CONTENTION-BASED QOS MAC MECHANISMS FOR VBR VOIP IN IEEE 802.11E WIRELESS LANS

INTRODUCTION

In recent years, Voice over IP (VoIP) has become an attractive alternative to the traditional public switched telephone network (PSTN). Providing QoS for VoIP on increasingly heterogeneous computer networks brings up many challenging issues. As an emergent component in computer networks, wireless local area network (WLAN) attracts wide attentions from both academia and industry. Furthermore, convergence of WLANs with cellular systems makes VoIP over WLAN (VoWLAN) an important research topic.Typically, VoWLAN is implemented as a networking protocol stack shown in Fig. 1.

At the top of this protocol stack, a number of popular VoIP codec, such as ITU G711, G729, and G723 may be adopted at the application layer. A real-time transport protocol (RTP) stack packs various sizes of audio payload in various intervals. When a VoIP packet, also known as payload, passes through RTP, UDP, IP, and MAC protocol layers, some extra bytes of overhead are added as the header or trailer. Finally, voice frames are transmitted according to MAC function over a radio link at the physical (PHY) layer.

Voice Model and Voice over IP (VoIP)

There are two types of VoIP: constant bit rate (CBR) VoIP and variable bit rate (VBR) VoIP. For CBR VoIP, a codec generates constant audio payload during the whole voice conversation period. Interactive voice conversations have two parties and each party has many talk spurts and silent periods alternately. A silence suppression technique is adopted to stop sending RTP packets during silent periods. One way tohandle VBR VoIP is to regard VBR VoIP as CBR traffic with its peak throughput. Thus the QoS MAC scheme studied in , can be directly applied. However, since each VBR VoIP call experiences silent/talk periods independently, some form of statistical multiplexing may be integrated into the QoS MAC to provide QoS for VBR VoIP with efficient bandwidth utilization. This is the primary objective of this research.IEEE802.11 WLAN and Related Works The IEEE802.11 WLAN is being deployed widely and rapidly for many different environments, including enterprise, home, and public access networking.

In a broadcast network, such as WLAN, the MAC sub-layer is responsible for arbitrating multiple stations to access a shared transmission medium. There are two channel access functions defined in the IEEE802.11 MAC: a mandatory Distributed Coordination Function (DCF), which is based on CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) with binary exponential backoff, and an optional Point Coordination Function (PCF),where the AP controls all the transmissions based on a centralized polling scheme. In order to enhance the IEEE 802.11 and provide QoS support over WLAN, the IEEE working group is currently finalizing the IEEE 802.11e standard . The IEEE 802.11e has a Hybrid Coordination Function (HCF), which includes a contention-based channel access part and a centrally controlled channel access part. The contention-based channel access of the HCF is referred as Enhanced Distributed Channel Access (EDCA); and the centrally controlled channel access is referred as HCF controlled channel access (HCCA).

QoS MAC is based on contention-based IEEE802.11e EDCA.There are increasing interests in providing QoS to VoIP sessions in WLANs. In the literatures, the focus was on designing a centrally controlled polling-based MAC to provide QoS to VoIP in WLANs. In the maximum number of VoIP sessions supported by contention-based MAC is evaluated in IEEE 802.11 (a/b) WLANs, and the unbalance problem of downlink/uplink traffics and the relationship between system capacity and VoIP codec are also studied. In , authors proposed a generic approach that relates VoIP performance with the dynamics of priority MAC. This method improves VoIP capacity in WLANs.