Question

Consider a router that is managing three flows, on which packets of constant size arrive at the following wall clock times: flow A: \(1,2,4,6,7,9,10\) flow B: \(2,6,8,11,12,15\) flow C: \(1,2,3,5,6,7,8\) All three flows share the same outbound link, on which the router can transmit one packet per time unit. Assume that there is an infinite amount of buffer space. (a) Suppose the router implements fair queuing. For each packet, give the wall clock time when it is transmitted by the router. Arrival time ties are to be resolved in order \(\mathrm{A}, \mathrm{B}, \mathrm{C}\). Note that wall clock time \(T=2\) is FQ-clock time \(A_{i}=1.5\). (b) Suppose the router implements weighted fair queuing, where flows \(\mathrm{A}\) and \(\mathrm{B}\) are given an equal share of the capacity, and flow \(\mathrm{C}\) is given twice the capacity of flow A. For each packet, give the wall clock time when it is transmitted.

Short answer

Fair Queuing: Flow A: \(1, 4, 7, 9, 10, 12, 14\), Flow B: \(6, 8, 13, 15\), Flow C: \(2, 3, 5, 11, 16-18\) Weighted Fair Queuing: Flow A: \(1, 5, 9, 13, 17, 19, 21\), Flow B: \(3, 7, 11, 15, 18, 20\), Flow C: \(2, 4, 6, 8, 10, 12, 14, 16\).

Step-by-step solution

- Understand Flow Arrival Times

Identify the arrival times for each flow and list them out. Flow A: \(1,2,4,6,7,9,10\) Flow B: \(2,6,8,11,12,15\) Flow C: \(1,2,3,5,6,7,8\)

- Fair Queuing: Calculate Transmission Times

In fair queuing, each flow gets an equal share of the bandwidth. We will transmit in round-robin order based on arrival times, resolving ties by order A, B, C. Packets will be transmitted as follows: Wall clock time 1: Flow A, Packet 1 Wall clock time 2: Flow C, Packet 1 Wall clock time 3: Flow C, Packet 2 Wall clock time 4: Flow A, Packet 2 Wall clock time 5: Flow C, Packet 3 Wall clock time 6: Flow B, Packet 1 Wall clock time 7: Flow A, Packet 3 Wall clock time 8: Flow B, Packet 2 Wall clock time 9: Flow A, Packet 4 Wall clock time 10: Flow A, Packet 5 Wall clock time 11: Flow C, Packet 4 Wall clock time 12: Flow A, Packet 6 Wall clock time 13: Flow B, Packet 3 Wall clock time 14: Flow A, Packet 7 Wall clock time 15: Flow B, Packet 4 Wall clock time 16: Flow C, Packet 5 Wall clock time 17: Flow C, Packet 6 Wall clock time 18: Flow C, Packet 7

- Weighted Fair Queuing: Calculate Weights

Here, Flow A and Flow B get equal bandwidth, and Flow C gets twice that. For every cycle of 4 transmissions, the distribution is: Flow A: 1 packet Flow B: 1 packet Flow C: 2 packets

- Weighted Fair Queuing: Calculate Transmission Times

Assign transmission slots according to the weights: Wall clock time 1: Flow A, Packet 1 Wall clock time 2: Flow C, Packet 1 Wall clock time 3: Flow B, Packet 1 Wall clock time 4: Flow C, Packet 2 Wall clock time 5: Flow A, Packet 2 Wall clock time 6: Flow C, Packet 3 Wall clock time 7: Flow B, Packet 2 Wall clock time 8: Flow C, Packet 4 Wall clock time 9: Flow A, Packet 3 Wall clock time 10: Flow C, Packet 5 Wall clock time 11: Flow B, Packet 3 Wall clock time 12: Flow C, Packet 6 Wall clock time 13: Flow A, Packet 4 Wall clock time 14: Flow C, Packet 7 Wall clock time 15: Flow B, Packet 4 Wall clock time 16: Flow C, Packet 8 Wall clock time 17: Flow A, Packet 5 Wall clock time 18: Flow B, Packet 5 Wall clock time 19: Flow A, Packet 6 Wall clock time 20: Flow B, Packet 6 Wall clock time 21: Flow A, Packet 7

