The two most popular simulators in the wireless networking space are ns2 and GloMoSim. The ns2 network simulator  has a long history with the networking community, is widely trusted, and has been extended to support mobility and wireless networking protocols. It is built as a monolithic, sequential simulator, in the library-systems simulator design. ns2 uses a clever ``split object'' design, which allows Tcl-based script configuration of C-based object implementations. This approach is convenient for users. However, it incurs a substantial memory overhead and increases the complexity of simulation code. Researchers have extended ns2 to conservatively parallelize its event loop . However, this technique has proved primarily beneficial for distributing ns2's considerable memory requirements. Based on numerous published results, it is not easy to scale ns2 beyond a few hundred simulated nodes. Simulation researchers have shown ns2 to scale, with difficulty and substantial hardware resources, to simulations of a few thousand nodes .
GloMoSim  is a newer simulator written in Parsec , a highly-optimized C-like simulation language. GloMoSim has has recently gained popularity within the wireless ad hoc networking community. It was designed specifically for scalable simulation by explicitly supporting efficient, conservatively parallel execution with lookahead. The sequential version of GloMoSim is freely available. The conservatively parallel version has been commercialized as QualNet. Due to Parsec's large per-entity memory requirements, GloMoSim implements a technique called ``node aggregation,'' wherein the state of multiple simulation nodes are multiplexed within a single Parsec entity. While this effectively reduces memory consumption, it incurs a performance overhead and also increases code complexity. The aggregation of state also renders speculative execution techniques impractical. GloMoSim has been shown to scale to 10,000 nodes on large, specialized multi-processor machines.