ALOHA in Computer Network

Introduction to Multiple Access Protocol

Multiple Access Protocols operate in the Medium Access Control sublayer (MAC sublayer) of the OSI(Open Systems Interconnection) model. These protocols enable several nodes or users to share a network channel. In general, Multiple Access Protocols are the set of protocols whose objective is to optimize the transmission line, avoid crosstalks and minimize the collisions in the channel.

When a sender and receiver share a dedicated link for data packet transmission, data link control is sufficient to handle the channel. Assume there is no specific path for two devices to communicate or transmit data. In that scenario, numerous stations access the channel and broadcast data over it at the same time. It may result in collisions and crosstalk. As a result, the multiple access technique is essential to limit collisions and eliminate channel crosstalk.

To understand the concept of Multiple Access Protocols more meaningfully, consider a scenario of a classroom full of students. When a teacher asks a question, all of the students (small channels) in the class begin addressing the question at the same time (transferring the data simultaneously). Since all students reply at the same moment, data is overlapping or lost. As a result, it is the job of a teacher (multiple access protocol) to control the students and force them to give one answer.

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

Random Access Protocols

In this protocol, each station has equal priority when it comes to sending data over a channel. One or more stations in a random access protocol cannot rely on another station, nor can one station control another. Each station broadcasts the data frame depending on the channel's status (idle or busy). A collision or conflict may occur if more than one station transmits data over the same channel. Data frame packets may be lost or altered due to the collision. As a result, it is not received by the receiving end.

Controlled Access Protocols

It is a technique for minimizing data frame collisions on a shared channel. Each station interacts and decides to send a data frame by a specific station that is approved by all other stations in the controlled access protocol. This means that until all other stations are not approved, a single station cannot deliver data frames. Reservation, polling, and token passing are the three methods of controlled access.

Channelization Protocols

A channelization system allows many stations to split the entire useable bandwidth in a shared channel based on their time, distance, and codes. It can access all stations simultaneously to send data frames to the channel. FDMA(Frequency Division Multiple Access), TDMA(Time Division Multiple Access), and CDMA(Code Division Multiple Access) are some of the methods of channelization protocols.

What is ALOHA in Computer Network?

ALOHA is a multiple-access protocol that allows data to be transmitted via a shared network channel. It functions in the open systems interconnection (OSI) model's medium access control sublayer (MAC sublayer). Several data streams from multiple nodes are transmitted across a multi-point transmission channel using this protocol. It was developed by Norman Abramson and his associates in the 1970s at the University of Hawaii.

Each node or station in ALOHA transmits a frame without attempting to determine whether the transmission channel is idle or busy. If the channel is idle, the frames will be successfully transmitted. If two frames try to occupy the channel at the same time, they will collide and be discarded. These stations may opt to retransmit the corrupted frames until successful transmission occurs.

Rules of ALOHA in Computer Network

1. At any time, any station can transmit data to a channel.
2. Collisions and data frames may be lost during data transmission over multiple stations.
3. There is no need for carrier sensing.
4. Aloha has no collision detection because the acknowledgment of the frames exists.
5. It necessitates data retransmission after a random amount of time.

Types of ALOHA in Computer Network

The two protocols of Aloha in computer networks are:

1. Pure Aloha
2. Slotted Aloha

Pure Aloha

We employ Pure Aloha whenever data is available for transmission over a channel at stations. In pure Aloha, if each station transmits data to a channel without determining if the channel is idle or not, a collision may occur, and the data frame may be lost. When any station sends a data frame to a channel, the pure Aloha waits for the receiver's acknowledgment. If the receiver's acknowledgment is not received within the given time, the station waits for a random amount of time, known as the backoff time (Tb). Furthermore, the station may believe the frame has been lost or destroyed. As a result, it retransmits the frame until all of the data is properly delivered to the receiver.

Four stations (an unreasonable example) compete with one another for access to the shared channel. The diagram demonstrates that each station sends two frames for a total of eight frames on the shared medium. Due to several frames competing for the same shared channel, some of these frames collide. The picture above reveals that just two frames survive: frame 1.1 from station 1 and frame 3.2 from station 3. Even if one bit from one frame coexists on the channel with one bit from another frame, a collision occurs, and both are destroyed.

Vulnerable Time for Pure Aloha

We assume that the stations send fixed-length frames, which takes Tt time( Transmission Time) to transmit the frames completely. The diagram given below shows the vulnerable time for station A. The Vulnerable time for any station must be twice the transmission time because to stop the collision of frames, we must make sure that no other station starts its transmission between Tt time before and Tt time after the transmitting station.

At the time t, Station A sends a frame. Assume station B has previously transmitted a frame between $t - T_{t}$ and $t$. As a result, the frames from station A and station B collide. The end of B's frame intersects with the start of A's frame. Assume, on the other hand, that station C sends a frame between $t$ and $t + T_{t}$. There is a collision between frames from stations A and C here. The start of the C's frame intersects with the conclusion of A's frame.

Where,

$V_{t}$ is a Vulnerable Time for Pure Aloha, $T_{t}$ is the Transmission Time of the station.

Protocol Flow Chart for ALOHA

The figure given below is the flow chart of the Aloha Protocol and describes the working of Aloha.

Explanation

1. A station having data to transmit will start its transmission.
2. It waits for some random time (depending on the type of Aloha Protocol). For example, it is $2*Transmission$ $Time$ in the case of Pure Aloha.
3. If the acknowledgment is received, then the transmission is successful.
4. If no acknowledgment is received, it means a collision has occurred. Then the station uses a backoff strategy to retransmit the packet again.
5. After various iterations, the station aborts the transmission if there is no acknowledgment.

Differences between Pure and Slotted ALOHA

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