Wireless networks are founded on the principles of collision avoidance. This paper makes an attempt to detect and abort collisions in wireless networks. Briefly, the receiver uses physical layer information to detect collisions, and immediately notifies the transmitter to abort transmission. The collision notification consists of a unique signature, sent on the same frequency channel as the data. The transmitter uses a second listener antenna to discern this notification through signature correlation. The transmitter aborts, freeing the channel for other productive transmissions. We call this Carrier Sense Multiple Access with Collision Notification (CSMA/CN). Early results from a small USRP/GNURadio testbed confirm the feasibility and performance gains with CSMA/CN.