Key differences between Pure ALOHA and Slotted ALOHA


Pure ALOHA is a fundamental network protocol for media access control, developed for early wireless local area networks, particularly satellite communications. Introduced in the 1970s by Norman Abramson and his team at the University of Hawaii, it was one of the first protocols to address the challenge of multiple users attempting to communicate over a shared communication channel.

The core concept of Pure ALOHA is simple: whenever a user has data to send, they transmit it without checking if the channel is currently in use. This approach is known as an unslotted system, where users are not restricted to specific time slots for sending data. Following transmission, the sender waits for an acknowledgment from the receiver. If the acknowledgment does not arrive within a predetermined time, it is assumed that the data packet collided with another transmission.

In such cases of collision, Pure ALOHA employs a random backoff algorithm. The sender waits for a random period before reattempting to transmit the data packet. This random delay helps reduce the likelihood of repeated collisions.

However, the simplicity of Pure ALOHA comes with significant drawbacks. The protocol has a low efficiency, particularly as the number of users increases. The probability of collisions rises sharply with more users, leading to a substantial amount of retransmissions and reduced throughput. The maximum channel utilization in Pure ALOHA is limited to approximately 18.4%, a figure derived from theoretical analysis of its performance under continuous heavy load.

Despite these limitations, Pure ALOHA set the groundwork for many subsequent developments in network protocols, particularly in the field of random access communication methods. Its core principles influenced the design of more efficient protocols, such as Slotted ALOHA and Carrier Sense Multiple Access (CSMA) protocols.

Pure ALOHA Functions:

  • Data Packet Transmission:

The primary function of Pure ALOHA is to allow network devices to transmit data packets over a shared communication channel. Whenever a device has data to send, it sends the data packet without waiting for a specific time slot or checking if the channel is currently free.

  • Collision Handling:

Pure ALOHA manages the possibility of data collisions, which occur when two or more devices transmit simultaneously over the same channel. Collisions result in corrupted data packets that cannot be properly received.

  • Acknowledgment of Receipt:

After sending a data packet, the sender expects an acknowledgment from the receiver. This acknowledgment confirms that the packet was received without collision and correctly decoded.

  • Random Backoff Algorithm:

In case of a collision (identified by the absence of an acknowledgment within a certain time frame), Pure ALOHA uses a random backoff algorithm. The sender waits for a randomly determined period before retransmitting the data packet. This approach attempts to minimize the likelihood of the packet colliding again upon retransmission.

  • Retransmission Attempts:

If a packet collides, Pure ALOHA allows the sender to attempt retransmission after the random backoff period. This process continues until the packet is successfully transmitted without collision or a set limit of retransmission attempts is reached.

  • Decentralized Control:

Pure ALOHA operates under a decentralized control mechanism. Each node in the network makes its own decision about when to transmit data, independent of a central controller or synchronization with other nodes.

  • Simplicity and Ease of Implementation:

One of the key functions of Pure ALOHA is its simplicity. It doesn’t require complex algorithms or hardware for time synchronization, making it easy to implement in various network environments, especially in the early days of computer networking.

Pure ALOHA Components:

  • User Nodes:

These are the devices or stations that participate in the network, each capable of sending and receiving data packets. In Pure ALOHA, any node can transmit data at any time without checking the status of the channel.

  • Shared Communication Channel:

The central medium through which data packets are transmitted. In the context of Pure ALOHA, this is typically a broadcast channel used by all nodes to send and receive data.

  • Data Packets:

The units of communication in the network. Each data packet typically includes the actual data being transmitted, along with necessary control information like source and destination addresses.

  • Transmitter:

Each node features a transmitter component to send data packets over the shared channel. The transmitter does not require any special mechanism to detect the channel status before transmission.

  • Receiver:

Each node also includes a receiver component to receive data packets sent by other nodes. The receiver attempts to detect and decode any incoming data packets.

  • Acknowledgment Messages:

After successfully receiving a data packet, the receiver sends back an acknowledgment message to the sender, indicating successful receipt of the packet.

  • Collision Detection:

While Pure ALOHA does not include a mechanism to prevent collisions, it implicitly involves the detection of collisions. This is typically inferred when an acknowledgment is not received within a certain time frame after transmission.

  • Random Backoff Algorithm:

A key component of Pure ALOHA’s method for handling collisions. When a node detects a collision (through the absence of an acknowledgment), it waits for a random period before reattempting transmission.

  • Timer:

Each node has a timer to keep track of the time elapsed after sending a data packet. This timer helps determine when to consider a packet lost due to collision (based on the lack of acknowledgment within a specific timeframe).

