Deployments of wireless networks (WLANs) have grown rapidly in the last few years, it can prove to be very useful in public places – libraries, guest houses, hotels, cafeterias, airports, and schools are all places where one might find wireless access to the Internet. Nowadays most of handheld devices such as notebooks, tablet PCs, and PDAs are provided by one or more built-in wireless interfaces, users can access internet without the need to install any wires.
IEEE 802.11 is the most popular protocol amongst other wireless protocols such as Bluetooth, WiMAX, and UWB. The 802.11 working group was established in 1990 by the IEEE Executive Committee. Their goal was to create a wireless local area network (WLAN) standard. The standard specified an operating frequency in the 2.4GHz ISM (Industrial, Scientific, and Medical) band. Seven years later (1997), the group approved IEEE 802.11 as the world's first WLAN standard with data rates of 1 and 2 Mbps. In 1999 the working group approved two extensions to 802.11, the first one is 802.11a operates on U-NII band (Unlicensed National Information Infrastructure) 5GHz, and with a maximum bandwidth of 54 Mbps (due to higher frequency), it only allow access to clients within 40 – 50 feet due to power limits enforced by the FCC, while the second extension is 802.11b which operates on 2.4GHz ISM band, with a maximum bandwidth of 11 Mbps, it allows client access up to well over 1000 feet. The whole Family of IEEE 802.11 is shown in Table 1.1.
Ad Hoc mode and Infrastructure mode are most common wireless configurations found today. Ad Hoc mode (which also referred to as “Independent Basic Service Set”
(IBSS)) provides peer-to-peer communication links between two or more wireless devices without the use of an AP, while Infrastructure mode (which is also known as
“Basic Service Set” (BSS)) requires an Access Point and at least one wireless client.
Table 1.1: The IEEE 802.11 family.
Standard Description 802.11a It operates in 5 GHz band, and uses a 52-subcarrier orthogonal
frequency-division multiplexing (OFDM) with a maximum raw data rate of 54 Mbps. The data rate is reduced to 48, 36, 24, 18, 12, 9 then 6 Mbps if required. It advantage is the less interference.
802.11b It operates on 2.4GHz ISM band, with a maximum bandwidth of 11 Mbps, it allows client access up to well over 1000 feet.
802.11c It is a specification to cover bridge operation with IEEE 802.11 MAC’s.
802.11d It is an amendment to 802.11 specification regarding telecommunication and information exchange between two systems and extensions to operate in additional regulatory domains
802.11e It expands support for applications with Quality of Service requirements
802.11f Inter Access Point Protocol (IAPP). It allows mobile devices roaming between multi-vendor APs.
802.11g It works in the 2.4 GHz band, and operates at a maximum raw data rate of 54 Mbps. The modulation scheme used in 802.11g is orthogonal frequency-division multiplexing (OFDM) for the data rates of 6, 9, 12, 18, 24, 36, 48, and 54 Mbps
802.11h It is standardized for certain zone spectrum (Europe). It is intended to resolve interference issues introduced by the use of 802.11a in some locations, particularly with military radar systems and medical devices.
802.11i It enhances the current 802.11 MAC to provide improvements in security (WEP and TKIP).
802.11j It is standardized for certain zone spectrum (Japan).
Recently, Voice over Internet Protocol (VoIP) became one of the most popular Internet applications. Some products of VoIP like Skype and MSN are able to guarantee a high level of voice quality over wired networks. The popularity of VoIP comes from its
low cost and good quality, thus, people can use their PCs to make calls instead of the expensive cell phone calls, statistics says that the number of residential VoIP users will rise from three million at 2005 to 27 million by the end of 2009[17].
To provide person-to-person (instead of place-to-place) connections anywhere and anytime, the Internet is expected to penetrate the wireless domain. One very promising wireless network is the wireless local area network (WLAN), which has shown the potential to provide high-rate data services at low cost over local area coverage. However, Voice over 802.11 (VO802.11) faces a lot of challenges to guarantee a high level of quality, such as low bandwidth, large interference, long latency, high loss rates, and jitter, so we need to enhance the 802.11 standard to be able to support VoIP with a high level of voice quality. Furthermore, the distributed coordination function (DCF) and the point coordination function defined in basic IEEE 802.11 are unsuitable to guarantee QoS effectively.
