The 18th Annual IEEE International Symposium on Personal, Indoor and Mobile Radio Communications (PIMRC’07) WIMAX MODULE FOR THE NS-2 SIMULATOR Juliana Freitag * and Nelson L. S. da Fonseca Institute of Computing State University of Campinas Campinas, Brazil ABSTRACT This paper presents the design and validation of an WiMAX module based on the IEEE 802.16 standard. The module, which was implemented, includes mechanisms for bandwidth request and allocation, as well as for QoS provision. Moreover, the implementation is standard-compliant. I I NTRODUCTION The IEEE 802.16 standard [1], widely known as WiMAX (Worldwide Interoperability for Microwave Access Forum), has been developed to accelerate the introduction of broad- band wireless access into the marketplace. Both industry and academia have been motivated to conceive novel mechanisms for 802.16 networks since some aspects of the standard are left to be defined by proprietary solutions. Several research groups have investigated QoS mechanisms, such as admission control and scheduling algorithms [2, 3, 4]. Simulation is an essential tool in the development and per- formance evaluation of communication networks. Among the available tools for networks simulation, the Network Simulator (ns-2) [5] is the most popular one in the research community. Much of this popularity is due to the fact that the ns-2 is a public domain tool which implements a rich set of Internet protocols, including wired and wireless networks. Recently, two modules were proposed for simulation of IEEE 802.16-based networks using ns-2. One of them, im- plemented by NIST [6], provides, among other features, WirelessMAN-OFDM physical layer with configurable mod- ulation, Time Division Duplexing (TDD), Point-to-Multipoint (PMP) topology, fragmentation and reassembly of frames, but it fails to implements MAC QoS support, namely, service flows and QoS scheduling. The other 802.16 simulation module, pro- posed by Chen et all [7], uses the wireless channel implemen- tation provided by the ns-2. It is also based on TDD duplexing mode and PMP topology, and it provides packet fragmentation and packing. Although this module implements the five ser- vice flow types specified in the IEEE 802.16 standard, the re- quest/grant mechanism defined for bandwidth management is not compliant to the MAC layer specification. Moreover, users cannot configure QoS requirements, such as maximum latency and minimum bandwidth, for the high priority service flows. There is another group developing an 802.16-based simulator for the OPNET tool [8], which is a private domain simulator; however, this module is available exclusively to the consortium members. * This research was sponsored by UOL (www.uol.com.br), through its UOL Bolsa Pesquisa program, process number 20060511022200a and by CNPq, process number 305076/2003-5. This paper presents the design and validation of a simulation module for 802.16-based networks in the ns-2 simulator. The focus of this implementation is the MAC layer and its mecha- nisms for bandwidth allocation and QoS support. The module implements the 802.16 five service flow types and their band- width request/grant mechanisms; moreover, it allows users to configure the QoS requirements of applications. Service flows are modeled by finite sate machines that capture how each ser- vice type react to different events. This module supports TDD mode and PMP topology. The wireless channel available in the ns-2 simulator is used. We believe that the module devel- oped is a significant contribution for the communication net- work research community since it allows research on 802.16 MAC layer specially those on bandwidth management and QoS provision. Although the code developed is large, containing 17 classes of objects and about 17,300 lines of code, the modular- ization provided by object oriented programming facilitates the inclusion of new functionalities. It is our best knowledge that no other module for WiMAX networks simulation implements bandwidth request/grant mechanism and QoS support accord- ing to the IEEE 802.16 standard. The remainder of this paper is organized as follows. Sec- tion II presents an overview of the IEEE 802.16 standard. Sec- tion III describes the proposed WiMAX module. Section IV presents the simulation experiments created to validate the module. Finally, Section V concludes the paper. II THE IEEE 802.16 STANDARD The physical channel defined in the IEEE 802.16 standard [1] operates in a framed format. Each frame is divided in two sub- frames: the downlink subframe is used by the BS to send data and control information to the SSs, and the uplink subframe is shared by all SSs for data transmission. In TDD mode, uplink and downlink transmissions occur at different times since both subframes share the same frequency. Each TDD frame has a downlink subframe followed by an uplink subframe. The 802.16 MAC protocol is connection-oriented. When a connection has backlogged data, the SS sends a bandwidth re- quest to the BS. The BS, in turn, allocates time slots to the SSs based on both bandwidth requests and QoS requirements of the requesting connection. A request for bandwidth can be sent as a stand-alone mes- sage, in response to a poll from the BS, or can be piggybacked in data packets. When the BS uses unicast polling, sufficient bandwidth to send a request is allocated to an SS. When a group is polled through multicast/broadcast polling, the members of the group which require bandwidth respond with a request. A contention resolution algorithm is used to resolve conflicts that arise when two or more transmission occur at the same time. 1-4244-1144-0/07/$25.00 c 2007 IEEE