  • Control Logic:

This encompasses the algorithms and protocols each node uses to determine when to send data, how to react to collisions, and when to reattempt transmissions.

Advantages of Pure ALOHA:

  • Simplicity:

Pure ALOHA is simple to understand and implement. It doesn’t require complex algorithms or mechanisms for coordinating transmissions among nodes.

  • Decentralization:

It operates in a completely decentralized manner. Each node makes its own decision about when to send data, without needing to synchronize with a central server or other nodes.

  • Flexibility in Access:

Nodes can transmit data at any time, leading to flexibility in communication. This is particularly useful in environments where it is not feasible to implement more structured access methods.

  • Robustness:

Since there is no central coordinating mechanism, the network is quite robust. The failure of one node does not significantly impact the ability of other nodes to communicate.

  • No Need for Synchronization:

Pure ALOHA does not require global time synchronization among nodes, which simplifies the network infrastructure.

Disadvantages of Pure ALOHA:

  • Low Efficiency:

The protocol is highly inefficient in terms of bandwidth usage. The probability of successful transmissions without collision is low, especially as network traffic increases.

  • High Collision Rate:

The probability of collisions is high due to the lack of coordination, which becomes more pronounced as the number of nodes increases.

  • Poor Scalability:

Pure ALOHA does not scale well with increasing network size. As more nodes join the network, collisions become more frequent, greatly reducing the network’s throughput.

  • No Collision Detection:

Unlike some other protocols, Pure ALOHA does not include a mechanism for detecting collisions. It relies on timeouts and acknowledgments, which can lead to inefficiencies.

  • Vulnerable to High Latency:

In situations where network latency is high, the performance of Pure ALOHA degrades further because of the increased likelihood of collision and the time it takes to detect them.

  • Throughput Limitations:

The theoretical maximum throughput of Pure ALOHA is only about 18.4% of the channel capacity, which is quite low compared to other more sophisticated protocols.

Slotted ALOHA

Slotted ALOHA is an improvement on the original ALOHA protocol, a method for controlling access in a telecommunications network. Developed by Norman Abramson in the 1970s, Slotted ALOHA divides time into discrete intervals or slots, each equal in length to the frame transmission time. This structure is a key difference from Pure ALOHA, where users can transmit at any time, leading to many collisions. In Slotted ALOHA, a node must wait for the beginning of the next slot to send its frame.

This synchronization of transmissions significantly reduces the chance of collisions because frames from different nodes are less likely to overlap. However, if two nodes transmit at the beginning of the same time slot, a collision still occurs. After a collision, nodes involved in the collision wait for a random number of time slots before attempting to retransmit, minimizing the chances of repeated collisions.

The advantages of Slotted ALOHA include improved efficiency over Pure ALOHA. Theoretically, it doubles the maximum throughput from about 18.4% to approximately 36.8%. However, it still suffers from issues of scalability and efficiency, especially in high-traffic environments. Slotted ALOHA is primarily significant in the historical development of network protocols, particularly for wireless communication systems and satellite networks, where coordinating transmission times is challenging.          

Slotted ALOHA Functions:

  • Time Slot Management:

Slotted ALOHA divides time into equal-sized slots. A key function is to manage these slots, ensuring that each data frame fits into a single slot.

  • Collision Reduction:

By allowing nodes to transmit only at the beginning of a time slot, Slotted ALOHA reduces the probability of collisions compared to Pure ALOHA. This function is critical for maintaining network efficiency.

  • Randomized Retransmission:

After a collision, nodes involved in the collision randomly choose a future time slot for retransmission. This randomness helps in further reducing the chances of subsequent collisions.

  • Frame Synchronization:

A crucial function of Slotted ALOHA is to synchronize the transmissions of nodes to the beginning of the time slots. This synchronization requires nodes to have some form of timing coordination.

  • Throughput Maximization:

By organizing the transmission process, Slotted ALOHA aims to maximize the throughput of the network within its constraints, making better use of the available bandwidth than Pure ALOHA.

  • Network Access Control:

It controls network access by providing a simple rule for nodes: they can transmit only at the beginning of a time slot. This rule is easier to follow than more complex access control methods.

  • Collision Detection and Notification:

While not a direct feature in the basic protocol, implementations of Slotted ALOHA often include mechanisms for detecting collisions and notifying nodes, so they know when to attempt retransmission.

Slotted ALOHA Components:

  • Time Slots:

The fundamental component of Slotted ALOHA is the division of time into equal-sized slots. Each slot is of a duration that matches the time required to send one packet or frame.

