1. A gateway selection method for a wireless mesh network, comprising: (a) building a spanning routing tree to a given mesh network; (b) calculating reference factors which indicate an efficiency of a routing path of a plurality of routers within the given mesh network to a total network capacity in association with the spanning routing tree from step (a); and (c) selecting the particular router as an optimal gateway, said particular router having the reference factor calculated in step (b) that is the highest.