However, users that are transmitting may listen to the channel to determine if a collision occurred. When this happens, Aloha employs a technique known as random back-off. When a user detects that his transmitted frame collided with another frame, the user waits a random amount of time and then retransmits the frame. The back-off time must be random in order to prevent the same users from colliding time after time. Users in a pure Aloha system may transmit instantly at any time, as shown in Figure 1 .
Aloha systems have the best performance when all devices use a constant frame size.
A pure Aloha system with a constant frame size has a maximum bandwidth utilization of about 18 percent. In a slotted Aloha system, time is divided into slots the size of a single frame. If a user wants to transmit, he must wait until the start of the next time slot. An example of a slotted Aloha system is shown in Figure 2 . By limiting the frame transmission to discrete time instances, a slotted Aloha system may achieve twice the bandwidth utilization of pure Aloha system, or about 37 percent.