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 :

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.

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:

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:

Therefore, the probability () of there being zero transmission-attempts between t-T and t+T (and thus of a successful transmission for us) is:

 it can be concluded that the throughput () is:

Vulnerable time=2*T.

then by using Poisson distribution, the probability that exactly x nodes begin transmission during period T is

Therefore, the probability that during any particular period from t=2nT to t=(2n+1)T,  exactly one node will begin transmission is

And the probability that during any particular period t=(2n+1)T to t=(2n+2)T, no node will begin transmission is

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

This is the throughput. Therefore, the throughput in pure ALOHA,

Similarly for slotted ALOHA, a frame will be successfully transmitted.

This is the throughput in slotted ALOHA. Thus,

2. Slotted ALOHA :

Slotted ALOHA protocol.

An improvement to the original ALOHA protocol was "Slotted ALOHA".

the probability of k packets is:

The throughput is:

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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