US Patent 7535931 Two-way timing protocol

A two-way time transfer protocol includes: sending a signal from a first node including a first clock and a first time interval counter coupled to the first clock over a transport physical layer coupled to the first node to a second node coupled to the transport physical layer, the second node including a second clock and a second time interval counter coupled to the second clock; then sending a last second node time interval counter value from the second time interval counter of the second node over the transport physical layer to the first node; and then comparing at the first node a last first node time interval counter value to the last second node time interval counter value. A first-way path latency from the first node to the second node is substantially equal to a second-way path latency from the second node to the first node, and all of a first node transmit delay, a first node receive delay, a second node transmit delay and a second node receive delay are substantially constant.