We need a protocol to decide which one can access the medium if more than one has information to send at the same time. We need a media access protocol (MAC). Some MAC protocols are:
CSMA/CA, Carrier Sense Multiple Access/Collision Avoidance:
Listen to see if the channel is in use. If it is, back off for a time and retry later. ("Carrier Sense" implies that a node can tell when another device is using the communication medium -- like a telephone busy signal).
CSMA/CD, Carrier Sense Multiple Access/Collision Detection:
If a collision is detected after transmitting a frame, back off for a time and retransmit it. (Collisions are detected when a node receives a garbled frame).
Polling:
The network has a master node and two or more slaves. The master node queries each slave in turn to see whether it has some data to transmit. If it does, it transmits the data, if not, the master moves on to the next slave node.
Token ring:
A token (a pattern of bits) is passed from one node to the next. A node can only transmit when it has the token. This is similar to polling, but the nodes are equal peers. There is no master node.
RTS/CTS, Request to Send/Clear to Send:
After the base station gives A permission to transmit, it tells B to hold off for a short time.