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Aloha in Computer Networks

Last Updated on January 8, 2024 by Ankit Kochar

In computer networks, the term "Aloha" is closely associated with a network protocol named after the traditional Hawaiian greeting. Aloha protocols are a family of network protocols that originated in the 1970s and are fundamental to the development of early packet-switched networks. This introduction provides an overview of the Aloha protocol and its significance in computer networks.

First, we’ll discuss Multiple Access protocols for a better understanding of the topic then we will deep delve into the main topic of the article followed by their types and the difference between them with the help of examples and figures.

Multiple Access Protocols

The OSI (Open Systems Interconnection) model’s Medium Access Control sublayer (MAC sublayer) is where Multiple Access Protocols function. These protocols make it possible for multiple people or nodes to share a network path. Multiple Access Protocols are a group of protocols whose main goals are to improve the communication line, prevent crosstalk, and reduce channel conflicts.

Data link management can handle the channel when a sender and recipient share a designated connection for data packet transmission. Assume that two devices cannot connect or send data along a particular path. In that case, a large number of sites connect to the channel and simultaneously transmit data over it. Collisions and interference could happen as a consequence. The numerous access method is therefore crucial to reducing accidents and removing channel crosstalk.
Imagine a classroom full of pupils in order to more completely comprehend the idea of multiple access protocols. All of the students (small channels) in the class start responding to the teacher’s query at the same moment. (transferring the data simultaneously). Since everyone responds at once, data can mix or be deleted. As a consequence, a teacher’s role (multiple access protocol) is to exert control over the pupils and compel them to provide a single response.

Let’s look at the types of various Multiple Access Protocols.

Random Access Protocols

In this protocol, transmitting data over a channel is given equitable precedence by each station. A random access procedure prohibits one or more locations from depending on or controlling others. Depending on the condition of the channel, each station transmits the data frame. (idle or busy). If multiple stations send data over the same channel, a collision or dispute may happen. The impact may cause data frame packets to be deleted or changed. As a consequence, the receiving end does not obtain it.

Controlled Access Protocols

On a common channel, is a method for reducing data frame conflicts. In the restricted access protocol, each station communicates with the others and chooses which station will transmit a data frame that has been authorized by every other station. This implies that a single station cannot send data frames until all other stations are authorized. The three types of restricted access are token transmission, voting, and reservations.

Channelization Protocols

Based on their time, location, and codes, numerous locations can divide the complete usable bandwidth in a common channel using a channelization system. To transmit data frames to the channel, it can concurrently reach all stations. Some of the channelization algorithms include FDMA (Frequency Division Multiple Access), TDMA (Time Division Multiple Access), and CDMA (Code Division Multiple Access).

What is ALOHA in Computer Network?

Using a common network channel, data can be transferred using the multiple-access protocol ALOHA. It operates in the medium access control sublayer of the open systems interconnection (OSI) architecture. (MAC sublayer). Using this protocol, numerous data streams from various locations are sent across a multi-point transmission path. It was created at the University of Hawaii in the 1970s by Norman Abramson and his colleagues.

Without checking to see if the broadcast route is active or not, each node or station in ALOHA sends a frame. The frames will be properly transferred if the channel is empty. Two frames will collide and be deleted if they attempt to fill the channel at the same moment. The corrupted frames may be retransmitted by these channels until the effective transfer takes place.

Rules of ALOHA in Computer Network

  • Any station may at any moment send information to a channel.
  • When sending data over several sites, collisions and data packets could be lost.
  • Carrier detection is not required.
  • Due to the existence of the acknowledgment of the frames, Aloha does not have impact recognition.
  • It demands data resend after an arbitrary interval of time.

Types of ALOHA in Computer Networks

The two protocols of Aloha in computer networks are

  • Pure Aloha
  • Slotted Aloha

Pure Aloha

When data can be transmitted over a channel at stations, we use Pure Aloha. In pure Aloha, a conflict may happen and the data frame may be lost if each station sends data to a channel without first checking to see if it is free or not. The pure Aloha waits for the recipient to acknowledge when any station transmits a data frame to a channel. The station watches for an arbitrary period of time, known as the backoff time, if the receiver’s response is not received within the allotted time. (Tb). The station might also think the frame has been misplaced or damaged. It consequently sends the frame again until all of the data is successfully transmitted to the recipient.

