NEST : Network Engineering and Simulation Tool. NEST is an integrated workbench that proposes several softwares and tools for the simulation and planning of large scale telecommunication networks. NEST incorporates 25 years of research results and experience in teletraffic modeling and optimisation-design of telecommunication networks. This research was conducted at LAAS-CNRS by QoS Design founders, and in collaboration with several companies and operators. NEST uses unique concepts such as Differential Traffic Theory and Distributed Hybrid Simulation, which is patented in Europe and in the USA, for modeling packet switched, circuit switched and virtual circuit switched networks. QoS Design has also developed several powerful algorithms for designing and optimising telecommunications networks : Routing optimisation, IP metric optimisation, resilience optimisation (backup LSPs, fast Reroute LSPs), dimensioning, topology design. QoS Design with its algorithms and its experience proposes unique solutions to solve QoS and Design problems in very large scale telecommunication networks.