Haider Khan M.T., Dmitriev V.N.

Astrakhan State Technical University,Astrakhan,Russia.

SYNCHRONIZATION OVER PACKET NETWORK USING PTP

The PTP periodically transmits synchronization messages to update the time on a master-slave basis. For that an algorithm from master election, management, and delay compensation is set on one of the upper layers of the communication stack. In the LXI case, this is the application layer where PTP communicates over User Datagram Protocol(UDP). The standard itself, however is implemented to the communication technology. Although the protocol can be implemented in software, high-precision timestamping has to be done in hardware in order to cancel out protocol stack jitter.

Distribution of timing(frequency and/or time of day) in packet network is achieved by a timing-over-packet protocol. Such protocols function by sending packets between a timing master and a timing slave. The timing master is usually connected to a highly accurate and stable frequency/time reference(PRC or PRTC). The timing packets must be formatted as valid packets for the network, and are transported from master to slave in the same fashion as all packets in the network.

PTP protocol can run in any networking environment. In a complicated system, however, there are usually some edge bridges or edge routers that do not support PTP. These devices cause relatively large delay and delay jitter. As a result, clock synchronization cannot achieve the expected precision.

IEEE 1588 provides this by defining a protocol known as the precision time protocol, or PTP. A heterogeneous network of clocks is a network containing clocks of varying characteristics, such as the origin of a clock’s time source, and the stability of the clock's frequency. The PTP protocol provides a fault tolerant method of synchronizing all participating clocks to the highest quality clock in the network. IEEE 1588 defines a standard set of clock characteristics and defines value ranges for each. By running a distributed algorithm, called the best master clock algorithm (BMC), each clock in the network identifies the highest quality clock; that is the clock with the best set of characteristics. The highest ranking clock is called the ‘grandmaster’ clock, and synchronizes all other ‘slave’ clocks. If the ‘grandmaster’ clock is removed from the network, or if its characteristics change in a way such that it is no longer the ‘best’ clock, the BMC algorithm provides a way for the participating clocks to automatically determine the current ‘best’ clock, which becomes the new grandmaster.
Slave clocks synchronize to the 1588 grandmaster by using bidirectional multicast communication . The grandmaster clock periodically issues a packet called a ‘sync’ packet containing a timestamp of the time when the packet left the grandmaster clock. The grandmaster may also, optionally, issue a ‘follow up’ packet containing the timestamp for the ‘sync’ packet. The use of a separate ‘follow up’ packet allows the grandmaster to accurately timestamp the ‘sync’ packet on networks where the departure time of a packet cannot be known accurately beforehand. For example, the collision detection and random back off mechanism of Ethernet communication prevents the exact transmission time of a packet from being known until the packet is completely sent without a collision being detected, at which time it is impossible to alter the packet’s content.

For Ethernet to become a carrier-grade technology in WANs, operators and vendors have introduced several key technologies for transport of timing and synchronization over packet networks, including adaptive clock recovery, synchronous Ethernet, and precision time protocol (PTP, IEEE 1588). Selection and verification of the appropriate technology requires careful analysis and test in labs and in the field. Verification must include the characterization of jitter/wander performance of the network at TDM and Ethernet interfaces.

References

1. IEEE, 1588-PTP, Standard for a Precision Clock Synchronization Protocol for Networked Measurement and Control System, 2008.

2. Introduction to Distributed Clock Synchronization and the IEEE 1588 Precision Time Protocol http://www.ni.com

3. Technical Communication on Synchronous Ethernet and IEEE 1588v2.