PURE ALOHA

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The Pure ALOHA just allows every station to transmit the data whenever they have the data to be sent. 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. If the acknowledgment arrived for the received frame, then it is ok or else if the two frames collide (Overlap), they are damaged.

If a frame is damaged, then the stations wait for a random amount of type and retransmits the frame till it transmits successfully. The waiting time of the each station must be random and it must not be same just to avoid the collision of the frames again and again. The throughput of the Pure ALOHA is maximized when the frames are of uniform length.

The probability of k frames transmission in t seconds and is given by the poisson distribution as follows :
P[k] = (2G)k * e-2G / k! , { k=0,1,2,3,.... }

The throughput for Pure ALOHA is : S = G * e-2G
G = One frame time
e-G = Probability of zero frames in t seconds

The maximum throughput S = 0.184 when G = 1/2

SLOTTED ALOHA

The Slotted ALOHA requires that time be segmented into slots of a fixed length exactly equal to the packet transmission time. Every packet transmitted must fit into one of these slots by beginning and ending in precise synchronisation with the slot segments. A packet arriving to be transmitted at any given station must be delayed until the beginning of the next slot.

If more than one station transmit in the same slot, it will lead to collision. Slotted ALOHA reduces this collision in the network system.

The probability of k frames transmission in t seconds and is given by the poisson distribution as follows :
P[k] = (G)k * e-G / k! , { k=0,1,2,3,.... }

The throughput for slotted ALOHA is: S = G * e-G
G = One frame time
e-G = Probability of zero frames in t seconds

The maximum throughput S = 0.368 when G = 1

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