Question: what are the different types of time servers?
Answer: NTP. The Network Time Protocol (NTP) is the most frequently used protocol for time synchronization in a network. ... 
SNTP. Simple Network Time Protocol (SNTP) is a less complex implementation of the network time protocol. ... 
PTP-IEEE-1588. ... 
Legacy Network Time Synchronization Protocols. ... 
GPS. ... 
DCF77. ... 
WWVB. ... 
MSF.