wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

18-8 Industrial Communication Systems
18.10 architecture, Protocol, and Algorithms
The idea behind the NTP is to distribute a common notion of time among all nodes of a network in
terms of a reference clock. This goal can be achieved in quite different strategies:
• Minimization of clock offset, which tries to minimize the sampled difference between the local
and the reference time.
• Alignment of the frequency. This is equal to the first strategy, with the exception that the absolute offset
of periodically generated clock signals is not considered, but the stability of the offset optimized.
• The alignment of the absolute clock value in terms of minimizing clock offset and optimizing the
difference between all nodes. This criterion is equal to internal clock synchronization (alignment
of all nodes of a network with no respect to the absolute time scale), with the exception that NTP
inherently assumes an alignment to an absolute correct time source of the overall master.
18.11 NTP Clock Synchronization Hardware Requirements
Like any other clock synchronization system, NTP has to make certain assumptions regarding the clocks.
As NTP aims to synchronize hosts within the Internet, one assumption is that the architecture of every
node is similar to a clock in a PC. One further assumption of NTP is that a network node does not necessarily
need to be powered up all time. However, this requires keeping up with the unstoppable progress of
time even during power down. In practice, this is usually done with a so-called real-time clock (RTC). This
clock is battery-powered and thus, running also during power-down times. For cost and energy efficiency
reasons, these clocks are usually equipped with 32,768.kHz crystals. This frequency is very convenient,
since 32,768 transitions can be represented as 2 15 , which means that a 15 bit counter can be used without
further fractional parts to represent 1.s. In common PC architectures, the content of this clock is loaded
during boot-up into a tick register. Usual clock registers for NTP are 48 bit wide, and updated by adding
the 32 bit wide clock-adjust register, with an interrupt typically scheduled every 1–20.ms. The content is
also called processor cycle counter (PCC). As these registers are updated by means of software routines
and without the possibility to schedule future updates (such as provided in high precision clocks like in
SynUTC), writing to these registers has to be done with high priority [Mills1998a]. Additionally, the 16 bit
wide drift-compensation register is used to compensate the frequency offset of the processor clock.
18.12 Synchronization Algorithms of NTP
Figure 18.6 shows the five components of the NTP:
• The data filter is instantiated on the client side for every server. The main task of this unit is to
calculate the delay through round-trip measurements and adjust the contents of each message
accordingly.
Network Data filter Peer selection Clock combining Loop filter Adjustable
clock
Data filter
Data filter
FIGURE 18.6
System concept of a typical NTP node (NTPSystemConcept.pdf).
© 2011 by Taylor and Francis Group, LLC