1. Quality of Service Support in Wireless Networks
Wireless Networks Laboratory (WINET)
Quality of Service Support in Wireless Networks
Hongqiang Zhai
Wireless Networks Laboratory
Department of Electrical and Computer Engineering
University of Florida
In Collaboration with Dr. Xiang Chen and my advisor Professor Yuguang ``Michale’’ Fang

2. Wireless Networks Laboratory (WINET)
Outline
Introduction
Performance analysis of the IEEE MAC protocol
A call admission and rate control scheme
Conclusion and future research issues

3. Wireless Networks Laboratory (WINET)
Wireless Landscape

4. Wireless Local Area Networks/ Wi-Fi Hot Spots
Wireless Networks Laboratory (WINET)
Wireless Local Area Networks/ Wi-Fi Hot Spots
Web traffic
Streaming video
Instant messaging
Gaming over IP
Voice over IP over Wi-Fi
Next Call May Come from a Wireless Hot Spot

5. Mobile Ad Hoc Networks and Wireless Mesh Networks
Wireless Networks Laboratory (WINET)
Mobile Ad Hoc Networks and Wireless Mesh Networks

6. Quality of Service (QoS) Requirements
Wireless Networks Laboratory (WINET)
Quality of Service (QoS) Requirements
Bandwidth
Delay and delay jitter
Packet loss rate

7. Challenges Unreliable physical channel
Wireless Networks Laboratory (WINET)
Challenges
Unreliable physical channel
Time-varying propagation characteristics
Interference
Limited bandwidth
Limited processing power and battery life
Distributed control
Mobility

8. Medium Access Control Coordinate channel access Reduce collision
Wireless Networks Laboratory (WINET)
Medium Access Control
Coordinate channel access
Reduce collision
Efficiently utilize the limited wireless bandwidth B D C A

9. IEEE 802.11 Distributed Coordinate Function (DCF) MAC Protocol
Wireless Networks Laboratory (WINET)
IEEE Distributed Coordinate Function (DCF) MAC Protocol
Carrier sense multiple access with collision avoidance (CSMA/CA)
Carrier sensing
Physical Carrier Sensing
Virtual Carrier Sensing
Interframe Spacing (IFS)
Short IFS (SIFS) < DCF IFS (DIFS)
Binary Exponential Backoff
Randomly chosen from [0, CW]
CW doubles in case of collision
Contention based MAC
Can it support QoS requirements of various applications?
Request to send
DIFS
DATA
Transmitter
Receiver
Others B A
ACK …
Backoff
NAV(RTS)
RTS
SIFS
DATA
SIFS
SIFS
NAV(CTS)
CTS
ACK
Clear to send
Acknowledge
DIFS
RTS …
Backoff

10. Previous Work on Performance Analysis of the IEEE 802.11 MAC Standard
Wireless Networks Laboratory (WINET)
Previous Work on Performance Analysis of the IEEE MAC Standard
Previous studies focus on saturated case
Each device always has packets in the system and keeps contending for the shared channel.
Collision probability is very high
Delay performance is very bad
Only throughput and average delay have been derived.
Related work
Bianchi, JSAC March 2000
Cali et al., IEEE/ACM Tran. Networking, Dec. 2000
QoS requirements of real-time services can not be guaranteed if there are many contending users?

11. Previous Work on Supporting QoS in WLANs
Wireless Networks Laboratory (WINET)
Previous Work on Supporting QoS in WLANs
Service differentiation
Provide different channel access priorities for different services by differentiating
Contention window
Interframe spacing (IFS)
IEEE e draft (based on b)
Related work
Ada and Castelluccia, Infocom’01 (CW, IFS)
Veres et al., JSAC Oct (real-time measurement in virtual MAC)
S.T. Sheu and T.F. Sheu, JSAC Oct (real-time traffic periods)
S. Mangold et al., Wireless Communications Dec (802.11e)
Service differentiation is still not enough to meet the strict QoS requirements
Can the IEEE MAC protocol do better than service differentiation?
Research issues
Performance in both non-saturated and saturated case
Probability distribution of medium access delay

12. MAC Service Time MAC service time is discrete in value
Wireless Networks Laboratory (WINET)
MAC Service Time
Packet arrival
MAC service time is discrete in value
SIFS, DIFS, EIFS
Backoff time is measured in time slots
Packet to be transmitted is also discrete in length
Transmit
queue 1 2 3 2 3 3
MAC 2 3 1
Probability Generating Function (PGF)
Pr{Ts=tsi}=pi (0 ≤ i < ∞)

13. MAC Service Time Widely used method
Wireless Networks Laboratory (WINET)
MAC Service Time
start
end
Widely used method
Calculate the average # of retransmissions NR = p/(1-p)
Average transition time is NR × τ1 + τ2=
Generalized state transition diagram (GSTD)
Mark the PGF of the transition time on each branch along with the transition probability
PGF of the transition time between two states is the corresponding system transfer function

14. MAC Service Time of IEEE 802.11
Wireless Networks Laboratory (WINET)
MAC Service Time of IEEE
State variable (j, k): j is the backoff stage, k is the backoff timer
Wj: the contention window at backoff stage j
p: collision probability perceived by a node
: maximum # of retransmissions

15. MAC Service Time of IEEE 802.11
Wireless Networks Laboratory (WINET)
MAC Service Time of IEEE
Collision probability p
payload size = 8000 bits, with RTS/CTS
MAC service time (ms)
MAC service time (ms)
PDF
Observation:
When p is small, both the mean and standard deviation of MAC service time are small.

