Sending 1000 bits over a 1 Gbps line takes 1 μsec. The speed of light in fiber optics is 200 km/msec, so it takes 0.5 msec for the request to arrive and another 0.5 msec for the reply to get back. In all, 1000 bits have been transmitted in 1 msec. This is equivalent to 1 megabit/sec, or 1/10 of 1% efficiency.