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

18-6 Industrial Communication Systems
and exiting, thus, enabling a system to calculate the processing time of a packet in devices. This is
required for switches, which do not have a predictable forwarding for packets or in ring and line
structures as implemented in PROFINET. This addition was introduced when it turned out that
the concept of boundary clocks of version 1 suffers from difficulties caused by cascaded control
loops, which may exhibit stability problems after several hops.
• In principle, the synchronization accuracy with IEEE 1588v2 can get better than 1.ns. Compared to
version 1, this was made possible by the increased length of several fields in the protocol allowing for
a higher resolution of parameters and timestamps. This extension reflects the increased demand for
accuracy in clock synchronization. Related to it and, among others, motivated by telecom applications
is the increased synchronization rate of the new version. Finally, fields to compensate asymmetry
have been included, which can be used to compensate inherently, different delays for send
and receive channels. However, also measures outside of the scope of the protocol can be made (e.g.,
usage of calibrated cables or knowledge of the channel as it is the case in powerline).
• Optional support of unicast messages. Generally speaking, IEEE 1588 assumes that one master is
elected and via that master several slaves are synchronized. Of course, one would take, if available,
multicast messages for synchronization, as the time to be distributed is the same for all
nodes attached to this master. However, in some cases, for example, if the communication technology
does not support multicast or different timescales need to be used, unicast is needed. This
will be possible in version 2.
• Profiles. Within the scope of standardization, profiles are in general needed to apply a special,
reduced set of values and ranges to a more general standard in order to apply it for a special use case.
In the case of IEEE 1588, profiles are needed in order to reflect the growing number of application
areas. For example, in the new version, a special profile for telecom applications, PROFINET, etc.,
can be defined. For all values not defined in a specific profile, the default values of the standard apply.
• Alternative best master clock algorithms. Especially, the telecom industry demands long holdover
times after a master failure. This is reflected by the possibility for alternate best master clock
(BMC) algorithms.
• Experimental specifications. As it can already be predicted that new features such as security
(which is currently not supported at all) or cumulative frequency offsets to the master are needed,
some experimental, non-normative clauses have been added. For some of those clauses, further
research and case studies are needed; however, the experimental annex eases the start for such
investigations [Treytl2007,Gaderer2006a].
18.8 Network Time Protocol
With the introduction of distributed applications in the Internet, a new requirement arose: the synchronization
between the participating computers. For example, simple applications like the synchronization
of files on two servers, like the well-known rsync application, use timestamps of files to distinguish
between older and newer. This problem was solved with the probably oldest application layer protocol
of the Internet still in use: The network time protocol (NTP). The open protocol standard NTP is a
specification for a TCP/IP-based clock synchronization scheme for network devices. The main work on
NTP was contributed by David L. Mills. The standardization was done via request for comment (RFC)
documents—the key contribution can be found in [Mills1991a]. NTP is a very carefully designed protocol
almost to every extent investigated. It covers, in opposite to IEEE 1588, all aspects like the control of
the clock and respective filter algorithms [Gurewitz2003,Mills1998b,Mills1998a] with a wide range
of application studies [Richards2006].
The latest version of NTP is 4; nevertheless, in the meanwhile, a simplified version called SNTP has
been specified and is in use.
© 2011 by Taylor and Francis Group, LLC