- Slotted ALOHA invented to improve the efficiency of pure ALOHA as chances of the collision in pure ALOHA are very high.
- In slotted ALOHAd, the time of the shared channel divided into discrete intervals called slots.
- The stations can send a frame only at the beginning of the slot and only one frame sent in each slot.
- In slotted ALOHA, if any station is not able to place the frame onto the channel at the beginning of the slot i.e. it misses the time slot then the station has to wait until the beginning of the next time slot.
What is the efficiency of an ALOHA channel?
Pure ALOHA à S = Ge-2 G
Slotted ALOHA à S = Ge-G
Fig: Throughput versus offered traffic for ALOHA systems
- As seen in figure both graphs have the same shape. If G is small so S, which means that if few frames generated few frames will be transmitted successfully.
- As G increases so do S but up to a certain point. As G continues to increase S approaches to 0 which means that if more frames generated there will more collisions and the success rate will fall to 0.
- Similarly, for pure ALOHA, the maximum occurs at G=0.5 for which S = 1/2e = 0.184 which means the rate of successful transmissions is approximately 18.4%.
- As seen from the graph the maximum for slotted ALOHA occurs at G=1 for which S=1/e=0.368. In other words, the rate of successful transmissions is approximately 0.368 frames per slot time or 37% of the time will spend on successful transmissions.
Advantages of Slotted Aloha
- A single active node can continuously transmit at the full rate of the channel.
- Slotted ALOHA also highly decentralized because each node detects collisions and independently decides when to retransmit.
- Slotted ALOHA also an extremely simple protocol.
Disadvantages of Slotted Aloha
- When there multiple active nodes, a certain fraction of the slots will have collisions and will, therefore, wasted.
- idle slots
- nodes may able to detect the collision in less than time to transmit packet clock synchronization.