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TimeHub 5500 User’s Guide
097-55501-01 Revision M – January 2009
Chapter 2 Engineering Ordering Information
NTP Server Card
Functional Overview
The NTP Server card exchanges NTP messages with NTP clients over the Ethernet
TCP/IP network. All NTP packet traffic goes through one or both NTP ports (A and B
in the diagram) on the NTP Server card. These two NTP ports are Gigabit Ethernet
ports that support both electrical and optical interfaces.
The NTP Server card supplies two of the four timestamps that the NTP client needs
to calculate the time offset between its local clock and the NTP Server clock. A
hardware-assisted timestamp mechanism in the NTP engine is based on a
125 MHz NTP system clock which has a resolution of 8 ns and an accuracy of a few
tens of nanoseconds. The NTP Server uses the TimeHub sync signal (1.544 MHz)
to derive the 125 MHz NTP clock. To maintain the time accuracy of its NTP system
clock, the NTP card either connects directly to a Stratum 0 TOD source (primary
server), or it derives the time correction using timestamps from external peer NTP
servers (secondary server). This is discussed in more detail in a later section.
The NTP Server card can process incoming packets with a sustained average rate
of 1500 unauthenticated packets or 1000 authenticated packets in a one-second
interval. When the number of incoming packets during each one-second interval
has reached this processing limit, subsequent packets will be dropped for the rest of
this one-second interval. The above packet processing capacity is based on the
assumption that incoming packets are evenly distributed in time.
The NTP Server card supports two redundancy modes. The two NTP ports can be
configured as two bonded ports sharing one IP address; one is active and the other
is on standby. On the system level, when two NTP Server cards are placed next to
each other in a port group, they are automatically configured as a redundant pair:
one card is active while the other is in standby mode. Redundancy is explained in
more detail in a later section.
The NTP Server card is managed like any other output card in the TimeHub shelf:
via the Ethernet or Serial Local port on the TimeHub main shelf. All management
messages are relayed to the CPU in the NTP Server card by the IMC and through
the TimeHub shelf backplane.
The Ethernet port on the NTP Connector card is only used for downloading software
to the NTP Server card.
Each TimeHub shelf (whether Master or Expansion) can support up to two pairs of
NTP Server cards. One output group can have a single NTP Server card or a
redundant pair of cards.
Functional Block Diagram
A simplified block diagram of the NTP implementation in the TimeHub is shown in
.