Question

Recall that with the CSMA/CD protocol, the adapter waits \(K \cdot 512\) bit times after a collision, where \(K\) is drawn randomly. For \(K=100\), how long does the adapter wait until returning to Step 2 for a 10 Mbps broadcast channel? For a \(100 \mathrm{Mbps}\) broadcast channel?

Short answer

0.00512 seconds for 10 Mbps and 0.000512 seconds for 100 Mbps.

Step-by-step solution

Determine Bit Time

The bit time is the duration it takes to transmit one bit on the network. For a 10 Mbps network, the bit time is \( \frac{1}{10^7} \) seconds, and for a 100 Mbps network, it is \( \frac{1}{10^8} \) seconds.

Calculate Total Waiting Time for 10 Mbps

The waiting time is calculated as \( K \cdot 512 \) bit times. Given \( K=100 \), the total waiting time for a 10 Mbps network is \( 100 \cdot 512 \cdot \frac{1}{10^7} = 0.00512 \) seconds.

Calculate Total Waiting Time for 100 Mbps

Similarly, for a 100 Mbps network, the total waiting time is \( 100 \cdot 512 \cdot \frac{1}{10^8} = 0.000512 \) seconds.

Key concepts

Collision Detection

At the heart of the CSMA/CD protocol is the concept of collision detection. This mechanism is crucial in networks to ensure data packets do not interfere with each other during transmission. Imagine a network as a busy road where cars (data packets) can collide if they follow the same path simultaneously. Collisions lead to data loss, causing delays as packets need to be resent.
During transmission, a device monitors the network for signals of a collision. If it detects one, it temporarily halts its transmission, waits for a random amount of time, and then retries. This strategy is essential because it minimizes the chances of repeated collisions, enhancing the efficiency and reliability of the communication process.

Broadcast Channel

A broadcast channel is a network communication method where a single transmission is sent to all devices on the network. Think of it like a radio station sending signals to multiple radios. All devices connected to the network share the same communication medium, which can be both an advantage and a challenge.
On a broadcast channel, data intended for one specific device is initially available to all, creating opportunities for data collision. Protocols like CSMA/CD help manage this by ensuring only one device transmits at any one time. This helps maintain data integrity and network efficiency, making the broadcast channel effective in environments with moderate levels of network traffic.

Network Speed

Network speed, often measured in Mbps (megabits per second), determines how quickly data can be transmitted over the network. It directly affects the duration of a bit time, which is the time required to send one bit of data.
For example, in a 10 Mbps network, a single bit time is \( \frac{1}{10^7} \) seconds, while for a 100 Mbps network, it is \( \frac{1}{10^8} \) seconds. Higher network speeds mean shorter bit times, allowing for more data to be transferred in a smaller timespan. While faster networks reduce the time taken to resend data after collisions, they can also increase the likelihood of collisions due to the higher frequency of transmissions.

Bit Time

Bit time is a fundamental concept within network communications, particularly in the context of CSMA/CD. It denotes the time it takes to transmit one bit of data on the network. The concept is crucial for calculating delays, especially after data collisions.
Bit time varies with network speed, such that a 10 Mbps network has a longer bit time compared to a 100 Mbps network. The formula for calculating bit time is simple: \( \frac{1}{\text{network speed in bps}} \). Consequently, in real-time scenarios, knowing the bit time allows network devices to synchronize their transmissions and manage collision recovery times accurately, ensuring efficient data flow across the network.