Question

Suppose nodes \(\mathrm{A}\) and \(\mathrm{B}\) are on the same \(10 \mathrm{Mbps}\) broadcast channel, and the propagation delay between the two nodes is 325 bit times. Suppose CSMA/CD and Ethernet packets are used for this broadcast channel. Suppose node A begins transmitting a frame and, before it finishes, node B begins transmitting a frame. Can A finish transmitting before it detects that B has transmitted? Why or why not? If the answer is yes, then A incorrectly believes that its frame was successfully transmitted without a collision. Hint: Suppose at time \(t=0\) bits, A begins transmitting a frame. In the worst case, A transmits a minimum-sized frame of \(512+64\) bit times. So A would finish transmitting the frame at \(t=512+64\) bit times. Thus, the answer is no, if B's signal reaches A before bit time \(t=512+64\) bits. In the worst case, when does B's signal reach A?

Short answer

Yes, A can finish transmitting before detecting B's frame because B's signal reaches A at 650 bit times, after A has completed its transmission at 576 bit times.

Step-by-step solution

Understanding the Propagation Delay

The problem provides a propagation delay of 325 bit times between nodes A and B. This means that any signal sent from node A will take 325 bit times to reach node B, and vice versa.

Minimum Frame Transmission Time for Node A

Node A starts transmitting a frame at time \( t = 0 \). According to the problem, A is transmitting a minimum-sized frame of \( 512 + 64 \) bit times. Therefore, A will finish transmitting this frame at \( t = 512 + 64 = 576 \) bit times.

Calculating When Node B's Signal Reaches Node A

Node B can start transmitting at any time after A starts. However, to cause the worst-case collision detection scenario, we assume B starts transmitting just before A finishes. If B starts its transmission after propagation delay of 325 bit times, its signal will reach A at \( 325 + 325 = 650 \) bit times.

Collision Detection Analysis

Since B's signal reaches node A at \( t = 650 \) bit times, which is after A has completed its transmission at \( t = 576 \) bit times, node A can indeed finish sending its frame before it detects a collision. This scenario illustrates that node A finishes its transmission and incorrectly assumes it was successful without a collision.

Key concepts

Ethernet

Ethernet is a widely used technology for local area networks (LANs). It defines how devices on a network can communicate with each other. Ethernet operates using cables as its primary medium, usually copper or fiber optic, to connect network devices such as computers, routers, and switches. Ethernet defines rules and standards such as speed and data frame sizes. Let's consider some key points:

• Ethernet typically operates at speeds like 10 Mbps, 100 Mbps, 1 Gbps, etc.
• The network is organized in packets or Ethernet frames, which have a defined structure including source and destination addresses.
• Ethernet uses a technique known as Carrier Sense Multiple Access with Collision Detection (CSMA/CD) to manage how data is communicated.

Understanding Ethernet is crucial for analyzing how data travels within a network and detects collisions on shared communication channels.

collision detection

Collision detection is an integral part of the CSMA/CD protocol used in Ethernet networks. It ensures that data packets do not interfere with each other during transmission. When two or more devices try to send data simultaneously, a collision occurs. Collision detection helps manage these collisions so data can be successfully transmitted. The main ideas of collision detection include:

• Before transmitting data, a device "listens" to the network channel to check if it's clear.
• If multiple devices transmit simultaneously, the mixed signals indicate a collision.
• When a collision is detected, each device stops transmitting, waits for a random backoff time, and tries again.

This process ensures that devices on an Ethernet network reduce the chances of repeated collisions, maintaining efficient data communication.

propagation delay

Propagation delay refers to the time taken for a signal to travel from the sender to the receiver across a network. It's an important factor in network performance and is influenced by the physical distance between devices and the speed of signal propagation. Here's what you need to know about propagation delay:

• In our example, the delay between nodes A and B is 325 bit times.
• This delay affects how quickly the receiving node can start processing incoming data.
• The longer the propagation delay, the longer it takes to detect a collision.

Understanding propagation delay is critical when analyzing network efficiency, especially in scenarios where collision detection timing is vital.

broadcast channel

A broadcast channel is a communication medium that allows data transmission from one node to all other nodes in a network. In Ethernet, the broadcast channel refers to the shared medium where all nodes can see the transmitted frames. Key features of a broadcast channel include:

• Every device connected to the network can potentially "hear" every message sent.
• Collisions are more likely in a broadcast channel because all devices share the same medium.
• Using protocols like CSMA/CD, devices can effectively manage data transmissions to avoid signal interference.

The concept of a broadcast channel is essential for understanding how data is handled in a network and why protocols like CSMA/CD are important to regulate traffic and prevent data loss from collisions.