When the load is very high, each group has just one station, i.e. Aloha is a multiple access protocol at the datalink layer and proposes how multiple terminals access the medium without interference or collision. At slot 1, only stations in group 1 can compete for channel access and so on. Aloha is a multiple access protocol at the data link layer and proposes how multiple terminals access the medium without interference or collision. The main difference between Pure ALOHA and Slotted ALOHA is that the time in Pure Aloha is continuous whereas, the time in Slotted ALOHA is discrete. Since, this works very good under light loads, limited contention protocols behave like slotted ALOHA under low loads.However, with the increase in loads, there occurs exponential growth in number of collisions and so the performance of slotted ALOHA degrades rapidly. Aloha in computer networks is an access control protocol. The main difference between Pure ALOHA and Slotted ALOHA is that the time in Pure Aloha is continuous whereas, the time in Slotted ALOHA is discrete. Collisions are avoided by mutual agreement among the contending stations on who gets the channel. In this, multiple stations can transmit data at the same time and can hence lead to collision and data being garbled. 23. As the probability of collision is less in Slotted ALOHA as compared to Pure ALOHA because the station waits for the next time slot to begin which let the frame in a previous time slot to pass and avoids the collision between the frames.It is very helpful article. Under low loads, only one group is there containing all stations, which is the case of slotted ALOHA. If first bit of a new frame overlaps with just the last bit of a frame almost finished, both frames will be totally destroyed and both will have to be retransmitted.• Slotted ALOHA was invented to improve the efficiency of pure ALOHA as chances of collision in pure ALOHA are very high.• In slotted ALOHA, the time of the shared channel is divided into discrete intervals called slots.• The stations can send a frame only at the beginning of the slot and only one frame is 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 • In slotted ALOHA, there is still a possibility of collision if two stations try to send at the beginning of the same time slot as shown in fig.• Slotted ALOHA still has an edge over pure ALOHA as chances of collision are reduced to one-half. All other frames are destroyed.• Whenever two frames try to occupy the channel at the same time, there will be a collision and both will be damaged. If collision occurs, it waits for a random period of time and re-sends it again – Sender can always find out if its frame was destroyed by listening to channel. ALOHA originally stood for Additive Links On-line Hawaii Area. He proposed to divide the time into discrete intervals called time slots. In Slotted Aloha, any station can transmit data only at beginning of any time slot. Let us discuss the other differences between Pure ALOHA and Slotted ALOHA in the comparison chart.Pure ALOHA is introduced by Norman Abramson and his associates at the University of Hawaii in early 1970. In this, multiple stations can transmit data at the same time and can hence lead to collision and data being garbled.
When every station transmits the data without checking whether the channel is free or not there is always the possibility of the collision of data frames. It is a very helpful article. ALOHA Class of Multiple Access Protocols • ALOHA, also called pure ALOHA: Whenever a user has a frame to send, it simply transmits the frame. The station senses the carrier or channel before transmitting a frame. Aloha Protocols History The Aloha protocol was designed as part of a project at the University of Hawaii. Aloha in computer networks is an access control protocol.
At slot 0, only stations in group 0 can compete for channel access. Pure ALOHA Offered Load vs. Throughput Max at G = 0.5, S = 1/2e, only about 0.184 (18%)! It is up to network architects to implement the layers according to their needs and resources available. There, the data proceeds to upper layers after processing till it reaches application layer.At the application layer, data or request is shared with the workstation. The Slotted ALOHA makes the station to wait till the next time slot begins and allow each data frame to be transmitted in the new time slot.The Slotted ALOHA is somewhat better than the Pure ALOHA.