There are two versions of ALOHA:
1. Pure ALOHA
2. Slotted ALOHA
The basic concept of an ALOHA is let users transmit whenever they have data to be sent. There will be collisions, and the colliding frames will be damaged. Due to the feedback feature of broadcasting, by listening to the channel destroyed frames can be identified by sender. If the frame was destroyed, the sender just waits a random amount of time and sends it again. The waiting time must be random because there will be a possibility that same frames will collide over and over, in lockstep. Systems which will be having multiple users and share a common channel in a way that can lead to conflicts which are called as contention systems.
A collision occurs whenever two frames try to occupy the channel at the same time, and both the frames are destroyed. If the first bit of a new frame overlaps with the last bit of almost transmitted frame, then both the frames will be destroyed completely and as a result of which both will have to be retransmitted later. The checksum cannot identify partially collided frames therefore even a partially qualified frame will be destroyed. To find out the efficiency of ALOHA channel. The user first sends a frame and waits for a reply. The station then transmits a frame containing the frame and checks the channel to see if it is free. If it was successful, the user sends the frame. If not, the user continues to wait and the frame is retransmitted over and over until it has been successfully sent.
Myriads of users generate new frames according to a Poisson distribution with mean N frames per frame time. If N > 1, then the users are generating frames at a higher rate than the channel can handle, and nearly every frame will suffer a collision. For reasonable throughput we would expect 0 < N < 1.
In addition to the new frames, the terminal will also re transmit frames that were destroyed before because of collisions. Let us assume that the probability of x frames attempted per frame time including new and old frames so new value will be G per frame time.
Clearly, G>=N. At low load (i.e., N~0), there will be less collisions, therefore less retransmissions, so G N. At high load there will be many collisions, therefore G > N. Under all loads, the throughput, S, is just the offered load, Y, times the probability, P0, of successful transmission S = GP0, where P0 is the probability that a frame does not suffer a collision.
A frame will not be collided if no other frames are sent within one frame time of its start, as shown in the figure. Let us assume t be the required time to send a frame. If other user has generated a frame between time t0 and t0 + t, the end of that frame will collide with the beginning of the shaded area in figure. In fact, the shaded frame’s result was already known even before the first bit was sent, but because in pure ALOHA a terminal does not communicate to the channel before sending, station by any means cannot know if another frame is already on it way.
Therefore the probability of zero frames is just e^-G. In time interval of two frames time, number of frames generated will be 2G. The possibility of no other traffic being initiated during the entire vulnerable period is thus given by P0=e^-2G. Using S=GP0 and S=Ge^-2G
The maximum throughput occurs at G = 0.5, with S = 1/2e, which is about 0.184 which means we can utilize a channel is 18%.
The idea of Slotted ALOHA was to divide time into discrete intervals, each interval respective to one frame. This approach demands that users to will be using slot boundaries. This can be achieved by setting a terminal which will tell other stations at the start of every interval.
In this method a computer is not permitted to send whenever a carriage return is typed but it is required to wait for the beginning of the next slot. So basically the continuous pure ALOHA has been 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=Ge^-G
As you can see from the figure slotted ALOHA peaks at G = 1, with a throughput of S =1/e or about 0.368, which is twice of pure ALOHA. If the system is operating at G = 1, and the possibility of an empty slot is 0.368. Performing at higher values of G will result in exponential increase in number of collisions.. To see how this rapid growth of collisions with G comes about, consider the transmission of a test frame. The probability that it will avoid a collision is e^-G, the probability that all the other users are silent in that slot. The probability of a collision is then just 1 – e^-G. The possibility of a transmission requiring exactly k attempts, (i.e., k – 1 collisions followed by one success) is