Difference between Pure ALOHA and Slotted ALOHA

 Main Difference

The main difference between Pure and Slotted ALOHA is that Pure ALOHA does not account for the changing of time duration whereas, Slotted ALOHA has several time limitations.

Comparison Chart

Basis of DistinctionPure ALOHASlotted ALOHA
DefinitionThe first version of the protocol for computer networking that was implemented by the Hawaii University for wireless communication.The latest version of the protocol for computer networking that was implemented by a person named Robertson.
TransmissionSends the messages any time they want.Send the information only during the prescribed time slots.
DelayTime keeps on changing, so the user has to wait.Time stays discrete, so the user sends and receives the messages.
NotificationNotified when they receive a message.Notified when they send or receive a message.
FormulaS= G* e^-2GS= G* e^-G.


It was introduced in 1971 and at that time, became the first proper wireless data network for the public. ALOHA stands for Additive Links Online Hawaii Area. The implementation of such a version did not create any problems since the commands were simple. For example, if a person wanted to send something, they just pressed send. Similarly, if they wanted to send the message during an active communication and another person transmits the data at the same time, the system keeps retrying for the information to deliver. What happens during this process is that Pure ALOHA does not consider the occupation of the full channel, even if the server becomes busy, it keeps on sending information from one device, and therefore the total system is not efficiently used. No time limit exists on how long a message takes from one device to reach the other and thus people had to wait for a long time before they got a reply. This action resulted in the efficiency being compromised. To meet this deficiency command such as “transmit later” comes in handy. One main property of such a protocol includes the length of the frames that always stays the same.

Slotted ALOHA

Several improvements were made on the initial system to remove the delays and therefore Slotted ALOHA is used currently with success. The difference between this and the previous system was the introduction of timeslots that helped to increase the efficiency. A person only can send information at the beginning of the conversation; a precise time exists, that ranges between few seconds in which the action must complete. While doing this, the collision between two values reduces significantly. Other factors are considered for the values to take effect such as the attempt to transmit data should be held between the first timeframe, if that does not happen, no option of using continuous timelines exist. It has many applications such as its use of the military facilities and satellite communication that require care and discreteness to make sure the correct information moves between the systems at the right time. Other examples of such a protocol include the set-top box communication, and mobile telephone call set up, once a person is dialing the number of the other, they cannot call anyone else. The other individual at the receiver also cannot dial the number of the individual who is calling them as the call always shows a busy sign to avoid any altercations.

Key Differences

  • The user can send the messages any time they want when it comes to pure Aloha whereas the user can send the information only during the prescribed period slots when it comes to slotted Aloha.
  • Time keeps on changing during the pure aloha, and therefore people must wait before one message sends, and they receive the other. On the other hand, time stays discrete, so the user sends and receives the messages during that period.
  • The user just has the receiver that tells them whenever they get a message during the Pure Aloha whereas the user has both the transmitter and the receiver which tells about the status, in slotted aloha.
  • The probability of successful transmission of the information within the pure aloha is S= G* e^-2G whereas the likelihood of transmission of the information within the slotted Aloha is S= G* e^-G.

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