Question

A TCP machine is sending full windows of 65,535 bytes over a 1-Gbps channel that has a 10 -msec one-way delay. What is the maximum throughput achievable? What is the line efficiency?

Short answer

Maximum throughput: 26.214 Mbps; Line efficiency: 2.62%.

Step-by-step solution

Calculate the Round-Trip Time (RTT)

The Round-Trip Time (RTT) is the total time it takes for a signal to go to the destination and back. Given the one-way delay is 10 ms, the RTT is twice this value:\[\text{RTT} = 2 \times 10 \text{ ms} = 20 \text{ ms} = 0.02 \text{ s}\]

Calculate the Bandwidth-Delay Product (BDP)

The Bandwidth-Delay Product tells us the number of bits that can be in transit in the network. The formula for BDP is:\[\text{BDP} = \text{Bandwidth} \times \text{RTT}\]Substituting the values:\[\text{BDP} = 1 \text{ Gbps} \times 0.02 \text{ s} = 0.02 \text{ Gb} = 20,000,000 \text{ bits}\]

Convert BDP to Bytes

Since 1 byte is 8 bits, we convert the BDP from bits to bytes:\[\text{BDP in bytes} = \frac{20,000,000 \text{ bits}}{8} = 2,500,000 \text{ bytes}\]

Determine the Maximum Throughput

The maximum throughput is equal to the amount of data that can be in transit at once, which for our TCP window is 65,535 bytes. Since it's less than the BDP in bytes, the throughput is limited by the TCP window size:\[\text{Maximum Throughput} = 65,535 \text{ bytes per RTT}\]To convert this to bps, use:\[\frac{65,535 \text{ bytes}}{0.02 \text{ s}} \times 8 = 26,214,000 \text{ bps} = 26.214 \text{ Mbps}\]

Calculate the Line Efficiency

Line efficiency is the ratio of the used capacity to the maximum capacity, expressed as a percentage:\[\text{Line Efficiency} = \frac{\text{Maximum Throughput}}{\text{Bandwidth}} \times 100\]Substitute the known values:\[\text{Line Efficiency} = \frac{26.214 \text{ Mbps}}{1000 \text{ Mbps}} \times 100 = 2.6214\%\]

Key concepts

Bandwidth-Delay Product

The Bandwidth-Delay Product (BDP) is a fundamental concept when working with network performance. It represents the maximum amount of data that can be in the pipeline of a network at any given moment. In simpler terms, it's the volume of data that can fill the link before the signal reaches the destination and comes back.

To calculate the BDP, we multiply the bandwidth of the network by the Round-Trip Time (RTT). For example, if you have a bandwidth of 1 Gbps and an RTT of 0.02 seconds, the BDP would be given by \(1 \text{ Gbps} \times 0.02 \text{ s} = 0.02 \text{ Gb}\).

Understanding the BDP helps determine how much data you can send before you should expect a feedback signal from the receiver. If your congestion window size in TCP is smaller than the calculated BDP, you will not fully utilize the available bandwidth, affecting throughput and efficiency.

• The higher the BDP, the more data can be in transit, leading to better utilization of the network.
• BDP is crucial for optimizing TCP window sizes and improving network efficiency.

Keep in mind that if the network delay increases or if the bandwidth capacity is high, the BDP becomes larger, requiring appropriately sized congestion windows.

Round-Trip Time

Round-Trip Time (RTT) is a critical networking measurement, referring to the total time it takes for a signal to travel from the sender to the receiver and back again. RTT reflects the latency of the network and is usually measured in milliseconds (ms).

In many network scenarios, knowing the RTT helps to appropriately adjust the TCP window size to maximize throughput without overwhelming the network. In our example, the RTT is calculated as twice the one-way delay time: \(2 \times 10 \text{ ms} = 20 \text{ ms} = 0.02 \text{ s}\).

A low RTT means that the network transmits information quickly, enhancing responsiveness and user experience. Conversely, high RTT may lead to delays in data transmission, affecting applications that rely on fast data exchange.

• A shorter RTT increases the efficiency of data exchanges.
• RTT is influenced by factors such as distance, congestion, and the number of hops in the network route.
• Accurate RTT measurement is essential for protocols like TCP to function optimally.

Maintaining a low RTT improves the speed and reliability of data communication, making it an essential consideration for network performance tuning.

Line Efficiency

Line efficiency is an important concept in understanding how effectively a data transmission pathway is utilized. It is defined as the ratio of the actual data throughput to the maximum possible throughput, expressed as a percentage.

In the context of our problem, the line efficiency can be calculated using the formula: \(\frac{\text{Maximum Throughput}}{\text{Bandwidth}} \times 100\). For instance, if the Maximum Throughput is 26.214 Mbps on a 1 Gbps line, the line efficiency would be \(\frac{26.214 \text{ Mbps}}{1000 \text{ Mbps}} \times 100 = 2.6214\%\).

Line efficiency indicates how well the bandwidth is being used in the network. If line efficiency is low, it suggests that the network isn't being fully utilized, which can happen if the congestion window of TCP is not optimally configured.

• Higher line efficiency indicates better utilization of the bandwidth and network resources.
• Factors such as network congestion, hardware limitations, and configuration issues can affect line efficiency.
• Improving line efficiency can often involve optimizing TCP window sizes or increasing the TCP Maximum Segment Size (MSS).

Overall, line efficiency is a key metric in evaluating network performance and can serve as a valuable guide for making improvements to data transmission processes.