16. Delay and Delay Variation
Wireless Networks Laboratory (WINET)
Delay and Delay Variation
Packet arrival TS
Transmit
queue 1 2 3 TW
MAC TR

17. Network Throughput (Tidl pi ) (Tcol pc ) (Tsuc ps )
Wireless Networks Laboratory (WINET)
Network Throughput
(Tidl pi ) (Tcol pc ) (Tsuc ps )
Channel utilization:
Normalized throughput :
Channel busyness ratio:
With RTS/CTS
Without RTS/CTS
n: # of nodes
: the prob. that a node transmits in any slot

18. Wireless Networks Laboratory (WINET)
Network Throughput
Maximum throughput
with good delay performance
Collision Probability p
Channel Busyness Ratio is an accurate, robust, and easily obtained sign of network status.

19. Wireless Networks Laboratory (WINET)
Packet Loss Rate
Given the collision probability p, the MAC layer may drop the packet with the probability
Avg. queue length
Pkt loss rate
Channel busyness ratio

20. Model Validation The optimal operating point denoted by Umax
Wireless Networks Laboratory (WINET)
Model Validation
Channel Busyness Ratio
The optimal operating point denoted by Umax
Simulation settings
50 nodes, RTS/CTS mechanism is used
Each node has the same traffic rate.
We monitor the performance at different traffic rates.

21. Call Admission and Rate Control (CARC)
Wireless Networks Laboratory (WINET)
Call Admission and Rate Control (CARC)

22. Call Admission Control
Wireless Networks Laboratory (WINET)
Call Admission Control
Channel utilization/channel busyness ratio for a flow
Admission control test
R: flow data rate (bps)
L: average packet length (bits)
Up to Urt (= γ Umax, 0<γ<1) can be assigned to real-time traffic

23. Rate control Notation: r: Channel resource r allocated to each node
Wireless Networks Laboratory (WINET)
Rate control
Notation:
r: Channel resource r allocated to each node
Allowable channel time occupation ratio
tp: channel time for packet p
Time that a successful transmission of packet p will last over the channel.
∆: scheduled interval
Time between two consecutive packets that DRA passes to the MAC layer
br: channel busyness ratio
brth = Umax

24. Rate control Initialization Procedure: r=rstart
Wireless Networks Laboratory (WINET)
Rate control
Initialization Procedure: r=rstart
Three-Phase Resource Allocation Mechanism:
multiplicative-increase if underloaded, i.e., br < BM=α×brth
Additive-increase if moderately loaded, i.e., BM ≤ br < brth
Multiplicative-decrease if heavily loaded, i.e., br ≥ brth

25. Theoretical Results of CARC
Wireless Networks Laboratory (WINET)
Theoretical Results of CARC
Convergence of Multiplicative-Increase Phase

26. Theoretical Results of CARC
Wireless Networks Laboratory (WINET)
Theoretical Results of CARC
Convergence to Fairness Equilibrium

27. Simulation Studies Simulation settings in ns2 Channel rate = 11 Mbps
Wireless Networks Laboratory (WINET)
Simulation Studies
Simulation settings in ns2
Channel rate = 11 Mbps
Voice traffic with an on-off model
The on and off periods are exponentially distributed with an average value of 300 ms each.
During on periods, traffic rate is 32kb/s with a packet size of 160 bytes.
Greedy best effort traffic
Saturated CBR traffic with a packet size of 1000bytes.

28. Throughput and MAC delay
Wireless Networks Laboratory (WINET)
Throughput and MAC delay
Each node is a source of greedy traffic
CARC improves the throughput by up to 71.62% with RTS/CTS,
and by up to % without RTS/CTS
CARC achieves up to 95.5% of maximum throughput with and without RTS/CTS

29. Fairness A new greedy node joins the network every other 10 seconds
Wireless Networks Laboratory (WINET)
Fairness
Throughput
Time (s)
A new greedy node joins the network every other 10 seconds
Higher aggregate throughput
Throughput
Time (s)
Fairness convergence speed: 0-2 s
Short term fairness

30. Quality of Service for Voice Traffic
Wireless Networks Laboratory (WINET)
Quality of Service for Voice Traffic
50 greedy nodes
A new voice node joins the network every other 10 seconds.
97%ile
99%ile s s

31. Wireless Networks Laboratory (WINET)
Conclusion
The IEEE MAC protocol can support strict QoS requirements of real-time services while achieving maximum throughput.
Channel busyness ratio is a good network status indicator of the IEEE systems.
An efficient call admission and rate control framework is proposed to provide QoS for real-time service and also to approach the maximum throughput.