ALOHA was developed in the 1970s by Norman Abramson at the University of Hawaii in i970 as a first demonstration of wireless networks. ALOHA is a system for coordinating and arbitrating access to a shared communication networks channel. A shared communication system like ALOHA requires a method of handling collisions that occur when two or more systems attempt to transmit on the channel at the same time. Slotted ALOHA was developed in 1972 by Robert to improve the efficiency of pure ALOHA.
In pure ALOHA, the stations transmit frames whenever they have data to send and in case two or more stations transmit simultaneously there will be a collision and frames will get destroyed. Therefore, every time a station transmits a frame, it expects the acknowledgement from the receiver and if the receiver does not get it within the specified time, the station assumes that the frame has been destroyed. The station has now to wait for a given amount of time before it sends again.
In slotted ALOHA, the stations can send a frame at the beginning of the slot and only one frame is sent in each slot and in case any station is unable to place the frame onto the channel at the beginning of the slot, it misses the time slot then the station has to wait until the beginning of the next time slot. Should the two stations try to send at the beginning of the same time slot then still there is a possibility of a collision.
Pure ALOHA Vs. Slotted ALOHA In Tabular Form
|BASIS OF COMPARISON
|Pure ALOHA was introduced by Norman Abramson in the association of the University of Hawaii in 1970.
|Slotted ALOHA was introduced by Robert 1972 in an effort to improve the capacity of pure ALOHA.
|Global Time Synchronization
|Pure ALOHA does not require global time synchronization.
|Slotted ALOHA requires the global time synchronization.
|Number of collisions
|Pure ALOHA does not reduce the number of collisions to half.
|Slotted ALOHA reduces the number of collisions to half and doubles the efficiency of pure ALOHA.
|In pure ALOHA, any station can transmit the data at any time.
|In slotted ALOHA, any station can transmit the data at the beginning of any time slot.
|In Pure ALOHA, station can send data in continuous time manner.
|In slotted ALOHA, station cannot send data in continuous time manner. It divides the time in slot.
|Probability of successful Transmission
|The probability of successful transmission of the data packet (frame) is S=G*e^-2G.
|In slotted ALOHA, the probability of successful transmission of data packet (frame) is S=G*e^-G.
|The maximum throughput in Pure ALOHA occurs at G=1/2 which approximately 18%.
|In Slotted ALOHA, the maximum throughput occurs at G=1 which equivalent to 37%.
|Vulnerable time for pure ALOHA = 2 x Tt.
|Vulnerable time for Slotted ALOHA =Tt.
|Pure ALOHA allows the user whenever they have data.
|Slotted ALOHA does not allow the user whenever they have data.
What is the main difference between Pure ALOHA and Slotted ALOHA?
The main difference between Pure and Slotted ALOHA is that, In pure ALOHA, a user can communicate at any time, but risks collisions with other users’ messages whereas in slotted ALOHA the chances of collisions have been reduced by dividing the channel into time slots and requiring that the user send only at the beginning of a time slot. This method requires synchronization between the sending nodes to prevent collisions.