Mode S transponders perform the same basic function as a traditional Mode A/C transponder and provides all the same data to the ground (and TCAS), however, the method by which these transponders are interrogated is much different. All so-called ATCRBS (Mode A/C) transponders reply when interrogated this happens when the beam of the interrogator dwells on the transponder. There is no selectivity between transponders they all reply. This can lead to saturation or garble when too many transponders attempt to reply to the same interrogation simultaneously. So a means of selectively interrogating specific transponders was needed. Mode S accomplishes just that. In fact, the S in Mode S stands for Select. Interrogators may now address their interrogations to specific transponders in their vicinity and no other transponder will reply. How does the Mode S Ground Station or TCAS know which address to interrogate for? All Mode S transponders squitter their unique, 24 bit, ICAO Mode S ID this address is assigned to each aircraft operating a Mode S transponder by the governing regulatory agency. A squitter is simply a transmission (unsolicited reply) from the transponder that TCAS systems listen for. When TCAS hears a squitter, it will add the address in the squitter to its roll call list and will discretely interrogate that transponder for its altitude and Mode A code along with other pieces of information. Mode S ground stations do not use these squitters the way TCAS does. They perform what is called a Mode S All Call interrogation and all Mode S transponders in the interrogation beam will respond with a reply that contains their Mode S address. The ground station then adds any new addresses to its roll call interrogation list.