The scenario of four channels competing for access to the common channel is illogical. The diagram shows that on the common medium, each location transmits two frames for a total of eight frames. Some of these frames clash as a result of multiple frames vying for the same common channel. Only two frames—frame 1.1 from station 1 and frame 3.2 from station 3—remain, as seen in the image above. Even if a bit from one frame and a bit from another cohabit on the channel, a collision takes place and both are annihilated.

Vulnerable Time for Pure Aloha
Since the stations broadcast fixed-length frames, it is assumed that it takes Tt time (Transmission Time) for each frame to be transmitted entirely. The susceptible period for station A is depicted in the figure below. Because we need to make sure that no other station begins broadcasting between Tt time before and Tt time after the transmitting station in order to prevent frame collisions, the Vulnerable time for any station must be twice as long as the transmission time.

Slotted Aloha

The slotted Aloha is meant to beat pure Aloha because pure Aloha has a very high probability of striking a frame. Slotted Aloha divides the common channel into discrete segments of time. As a consequence, only at the beginning of the slot and only one frame can be sent to each slot by a station if it wishes to transmit a frame to a shared channel. If the station is unable to transmit data at the start of the slot, it must also wait until the beginning of the slot for the following broadcast. However, there is still a chance of a collision when transmitting a frame at the beginning of two or more station time periods.

The channel’s time division is depicted in the image. Only at the commencement of the slot can a station begin its broadcast. The only scenario that could result in a crash is if two or more sites begin transmitting during the same time period. Frame 1.2 of Station 1 and Frame 4.1 of Station 4 both display this state.

Vulnerable Time for Slotted Aloha
In the instance of Slotted Aloha, the Vulnerable period is equivalent to the station’s transmission period. It’s because we tied networks’ broadcasts to specific timeslots. The vulnerable period for Slotted Aloha is depicted in the image below.

Protocol Flow Chart for ALOHA
The Aloha Protocol’s flowchart, which is shown in the image below, explains how it operates.

Difference between Pure Aloha and Slotted Aloha

Pure Aloha Sorted Aloha
Any node can send data at any moment in this Aloha. In this, the data can be transmitted at the start of any time period by any location.
In this, time is constant and not synchronized worldwide. The clock is synchronized worldwide and is discrete in this.
Vulnerable time for Pure Aloha = 2 x Tt Vulnrable time for Slotted Aloha = Tt
Probability of a data payload being successfully transmitted in Pure Aloha \= G x e-2Greduce Probability of the data payload being successfully transmitted in Slotted Aloha \= G x e-G
In Pure Aloha, Maximum efficiency \= 18.4% In Slotted Aloha, Maximum efficiency \= 36.8%
The Number of collisions is not cut in half by practicing pure aloha. By halving the number of collisions, Slotted Aloha doubles the effectiveness of Pure Aloha.

In conclusion, Aloha protocols have played a pivotal role in the evolution of computer networks, serving as a foundational concept for early packet radio networks and influencing subsequent networking technologies. While the original Aloha protocol had its limitations, it laid the groundwork for the development of more sophisticated protocols that have become integral to modern communication systems.

FAQs Related to Aloha in Computer Networks

Below are some FAQs related to Aloha in Computer Networks:

Q1: What is the Aloha protocol in computer networks?
The Aloha protocol is a network protocol designed for packet radio systems, allowing multiple stations to communicate over a shared communication channel. It introduced the concept of random access and laid the foundation for later collision resolution protocols.

Q2: What are the two types of Aloha protocols?
There are two main types of Aloha protocols: Pure Aloha and Slotted Aloha. Pure Aloha allows stations to transmit at any time, while Slotted Aloha divides time into slots, with stations transmitting only at the beginning of a slot.

Q3: What is the significance of Aloha protocols in networking?
Aloha protocols were pioneering in the development of early packet-switched networks. They introduced the concept of decentralized access to a shared communication medium, influencing the design of subsequent network protocols and technologies.

Q4: What are the drawbacks of the original Aloha protocol?
The original Aloha protocol suffered from inefficiencies due to collisions, where two or more stations transmitted simultaneously, leading to data loss. This limitation was addressed by later protocols that implemented collision detection and resolution mechanisms.

Q5: How have Aloha protocols evolved in modern networking?
While the original Aloha protocol is largely obsolete, its concepts influenced the development of modern protocols and technologies, including Carrier Sense Multiple Access (CSMA) protocols. These advancements have significantly improved the efficiency and reliability of communication in contemporary computer networks.

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