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Wireless Ad Hoc and Sensor Networks

Wireless Ad Hoc and Sensor Networks

Wireless Ad Hoc and Sensor Networks

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288 <strong>Wireless</strong> <strong>Ad</strong> <strong>Hoc</strong> <strong>and</strong> <strong>Sensor</strong> <strong>Networks</strong>SyncbytesPreambleMAC Destinationlength addressSourceaddressTransmissionpowerExtendablefieldsData ...CRCPHYMAC headerFIGURE 6.29Protocol frame format.network. The ADF7020 provides the hardware preamble detection <strong>and</strong>interrupt source to the microprocessor.The preamble is followed by the MAC header. The length of the MACheader can be programmable using its first byte, therefore allowing multipleextensions for the future. For the DAPC, only transmission powerfield is required. After the MAC header, data <strong>and</strong> CRC are transmitted.6.11.2.2 Baseb<strong>and</strong> ControllerA baseb<strong>and</strong> controller is implemented to interface with the physical layer.It also provides an API for higher layers to access all functionalities offeredby the radio transceiver. In the implementation of the DAPC MAC, onlyRSSI read-back <strong>and</strong> power control are used. Other options are available <strong>and</strong>can be utilized easily for future implementations of different protocols.6.11.2.3 Operation ModesThe baseb<strong>and</strong> controls the radio in three operational modes, transmit,receive, <strong>and</strong> idle, which are h<strong>and</strong>led by the Tx/Rx state machine. The radioshould always operate in the idle mode unless a packet is ready fortransmission or a preamble is detected indicating the beginning of apacket reception.Idle mode: In the idle mode, the radio is still listening to the channelhowever any incoming data from the radio is ignored.Receive mode: During the idle mode, when a preamble is detected bythe radio, an interrupt is sent to the microprocessor. Upon interrupt,the baseb<strong>and</strong> switches to the receive mode, <strong>and</strong> begins bufferingincoming bytes; the length of the packet is prefixed between thetransmitter <strong>and</strong> receiver.Transmit mode: When a packet is ready for transmission, the baseb<strong>and</strong>switches to the transmit mode by appending the preamble <strong>and</strong> itsends out the entire packet with no interruptions.6.11.2.4 RSSI ReadingThe implementation of DAPC requires RSSI readings to calculate the SNRfor every packet. To provide accurate SNR values, RSSI readings are taken

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