Time servers are dedicated network computers that provide time-synchronization functions for all of the devices in the network. They provide a common frame of reference (time) for network routers and help ensure the accuracy of network logs.
Time servers contain an internal oscillator, a device that is used to generate repetitive signals. Normally, the oscillator’s active component is in the feedback loop. Time servers use several different types of oscillators. Examples include TCXO, OCXO, MCXO and rubidium. Temperature-compensated crystal oscillators (TCXO) use a thermistor network to generate a correction voltage to reduce frequency deviations over temperature. Oven-controlled crystal oscillators (OCXO) use temperature-control circuitry to hold the crystal and its circuitry at a precise, constant temperature. Time servers with microcomputer-compensated crystal oscillators (MCXO) and rubidium oscillators are also available.
Time Sources for Time Servers
There are many time sources for time servers. Network time protocol (NTP) is an Internet-standard that enables client computers to synchronize their times with master clocks at the U.S. Naval Observatory. NTP runs as a client program and sends periodic time requests to one or more servers. Server time stamps are used to adjust each client’s clock accordingly. The National Institute of Standards and Technology (NIST) automated computer time service (ACTS) is designed for analog modems that use regular telephone lines. NIST ACTS transmits ASCII time codes which are used to set computer clocks. The NIST Internet time service (ITS) works with digital modems, digital subscriber lines (DSL), cable modems, and wireless modems. NIST ITS uses multiple stratum-1 or straum-2 time servers. Internet time code protocols are defined by requests for comments (RFC). The International Telecommunications Union radio bureau (ITU-R) provides another time source for time servers. Global positioning system (GPS) time transfers use GPS satellites and indoor or outdoor antennas.
Specifications for time servers include outputs, network protocols, and security protocols. Choices for outputs include Ethernet 10 Base-T or Ethernet 100 Base-T; serial time codes via RS-232 or RS-485 communications; lettered IRIG time codes from the Inter-Range Instrumentation Group (IRIG); and alarms, events, timers, and relays. Time servers with one pulse per second (1PPS) and 10-MHz outputs are also available. Network protocols for time servers include NTP versions 2, 3 and 4; simple network management protocol (SNMP) versions 1, 2, and 3; Internet protocol versions 4, 5, and 6 (IPv4, IPv5, and IPv6); message digest 5 (MD5); dynamic host control protocol (DHCP) and DHCP version 6 (DHCP6); lightweight directory access protocol (LDAP), remote authentication dial-in user service (RADIUS), and Syslog. Hypertext transfer protocol (HTTP), file transfer protocol (FTP) and Telnet are common security protocols for time servers.