As a solution of the Quality of Service problem facing the VO802.11, the IEEE 802.11 group chartered the 802.11e task group with the responsibility of enhancing the 802.11 Medium Access Control (MAC) to include bidirectional QoS to support latency-sensitive applications such as voice and video. The new standard of IEEE 802.11e [1] is expected to solve the QoS problem of real-time applications over wireless local area networks.
IEEE 802.11e has brought many enhancements, some are general and some are specific. The first general enhancement is the option to allow stations to talk directly to other stations, bypassing the AP, even when there is one. The second general enhancement is the introduction of negotiable acknowledgements, IEEE 802.11e introduces the notion of negotiable acknowledgements whereby a packet in a stream need not to be acknowledged, or such acknowledgements can be aggregated depending on the parameterization of a stream, such negotiable acknowledgements lead to a more efficient utilization for the available channel and enable applications such as a reliable multicast.
And the third general enhancement is the introduction of traffic parameterization and traffic prioritization. These are central to any QoS mechanism. The major novelty developed by IEEE 802.11e is the new coordination function called Hybrid-Coordination Function (HCF). It is a coordination function that combines aspects of the distributed
coordination function and the point coordination function. HCF uses a contention-based channel access method, called the Enhanced Distributed Channel Access (EDCA).
Because it operates in the contention period, EDCF is similar to DCF. QoS support is realized with the introduction of the Traffic Categories (TCs). However, EDCA doesn’t guarantee QoS of time sensitive applications. The second mechanism for access to the wireless medium, which is part of the HCF, is called HCF-Controlled Channel Access (HCCA). It provides the capability for reservation of transmission opportunities (TXOPs) with the HC. This mechanism is expected to be used for the transfer of periodic traffic such as voice and video. HCCA operates on Contention Free Periods (CFPs) or it can be invoked during Contention Periods (CPs). HCF is based on polling, so it is a modified version of PCF, it is promising to guarantee QoS of real-time applications. HCCF is a little complex to be implemented and many researches are still in progress.
HCCA polling mechanism is still unspecified in the standard, polling scheduling in HCCA is left to designers till now. The study in [2] shows the polling overhead problem and its negative effect on the QoS of voice streams in WLANs. An effective polling scheduler will extremely enhance the performance, and assure a high level of QoS for the time sensitive applications. The most popular polling mechanism mentioned in [6] is the round robin mechanism, it is popular due to the simplicity if its implementation. Many other polling schemes have been proposed such as [4], [5], and [7].
In our study we focus on HCCA, on its polling scheduler in particular. We proposed a new scheduler called Adaptive Polling Scheme (APS), the proposed scheduler showed a high performance against the other schedulers. APS depends on prioritizing voice stations according to their state using a simple mechanism of silence detection, and on the queue occupancy at both the AP side and the station side. In addition to its role in scheduling the order of stations and calculating TXOPs, it works as a queue management scheme to prevent full queues on the AP and to prevent stations from dropping packets of their voice traffic category. In APS we maximized the usage of piggybacking feature of IEEE 802.11e, to increase the total throughput of the wireless channel. Here we proposed two dynamic polling lists, one contains the information of the stations in talking state while the other contains the information of the stations in the silent state, each station has its own order in the polling lists, since each one of the two polling list is sorted according
to different criteria, the talking list is sorted according to the occupancy percentage at the AP transmission queues and at the station traffic categories calculated by giving those occupancies different weights, while silence list is sorted according to the occupancy percentage of the transmission queues at the AP only.
The rest of this thesis is organized as follows. Chapter 2 introduces the background of IEEE 802.11 and IEEE 802.11e, voice transmission requirements on WLANs, and a brief survey of current polling mechanisms. Chapter 3 points to the problem definition that motivated us to do this work. In chapter 4, we discuss the proposed adaptive polling scheme. We demonstrate our simulation and analytical results in chapter 5. Finally the conclusion and future works are presented in chapter 6.