What is ALOHA in Computer Network

Aloha is a data link layered multiple access protocol that describes the way in which multiple terminals can access without interference or conflict.

“Aloha” in the context of computer networks refers to the ALOHA protocol, which is one of the earliest communication protocols used for computer networking. It was developed at the University of Hawaii in the early 1970s to support communication between the university’s computers on different islands using UHF wireless radios.

Table of Contents

• Difference between Pure and Slotted Aloha in Computer Networks
• What is ALOHA in computer networks?
• Types of ALOHA
• Conclusion

Difference between Pure Aloha and Slotted Aloha

The difference between Pure Aloha and Slotted Aloha are as follows:

What is ALOHA in Computer Network?

ALOHA in computer network is referred to as a multiple-access protocol that is used for transmitting data through shared network channels. It operates within the medium access control sublayer of the OSI model. Through this protocol, several data streams that originate from multiple nodes are transferred via a multi-point transmission channel.

Every node and station transmit frame without detecting whether the transmission channel is busy or idle. When the channel is idle, the frames are successfully transmitted, and when two frames simultaneously try to occupy the channel, frames collision will occur, and they will be discarded. These stations will retransmit corrupted frames repeatedly till successful transmission will occur.

Types of ALOHA

Over the years, ALOHA has been developed to improve efficiency and effectiveness. The following are notable versions of ALOHA:

1. Pure ALOHA

In the case of Pure Aloha, transmission time remains continuous. When a station has the availability of frames, it will send the frame. In case there is a collision and the frame gets destroyed, sender will wait for random amount of time before retransmission. Let us now understand this technique:

Step 1: In the case of Pure ALOHA, the nodes transmit frames whenever the data is available for sending.

Step 2: Whenever two or more nodes transmit data simultaneously, there is a chance of collision and frames get destroyed.

Step 3: The sender will expect an acknowledgement from the receiver. 

Step 4: When the acknowledgement is not received within a specified time, the sender node will assume that the frame has been destroyed. 

Step 5: If the frame is destroyed by a collision, the node will wait for a random amount of time and sends it again. The waiting time will be random since otherwise same frames will collide multiple times.

Step 6: As per Pure ALOHA, when the time-out period passes, every station must wait for a random time before resending frame. This randomness will help in avoiding more collisions.

2. Slotted ALOHA 

Slotted ALOHA was brought in 1972 as an improvement to pure ALOHA. The time in the case of slotted ALOHA is divided into discrete intervals called slots that correspond to a frame. Here, slotted ALOHA overcomes the possibility of frame hitting in pure ALOHA. Unlike pure ALOHA, this type of ALOHA does not allow data transmission whenever the station wants to send it. 

If a station wants to send the frame to a shared channel, frames can only be sent at the beginning of the slot. Only one frame is allowed to be sent to every slot. In case the station fails to send data, it will have to wait till the next slot. Still, there is a possibility of collision in case two stations try to send a frame at the beginning of the time slot. 

Throughput of Slotted ALOHA

Let us consider that T is a time frame where the required time for a single frame to be transmitted. Now, let us consider that G represents the number of transmission attempts per frame time. Here, using the Poisson distribution, one can discover the probability that k frames are generated during the frame time. 

Here we can say that the probability of 0 frames being generated (when k = 0) during frame time is e^-g

In slotted ALOHA, the vulnerable time period for collision between two frames is equal to the time duration of 1 slot. Here, one slot is equal to 1 tine frame where T is the time during which an average number of transmission attempts is equal to G. 

Here, the probability that 0 frames are initiated in the vulnerable time period is P(0) = e^-G. ‘S’ is the throughput which is calculated as the number of transmission attempts per frame G multiplied by the probability of success PP(0). S – G.P(0) or S = Ge^-G. 

3. Carrier Sense Multiple Access with Collision Detection

It is a medium access control (MAC) method that is used in Ethernet technology for LAN. CSMA/CD uses a carrier sensing for deferring transmissions until no other station is transmitting. CSMA/CD is used in combination with collision detection, where the transmitting station detects collision by sensing transmission from other stations. 

It senses and listens to whether the shared channel for transmission is busy and defers transmissions till the time channel becomes free. Here, the collision detection technology detects collisions by sensing transmission from other stations. On detecting a collision, the station stops transmitting, sends the jam signal and waits for a random time interval before retransmission.

Principles Governing Aloha

Aloha is a networking proptocol that is used for satellite and ground-based radio transmissions. It was one of the foremost protocols that was designed for managing packet switching over shared medium. Following principles govern both types of Aloha:

1. Pure ALOHA

• No time boundation: Send Whenever: In Pure ALOHA, a user can transmit data packets whenever data is ready to be sent, without checking if the channel is free.
• Listen After Sending: After transmitting a packet, the sender listens to the network to determine if the transmission was successful.
• Collision Handling: If a packet collides with another (i.e., if another user was transmitting at the same time), it becomes garbled and needs to be resent.
• Random Delay Before Retransmitting: When a collision occurs, the sender waits for a random amount of time before retransmitting the packet. This random delay helps to reduce the likelihood of another collision.
• No Coordination Between Users: There is no central coordination; each user transmits at will, leading to a high possibility of collisions, especially as network load increases.

2. Slotted ALOHA

Slotted ALOHA is an improved version of Pure ALOHA which increases the network's efficiency. It introduces time slots to organize transmissions:

• Time-Slotted Transmission: The time is divided into discrete intervals or slots, and users are only allowed to send packets at the beginning of a time slot.
• Synchronized Network: This method requires all users to be synchronized with the time slots to avoid overlaps partially.
• Collision Handling: Like Pure ALOHA, if a collision occurs, the sender will wait for a random number of time slots before trying again.
• Reduced Collision Probability: By restricting transmissions to specific times, Slotted ALOHA reduces the likelihood of collisions compared to Pure ALOHA. It effectively doubles the throughput of the network.
• No Central Coordination: Even with time slots, there's no central coordination, and users independently decide when to retransmit.

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Conclusion

Thus, the ALOHA protocol, originating in the 1970s, was a foundational computer networking method. Its approach to handling data collisions influenced future protocols. While not prevalent today, its principles remain significant in networking history.