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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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30 <strong>Wireless</strong> <strong>Ad</strong> <strong>Hoc</strong> <strong>and</strong> <strong>Sensor</strong> <strong>Networks</strong>DATA, <strong>and</strong> ACK transmissions. Similar is the case with duration fields inCTS <strong>and</strong> DATA frames.The users in the IEEE 802.11 network maintain a NAV (network allocationvector), which indicates the remaining time of the ongoing transmissionsessions. The NAVs are updated using the duration field informationfrom RTS, CTS, <strong>and</strong> DATA frames. The channel is considered busy if eitherphysical or virtual carrier sensing indicates that the channel is busy.Figure 1.4 shows the way users in the network in the carrier sensingrange adjust their NAV during the RTS, CTS, DATA-ACK exchange. Thereare four IFS (interframe space) <strong>and</strong> they are: SIFS (short interframe space),DIFS (DCF interframe space), PIFS (PCF interframe space), <strong>and</strong> EIFS(extended interframe space). The IFS provides priority levels for accessingthe channel. The SIFS is used after RTS, CTS, <strong>and</strong> DATA frames to givethe highest priority to CTS, DATA, ACK, respectively. In DCF, when thechannel is idle, a node waits for the DIFS duration before transmittingany packet.As shown in Figure 1.4, nodes in the transmission range correctly settheir NAVs when receiving RTS or CTS. However, because nodes in thecarrier sensing zone cannot decode the packet, they do not know theduration of the packet transmission. To prevent a collision with the ACKSourceRTSDATADIFSSIFSDestinationCTSACKSIFSSIFSDIFSNodes intransmissionRangeNAV (RTS)NAV (CTS)Nodes incarrierSensing rangeNAV (EIFS)NAV (EIFS)NAV (EIFS)Defer accessFIGURE 1.4NAVs during RTS-CTS-DATA-ACK transmission.

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