QoS In WLAN
By
Abdulbaset Hassan
Muneer Bazama
Outline
• Introduction
• QoS Parameters.
• 802.11 medium access control schemes
(MAC).
• 802.11e medium access control schemes
(MAC).
• Conclusion.
• Reference.
Introduction
• What is 802.11?
• 802.11 refers to a family of specifications
developed by the IEEE for wireless LAN
technology.
• 802.11 Standards.
• 1997 - 802.11 up to 2Mbps (900MHZ).
• 1999 - 802.11b 2.4GHZ and up to 11Mbps.
• 1999 – 802.11a 5GHZ and up to 54Mbps.
• 200x – 802.11g 2.4GHZ and up to 54Mbps.
• 200x – 802.11e for QoS.
Quality of Service (QoS)
•
•
QoS is the capability to provide resource
assurance in a network.
QoS Parameters:
Bandwidth.
Delay.
Packet loss rate.
Jitter.
802.11 medium access control
schemes (MAC).
There are two basic medium
access control (MAC)
modes:
• Distributed Coordination
Function (DCF).
• Point Coordination Function
(PCF).
Distributed Coordination
Function (DCF)
• Contention-Based.
• Based on Carrier Sense Multiple Access with
Collision Avoidance (CSMA/CA) algorithm.
• Uses a Contention (Backoff) algorithm.
• Designed for a best-effort service.
• Supports Asynchronous transmission.
Example of DCF
B1 = 25
B1 = 5
wait
data
data
B2 = 20
wait
B2 = 15
B1 and B2 are backoff intervals
at nodes 1 and 2
B2 = 10
Limitation of Distributed
Coordination Function (DCF)
•
•
•
•
Does not provide any QoS guarantees.
Does not support real-time application.
Designed for equal priorities.
Does not support the concept of
differentiating frames with different user
priorities.
Point Coordination Function
(PCF)
•
•
•
•
•
Contention free-based.
Uses a centralized polling scheme.
Differentiation Considered based on priority.
Supports synchronous transmission.
Supports time-bounded multimedia
applications.
Limitation of Point Coordination
Function (PCF)
• poor QoS performance.
• Uses a simple round-robin
algorithm, which can not handle the
various QoS requirements.
• Transmission time of the polled
stations is unknown.
Hybrid Coordination Function
(HCF)
There are two methods of channel
access:
• Enhanced DCF channel access
(EDCA).
• HCF controlled channel access
(HCCA).
Enhanced DCF Channel
Access (EDCA)
• Contention-Based channel access.
• Provides service differentiation.
• Classifies the traffic into 8 different
classes.
• Each station has 4 access categories to
provide service differentiation.
Pri 8
Backoff
(AIFSN0)
AC1
Backoff
(AIFSN1)
AC2
Backoff
(AIFSN2)
AC3
Backoff
(AIFSN3)
Transmission Opportunity (TXOP): is the time interval permitted for a particular STA to transmit packets.
Transmission Attempt
Pri 1
AC0
Scheduler (resolves virtual collisions by
granting TXOP to highest priority
8 User priorities per QSTA
Pri 0
8 User priorities mapping to 4 Access Categories
EDCA (Cont.)
EDCA (Cont.)
Priority
Access
Category (AC)
Designation
(Informative)
0
0
Best Effort
1
0
Best Effort
2
0
Best Effort
3
1
Video Probe
4
2
Video
5
2
Video
6
3
Voice
7
3
Voice
HCF Controlled Channel
Access ( HCCA)
• Operates in CFP and CP.
• Provides Guaranteed Services
with a much higher probability
than EDCA.
• Combines the advantages of PCF
and DCF.
• Coordinates the traffic in any
fashion (not just round- robin).
Conclusion
• The IEEE 802.11 wireless LAN (WLAN) has
become one of the most widely used wireless
technologies in these days.
• The 802.11 standards can not provide any
QoS guarantees due to poor performance.
• The 802.11e offers QoS support to timesensitive applications, such as, audio and
video communications.
References
1.
2.
3.
4.
5.
6.
Paal E. Engelstad and Olav N osterbo, Analysis of QoS in WLAN,
Telektronikk 1.2005. (http://folk.uio.no/paalee)
Priyank Garg, Rushabh Doshi, Russell Greene, Mary Baker, Majid
Malek, Xiaoyan Cheng, Using IEEE 802.11e MAC for QoS over
Wireless, 2003 IEEE.
Jie MaYuan'an Liu,Bihua Tang, QoS Research and Design for
WLAN, Proceedings of ISCIT2005.
J.K.Choi, J.S.Park, J.H. Lee, K.S. Ryu, Review on QoS issues in
IEEE 802.11 W-LAN, Feb. 20-22, 2006 ICACT2006.
Jose Villalon, Pedro Cuenca and Luis Orozco-Barbosa, Limitations
and capabilities of QoSSupport in IEEE 802.11 WLANS, 2005 IEEE.
http://en.wikipedia.org/wiki/IEEE_802.11e.