There is reason why ACK timeout is 2 µs by default, because in indoor environment, all wireless devices are close with each other (5m or 10m), which means 2 µs is already enough for ACK message to get back to the sender. But in outdoor environment, wireless devices (sender & receiver) are far away from each other (5km or 10km), so ACK message may need more than 2 µs to get back to the sender.