Question

Suppose the round-trip propagation delay for Ethernet is \(46.4 \mu \mathrm{s}\). This yields a minimum packet size of 512 bits ( 464 bits corresponding to propagation delay \(+\) 48 bits of jam signal). (a) What happens to the minimum packet size if the delay time is held constant, and the signalling rate rises to \(100 \mathrm{Mbps}\) ? (b) What are the drawbacks to so large a minimum packet size? (c) If compatibility were not an issue, how might the specifications be written so as to permit a smaller minimum packet size?

Short answer

a) New minimum packet size: 4688 bits. b) Drawbacks: Inefficiency, wasted bandwidth, increased latency for small transmissions. c) Solutions: Frame bursting, refined collision detection, faster medium, optimized packet structure.

Step-by-step solution

Understanding the Current Scenario

For the given Ethernet setup, the round-trip propagation delay is 46.4 microseconds, and this results in a minimum packet size of 512 bits. This is a sum of 464 bits (to account for the delay) and 48 bits (jam signal).

Minimum Packet Size at 100 Mbps

First, convert the signaling rate to the new rate: 100 Mbps. The delay time is still 46.4 microseconds. The number of bits needed for delay: \( 100 \text{ Mbps} \times 46.4 \times 10^{-6} \text{ s} = 4640 \text{ bits} \). Adding the jam signal (48 bits), the new minimum packet size is \(4640 + 48 = 4688 \text{ bits}\).

Drawbacks of Large Minimum Packet Size

A large minimum packet size can create inefficiencies, especially for small data transmissions. It increases the amount of padding required for small messages, resulting in wasted bandwidth. Additionally, it can lead to increased latency when transmitting small packets.

Revising Specifications for Smaller Packet Size

To permit a smaller minimum packet size without compatibility issues, the specifications could incorporate features like frame bursting or enable more refined collision detection mechanisms. Additionally, using a faster medium with lower propagation delay or optimizing the structure of packets to reduce the overhead might be suitable approaches.

Key concepts

Propagation Delay

Propagation delay is the time it takes for a signal to travel from the sender to the receiver across a network. It is a crucial factor in determining network performance. In the context of Ethernet, the propagation delay impacts the minimum packet size. When the round-trip propagation delay is 46.4 microseconds, it translates to 464 bits at a signaling rate of 10 Mbps. However, if the signaling rate increases to 100 Mbps, the number of bits needed to cover the same propagation delay also rises. This affects the minimum packet size required. Understanding propagation delay is key to optimizing network efficiency and ensuring reliable data transmission.

Signaling Rate

The signaling rate, or data rate, measures how quickly data can be transmitted over a network. It is often measured in megabits per second (Mbps). A higher signaling rate means more bits can be sent per second. For example, at 100 Mbps, much more data can be transmitted in the same amount of time compared to 10 Mbps. However, this also means that the packet size increases because more bits are needed to account for the same propagation delay. Managing the signaling rate efficiently helps optimize network performance and minimize latency, especially in high-speed networks.

Network Efficiency

Network efficiency refers to the ratio of useful data transmitted to the total data sent, including overheads like headers and control signals. Larger minimum packet sizes can reduce network efficiency because they require more padding for small messages, leading to wasted bandwidth. This is a significant drawback when dealing with small data transmissions. Inefficiencies can also increase transmission time, leading to higher packet latency and potentially slowed network performance. Optimizing network efficiency involves balancing packet size, data rate, and minimizing overhead to maximize the proportion of useful data sent.

Packet Latency

Packet latency is the time it takes for a packet to travel from the sender to the receiver. Higher packet sizes and increased propagation delays contribute to greater packet latency. When the minimum packet size is large, as seen when the signaling rate increases, it results in longer transmission times for each packet. This can be problematic for applications requiring quick, real-time data transmission. Reducing packet latency involves optimizing packet sizes, minimizing delays, and improving network technologies. Strategies like frame bursting or advanced collision detection can help mitigate these issues and enhance overall network performance.