  • Nodes:

These are the devices (like computers, sensors, or satellites) that wish to transmit data over the network. Each node operates independently and decides when to transmit based on the protocol rules.

  • Shared Medium:

This is the communication channel (like a radio frequency or a network segment) over which the nodes transmit their data. It is ‘shared’ because multiple nodes use the same medium for transmission.

  • Frame (or Packet):

The data unit that nodes transmit during a time slot. The frame typically includes the actual data payload along with necessary header and trailer information.

  • Clock or Timing Mechanism:

This is essential for synchronizing the time slots across all nodes. Each node must be able to identify the start of a new time slot to follow the protocol’s rules.

  • Randomization Mechanism:

After a collision, nodes use a randomization mechanism (like a random backoff algorithm) to decide when to re-attempt transmission. This helps to reduce the likelihood of repeated collisions.

  • Collision Detection:

While not intrinsic to the basic protocol, many implementations of Slotted ALOHA include a method for detecting when collisions occur. This can be either through feedback from the medium or by other means.

  • Control Algorithm:

This component governs how nodes behave under the protocol, including when and how to transmit, respond to collisions, and manage retransmissions.


  • Simplicity:

Slotted ALOHA is straightforward to implement, making it suitable for networks with limited complexity.

  • Decentralization:

It does not require centralized control, allowing for flexible and independent operation of each node.

  • No Coordination Required:

Nodes do not need to coordinate with each other to transmit, which simplifies the process.

  • Better Performance than Pure ALOHA:

By dividing time into slots, Slotted ALOHA reduces the chances of collision, offering better throughput than Pure ALOHA.

  • Robust in High-Latency Networks:

It works well in environments with high propagation delays, such as satellite communication.


  • Time Synchronization Requirement:

Nodes must be synchronized with time slots, which can be challenging, especially in distributed or large-scale networks.

  • Lower Efficiency:

The protocol is still prone to collisions, leading to lower efficiency compared to more advanced protocols.

  • Wastage of Bandwidth:

If only a few nodes have data to send, much of the bandwidth goes unused due to the rigid time-slot structure.

  • Collisions and Retransmissions:

When collisions occur, retransmissions are necessary, which can lead to further collisions and reduced network throughput.

  • Not Scalable:

The performance of Slotted ALOHA degrades as the number of active nodes increases, making it less suitable for large or busy networks.

  • Overhead:

The requirement for time slots introduces some overhead, as nodes must wait for the next slot to transmit, even if the channel is idle.

Key differences between Pure ALOHA and Slotted ALOHA

Basis of Comparison


Slotted ALOHA

Time Slot Structure

No time slots Time slots used
Transmission Start Anytime Start at slot beginning
Complexity Simpler Slightly more complex
Efficiency Less efficient More efficient
Throughput Max 18.4% efficiency Max 36.8% efficiency
Collision Probability Higher Lower
Time Synchronization Not required Required
Frame Transmission Entire frame anytime Frame in specific slot
Vulnerable Time Period 2x frame time 1x frame time
Ideal Environment Low network traffic Moderate network traffic
Bandwidth Utilization Lower Higher
Collision Resolution More collisions Fewer collisions
Network Scalability Less scalable More scalable
Implementation Easier Requires more control
Suitability Small/simple networks Slightly busier networks

Key Similarities between Pure ALOHA and Slotted ALOHA

  • Random Access Protocols:

Both are random access protocols used in wireless and satellite networks, allowing for spontaneous transmission of data by nodes.

  • Basis for Wireless Communication:

They serve as foundational protocols for understanding and developing wireless communication techniques.

  • Collision Handling:

Both protocols deal with the issue of collisions when two or more nodes transmit simultaneously.

  • No Central Coordination:

Neither protocol requires centralized control for managing the transmissions, as each node operates independently.

  • Retransmission Strategy:

In both Pure and Slotted ALOHA, when a collision occurs, nodes retransmit their frames after a random time delay.

  • Suitable for Multi-Access Channels:

They are designed for multi-access channel environments where multiple users share a common communication medium.

  • Simple to Understand and Implement:

Both protocols are conceptually simple and easy to understand, making them ideal for educational purposes.

  • Use in Satellite Networks:

Both are historically significant in satellite networks, particularly in the context of early developments in wireless communications.

  • Collision Detection:

Neither protocol incorporates collision detection; they only deal with collision resolution.

  • Applicability:

Both are more suited to networks with low to moderate traffic conditions, where the efficiency is not a critical concern.

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