Specifications

CHAPTER 2. NETWORK PERFORMANCE MEASUREMENTS SYSTEMS 9
• Some of them can be plugged directly into a computer (Meinberg PCI card)
Instead of a GPS receiver, one can use a CDMA receiver. The main advantage over GPS is
that it does not need a roof-mounted antenna because CDMA signals can be received inside
buildings. The system uses the CDMA cellular mobile communications network which, for
time and frequency applications, acts as a GPS repeater. The accuracy that can be obtained
is around 10 microseconds. One such system is the Praecis C t from EndRun Technologies.
This device can provide the PPS output and c an emulate a GPS receiver so it can be used
in conjunction with NTP (in places where a GPS antenna cannot be installed).
2.2.4 Sample implementations
In the next paragraphs we present two systems that use clock synchronization mechanisms:
one is for network measurements and the other is for accurate time-stamping of events.
High precision solution using NTP and GPS
A high precision architecture using NTP and a GPS receiver is shown in Figure 2.5.
It use s an NTP server that reads the timing from a Stratum 0 source - a GPS receiver.
The workstations run NTP client software and a modified Unix kernel – a nanokernel.
An accuracy of the order of microsecond is obtained by distributing the PPS signal
from the GPS to all workstations (via the serial ports) - the NTP software can use
this signal to improve the acc uracy.
GPS PPS Signal
NTP via Ethernet
10 miliseconds accuracy 1 − 10 microseconds accuracy
using NTP only using NTP and the GPS Pulse Per Second signal
Network Time Server GPS Receiver
Figure 2.5: High precision NTP implementation
An implementation of the setup from Figure 2.5 was done at the Tampere University of
Finland [9] where some researchers developed a measuring system for QoS parameters
that involves synchronized clocks and that can work over Gigabit Ethernet links. The
system uses NTP as the primary mechanism. The accuracy comes from a low-cost
Garmin GPS receiver that can provide the PPS output with microsecond accuracy.