QuitExams

🔹 Introduction

Scheduling mechanisms are algorithms used in networks to decide **which frame or packet should be transmitted next**, especially when multiple queues or priorities exist. These mechanisms ensure fairness, optimized performance, quality of service (QoS), and efficient bandwidth utilization.

Scheduling is crucial in:

• Routers
• Switches
• QoS-enabled networks
• Wireless access points
• Real-time audio/video applications

1️⃣ FIFO (First In, First Out)

FIFO transmits packets in the order they arrive. It is simple but does not support priority or QoS.

✔ Features

• No prioritization
• No fairness guarantees
• No QoS

✔ Example

Packets: A → B → C  
Transmission order: A → B → C

2️⃣ Priority Queuing (PQ)

Packets are placed in different queues based on priority (High, Medium, Low). The scheduler always serves higher priority queues first.

✔ Diagram

High Priority Queue    → served first  
Medium Priority Queue  → served after high  
Low Priority Queue     → last

✔ Advantages

• Good for real-time traffic (VoIP, video)

✔ Disadvantages

• Low-priority traffic may starve

3️⃣ Round Robin (RR)

Each queue is served in a cyclic order ensuring fairness.

✔ Example

Queues: Q1, Q2, Q3  
Order: Q1 → Q2 → Q3 → Q1 → Q2 → Q3 → ...

✔ Advantages

• Fair distribution

✔ Disadvantages

• No support for priorities

4️⃣ Weighted Round Robin (WRR)

Each queue is assigned a weight. Higher weights → more packets sent per cycle.

✔ Example

Weights: Q1=3, Q2=2, Q3=1  
Order: Q1 Q1 Q1 → Q2 Q2 → Q3 → repeat

✔ Use Case

• Bandwidth allocation in routers

5️⃣ Fair Queuing (FQ)

FQ divides the available bandwidth equally among all active flows.

✔ Concepts

• Each flow has its own queue
• Served in small round-robin manner

✔ Use Case

• Routers to prevent bandwidth domination

6️⃣ Weighted Fair Queuing (WFQ)

Weighted version of FQ. Assigns different bandwidth shares to different flows.

✔ Formula (Simplified)

Bandwidth per flow = (Weight_of_flow / Total_weight) × Total_bandwidth

✔ Example

Flow A weight = 2  
Flow B weight = 1  
Bandwidth → A gets 2x more than B

7️⃣ GPS (Generalized Processor Sharing)

Theoretical ideal model where flows are served simultaneously with proportional weights.

✔ Why Important?

Real schedulers like WFQ approximate GPS.

8️⃣ DRR (Deficit Round Robin)

Advanced scheduling that handles variable packet sizes.

✔ Advantages

• No starvation
• Works for variable packet lengths

9️⃣ Comparison of Scheduling Techniques

📝 Conclusion

Scheduling mechanisms ensure efficient, fair, and QoS-aware packet transmission in networks. From simple FIFO and RR to advanced WFQ and DRR, scheduling plays a crucial role in routers, switches, wireless networks, and real-time audio/video systems. Choosing the right scheduling algorithm improves performance, reduces delay, and meets QoS guarantees.