Fair Sharing of MAC under TCP inWireless Ad Hoc NetworksKen Tang, Mario Gerla{ktang, gerla}@cs.ucla.eduWireless Adaptive Mobility LaboratoryComputer Science DepartmentUniversity of California, Los AngelesLos Angeles, CA 90095http://www.cs.ucla.edu/NRL/wirelessAbstractIn this study we investigate the performance of TCP and MAC layer in a wireless multi-hopnetwork. Using simulation, we provide new insights into the interactions between TCP and variousMAC layer protocols, including CSMA, FAMA and 802.11. These MAC protocols were chosenbecause they provide an evolution of wireless medium access schemes, starting with carrier sensing(CSMA), then evolving to the utilization of RTS/CTS control frames (FAMA) and finally progressingto collision avoidance and acknowledgements (802.11). We examine these interactions in variousnetwork topologies and in a mobile environment where node movements are unpredictable. Inparticular, we address the issue of fair sharing of MAC with multiple TCP flows.1. IntroductionThe rapid advancement in portable computing platforms and wireless communication technologyhas led to significant interest in the design and development of protocols for instantly deployable,wireless networks often referred to as "ad hoc networks". Ad hoc networks are required in situationswhere a fixed communication infrastructure, wired or wireless, does not exist or has been destroyed.The applications span several different sectors of society. In the civilian environment, they can beused to interconnect workgroups moving in an urban or rural area or a campus and engaged incollaborative operation such as distributed scientific experiments and search and rescue. In the lawenforcement sector, applications such as crowd control and border patrol come to mind. In themilitary arena, the modern communications in a battlefield theater require a very sophisticated instantinfrastructure with far more complex requirements and constraints than the civilian applications [8].In such environments, reliable data transfer and congestion control is paramount. TCP isgenerally used to support these features. However, as shown in [9], depending on the MAC layerbeing used, TCP can exhibit capture behavior that is unacceptable in these critical environmentswhere multiple TCP flows on a single node are common. One node capturing the wireless channelwhile the other nodes are being locked out can result in catastrophic outcomes in the search andrescue or military operations. Thus, an important issue that we address in our study is the effect ofTCP performance with multiple flows on such ad hoc networks under various MAC.In this paper, we study the TCP/MAC layer interaction via simulation, specifically emphasizingon fair sharing of MAC under TCP. The simulation platform used is GloMoSim [12]. GloMoSim isa discrete event, parallel simulation environment implemented in PARSEC, PARallel SimulationEnvironment for Complex Systems [1]. It includes various wireless protocols in its library (radiopropagation, mobility, MAC, network, transport and applications). In addition, GloMoSim providesa valuable and useful feature that facilitates different protocols at a given layer to be swapped in andout of the protocol stack and thus allows for comparison between these different protocols. Mostimportantly, GloMoSim permits the detailed modeling of several layers and the study of theirinteraction, yet preserving very good runtime efficiency and yielding manageable execution time.The rest of the paper is organized as follows: Section 2 reports the configuration and parameterswe used for our simulation. TCP over the MAC layer simulation experiments are examined insection 3. Finally, section 4 concludes the paper.2. Experimental Configuration and ParametersFor our simulation experiments, we consider several topologies (Fig. 1 – Fig. 4): string, hiddenterminal, ring, and grid. The arrows represent the direction of data packet transmissions. FTP withinfinite backlog running on top of TCP is used for the application. We utilize static routing to routepackets when mobility is not considered and use Bellman-Ford routing when mobility is introduced.Three MAC protocols are considered: CSMA, FAMA and IEEE 802.11. Radios with no captureability are modeled with a channel bandwidth of 2Mbps. Furthermore, the channel uses free-spacewith no external noise (perfect channel).Fig. 1: String TopologyFig. 2: Hidden Terminal Topology Fig. 3: Ring Topology Fig. 4: Grid Topology01654327 10 19 11023415012CSMA, FAMA and IEEE 802.11 are chosen because they represent a progression of carriersensing methods. CSMA (Carrier Sense Multiple Access) requires carrier sensing beforetransmission. If the channel is free, the packet is transmitted immediately. Otherwise, it isrescheduled after a random timeout. The major limitation of CSMA is the "hidden terminal” and“exposed terminal” problem [11]. The hidden terminal problem illustrates that collision of datapackets occurs at the receiver, rather than at the sender. Even if the channel is free within thesender’s range, it may not be free at the receiver. In the exposed terminal problem, although themedium is sensed busy near the transmitter, the medium may be free near the intended receiver.CSMA was used first in the Packet Radio network in the mid 1970's [10].FAMA (Floor Acquisition Multiple Access) is an experimental MAC protocol specificallydeveloped for the Glomo DARPA program. In addition to carrier sensing, FAMA features the RTS(Request To Send) and CTS (Clear To Send) exchange to prepare the floor for data transmission (thusavoiding hidden terminal collision in most cases) [7]. A node wanting to transmit first senses thechannel. If the channel is busy, the node backs off a random amount of time and tries again. If thechannel is idle, the node initiates a Request To Send (RTS) control frame, and sets a timer to awaitthe Clear To Send (CTS) control frame from the intended receiver. Any node overhearing the RTScontrol frame is within range to collide with the expected incoming CTS frame, and must defertransmission for a period of time long enough to ensure the return delivery of the CTS. If the nodeinitiating the RTS does not receive the CTS within a specified timeout interval, it assumes a collisionhas occurred, and initiates a backoff before attempting to retransmit. Any node overhearing the CTScontrol frame is within range to collide with the upcoming
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