A unified performance model for best-effort services in WiMAX networks

Based on the work from the IEEE Working Group 802.16 and ETSI HiperMAN Working Group, the WiMAX (Worldwide Interoperability for Microwave Access) technology is defined by the WiMAX Forum to support fixed and mobile broadband wireless access. In the standard (IEEE 802.16 standard, 2009), it defines several air interface variants, including WirelessMAN-SC, WirelessMAN-OFDM, WirelessMAN-OFDMA and WirelessMAN-HUMAN. WiMAX networks can be operated in two different modes: point to multi-point (PMP) mode and mesh mode. Under the PMP mode, all traffics from subscriber stations (SSs) are controlled by the base station. Mesh mode is a distributed architecture where traffics are allowed to route not only between SSs and the base station but also between SSs. In this chapter, we focus on the WirelessMAN-SC air interface operating in the PMP mode. In WiMAX networks, quality of service (QoS) is provided through five different services classes in the MAC layer (Andrews et al., 2007): 1. Unsolicited grant service (UGS) is designed for real-time applications with constant data rate. These applications always have stringent delay requirement, such as T1/E1. 2. Real-time polling service (rtPS) is designed for real-time applications with variable data rate. These applications have less stringent delay requirement, such as MPEG and VoIP without silence suppression. 3. Extended real-time polling service (ertPS) builds on the efficiency of both UGS and rtPS. It is designed for the applications with variable data rate such as VoIP with silence suppression. 4. Non-real-time polling service (nrtPS) is designed to support variable bit rate non-real-time applications with certain bandwidth guarantee, such as high bandwidth FTP. 5. Best effort service (BE) is designed for best effort applications such as HTTP. To meet the requirements of different service classes, several bandwidth request mechanisms have been defined, namely, unsolicited granting, unicast polling, broadcast polling and piggybacking. In this chapter, we present a performance model for services, such as BE service, based on the broadcast polling mechanism which is contention based and requires he SSs to use the truncated binary exponential backoff (TBEB) algorithm (Kwak et al., 2005) to resolve contention.