近幾年來，無線區域網路(wireless local area network)的發展已日趨成熟，各種無線區域網路的規格也相繼出現。無線網路的優點為不需經由實體的纜線連接，架線的成本可大大的減少。尤其在一些廣大且無基礎佈線的區域更能顯出無線網路的便利性。此外，具有無線網路功能的裝置更能夠享受在移動中使用的好處。無線網路的應用不僅僅是連上網際網路，更可以作為各種無線裝置的連通與通訊。因此，無線網路所帶來的好處和實用性將會對傳統的網路環境有一定的衝擊。然而，無線網路的瓶頸在於它的有限頻寬及易受干擾的特性，因此要達到對資料流(data flow)有服務保證品質(quality of service, QoS)和增加系統的生產率(throughput)，仍然有待更新的技術出現。
美國電機電子工程師協會(Institute of Electrical and Electronic Engineers，IEEE)的802委員會在1997年正式制定802.11無線區域網路的標準。在最初的制定中，所傳送的資料傳送速率為每秒鐘1百萬位元(Mbps)或2百萬位元，但隨著各種應用程式與需求的成長，每秒鐘2百萬位元的資料傳送速率逐漸不敷使用。1999年，IEEE 802.11a及IEEE 802.11b的規格相繼被提出。IEEE 802.11a為在5GHz頻帶的高速無線區域網路規格，其資料傳送速率最高可達每秒鐘54百萬位元；IEEE 802.11b為在2.4GHz頻帶的高速無線區域網路規格，其資料傳送速率可達每秒鐘11百萬位元。

在此論文中我們提出一個在IEEE 802.11無線區域網路的環境下使用以等級限定為基礎的媒體存取機制來提供服務等級區分(service differentiation)，並且更進一步達到降低擁塞度(congestion)及減少碰撞機率(collision probability)，提高整個系統的生產率及對即時性(real-time)資料支援短延遲(latency)和減少變異度(jitter)。

經由模擬實驗的結果顯示，所提出的方法無論在網路交通擁塞與否的情況下都能夠對即時性的資料流有效率的達到短延遲和減少變異度，並且能使得生產率有一定程度的改善。

The wireless access network with the capability for people to access information anytime and anywhere has attracted much attention in recent years. The IEEE 802.11 wireless local area network (WLAN) can offer high data rates through IEEE 802.11a and IEEE 802.11b technologies [5].
In this thesis, we emphasize how to improve the performance of the 802.11 MAC protocol. We present a Priority-Based Contention Control (PCC) scheme to well manage the contention for the medium among the wireless stations (STA) in the IEEE 802.11 wireless LAN, and, the PCC scheme has the following advantages: providing service differentiation among the traffic flows with different priorities and easing off the serious collisions in high traffic loads.

