest medium for transport but also that it has the fewest hops.

Fiber also has to move burrow under streets and wind around obstacles, lengthening the overall distance over which signals must travel. Meanwhile, microwave, in most cases, can take the most direct path as transmitters must only be within each other’s line of sight. The only fiber components in McKay’s network will be in the end cities itself, completing the final mile link between the end-point transmission towers and McKay’s financial firm customers.