Specifications

Chapter 3
System Architecture
3.1 General overview
The network tester that we use can generate Gigabit Ethernet traffic and can measure all
the important performance parameters of a network. The functionality is s imilar to other
products that can be found on the market, but our main advantage is the high degree
of flexibility and accuracy. The system uses mainly off-the-shelf hardware and custom
associated software.
The main hardware components are the Alteon Acenic Gigabit Ethernet network interface
cards. These network interfaces can be programmed by the user and their firmware can
be easily changed to suit any particular application or purpose (the source c ode of the
original firmware was freely available from the manufacturer) — so they give the possibility
to create a flexible network traffic generator and measurement tool.
The cards contain two programmable MIPS compatible proc es sors, some Random Access
Memory and a PCI interface to the host computer. We have modified the code supplied by
the vendor in order to utilize the NIC as a tester. The cards were programmed to generate
frames (at layer 2) according to a pre-defined pattern up to Gigabit Ethernet line speed
for all packet sizes. For latency computation all outgoing packets are marked with a time
stamp from a global clock card (which was designed and built at CERN). The software
running on the network cards is written in C and compiled with tools adapted from the
GNU C compiler (gcc).
All the operations involving the real-time traffic (sending and receiving) are done by the
two CPUs on the card. This avoids the delays related to PCI transfers and allows the
system to generate traffic at full Gigabit line speed, even when multiple NICs are installed
in the same computer.
Prior to s tarting any tests, each card receives a traffic description table containing the full
header information of the packets to be generated, the size of the packets and the time
between two consecutively sent packets. We thus have full control over all the fields in the
headers (including the Priority from the VLAN field of the Ethernet packet). For the time
between consecutive packets and for all other fields of the packets the user can choose a fixed
value or any other random number distribution. After the traffic descriptors are loaded and
the test started, the host computer in not involved anymore in the traffic generation.
11