Abstract: Performance evaluation is an important stage during the development of network protocols．Algorithms and Protocols are usually tested in experimental network or simulation environments．Simulation level of abstraction may be too high to capture low-level effects．Experimental networks are neither controllable nor repeatable．Emulation provides a third point for protocol testing and evaluation；it is expected to be a low-cost,controllable and repeatable testing platform with high degree of realism．In this paper,we make a survey on network emulation systems,including their concepts,classification,focus on design methodology of the Virtual Machines and the Resource Mapping emulation system,the developing trends and main challenges are also summarized.

Key words: Network emulation,Protocol evaluation,Design methodology,MANET