Symmetrical latency with TDM circuit emulated service
View Patent ↗A method and system are provided for allowing time-alignment of teleprotection measurements of power signals. Teleprotection observations are communicated between teleprotection ends through a packet switched network. At each end of a teleprotection segment, a teleprotection device communicates with the network through a router providing CES and located at the edge of the network. Clocks within the two routers are synchronized using IEEE 1588v2 signals. Using this synchronization, accurate one-way latency of data through the network between the two routers in each direction can be determined. The router at the incoming end of the faster path delays playout of packets by the difference between the two one-way latencies, thereby ensuring time-alignment of observations as they are sent from the routers to the teleprotection devices.
1. A method of providing Circuit Emulation Service over Packet (CESoP) over a packet switched network, the packet switched network including a first router in TDM communication with a first device and a second router in TDM communication with a second device, the first router and the second router being separated by the packet switched network, the method comprising:
determining the one-way latency from the second router to the first router, this latency being termed the inbound latency;
determining the one-way latency from the first router to the second router, this latency being termed the outbound latency;
comparing the inbound latency and the outbound latency;
if the outbound latency is lower than the inbound latency, initiating TDM playout to the first device when a jitter buffer reaches a playout level; and
if the inbound latency is lower than the outbound latency, initiating TDM playout to the first device after the jitter buffer reaches the playout level and then a duration equal to the difference between the inbound latency and the outbound latency has elapsed.
2. The method of claim 1 further comprising:
synchronizing the first router with a reference clock using IEEE 1588v2.
3. The method of claim 1 wherein initiating TDM playout to the first device after the jitter buffer reaches the playout level and a duration equivalent to the difference between the inbound latency and the outbound latency comprises:
waiting for the jitter buffer to reach the playout level;
then running a timer for the duration; and
initiating TDM playout upon expiry of the timer.
4. The method of claim 1 wherein determining each one-way latency comprises:
sending a first ping packet to the second router;
noting the timestamp of a second ping packet sent by the second router;
determining the inbound latency as the difference between the timestamp of the second ping packet and a local time of the router; and
determining the outbound latency as the difference between the round-trip time of the first ping packet and the inbound latency.
5. The method of claim 1 wherein each device is a teleprotection device, and wherein TDM playout comprises TDM playout of teleprotection measurements.
6. A first router providing Circuit Emulation Service over Packet (CESoP) to a first device, comprising:
a CESoP processor for receiving packets received over a packet switched network and playing out the packets into a TDM bitstream to the first device;
a jitter buffer; and
a symmetry enforcer for determining a difference between (1) the one-way latency from the first router to a second router providing CESoP to a second device and with which the first router is in communication over the packet switched network and (2) the one-way latency from the second router to the first router, and for initiating TDM playout to the first device after the fill level of the jitter buffer reaches a playout level and then a duration equal to the difference between the one-way latencies has elapsed.
7. The first router of claim 6 , wherein the symmetry enforcer is configured to synchronize the first router with a reference clock using IEEE 1588v2.
8. The first router of claim 6 wherein the symmetry enforcer initiates TDM playout by notifying the CESoP processor that TDM playout is to commence.
9. The first router of claim 6 wherein the symmetry enforcer initiates TDM playout by:
waiting until the fill level of the jitter buffer reaches the playout level;
then running a timer of duration equal to the difference between the one-way latencies; and
upon expiry of the timer, initiating TDM playout.
10. The first router of claim 6 wherein each device is a teleprotection device, and wherein TDM playout comprises TDM playout of teleprotection measurements.
11. A method of providing Circuit Emulation Service over Packet (CESoP) over a packet switched network, the packet switched network including a first router in TDM communication with a first device and a second router in TDM communication with a second device, the first router and the second router being separated by the packet switched network, the method comprising:
at each router, determining the one-way latency from the second router to the first router, this latency being termed the inbound latency for the first router and the outbound latency for the second router;
at each router, determining the one-way latency from the first router to the second router, this latency being termed the outbound latency for the first router and the inbound latency of the second router;
at each router, comparing the inbound latency and the outbound latency of the router;
at each router, if the outbound latency of the router is lower than the inbound latency of the router, initiating TDM playout to the device in TDM communication with the router when the jitter buffer of the router reaches a playout level; and
at each router, if the inbound latency of the router is lower than the outbound latency of the router, initiating TDM playout to the device in TDM communication with the router after the jitter buffer of the router reaches the playout level and then a duration equal to the difference between the inbound latency and the outbound latency has elapsed.
12. The method of claim 11 further comprising:
synchronizing each router with a reference clock using IEEE 1588v2.
13. The method of claim 11 wherein at each router, initiating TDM playout to the device after the jitter buffer reaches the playout level and then a duration equivalent to the difference between the inbound latency and the outbound latency has elapsed comprises:
waiting for the jitter buffer to reach the playout level;
then running a timer for the duration; and
initiating TDM playout upon expiry of the timer.
14. The method of claim 11 wherein each device is a teleprotection device, and wherein TDM playout comprises TDM playout of teleprotection measurements.
15. A system for providing CESoP between two devices, comprising:
a first router at the edge of a packet switched network and in TDM communication with a first of the devices, the first router having a jitter buffer, the first router initiating TDM playout of packets to the first device after the fill level of its jitter buffer reaches a playout level; and
a second router at the edge of the packet switched network and in TDM communication with the second of the devices, the second router having a jitter buffer, the second router initiating TDM playout of packets to the second device only after the fill level of its jitter buffer reaches the playout level and then a duration equal to the difference between the two one-way latencies between the routers has elapsed.
16. The system of claim 15 , wherein each router calculates each one-way latency.
17. The system of claim 16 wherein as part of calculating each one-way latency, each router sends a ping packet to the other router.
18. The system of claim 15 , wherein the first router and the second router are each synchronized with a reference clock using IEEE 1588v2.
19. The system of claim 15 each device is a teleprotection device, and wherein TDM playout comprises TDM playout of teleprotection measurements.