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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?

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qos in wlan



Abdulbaset Hassan

Muneer Bazama

  • Introduction
  • QoS Parameters.
  • 802.11 medium access control schemes (MAC).
  • 802.11e medium access control schemes (MAC).
  • Conclusion.
  • Reference.
  • 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
Quality of Service (QoS)
  • QoS is the capability to provide resource assurance in a network.
  • QoS Parameters:



Packet loss rate.


802 11 medium access control schemes mac
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
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





B2 = 10

B2 = 20

B2 = 15

Example of DCF

B1 and B2 are backoff intervals

at nodes 1 and 2

limitation of distributed coordination function dcf
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
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
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
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
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.
edca cont
Pri 0




Pri 1




Transmission Attempt

Scheduler (resolves virtual collisions by granting TXOP to highest priority

8 User priorities per QSTA

8 User priorities mapping to 4 Access Categories







Pri 8

EDCA (Cont.)

Transmission Opportunity (TXOP): is the time interval permitted for a particular STA to transmit packets.

hcf controlled channel access hcca
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).
  • 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 time-sensitive applications, such as, audio and video communications.
  • 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.