A transmitter is a plastic shell with control sticks and various switches, internal control logic, and perhaps an LCD. It also has a self-contained radio module that plugs into the back, which takes the serial communication stream from the transmitter and generates a wireless communications signal, which it outputs to whatever antenna is plugged into it, and transmits it to the receiver. Most 'narrow-band' modules operate at a selectable frequency channel within their band, while 2.4Ghz spread spectrum modules intelligently manage channels, communicating over several of them at once and 'hopping' to different ones when they encounter interference. The variety of schemes that they use means there are many 'flavors' of spread spectrum, rather than the somewhat standardized compatibility encountered in other bands. A module change may be used to switch to a different band entirely, or may be used to switch flavors of 2.4ghz.