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097-55501-01 Revision M – January 2009
TimeHub 5500 User’s Guide
75
Chapter 2 Engineering Ordering Information
NTP Server Card
The TimeHub system is responsible for providing the 1.544 MHz signal. When all
frequency reference inputs to TimeHub are lost or disqualified, the stability of the
125 MHz NTP clock is determined by the holdover characteristic of the active
TimeHub clock card.
When the TOD input is lost, the NTP Server itself goes into a time holdover mode.
The 1 PPS is no longer available for phase alignment of the 125 MHz NTP clock.
The quality of the NTP clock in this time holdover mode will depend on the stability
of the active TimeHub clock card.
NTP Server or Client
A NTP Server can be configured to be either a primary server (stratum 1) or a
secondary server (stratum 2 and lower).
If the NTP Server card is connected to a stratum 0 TOD source (GPS), it is
configured to be primary NTP server. A primary NTP server cannot be a NTP client.
If the NTP Server card is not connected to a stratum 0 TOD source, it is configured
to be a secondary server. A secondary server is both a client and a server. It obtains
its TOD by being a client to peer NTP servers of the same or higher stratum level. It
is also a time server to lower stratum level servers or clients.
The TimeHub NTP Server (whether configured as a primary or secondary server)
supports both unicast and broadcast/multicast server functions.
Server Unicast
In the Server Unicast role the TimeHub NTP Server responds to individual clients'
requests and exchanges NTP packets with them. User sets the IP addresses of port
A and port B. If a pair of NTP cards is installed in a group, the IP address of port A
will apply to both cards, and the IP address for port B will apply to both cards.
Server Broadcast/Multicast
In the Broadcast/multicast role, the NTP Server card sends NTP packets
periodically without being requested. The user sets the TimeHub NTP Server card's
broadcast/multicast addresses, the MD5 key IDs if authentication is used, and
broadcast time interval (from once every 16 seconds to once every 17 minutes). For
each active TimeHub NTP Server card the user can define up to a total of four
broadcast and multicast addresses. Which NTP port is used to transmit the
broadcast or multicast packets will depend on two things: the bonding status of the
NTP card and whether a broadcast/multicast address is on the same subnet as one
of the two NTP ports:
When the two ports are bonded:
Multicast will use the active port
Broadcast will use the active port