Specifications

CHAPTER 2. NETWORK PERFORMANCE MEASUREMENTS SYSTEMS 7
hierarchical configuration where c locks are synchronized to each other and to world-wide
time standards (UTC time).
There is a hierarchy of NTP time servers, depending on the accuracy of their time references
(from stratum 0 – atomic clocks, up to stratum 15). NTP operates in a client/server fashion.
The client queries periodically the time se rver and it dynamically adjusts its clock to match
the o ne from the server. Due to unpredictable network delays that can’t be easily modeled
and due to the timing hardware found in common computers, in most cases the accuracy
of NTP synchronized clocks is of the order of milliseconds. However this can be improved
in certain conditions.
For the Unix operating systems there are kernel modifications — nanokernel patches —
that can improve the resolution of computer clocks to the order of nanoseconds. The goal
is to offer a function for reading time with the resolution up to a nanosecond. The principle
is to use the Proce ss Cycle Counter found in mo dern processors – this counts CPU ticks,
at the frequency of the CPU. Therefore, the resolution is dependent on the CPU clock and
it is really at the order of 1 nanosecond with modern processors. There are three issues
concerning this system:
• The clock can’t be easily controlled or reseted on request
• There might appear indeterminations in the time-stamping process because of the
way the clock is read
• It does not work very well on multiprocessor systems
The accuracy can be greatly improved if NTP is used together with a very accurate PPS
signal (Pulse per Second) that comes from a GPS receiver or cesium oscillator. The NTP
client can use these pulses to adjust the clocks with much better precision. In this way one
can reach an accuracy of the order of microseconds. See Sec tion 2.2.4 for more information.
2.2.2 Network Time Servers
An NTP time server can be a normal workstation running NTP server software and that
is connected to a primary time reference (other time server or a GPS receiver). There are
companies offering network appliances that are dedicated time servers. Some of them are
presented below:
• TymServe 2100, Trusted Time SyncServer S100, Epsilon clock NTP, Lantronix CoBox
NTP
Figure 2.3: A network time server
The common features found in these time servers are: