Slotted ALOHA

Slotted ALOHA

In 1972, Roberts published a method for doubling the capacity of an ALOHA system (Robert, 1972). His proposal was to divide time into discrete intervals, each interval corresponding to one frame. This approach requires the users to agree on slot boundaries. One way to achieve synchronization would be to have one special station emit a pip at the start of each interval, like a clock.
In Roberts' method, which has come to be known as slotted ALOHA, in contrast to Abramson's pure ALOHA, a computer is not permitted to send whenever a carriage return is typed. Instead, it is required to wait for the beginning of the next slot. Thus, the continuous pure ALOHA is turned into a discrete one. Since the vulnerable period is now halved, the probability of no other traffic during the same slot as our test frame is e-G which leads to
S=G e ^-2G
The slotted ALOHA peaks at G = 1, with a throughput of S =1/e or about 0.368, twice that of pure ALOHA as shown in figure 3.4. If the system is operating at G = 1, the probability of an empty slot is 0.368. The best we can hope for using slotted ALOHA is 37 percent of the slots empty, 37 percent successes, and 26 percent collisions. Operating at higher values of G reduces the number of empties but increases the number of collisions exponentially. To see how this rapid growth of collisions with G comes about, consider the transmission of a test frame.

Slotted ALOHA

Advantages

» Doubles the efficiency of Aloha.
» Adaptable to a changing station population.

Disadvantages

» Theoretically proven throughput maximum of 36.8%.
» Requires queuing buffers for retransmission of packets.

Synchronization required

» Synchronous system: time divided into slots
» Slot size equals fixed packet transmission time
» When Packet ready for transmission, wait until start of next slot
» Packets overlap completely or not at all