Ever since the iPhone crashed AT&T’s network, it has become common cause that the increase in mobile data, or rather data that is transmitted over the air, is going to grow. A lot. Networks are already straining to cope. But that’s not the big problem, says Andrew Herbert, director of Microsoft’s Cambridge Lab. The real problem, he says, is latency. Latency is the delay between sending a signal and receiving an acknowledgment of receipt. The time it takes light to circumnavigate the Earth, about 133 milliseconds, is a fundamental constraint on network speed.