Question

Agree or disagree with the following assertion and state why: In an Ethernet network, even though there are collisions, every message is guaranteed to be delivered in some maximum amount of time \(T\).

Short answer

Disagree. Ethernet networks have no guaranteed maximum delivery time due to potential persistent collisions and lack of acknowledgment.

Step-by-step solution

Understand Ethernet Collision

Ethernet networks use a method called Carrier Sense Multiple Access with Collision Detection (CSMA/CD). In this method, devices listen to the network before transmitting to avoid collisions. However, collisions can still occur, requiring retransmission.

Retransmission Process

When a collision is detected, the Ethernet protocol specifies a backoff algorithm, where each device waits a random amount of time before attempting to retransmit, exponentially increasing wait times (exponential backoff) after each subsequent collision.

Concept of Acknowledgment

Unlike protocols such as TCP, Ethernet does not have an inherent mechanism to guarantee message delivery with acknowledgment. Ethernet deals with frames and attempts delivery but does not confirm if the message reached its destination.

Analyze Maximum Time Guarantee

Due to the nature of the backoff algorithm, there is no fixed maximum time within which delivery is guaranteed. Indefinite delays can happen if the network is busy and collisions persist.

Conclusion on Guarantee

The assertion that every message is guaranteed to be delivered in some maximum time T is incorrect because Ethernet does not provide such a guarantee. Persistent collisions and lack of acknowledgment mean there is no upper time bound on delivery.

Key concepts

CSMA/CD

Ethernet networks use a protocol known as Carrier Sense Multiple Access with Collision Detection (CSMA/CD). This protocol helps manage how devices share a single communication channel. Each device must "listen" to the network first, ensuring it's free, before attempting to send data. This is like checking that nobody is talking on a walkie-talkie channel before you start speaking, to avoid interruptions.

However, even with CSMA/CD in place, collisions can still occur. A collision happens when two devices accidentally attempt to transmit data simultaneously, intersecting signals. The beauty of CSMA/CD is that once a collision is detected, both devices will stop transmitting, "back off," and retry sending their data after a random delay. The protocol effectively reduces collisions and ensures a more orderly communication process.

collisions

Collisions in an Ethernet network are a natural byproduct of multiple devices communicating over the same channel. Imagine a busy highway where two lanes suddenly merge into one. If two vehicles try to merge at the same time, they'll collide. Similarly, when devices try to send data simultaneously, a collision occurs, causing both signals to interfere with one another.

The Ethernet network can detect these collisions through CSMA/CD. It efficiently manages them by signaling the devices to cease transmission the moment a collision is sensed. This prompt detection ensures that network time isn't wasted on overlapping messages that won’t be correctly received. Once the collision is detected, the devices follow a process to mitigate the collision and attempt to resend their messages later.

exponential backoff

After a collision, Ethernet devices employ a strategy known as exponential backoff. When two devices detect a collision, they each wait a random amount of time before trying to retransmit their messages. But there’s a twist! Each time a device experiences a collision, the delay before attempting to retransmit gets exponentially longer. Hence, the name 'exponential backoff'.

This method minimizes the chance of repeated collisions by strategically spreading out transmission attempts over time. The algorithm calculates this by adjusting the waiting time using a mathematical function that doubles the wait time with every successive collision. This randomized delay helps prevent too many devices from trying to retransmit at the same moment, thereby managing network traffic more effectively.

message delivery guarantee

The concept of a message delivery guarantee within Ethernet networks is complex. Ethernet's design focuses on efficient packet delivery, but it doesn't inherently provide a guarantee. This is because Ethernet lacks a built-in feedback system to confirm if a message has been successfully delivered.

Unlike some other protocols, Ethernet does not use acknowledgments. In other words, there is no mechanism to ensure the recipient received, acknowledged, or even actively heard the sent message. This relatively simple design allows for fast transmissions but doesn’t promise delivery within a fixed period, especially under conditions of network congestion. Therefore, the statement that every message is delivered within a certain time frame is misleading.