Page "I²C" Paragraph 65

Every master monitors the bus for start and stop bits, and does not start a message while another master is keeping the bus busy.

However, two masters may start transmission at about the same time ; in this case, arbitration occurs.

Slave transmit mode can also be arbitrated, when a master addresses multiple slaves, but this is less common.

In contrast to protocols ( such as Ethernet ) that use random back-off delays before issuing a retry, I²C has a deterministic arbitration policy.

Each transmitter checks the level of the data line ( SDA ) and compares it with the levels it expects ; if they don't match, that transmitter has lost arbitration, and drops out of this protocol interaction.