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87 A multi channel MAC scheme Pedram Samadi Dinani psamadi@ec iut ac ir Date of Submission May 15 2009 Department of Electrical and Computer Engineering Isfahan University of Technology Isfahan 84156 83111 Iran Degree M Sc Language FarsiSupervisor S Jamaloddin Golestani golestani@ieee orgAbstract In this thesis a new multi channel medium access control MAC scheme is proposed The scheme isbased on a common control channel to coordinate the use of multiple data channels among the terminals anda request pulse RP channel in which narrow request pulses are transmitted to help avoid occurrence ofcollisions in the common control channel The transmission of each data packet on a data channel is preceded by the exchange of three controlmessages RTS CTS and RES between the transmitting and receiving terminals using the common controlchannel These control messages allow the transmitter and receiver to agree on the channel to be used for datatransmission Furthermore they provide the necessary mechanism for avoiding collisions in data channels while resolving the well known multi channel hidden node and multi channel deafness problems The idea of a common control channel while resolving the challenges faced in a multi channleenvironment introduces a new problem of its own as the number of data channels and the number of datapackets sent on these channels go up the volume of control messages that need to be exchanged in thecontrol channel would also increase ultimately making the control channel a system bottleneck We addressthis problem in the following way each terminal seeking to transmit data before sending its RTS controlmessage in the control channel announces its desire to do so by sending a narrow request pulse in a separateauxiliary control channel called request pulse channel Other terminals hearing this request pulse refrainfrom sending their request pulses for a prescribed period of time Using this approach we are able tominimize the number of collisions of RTS messages in the control channel thereby improving its efficiencyand enabling a single control channel to suport a larger number of data channels The idea of sending request pulses on an auxiliary channel in order to prevent collisions in the mainchannel was introduced by Bateni Here we have modified the original scheme as described below in orderto optimize it for the present application where the channel benefiting from the request pulses is the commoncontrol channel with very short control messages instead of the data channel If two nodes contend to access the medium in a time period shorter than the propagation time betweenthem they face a challenging problem called late hearing Late hearing may degrade the effectiveness ofrequest pulses to reserve the control channel and avoid collision between control packets To overcome thisproblem the idea of random transmission in the case of late hearing is proposed which reduces theprobability of collision between control packets and increases the achievable throughput of the controlchannel In the presence of a common control channel two approaches are possible to send acknowledgment Ack for a given data packet send the Ack in the data channel itself or send it in the common controlchannel In the former case the spatial reservation needed in the corresponding data channel would be larger

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