In packet mode communication networks, contention is a condition that arises when two or more data stations attempt to transmit at the same time over a shared channel, or when two data stations attempt to transmit at the same time in a half duplex communication link. A contention based channel access (multiple access) protocol is a protocol where data packet collisions may occur. Examples of such protocols are: * The Aloha protocol * Carrier Sense Multiple Access (CSMA) * Multiple Access with Collision Avoidance