Pure Aloha

Key Concepts

Multiple Access Protocols, Random Access Protocols, Controlled Access Protocols, Channelization Protocols, Aloha Protocol, Pure Aloha, Slotted Aloha, Collision, Shared Medium, Frame Transmission Time (Tfr), Vulnerable Time, Throughput, Backoff Time, Acknowledgement.

Multiple Access Protocols

The lecture begins by outlining the three types of multiple access protocols:

1. Random Access Protocols: Stations can transmit data at any time. Aloha falls under this category.
2. Controlled Access Protocols: Access to the channel is controlled to avoid collisions.
3. Channelization Protocols: The channel is divided into slots or sub-channels.

Aloha Protocol

• Definition: Aloha is a random access protocol designed for wireless LANs (WLANs) but applicable to shared mediums.
• Random Access: Any station can transmit data at any time.
• Collision: If two or more stations transmit simultaneously, a collision occurs, garbling the data.

Collision Explained

• Scenario: Two stations, A and B, share a common medium.
• Simultaneous Transmission: If A and B transmit data (A frame and B frame) at the same time, their frames collide.
• Result: The frames are lost or corrupted, becoming unusable.

Types of Aloha

There are two main types of Aloha:

1. Pure Aloha: The focus of the lecture.
2. Slotted Aloha: Not covered in detail in this lecture.

Pure Aloha: A Detailed Scenario

• Setup: Four stations (1, 2, 3, and 4) share a common medium and a common timeline.
• Successful Transmission: If only one station transmits (e.g., station 1), the transmission is successful.
• Collision Example: Station 3 starts transmitting, then station 2 and station 4 begin transmitting during station 3's transmission. Station 1 also transmits. This results in a collision involving all four frames. The collision duration is the period where multiple stations are transmitting simultaneously.
• Another Collision Example: Station 2 transmits successfully, then station 4 starts transmitting while station 2 is still transmitting, leading to a collision between their frames.
• No Collision Example: Station 3 transmits when no other station is transmitting, resulting in a successful transmission.

Pure Aloha: Theoretical Aspects

• Transmission Rule: Stations transmit whenever they have data to send.
• Acknowledgement: After transmitting, a station waits for an acknowledgement (ACK).
• Retransmission: If an ACK is not received within an allotted time, the station waits for a random amount of time (backoff time) and then retransmits the data.
• Backoff Time: Using a random backoff time reduces the probability of repeated collisions.
• Throughput Maximization: Throughput is maximized when frames are of uniform length (fixed-size framing).

Collision in Detail

• Garbled Frames: When two frames overlap in time, a collision occurs, and both frames are garbled.
• Example: Station B is about to finish transmitting when station A starts transmitting. This causes a collision, rendering both frames unusable.
• Another Example: Frames A, B, and C are transmitted in such a way that A collides with B's end, and A's end collides with C's beginning, garbling all three frames.

Vulnerable Time

• Definition: The vulnerable time is the time period during which a transmission is susceptible to collision.
• Calculation: The vulnerable time for pure Aloha is twice the frame transmission time (2 * Tfr).
• Explanation: For a frame to be transmitted successfully, the channel must be free for the duration of the frame transmission time (Tfr) and also for a Tfr period before the frame's start time. This ensures that no other frame interferes with the transmission.

Throughput Formula

• Formula: Throughput (S) = G * e^(-2G), where G is the number of stations that wish to transmit at the same time.
• Maximum Throughput: The maximum throughput for pure Aloha is approximately 0.18 (or 18%) when G = 0.5 (half a station transmitting during Tfr). This means that maximum efficiency is achieved when, on average, half a station is transmitting during a frame transmission time.

Conclusion

Pure Aloha is a simple random access protocol where stations transmit data whenever they have it. Collisions are a significant issue, and the protocol uses acknowledgements and random backoff times to handle them. The maximum throughput is relatively low (18%) compared to other protocols, achieved when the channel utilization is at a specific level (G=0.5). The vulnerable time is twice the frame transmission time, highlighting the period where collisions can occur.