Wireless communications have facilitated human beings over half a century. From people’s aspects, it is convenient to connect to Internet using electronic device without obstructive wired line. Therefore, in recent years most of mobile devices such as tablet PC and PDA are equipped with at least one type of wireless communication interface. Most recently, several wireless communication protocols, such as WLAN, Bluetooth, WiMAX, and UWB were proposed for various application segments.
Among them IEEE 802.11 WLAN standard is the most popular protocol that provides wireless communications services. WLAN technology supports high bandwidth with affordable cost and easy installation, thus it spreads quickly and widely.
IEEE 802.11 WLAN standard is a large family. All beginning in 1997, the original 802.11 standard was established by IEEE. 802.11 supports only 1 and 2Mbps transmission rates through three PHY medium, which are infrared (IR), frequency hopping spread spectrum radio (FHSS), and direct sequence spread spectrum radio (DSSS). The IEEE 802.11 MAC sub-layer defines two medium access coordination functions, the basic Distributed Coordination Function (DCF) and the optional Point Coordination Function (PCF). Two high rate extended WLAN protocol standards were introduced by IEEE in 1999. One is 802.11b that is based on DSSS technology and works in 2.4GHz with data rate up to 11Mbps. The other is 802.11a which is based on orthogonal frequency-division multiplexing (OFDM) technology and works in 5.4GHz with data rate up to 54Mbps. Several years later, 802.11g standard which extends data rate of 802.11b into 54Mbps was finalized in 2003. An even higher speed WLAN standard 802.11n that supports more than 100Mpbs data rate through MIMO technology is on the way now. Besides, several WLAN standards were
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standardized for certain zone spectrum such as 802.11j for Japan and 802.11h for Europe. Moreover, there were some 802.11 standards developed for special purposes.
802.11e was introduced for quality of service (QoS) provisioning over WLAN. Inter Access-Point Protocol (IAPP), 802.11f allows mobile devices roaming between multi-vendor APs. Enhanced security and authentication mechanisms were concerned in 802.11i. The motivation of 802.11s standard was mesh networking.
Recently, voice over IP（VoIP）becomes another popular Internet applications.
Some products, such as Skype, are able to support good quality of VoIP over the wired Internet. VoIP uses conventional IP network to provide voice communication service between end users. It is much cost effective and easier to construct IP network than to construct traditional telecommunication networks, and it also can provide more facility voice services for users, so VoIP business grows up quickly. Taking an example, according to the statistics, there are more than 150,000 new Skype registers worldwide every day.
Therefore, people try to use handset devices to make VoIP calls through WLAN in place of cell phones or traditional telephones. This means that we need to provide QoS in WLAN. However, in particular, wireless links feature specific characteristics such as low bandwidth, large interference, high loss rates, bursts of frame loss, long latency, and jitter, so it is much harder to provide QoS for time-bounded multimedia services in wireless environment than in wired networks. Furthermore, the distributed coordination function (DCF) and point coordination function (PCF) defined in basic 802.11 are unsuitable to provide QoS effectively [3] [7] .
To solve the QoS provisioning problem, IEEE 802.11e standard [1] has been established. 802.11e standard introduced some MAC mechanisms to strengthen functions of original 802.11 to support QoS for time-sensitive applications, such as VoIP, multimedia streaming and so on. Hybrid coordination function (HCF), direct
link protocol (DLP), and BlockACK, are three major proposed schemes in 802.11e to achieve the goal of provisioning high quality services for real-time traffic [3].
However, although 802.11e has almost been finalized, it is still unable to provide satisfactory QoS for all real-time applications. For example, EDCA only provides probabilistic QoS instead of deterministic QoS. Specifically, in some worst cases, the quality of the delay-sensitive traffic is even unacceptable. Furthermore, when a lot of STAs try to compete at the same time, the packet collision rate may be vitally high.
On the other hand, HCCA is a little complex to implement and the actual effect is still unknown. As a consequence, many research are still in progressing, because we couldn’t find an operative, simple, robust and total solution for QoS provisioning in WLAN.
Regarding EDCA enhancement, the study in [21] shows that differentiating the initial CW size is better than differentiating the interframe space (IFS) in terms of total throughput and delay. Intuitively, different arbitration IFS has the function of providing priorities, but it can not reduce collisions. Whereas differentiating initial CW size features not only the function of providing priorities but also the function of reducing collisions. Because of that, almost all researches of EDCA improvement were focused on adjusting the initial value or the varying behavior of CW.
Even so, we amend a new IFS-based scheme that is based on typical EDCA to accommodate QoS issue over WLAN. In our scheme, AP plays both the roles of global transmission parameters manager and admission controller. An AP is allowed to adjust the transmission parameters, such as AIFSN and CWmin, for a certain access category (AC) in every STAs associated with the AP dynamically, based on to the number of admitted QoS AC. Moreover, in order to accommodate various kinds of network environments, AP may set two parameters configurations. One is called unique AIFSN assignment (UAA) which is designed for wireless network
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environments with small number of QoS ACs. As its name, UAA allows that each QoS AC own a unique AIFSN which is assigned by the associated AP and transmits packets without backoff after deferring AIFS. Besides, UAA insures that each AIFSN[QoS AC] < AIFSN[BE AC] strictly, therefore QoS-aware traffic can always be sent before best effort traffic. Through this method, QoS-aware packets can be transmitted as soon as possible, and collision can also be avoided too. The other configuration is called contention window partitioning (CWP) which is designed for high data rate wireless network with large number of QoS ACs. CWP is similar to the original EDCA, except that we enlarge the AIFSN difference between ACs and use a fixed CW for each QoS AC. Our proposed scheme provides effective, effortless, flexible and reliable mechanism and simulation result shows that our scheme allows time-sensitive packets to be sent with guaranteed time-bound.
To implement our scheme, we need to add a management function on top of AP MAC for control the parameters setting, as well as steal a field for AIFSN information in QoS ADDTS response, which is sent by AP. Each STA should be able to recognize the information. Consequently, we can see that the total complexity of proposed scheme only increases a little and can be implemented easily.
The rest of this thesis is organized as follows. Chapter 2 introduces the background of 802.11 and 802.11e, multimedia traffic attribute and WLAN MAC mechanism. We also discuss some related solutions which have been proposed. In Chapter 3, we discuss the proposed mechanism and two parameter configurations as well as compare our scheme with EDCA and HCCA. The public hidden node problem and some mitigating approaches will also be addressed. Simulation and numerical results are demonstrated in Chapter 4. Finally, the conclusion and future works are presented in Chapter 5.