Fair Realtime Traffic Scheduling over A Wireless Local Area Network

1
Fair Real-time Traffic Scheduling over A
Wireless Local Area Network

• Maria Adamou, Sanjeev Khanna,
• Insup Lee, Insik Shin, and Shiyu Zhou
• Dept. of Computer Information Science
• University of Pennsylvania, USA

2
Real-time Communication over Wireless LAN

MH1
BS
MH2
MH3
3
Wireless LAN MAC Protocol

• IEEE 802.11 standard
• DCF (distributed)
• Contention-based transmission
• PCF (centralized)
• Contention-free (CF) transmission
• BS schedules CF transmissions by polling

4
Wireless Network Characteristics

• Unpredictable Channel Error
• location dependent
• bursty

MH1
BS
?
MH2
MH3
5
Challenges

• How do channel errors affect real-time
  transmissions?
• QoS degradation
• Wireless channel error model
• How does BS schedule real-time transmissions with
  unpredictable errors?
• Real-time scheduling objective considering QoS
  degradation with errors
• Real-time scheduling algorithm

6
Outlines

• Real-time traffic model
• Scheduling objectives
• Theoretical results
• Online scheduling algorithms
• Simulation results
• Conclusion

7
Real-time Traffic Model

• Periodic packet generation (release time)
• Soft deadline
• Upon missing deadline, a packet is dropped
• Acceptable packet loss (deadline miss) rate
• Degradation actual loss rate acceptable loss
  rate
• The same packet length (execution time)

8
Scheduling objectives

• 1. Fairness (considering each flow)
• Location dependent channel errors
• Minimizing the maximum degradation
• 2. Throughput (considering the system)
• Maximizing the overall system throughput
  (fraction of packets meeting deadlines)
• Online scheduling algorithm
• without knowledge of error in advance

9
Theoretical results

• No online optimal algorithm
• Performance ratio of an online algorithm w.r.t.
  optimal
• for throughput maximization, two
• for achieving fairness, unbounded
• For the combined objectives, unbounded
• A polynomial time offline algorithm that
  optimally achieves our scheduling objectives

10
Online scheduling algorithms

• EDF (Earliest Deadline First)
• GDF (Greatest Degradation First)
• EOG (EDF or GDF)
• LFF (Lagging Flows First)

11
EDF (Earliest Deadline First)
when a new packet is available
when it dispatches
Scheduler
12
GDF (Greatest Degradation First)
when a new packet is available
when it dispatches
Scheduler
13
EOG (EDF or GDF)
when a new packet is available
If there is a packet that will miss its deadline
after next slot
when it dispatches
Scheduler
Otherwise
14
LFF (Lagging Flows First)
when a new packet is available
index
4
3
2
1
LFF Array
15
LFF (Lagging Flows First)
when a new packet is available
index
4
3
2
when it dispatches
1
Scheduler
LFF Array
16
LFF (Lagging Flows First)
when a new packet is available
If there is a packet that will miss its deadline
after next slot
when it dispatches
Scheduler
Otherwise
17
Simulation Performance Metrics

• Degradation (for each flow)
• Fraction of packets lost beyond the acceptable
  packet loss rate
• Throughput (over all flows)
• Fraction of successfully transmitted packets

18
Simulation Error Modeling

• Random blackouts (wi) for error period
• Error duration rate

tmax
t0
wi
MH1
BS
MH2
?
?
?
MH3
MH2
MH3
19
Results Max Degradation
20
Results Throughput Ratio
21
Related Work

• QoS guarantees over wireless links
• No consideration of fairness issue
• WFQ over wireless networks
• No consideration of deadline constraint
• QoS degradation considering deadline
• Imprecise computation
• IRIS (Increased Reward with Increased Service)
• (m,k)-firm deadline model
• DWCS (Dynamic Window-Constrained Scheduling)

22
Conclusion

• Scheduling objectives
• Fairness minimizing the maximum degradation
• Overall throughput maximization
• Theoretical results
• No online algorithm can be guaranteed to achieve
  a bounded performance ratio for the scheduling
  objective

23
Conclusion

• Online algorithms
• For fairness objective
• 1. LFF 2. GDF 3. EOG 4.EDF
• For maximum throughput objective
• 1. EDF 2. LFF 3. EOG 4.GDF
• Future work
• Variable length packets
• Other measures of fairness