the Speedlite 600EX-RT facilitates radio controlled, two-way wireless transmission up to 98.4 feet, among up to 5 groups with a total of 15 individual Speedlites. Communicating on 2.4 GHz frequencies, the Speedlite 600EX-RT does not have the same directional limitations of traditional wireless optical transmitters. Where other wireless signals can be interrupted by physical obstacles, radio controlled systems excel. 15 transmission channels are available, selectable manually or automatically, and radio transmission IDs can be set to prevent misfiring in the event of signal interference on the same channel.