  The LHCb (Large Hadron Collider beauty) experiment is designed to study
differences between particles and anti-particles as well as very rare decays in
the charm and beauty sector at the LHC (Large Hadron Collider). The detector
will be upgraded in 2019 and a new trigger-less readout system will be
implemented in order to significantly increase its efficiency and take
advantage of the increased machine luminosity. In the upgraded system, both
event building and event filtering will be performed in software for all the
data produced in every bunch-crossing of the LHC. In order to transport the
full data rate of 32 Tb/s we will use custom FPGA readout boards (PCIe40) and
state of the art off-the-shelf network technologies. The full event building
system will require around 500 nodes interconnected together. From a networking
point of view, event building traffic has an all-to-all pattern, therefore it
tends to create high network congestion. In order to maximize the link
utilization different techniques can be adopted in various areas like traffic
shaping, network topology and routing optimization. The size of the system
makes it very difficult to test at production scale, before the actual
procurement. We resort therefore to network simulations as a powerful tool for
finding the optimal configuration. We will present an accurate low level
description of an InfiniBand based network with event building like traffic. We
will show comparison between simulated and real systems and how changes in the
input parameters affect performances.
