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.
1. A method of estimating a time error between a first clock at a first node and a second clock at a second node, comprising:
sending a first node time value from the first node to the second node over a physical layer transport that is coupled to the first and second nodes;
sending a second node time value from the second node to the first node over the physical layer transport; and
determining an estimate of the time error between the first clock and the second clock based on a first time interval value and a second time interval value, wherein the first time interval value represents a difference between the second node time value and a clock value of the first clock at the time the first node received the second node time value, and the second time interval value represents a difference between the first node time value and a clock value of the second clock at the time the second node received the first node time value.
2. The method of claim 1 , wherein the estimated time error between the first clock and the second clock, as determined, is further based on a first node transmit delay (dt1), a first node receive delay (dr1), a second node transmit delay (dt2), and a second node receive delay (dr2).
3. The method of claim 2 , wherein the estimated time error is calculated as: (TIC1−TIC2)/2−(dt2−dt1+dr1−dr2)/2, wherein TIC1 corresponds to the first time interval value and TC2 corresponds to the second time interval value.
4. The method of claim 3 , wherein the estimated time error is calculated using a field programmable gate array.
5. The method of claim 1 , wherein the second node sends the second time interval value to the first node and the estimated time error is calculated at the first node.
6. A method of estimating a latency in a physical layer transport that is coupled between a first node having a first clock and a second node having a second clock, comprising:
sending a first node time value from the first node to the second node over the physical layer transport;
sending a second node time value from the second node to the first node over the physical layer transport; and
determining an estimate of the latency in the physical layer transport based on a first time interval value and a second time interval value, wherein the first time interval value represents a difference between the second node time value and a clock value of the first clock at the time the first node received the second node time value, and the second time interval value represents a difference between the first node time value and a clock value of the second clock at the time the second node received the first node time value.
7. The method of claim 6 , wherein the estimated latency in the physical layer transport, as determined, is further based on a first node transmit delay (dt1), a first node receive delay (dr1), a second node transmit delay (dt2), and a second node receive delay (dr2).
8. The method of claim 7 , wherein the estimated latency is calculated as: (TIC1+TIC2)/2−(dt2+dt1+dr1+dr2)/2, wherein TIC1 corresponds to the first time interval value and TC2 corresponds to the second time interval value.
9. The method of claim 8 , wherein the estimated latency is calculated using a field programmable gate array.
10. The method of claim 6 , wherein the second node sends the second time interval value to the first node and the estimated latency is calculated at the first node.