What is ALOHA?What are the types of ALOHA?Describe it


ALOHA : ALOHAnet, also known as the ALOHA System, or simply ALOHA, was a pioneering computer networking system.

The ALOHAnet used a new method of medium access (ALOHA random access) and experimental ultra high frequency (UHF) for its operation

There are two types of ALOHA : 

1. Pure ALOHA :

Graph of frames being sent from 4 different stations according to the pure ALOHA protocol with respect to time, with overlapping frames shaded to denote collision.
Pure ALOHA protocol. Boxes indicate frames. :
  • If you have data to send, send the data
  • If, while you are transmitting data, you receive any data from another station, there has been a message collision. All transmitting stations will need to try resending "later".
Let "T" that is, on average, there are G transmission-attempts per frame-time.
Graph of 3 frames with respect to time. The earlier green frame overlaps with the yellow frame sent at time t0, which overlaps with the later purple frame.




Overlapping frames in the pure ALOHA protocol. Frame-time is equal to 1 for all frames.
For any frame-time, the probability of there being k transmission-attempts during that frame-time is:
\frac{G^k e^{-G}}{k!}
Throughput vs. Traffic Load of Pure Aloha and Slotted Aloha.
Comparison of Pure Aloha and Slotted Aloha shown on Throughput vs. Traffic Load plot.
The average amount of transmission-attempts for 2 consecutive frame-times is 2G.  two frame-times is:
\frac{(2G)^k e^{-2G}}{k!}
Therefore, the probability (Prob_{pure}) of there being zero transmission-attempts between t-T and t+T (and thus of a successful transmission for us) is:
Prob_{pure}=e^{-2G}
 it can be concluded that the throughput (S_{pure}) is:
S_{pure}=Ge^{-2G} Vulnerable time=2*T.

then by using Poisson distribution, the probability that exactly x nodes begin transmission during period T is
P[X=x]=\frac{G^x e^{-G}}{x!}
Therefore, the probability that during any particular period from t=2nT to t=(2n+1)T,  exactly one node will begin transmission is
P[X=1] = \frac{G^1 e^{-G}}{1!} = Ge^{-G}
And the probability that during any particular period t=(2n+1)T to t=(2n+2)T, no node will begin transmission is
P[X=0] = \frac{G^0 e^{-G}}{0!} = e^{-G}
That is during period t=2nT to t=(2n+1)T, exactly one node begins transmission and during t=(2n+1)T to t=(2n+2)T no node begins transmission
P = P(0) \times P(1) = Ge^{-G} \times e^{-G} = Ge^{-2G}
This is the throughput. Therefore, the throughput in pure ALOHA,
S_{pure}=Ge^{-2G}
Similarly for slotted ALOHA, a frame will be successfully transmitted.
P[X=1]= \frac{G^1 e^{-G}}{1!} = Ge^{-G}
This is the throughput in slotted ALOHA. Thus,
S_{slotted} = Ge^{-G}

2. Slotted ALOHA :

Graph of frames being sent from 8 different stations according to the slotted ALOHA protocol with respect to time, with frames in the same slots shaded to denote collision.
Slotted ALOHA protocol.
An improvement to the original ALOHA protocol was "Slotted ALOHA".
Prob_{slotted} = e^{-G}
the probability of k packets is:
Prob_{slotted} k = e^{-G} ( 1 - e^{-G} )^{k-1}
The throughput is:
S_{slotted}=Ge^{-G}
The maximum throughput is 1/e frames per frame-time (reached when G = 1), which is approximately 0.368 frames per frame-time, or 36.8%.



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