An 802.11 probe response frame is very similar to a beacon frame, except that probe responses don't carry the TIM info and are only sent in response to a probe request. A station may send a probe request frame to trigger a probe response when the station needs to obtain information from another station. A radio NIC, for instance, will broadcast a probe request when using active scanning to determine which access points are within range for possible association. Some sniffing software (e.g., NetStumbler) tools send probe requests so that access points will respond with desired info