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16.3.8.3 Synchronization alternativesTwo main alternatives of external time synchronization are available. Thesynchronization message is applied either via any of the communication ports ofthe IED as a telegram message including date and time or via IRIG-B.Synchronization via SNTPSNTP provides a ping-pong method of synchronization. A message is sent from anIED to an SNTP server, and the SNTP server returns the message after filling in areception time and a transmission time. SNTP operates via the normal Ethernetnetwork that connects IEDs together in an IEC 61850 network. For SNTP tooperate properly, there must be an SNTP-server present, preferably in the samestation. The SNTP synchronization provides an accuracy that gives 1 ms accuracyfor binary inputs. The IED itself can be set as an SNTP-time server.SNTP server requirementsThe SNTP server to be used is connected to the local network, that is not more than4-5 switches or routers away from the IED. The SNTP server is dedicated for itstask, or at least equipped with a real-time operating system, that is not a PC withSNTP server software. The SNTP server should be stable, that is, eithersynchronized from a stable source like GPS, or local without synchronization.Using a local SNTP server without synchronization as primary or secondary serverin a redundant configuration is not recommended.Synchronization via IRIG-BIRIG-B is a protocol used only for time synchronization. A clock can provide localtime of the year in this format. The “B” in IRIG-B states that 100 bits per secondare transmitted, and the message is sent every second. After IRIG-B there numbersstating if and how the signal is modulated and the information transmitted.To receive IRIG-B there are one dedicated connector for the IRIG-B port. IRIG-B00x messages can be supplied via the galvanic interface, where x (in 00x) means anumber in the range of 1-7.If the x in 00x is 4, 5, 6 or 7, the time message from IRIG-B contains informationof the year. If x is 0, 1, 2 or 3, the information contains only the time within theyear, and year information has to come from the tool or local HMI.The IRIG-B input also takes care of IEEE1344 messages that are sent by IRIG-Bclocks, as IRIG-B previously did not have any year information. IEEE1344 iscompatible with IRIG-B and contains year information and information of the time-zone.It is recommended to use IEEE 1344 for supplying time information to the IRIG-Bmodule. In this case, send also the local time in the messages.Synchronization via DNPThe DNP3 communication can be the source for the course time synchronization,while the fine time synchronization needs a source with higher accuracy. See thecommunication protocol manual for a detailed description of the DNP3 protocol.Section 16 1MRK 502 034-UEN -Basic IED functions494Technical Manual