Network clock sources are timing devices that use a signal from an atomic clock or other external reference source to set and maintain a central time for all of the elements in a network. They minimize the number of external time-synchronization links, provide a common frame of reference (time) for all network devices, and help ensure the accuracy of network logic.
Network clock sources can use an asynchronous transfer mode (ATM) switch to receive clocking signals from a private branch exchange (PBX), a building integrated timing supply (BITS), or another ATM switch. Typically, a PBX receives its timing signal from a public switched telephone network (PSNT). This method that is suitable for applications in which the network is already connected to telephone equipment via circuit emulation services (CES).
Network clock sources that use a BITS can use a T1, E1, or other high-speed connection to provide DS1 timing to all of the network devices within a building, office, or facility. DS1 timing outputs are derived from the optical line rate and are relatively jitter-free.
Specifications for network clock sources 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. Network clock sources with one pulse per second (1PPS) and 10-MHz outputs are also available. Network protocols for network 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 network clock sources.