History of Aloha System in Computer Networking: “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.
The multiple access protocol ALOHA (Advocates of Linux Open-source Hawaii Association) is used to transmit data over a public network channel. It functions within the Open Systems Interconnection (OSI) model’s Medium Access Control (MAC) sublayer.
The term Aloha in computer network means hello. Aloha is a data-link layer multiple access protocols that describes how multiple terminals can access the medium without interfering or conflicting. Norman Abramson and his associates developed it 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.
What is ALOHA in Computer Network
Table of Contents
- What is ALOHA in Computer Network?
- ALOHA Means “Hello”
- Versions of ALOHA
- Pure ALOHA
- Slotted ALOHA
What is ALOHA in Computer Network?
Access to the shared communication network channel can be coordinated and arbitrated using the ALOHA system. It was created in the 1970s at the University of Hawaii by Norman Abramson and his associates. The system was originally designed for ground-based radio broadcasting, but satellite communication systems now use it.
When two or more systems seek to transmit on the same channel at the same time, a shared communication system such as ALOHA needs a way to handle collisions. A node in the ALOHA system transmits whenever data is ready to be sent. A collision takes place and the sent frames are lost if the other node transmits at the same time. However, a node can check to see if the frames were transferred by listening to broadcasts on the medium, including its own.
ALOHA Means “Hello”
ALOHA is basically a multiple access protocol which describes how all the terminals can access a medium without interfering at all with one another or even colliding. It operates at the data-link layer. Roberts created a protocol in 1972 that would double the capacity of ALOHA. The Slotted ALOHA protocol divides the time interval into discrete slots, each of which is equal to one frame’s worth of time. To avoid collisions, this approach needs synchronisation between the sending nodes.
Versions of ALOHA
Pure ALOHA
The total time of transmission is continuous in pure ALOHA. A station sends a frame whenever one is available. The sender waits for an arbitrary period of time before retransmitting the frame if there is a collision and it is destroyed.
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.
Slotted ALOHA
Slotted ALOHA increases the capacity of pure ALOHA while reducing collisions. Slots are discrete time periods that are used to divide up the shared channel. Only at the start of each time slot is the station able to transmit data. If multiple stations attempt to transmit at the start of the same time slot, there may still be collisions.
Pure Aloha has a very high chance of hitting a frame, hence the slotted Aloha is intended to outperform its efficiency. In slotted Aloha, the shared channel is split into fixed time intervals called slots. As a result, if a station wants to send a frame to a shared channel, it can only do so at the start of the slot, and only one frame can be sent to each slot. Additionally, the station must wait until the beginning of the slot for the subsequent transmission if it is unable to transfer data at the beginning of the slot. However, sending a frame at the start of two or more station time slots still carries the risk of a collision.
Differences between Pure and Slotted ALOHA
| Pure Aloha | Slotted Aloha |
|---|---|
| Any station can start transmission at any time. | Any station is allowed to start transmission only at the beginning of the slot. |
| Time is continuous and not globally synchronized. | Time is discrete and globally synchronized. |
| Vulnerable Time is equal to twice the transmission time of the station. Vt=2∗Tt | Vulnerable time is equal to the transmission time and given by: Vt=Tt |
| The average number of successful ALOHA transmissions in the case of Pure Aloha is given by: S=G∗e−2G | The average number of successful ALOHA transmissions in the case of Slotted Aloha is given by: S=G∗e−G |
| The maximum efficiency is 18.4%. | The maximum efficiency is 36.8%. |
| The chance of collision is high. | The chance of collision is relatively less as compared to Pure Aloha. |
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.
Rules of ALOHA in Computer Network
- At any time, any station can transmit data to a channel.
- Collisions and data frames may be lost during data transmission over multiple stations.
- There is no need for carrier sensing.
- Aloha has no collision detection because the acknowledgment of the frames exists.
- It necessitates data retransmission after a random amount of time.
