# ALOHA Protocol

ALOHA Protocol is the important topic of the Computer Network. Moreover, freestudy9 has all kind of important information and topic related to it.

**Pure ALOHA**

- Pure ALOHA allows users to transmit whenever they have data to send.
- Senders wait to see if a collision occurred (after the whole message has send).
- If the collision occurs, each station involved waits a
**random amount of time**then tries again. - Systems in which multiple users share a common channel in a way that can lead to conflicts are widely known as
**contention systems**.

##### Fig: In pure ALOHA, frames transmitted at completely arbitrary times

- Whenever two frames try to occupy the channel at the same time, there will a collision and both will garble.
- If the first bit of a new frame overlaps with just the last bit of a frame almost finished, both frames will totally destroy and both will have to retransmitted later.

**What is the efficiency of an ALOHA channel?**

- Let the
**”frame time”**denote the amount of time needed to transmit the standard, fixed-length frame. - Also, assume that the infinite population of users generates new frames with mean N frames per frame time.
- If N > 1, the user community is 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.
- Let us further assume that the probability of k transmission attempts per frame time, old and new combined, is also Poisson, with mean G per frame time.
- Under all loads, the throughput, S, is just the offered load, G, times the probability, P
, of a transmission succeeding-that is, S = GP_{0}, where P_{0}is the probability that a frame does not suffer a collision._{0 } - The probability that k frames generated during a given frame time is given by the Poisson distribution.
- The maximum throughput occurs at G = 0.5, with S = 1/2e, which is about
**0.184**.

**Related** **Terms**

Introduction Computer Network, TDMA, Error Detection, Multiple Access Links.

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