What is the use of ALOHA?
ALOHA is used to transmit data over a public network channel.
What is ALOHA and its type?
ALOHA is a basic method for sending data over a shared network. It was created to manage how data is transmitted and to deal with collisions when two devices send data at the same time.
There are two types of ALOHA:
Frequently Asked Questions on Pure ALOHA – FAQs
Pure aloha v/s slotted aloha
Now, let's see the comparison chart between pure aloha and slotted aloha. We are comparing both terms on the basis of characteristics to make the topic more clear and understandable.
From the above discussion, it can be said that slotted aloha is somewhat better than pure aloha. It is because there is less possibility of collision in slotted aloha.
So, that's all about the article. Hope it will be helpful and informative to you.
Plot the characteristic curve of throughput versus offered traffic for a Pure and Slotted ALOHA system
NOTE: NetSim Academic supports a maximum of 100 nodes and hence this experiment can only be done partially with NetSim Academic. NetSim Standard/Pro would be required to simulate all the configurations.
ALOHA provides a wireless data network. It is a multiple access protocol (this protocol is for allocating a multiple access channel). There are two main versions of ALOHA: pure and slotted. They differ with respect to whether or not time is divided up into discrete slots into which all frames must fit.
In Pure Aloha, users transmit whenever they have data to be sent. There will be collisions and the colliding frames will then be retransmitted. In NetSim's Aloha library, the sender waits a random amount of time per the exponential back-off algorithm and sends it again. The frame is discarded when the number of collisions a packet experiences crosses the the "Retry Limit" - a user settable parameter in the GUI.
Let ''frame time'' denotes the amount of time needed to transmit the standard, fixed-length frame. In this experiment point, we assume that the new frames generated by the stations are modeled by a Poisson distribution with a mean of N frames per frame time. If N > 1, the nodes 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 stations also generate retransmissions of frames that previously suffered collisions.
The probability of no other traffic being initiated during the entire vulnerable period is given by$\ e^{- 2G}\ $which leads to $S = \ G \times e^{- 2G}$ where, S is the throughput and G is the offered load. The units of$\ S$ and $G$ is frames per frame time.
G is the mean of the Poisson distribution followed by the transmission attempts per frame time, old and new combined. Old frames mean those frames that have previously suffered collisions.
The maximum throughput occurs at $G = 0.5$, with $S = \frac{1}{2e}$, which is about 0.184. In other words, the best we can hope for is a channel utilization of 18%. This result is not very encouraging, but with everyone transmitting at will, we could hardly have expected a 100% success rate.
In slotted Aloha, time is divided up into discrete intervals, each interval corresponding to one frame. In Slotted Aloha, a node is required to wait for the beginning of the next slot in order to send the next packet. The probability of no other traffic being initiated during the entire vulnerable period is given by $e^{- G}$ which leads to $S = G \times e^{- G}$. It is easy to compute that Slotted Aloha peaks at G = 1, with a throughput of $s = \frac{1}{e}$ or about 0.368.
ALOHA is a medium access control (MAC) protocol for transmission of data via a shared network channel. Using this protocol, several data streams originating from multiple nodes are transferred through a multi-point transmission channel. There are two types of ALOHA protocols – Pure ALOHA and Slotted ALOHA.
In pure ALOHA, the time of transmission is continuous. Whenever a station has an available frame, it sends the frame. If there is collision and the frame is destroyed, the sender waits for a random amount of time before retransmitting it.
After transmitting a frame, a station waits for a finite period of time to receive an acknowledgement. If the acknowledgement is not received within this time,the station assumes that the frame has been destroyed due to collision and resends the frame.
A collision occurs if more than one frame tries to occupy the channel at the same time. The situation is depicted in the following diagram−
Maximum Throughput of Pure ALOHA
The maximum throughput occurs when G𝐺=0.5.
The maximum throughput is thus−
$S_{max}$𝑎𝑥 = $0.5𝑒e^{-2*0.5}$ = $\frac{1}{2e}= 0.184$
Thus, it can be seen that the maximum throughput is only 18.4% in pure ALOHA.
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The ALOHA protocol was first developed at the University of Hawaii in the early 1970s for packet radio networks. However, it can be used in any situation where multiple devices share a common communication channel. This protocol allows devices to transmit data at any time, without a set schedule. This is known as a random access technique, and it is asynchronous because there is no coordination between devices. When multiple devices attempt to transmit data at the same time, it can result in a collision, where the data becomes garbled. In this case, each device will simply wait a random amount of time before attempting to transmit again. The basic concept of the ALOHA protocol can be applied to any system where uncoordinated users are competing for the use of a shared channel.
ALOHA is an early computer networking method created at the University of Hawaii in the early 1970s. It’s a straightforward way to send data over a shared medium, like a wireless or wired network. The main idea of ALOHA is how it handles collisions, which happen when two devices try to send data at the same time, causing interference.
There are two main versions of the ALOHA protocol:
In this article we going to discuss pure ALOHA in detail.
Vulnerable period for packet A
Throughput of Pure ALOHA
Let 𝑇 be the frame time, i.e. the time required for 1 frame to be transmitted.
Let G be the number of transmission attempts per frame time.
The probability that 𝑘 frames are generated during the frame time is given by the Poisson distribution−
$$P(k)=\frac{G^k e^{-G}}{k!}$$
From this we can say that the probability that 0 frames are generated ( 𝑘k = 0 ) during the frame time is $e^{-G}$.
In case of pure ALOHA, the vulnerable time period so that collision does not occur between two frames is equal to two frame times, i.e. 2T𝑇. In 2T time,average number of transmission attempts is 2G.
The probability that 0 frames are initiated in the vulnerable time period will be −
The throughput, 𝑆, is calculated as the number of transmission attempts per frame time, 𝐺, multiplied by the probability of success, 𝑃(0).
What is the full form of ALOHA?
The full form of ALOHA is Advocates of Linux Open-source Hawaii Association.
Throughput of Pure ALOHA
The probability of successful transmission (S) can be derived from the probability that no other packets are sent during the vulnerable time period. This is given by:
Key Features of Pure ALOHA
For more details please refer Derive the efficiency of Pure ALOHA protocol and Differences between Pure and Slotted Aloha article.
Pure ALOHA is an early and simple method for sending data over a shared network, where devices transmit whenever they have data to send. It doesn’t check if the channel is free, leading to frequent collisions when two devices send data at the same time. This results in a maximum efficiency of about 18.4%, meaning many transmissions are lost due to these collisions.
