Pure aloha is used when stations have data to send over a channel, whereas slotted aloha improves on pure aloha by reducing the chances of collisions between frames.
In this article, we will discuss the difference between Pure Aloha and Slotted Aloha, which are the types of ALOHA.
What is Aloha?
ALOHA is a MAC (Medium Access Control) protocol in which every station is equally allowed to transmit data. As there is more chance of collision, this Aloha protocol is mainly used in fewer stations. In this protocol, any user can send the data in any priority, and if there is no other transmission, the data will be transmitted successfully. However, if there is more than one transmission together, a collision will occur, and the transmission will fail.
There are two types of Aloha protocol
Pure Aloha
Slotted Aloha
Let's see them in detail and the difference between Pure Aloha and Slotted Aloha.
What is Pure Aloha?
Pure Aloha was first used at the University of Hawaii in 1970 and is the most basic form of Aloha. There is no restriction for the transmission, so a station sends the frame whenever it wants without taking care of any collisions.
Pure Aloha has a high risk of collision, so this is not used in the more traffic system to get the most negligible probability of collisions. The vulnerable time of this Aloha is equal to double the transmission time, which means that this is a time at which there will be a collision.
So for no collision, the Transmission should not have to be before the transmission time of starting the subsequent Transmission to the end of that Transmission.
Vulnerable time = 2 * Transmission Time
Stations wait for any finite time for acknowledgment. If not received, the transmission is assumed to be destroyed due to collision.
If the collision occurs more than once, the user has to wait for a random time after any collision, known as the Back-off time.
The maximum efficiency of Pure Aloha is 18.4%.
What is Slotted Aloha?
Pure Aloha was improved in 1972, and Slotted Aloha was formed. This is enhanced by a scientist named Robert. In this Aloha, the time is divided into fixed intervals known as Slots instead of any time transmission. Since if any user wants to transmit, it has to send at the beginning of the next slot.
The time of a slot is equal to the transmission time.
As the time is divided into slots, this is called Slotted Aloha. In this Aloha, we have to wait for a slot for no collision since the vulnerable time of Slotted Aloha is the time of a slot equal to the transmission time.
Vulnerable time = Transmission Time;
The collision can also occur in this Aloha if more than one transmission occurs at the beginning of the slot. Due to the decrement in the vulnerable time, the maximum efficiency of Slotted Aloha is 36.8% which is the main difference between Pure Aloha and Slotted Aloha.
Difference between Pure Aloha and Slotted Aloha
There are many differences between Pure Aloha and Slotted Aloha.
Some of these differences are given below in the table.
Pure Aloha
Slotted Aloha
Transmission can occur at any time from any of the stations.
Transmission can only occur at the start of any particular slot.
There is a chance of more collisions as anyone can transmit at any time.
The chances of collisions are significantly lower due to the discrete-time system.
The vulnerable time of this aloha is twice the transmission time (Tt).
The vulnerable time of this aloha is equal to the transmission time (Tt).
What is difference between pure ALOHA and slotted aloha?
Pure ALOHA allows transmissions at any time, while slotted ALOHA divides time into slots, improving efficiency by synchronizing transmissions.
What is the difference between pure ALOHA and CSMA?
Pure ALOHA has no carrier sensing, leading to collisions, while CSMA senses the carrier before transmitting, reducing collision probability.
What is the pure ALOHA protocol?
Pure ALOHA allows stations to transmit data whenever they have it, without synchronization, leading to collisions and retransmissions.
What is ALOHA and its types?
ALOHA is a random access protocol. Types include Pure ALOHA, allowing any-time transmissions, and Slotted ALOHA, which synchronizes transmissions into time slots.
Conclusion
In this article, we learned about some basics of Aloha and the terms used in these protocols, and mainly we discussed the difference between Pure Aloha and Slotted Aloha.
The main difference between Pure Aloha and Slotted Aloha is that the vulnerable time of slotted Aloha is less, and the efficiency is more than pure one.