Key concepts

fair queuing

Fair queuing is a method used in routers to ensure that multiple data flows are handled in an equitable manner. Each flow is given an equal share of the bandwidth.
In the given exercise, we have three flows. The router must transmit packets in a round-robin fashion based on their arrival times. When two packets arrive at the same time, ties are resolved in the order of flows A, B, and C.
Here’s a quick breakdown:

• Flow A packets arrive at: 1, 2, 4, 6, 7, 9, 10
• Flow B packets arrive at: 2, 6, 8, 11, 12, 15
• Flow C packets arrive at: 1, 2, 3, 5, 6, 7, 8

The transmission schedule would be:

• Time 1: Flow A
• Time 2: Flow C
• Time 3: Flow C
• Time 4: Flow A
• Time 5: Flow C
• Time 6: Flow B
• Time 7: Flow A
• Time 8: Flow B
• Time 9: Flow A
• Time 10: Flow A
• Time 11: Flow C
• Time 12: Flow A
• Time 13: Flow B
• Time 14: Flow A
• Time 15: Flow B
• Time 16: Flow C
• Time 17: Flow C
• Time 18: Flow C

weighted fair queuing

Weighted Fair Queuing (WFQ) allows different flows to have different bandwidth shares based on assigned weights. This is particularly useful for prioritizing certain types of traffic.
In the given exercise, Flows A and B are assigned equal bandwidth, while Flow C is assigned twice the bandwidth of Flow A.

This means that Flow C will get more chances to transmit its packets compared to Flows A and B in every cycle. The distribution would look something like this:

• Flow A: 1 packet
• Flow B: 1 packet
• Flow C: 2 packets

The resulting transmission schedule would be:

• Time 1: Flow A
• Time 2: Flow C
• Time 3: Flow B
• Time 4: Flow C
• Time 5: Flow A
• Time 6: Flow C
• Time 7: Flow B
• Time 8: Flow C
• Time 9: Flow A
• Time 10: Flow C
• Time 11: Flow B
• Time 12: Flow C
• Time 13: Flow A
• Time 14: Flow C
• Time 15: Flow B
• Time 16: Flow C
• Time 17: Flow A
• Time 18: Flow B
• Time 19: Flow A
• Time 20: Flow B
• Time 21: Flow A

flow control

Flow control is the technique used to manage the rate of data transmission between two nodes to prevent a fast sender from overwhelming a slow receiver. In the context of the exercise, all flows share the same outbound link and have infinite buffer space.
This practice ensures that no matter how quickly packets arrive, there is no packet loss due to buffer overflow.
Effective flow control ensures smooth data transmission, minimizing packet delay and optimizing the utilization of the network resources. When combined with queuing mechanisms like fair queuing or weighted fair queuing, network performance significantly improves.

packet scheduling

Packet scheduling refers to the algorithm or method used to determine the order in which packets are transmitted over the network. It’s a critical component in managing network traffic, ensuring timely delivery of packets, and optimizing network performance.
In our example, the router uses fair queuing and weighted fair queuing as its packet scheduling strategies.
Fair queuing aims for equal bandwidth distribution among flows, while weighted fair queuing gives different weights to different flows based on priority or required bandwidth.
This ensures real-time applications get better service, enhancing user experience. By scheduling packets efficiently, networks can minimize latency, avoid congestion, and ensure fair resource allocation.

network traffic analysis

Network traffic analysis involves monitoring and examining data flows in a network to understand its behavior and optimize performance. This can help identify bottlenecks, detect anomalies, and enhance security.
In our exercise, analyzing the arrival times and transmission schedules of packets across different flows can help us understand how efficiently the network handles multiple traffic streams.
By employing different queuing mechanisms like fair queuing and weighted fair queuing, one can assess which method delivers better performance under varying traffic conditions.
This analysis is crucial for improving network policies and ensuring robust and efficient network operations.
Understanding the flow patterns and transmission times can help network managers make informed decisions to enhance overall network quality of service (QoS).