Question

Suppose five stations are waiting for another packet to finish on an Ethernet. All transmit at once when the packet is finished and collide. (a) Simulate this situation up until the point when one of the five waiting stations succeeds. Use coin flips or some other genuine random source to determine backoff times. Make the following simplifications: Ignore interframe spacing, ignore variability in collision times (so that retransmission is always after an exact integral multiple of the \(51.2-\mu\) s slot time), and assume that each collision uses up exactly one slot time. (b) Discuss the effect of the listed simplifications in your simulation versus the behavior you might encounter on a real Ethernet.

Short answer

After two backoff attempts, Station E successfully sends its packet while avoiding further collisions.

Step-by-step solution

- Initial Setup

Identify the given scenario. Five stations are waiting to send packets, and they will all transmit simultaneously after the current packet finishes, resulting in a collision.

- Understand Backoff Algorithm

In Ethernet, after a collision, each station chooses a random backoff time from a range of slot times. The range doubles after each collision.

- First Collision

After the first collision, each station will choose a random number of slot times from [0, 1]. Use a random process (like coin flips) to simulate this step.

- Random Selection of Backoff Times for First Collision

Simulate each station choosing a random backoff time (0 or 1). For example: Station A: 0, Station B: 1, Station C: 1, Station D: 0, Station E: 1.

- Check for Collisions After First Attempt

Stations with the same backoff time will collide again. In this case, Station A and D will collide, and Stations B, C, and E will collide.

- Second Collision Backoff

Stations that collided will now choose from the range [0, 1, 2, 3]. Use a random process again to simulate these choices.

- Random Selection of Backoff Times for Second Collision

Simulate each station choosing a new random backoff time: Station A: 1, Station D: 2, Station B: 2, Station C: 3, Station E: 0.

- Check for Successful Transmission and Collisions

Station E does not collide and sends its packet successfully. Stations A and D collide again, Stations B and C do not collide but have higher backoff times.

- Discuss Simplifications

Discuss how ignoring interframe spacing, variability in collision times, and fixed slot times may differ from actual Ethernet operation where backoff times can vary more significantly and collisions may take different amounts of time.

Key concepts

Ethernet protocol

The Ethernet protocol is fundamental in local area networks (LANs). It governs how devices communicate over a shared medium. Each device, or 'station', listens for network traffic before sending its data to avoid collisions. Ethernet frames carry data, source, and destination information across the network.
The protocol uses a method known as Carrier Sense Multiple Access with Collision Detection (CSMA/CD). This means stations listen (carrier sense) to detect collision presence on the network. Multiple stations may attempt access, and collisions can occur.
When a collision happens, Ethernet uses the collision detection (CD) part to manage it and ensure proper data transmission.

collision detection

Collision detection in Ethernet is like traffic management. When multiple stations send data simultaneously, their signals interfere, causing a collision.
CSMA/CD comes into play. Stations sense the carrier signal on the network and can detect if their transmitted signal collides with another. If a collision is detected, the sending stations stop transmitting immediately to minimize data corruption.
This process is critical because without detecting collisions, several retransmissions will fall into conflicts, leading to increased network congestion and inefficient use of bandwidth.

network simulation

Network simulation is a technique to model and analyze network behavior without needing an actual setup. It helps to visualize and understand how different scenarios may unfold.
In our exercise, a network simulation can be used to predict how collisions and backoffs will occur with five stations trying to send data simultaneously. The simplifications used include assumptions like fixed slot times and no variance in collision times.
Such a simulation can expose theoretical performance under ideal conditions but might differ from real-world behavior due to various factors such as more complex timing, real-time traffic, and hardware differences.

backoff algorithm

The backoff algorithm helps manage the retransmission of packets after a collision. When stations collide, they don't retransmit immediately. Instead, they wait for a random 'backoff' time before attempting to resend.
In Ethernet, the backoff time doubles with each collision. After the first collision, stations pick from a range of 0 to 1 slot times. After a second collision, the range increases to 0 to 3, and so on.
This method helps to reduce the probability of repeated collisions because it spreads out the retry times, giving each station a fair chance to access the medium and transmit its data.

random backoff time

Random backoff time is a crucial component in the backoff algorithm for collision handling. The idea is to introduce randomness in retransmission attempts to avoid continuous collisions.
For instance, if five stations collide and they all wait for a fixed time before retrying, they will collide again. Instead, each station selects a random time (0 or 1 slot) for the first collision.
If another collision occurs, the range expands (0 to 3 slots), making it less likely for stations to select the same backoff time and collide again. Random backoff times enhance overall network throughput by minimizing persistent collisions.