Specifications

CHAPTER 2. NETWORK PERFORMANCE MEASUREMENTS SYSTEMS 10

The computers involved in the system run a Linux with the nanokernel patches. This

allows a nanosec ond resolution of the system clock. The PPS signal is distributed

from the GPS to the serial ports of all computers via standard Cat5 cabling. The

PPS distribution contains also a voltage level converter from TTL to RS-232. The

precision obtained with this setup is of 10 microseconds.

The time synchronization system for K2K experiment

The K2K is a physics experiment developed at the Japanese national high energy

physics laboratory [10]. The experiment needs synchronized clocks in two sites that

are at a distance of 250km one from the other. The solution involved GPS receivers

at each site and some custom made local clock boards (LTC). The clock boards have

a 50MHz free-running counter. The GPS has two outputs: the PPS signal whose

leading edge is correlated with the UTC second, and an Ascii stream with the full

GPS data (time and position). The PPS signal is used to calibrate the clock board

(to see the average number of clock ticks per UTC second).

Upon receipt of each event that needs a time stamp, the LTC count and the GPS

Ascii data are latched and recorded. The LTC count provides the fractional second

of the time, down to 20 nsec precision, accurately synchronized with UTC within

100 nsec. The Ascii data provides the date and coarse time down to seconds. The

accuracy that is obtained is of the order of hundreds of nanosec onds.

2.3 Some observations

Most commercial testers that were presented are dedicated devices that oﬀer a set of services

but that can’t be easily modiﬁed by the user without the help of the manufacturer. We

are interested in a system that is powerful enough but still gives full access to its internal

functioning. All the systems described above provide similar functionality for network

testing.

For accurate long distance measurements the solution is the GPS for time synchronization.

If one wants to build a testing system using oﬀ-the-shelf PCs and network cards the solution

for accurate timing is the NTP. This protocol can transfer time from host workstations con-

ﬁgured as a time server or from oﬀ-the-shelf time server products. These network transfer

techniques oﬀer performance in the 1-10 millisecond synchronization accuracy range. This

performance can be extended further, to the 1- 10 microsecond range, by employing direct

connection time transfer techniques: time code, serial time messaging, and 1 PPS/reference

frequency signals. The time resolution on the host computers can be also improved up to

the nanosecond le vel by using some internal registers found in modern processors. The

accuracy obtained is of the order of tens of microseconds.

The most accurate method is the one that uses directly a GPS reference and special clock

cards (as in the K2K experiment) — this is very similar to the method that will be described

in this report.

In the following we shall describe the architecture of the testing system – we shall see

that the most important parts use customized hardware. This is because a pure software

solution cannot handle the performance measurements at full Gigabit line speeds.