1. Introduction to Packet Scheduling
ECE 1545

2. Packet Switch Output Scheduling Switch Fabric Routing Table
Forwarding Decision
Routing Table
Forwarding Decision
Routing Table
Forwarding Decision

3. Packet Scheduling
Packet scheduling algorithm determines the order in which backlogged packets are transmitted on an output link
Allocates output bandwidth
Controls packet delay
scheduler

4. Flows and Classes A flow is a single end-to-end data stream
In a virtual circuit network, it is a connection
In a datagram network it is a transport level flow
A class is a group of flows with common characteristics
Belong to the same application (video, data)
Belong to the same service (port number)
Have common header fields (e.g., DiffServ field)

5. Desirable Properties of a Packet Scheduler
Protection among flows
Misbehaving flows do not affect well-behaving flows
Guarantees
Differentiate between different types of traffic
Give guarantees on delay and rate
Flexible
Accommodate wide range of service requests
Simple
Easy to implement

6. Packet Scheduling
Packet scheduling algorithms are mostly work-conserving, i.e., the scheduler transmits packets as long as there are packets waiting
Non-workconserving: Even though there are packets waiting, scheduler may not transmit

7. First-Come-First Served (FIFO)
Packets are transmitted in the order of their arrival
Advantage:
Very simple to implement
Disadvantage:
Cannot give different service to different types of connections
Each flow (even with low data rate) can experience long delays

8. Static Priority Also called Head-of-Line (HOL) queueing:
Each traffic flow belong to a class
Each class has a priority
One FIFO queue for each class
Transmit from the highest priority queue with a backlog
Advantage:
Simple
Disadvantage:
Tends to “starve” the lower priority classes

9. Earliest Deadline First (EDF)
Each flow is associated with a delay index (d1, d2, d3)
Packet from flow i that arrives at time t is assigned deadline t+di
Packets are transmitted in the order of their deadlines

10. Fair Queueing
Attempts to implement a scheduler that serves all flows with a backlog at the same rate
Emulates a bitwise Round Robin scheduling algorithm
Not easy to implement Fair Queuing in a packet network
FIFO
Fair Queueing

11. Weighted Round Robin (WRR) Deficit Round Robin (DRR)
An approximation of fair queueing:
One FIFO queue for each flow
Operates in “rounds”, where each queue with a backlog is visited once in a round
Packet sizes are considered when determining the amount of traffic that can be sent in a round