[1] I. Aad and C. Castelluccia, “ Differentiation mechanisms for IEEE 802.11 ”, IEEE Procs. INFOCOM 2001. Volume: 1 , 2001 Page(s): 209 -218
[2] K. Kim, S. Shin, and K. Kim,” A novel MAC scheme for prioritized services in IEEE 802.11a wireless LAN”, Joint 4th IEEE International Conference on ATM (ICATM 2001) and High Speed Intelligent Internet Symposium, 2001. Page(s): 196 -199
[3] G. Bianchi “Performance analysis of the IEEE 802.11 distributed coordination function”, IEEE Journal on Selected Areas in Communications, Volume: 18 Issue: 3 , March 2000, Page(s): 535 –547.
[4] “3rd Generation Partnership Project; Technical Specification Group Services and System Aspects; QoS Concept and Architecture (Release 1999)”, 3GPP.
[5] ISO/IEC 8802-11; ANSI/IEEE Std 802.11, 1999 edn , 20 Aug. 1999 ”Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications”
[6] IEEE P802.11 TASK GROUP E, http://www.ieee802.org/11/
[7] J.-M. Ho, ”Traffic Descriptions For 802.11E Performance Simulation”, IEEE Document, 802.11-00/363, November 2000.
[8] B. P. Crow, I. Widjaja, J. G. Kim, and P. T. Sakai, “IEEE 802.11 Wireless Local Area Networks,” IEEE Communications Magazine, Vol. 35, Issue 9, pp. 116-126, September 1997.
[9] M. Natkaniec and A. R. Pach, “An Analysis of the Backoff Mechanism used in IEEE 802.11 Networks,” Proc. the fifth IEEE Symposium on Computers and Communications, pp. 444-449, 2000.
[10] F. Cali, M. Conti, and E. Gregori, “Dynamic Tuning of the IEEE 802.11 Protocol to Achieve a Theoretical Throughput Limit,” IEEE/ACM Trans. on Networking, Vol. 8, Issue 6, pp. 785-799, December 2000.
[11] S. S. Kang, and M. W. Mutka, “Provisioning service differentiation in ad hoc networks by modification of the backoff algorithm” Proc. Tenth International Conference on Computer Communications and Networks, 2001, Page(s): 577 –580
[12] T. Ozugur, “Weighted hierarchical backoff algorithm for wireless ad hoc networks” IEEE Global Telecommunications Conference, 2001, Volume: 5, 2001, Page(s): 2937 –2943
[13] L. G. Martinez, B. Jafarian, and H. Aghvami, “Performance evaluation of PRMA with priorities for wireless ATM “ IEEE 49th Vehicular Technology Conference, 1999, Volume: 2, Page(s): 1136 -1140 vol.2
[14] D. J. Goodman, R. A. Valenzuela, K. T. Gayliard, and B. Ramamurthi, “Packet reservation multiple access for local wireless communications” IEEE Transactions on Communications, Volume: 37 Issue: 8, Aug. 1989, Page(s): 885 -890
[15] S. T. Shen, and T. F. Sheu, “A bandwidth allocation/sharing/extension protocol for multimedia over IEEE 802.11 ad hoc wireless LANs” IEEE Journal on Selected Areas in Communications, Volume: 19 Issue: 10, Oct. 2001, Page(s): 2065 -2080
[16] ETSI STC-RES 10 Committee, ”HIPERLAN Functional Specifications”, Draft standard ETS 200-652, 1995.
[17] S. Cevrel, A. Aghvami, et al. “Analysis and optimisation of the HIPERLAN Channel Access Contention Scheme,” Wireless Personal Communications, Volume 4, Issue 1, January 1997, pp. 27-39, Kulwer Acadamic Pubilshers, 1997.
[18] P. Jacquet, P. Minet, P. Mühlethaler, and N. Rivierre, “Priority and Collision Detection with Active Signaling – The Channel Access Mechanism of HIPERLAN”, Wireless Personal Communications, Volume 4, Issue 1, January 1997, pp. 11-25, Kulwer Acadamic Pubilshers, 1997.
[19] http://www.bluetooth.com/
[20] A. □Banchs, X. Perez, M. Radimirsch and H. J. Stuttgen, “Service differentiation extensions for elastic and real-time traffic in 802.11 wireless LAN”, IEEE Workshop on High Performance Switching and Routing, 2001 Page(s): 245 –249
[21] C. Fragouli, V. Sivaraman, and M. B. Srivastava, “Controlled Multimedia Wireless Link Sharing via Enhanced Class-based Queuing with Channel-state-dependent Packet Scheduling”, IEEE Procs. INFOCOM, April 1998, Vol.2, pp. 572 –580.
[22] G. Bianchi, F. Borgonovo, L. Fratta, L. Musumeci, and M. Zorzi, “C-PRMA: A Centralized Packet Reservation Multiple Access for Local Wireless Communications”, IEEE Transactions on Vehicular Technology, Volume: 46 Issue: 2 , May 1997 Page(s): 422 –436
[23] W. T. Chen, Jun-Jia Dai, and Shou-Chih Lo, 2001, “A MAC Protocol with QoS Guarantee for Multiclass Traffics in Wireless LANs,” Proceedings of the 13th International Conference on Wireless Communications, Calgary, Canada, July 2001.
[24] M. Barry, A. T. Campbell, A. Veres, “Distributed Control Algorithms for Service Differentiation in Wireless Packet Networks” IEEE Procs. INFOCOM, Volume: 1, 2001 Page(s): 582 –590