A hybrid topology is always produced when two different basic network topologies are connected. Expanders exist to allow more complex interconnect topologies. Early work included simulating ad hoc mobile networks on sparse and densely connected topologies. This is critically important in redundant multicast topologies. Different protocols are often used for different topologies or different application areas. Cable types, connector types and cabling topologies are defined by TIA/EIA-568-B. Some significantly different circuit topologies exist compared to transistor designs. This allows RS-485 to implement linear bus topologies using only two wires. There exist numerous topologies on any given finite set. There are several different filter topologies available to implement a linear analogue filter. There are many different power circuit topologies and control strategies used in inverter designs. Other sources state that various e-machine topologies have differing torque density. There are close connections between these topologies and the concepts of order theory. Outdoor applications may use mesh topologies. Plasma railguns appear in two principal topologies, linear and coaxial.