In Aloha systems there is No Carrier Sense.

1. Pure Aloha:

   a. Time is not slotted and stations can be transmitted whenever they want to.

   b. There is possibility of collision. The colliding frames will be destroyed.

   c. If frames collide and get destroyed then sender waits for a random amount of time and resends the frame.

2. Slotted Aloha:

   a. Time is divide into slot.

   b. Frame transmission begins at the start of a slot.

   c. If two or more nodes transmit in the same slot a collision is detected.