The network using CSMA/CD has bandwidth of 20 mbps. If the maximum propagation time is 25 μsect (microsecond), what is the minimum size of the frame?

Correct option is B

To calculate the minimum size of the frame for a network using CSMA/CD (Carrier Sense Multiple Access with Collision Detection), we use the formula:
​$$\text{Minimum Frame Size} = 2 \times \text{Propagation Time} \times \text{Bandwidth}$$​​
Where:
• Propagation Time ($$t_p$$​) = 25 μsec = $$\mathrm{25 \times 10^{-6} seconds}.$$​​
• Bandwidth (B) = 20 Mbps = $$20 \times 10^6$$​ bits per second.
Substitute the values into the formula:
​$$\text{Minimum Frame Size} = 2 \times (25 \times 10^{-6} \, \text{s}) \times (20 \times 10^6 \, \text{bits/sec})$$​​
​$$\text{Minimum Frame Size} = 2 \times 25 \times 10^{-6} \times 20 \times 10^6$$​​
​$$\text{Minimum Frame Size} = 1000 \, \text{bits}$$​​
So, the minimum size of the frame is 1000 bits.
Information Booster:
1. CSMA/CD:
o CSMA/CD is a network protocol that helps manage access to the shared medium, detecting and resolving collisions. It is commonly used in Ethernet networks.
o The minimum frame size is crucial to ensure that the sender can detect a collision before it finishes sending the frame.
2. Collision Detection: For CSMA/CD to work correctly, the time it takes for a signal to travel to the farthest point on the network and back (the propagation time) must be less than the time it takes to transmit the frame. This ensures that the sender can detect collisions while still transmitting the frame.
3. Frame Size Calculation: The minimum frame size ensures that the sender can listen for a collision during transmission, which is essential for the collision detection mechanism to work effectively.
Additional Knowledge:
• Bandwidth and Propagation Time: The bandwidth of a network determines how much data can be transmitted per second, while the propagation time is the time it takes for the signal to travel across the network. Together, these factors influence the efficiency and collision detection in CSMA/CD networks.
• Ethernet Standards: The minimum frame size for Ethernet using CSMA/CD is 512 bits, which is derived from similar calculations. If the frame size is smaller than the minimum required, the sender might finish transmitting before the collision is detected, leading to errors.