Hidden Terminal problem How does c know how long to wait before it can attempt a transmission a includes length of data that it wants to send in the rts packet B includes this information in the Cts packet C, when it overhears the CTs packet, retrieves the length information and uses it to set the inhibition time CS5602: Principles and Techniques for Sensors and Information Perception
Hidden Terminal Problem How does C know how long to wait before it can attempt a transmission? ▪ A includes length of DATA that it wants to send in the RTS packet ▪ B includes this information in the CTS packet ▪ C, when it overhears the CTS packet, retrieves the length information and uses it to set the inhibition time CS5602: Principles and Techniques for Sensors and Information Perception 36
Exposed Terminal problem RTS RTS B Tx not cTS ℃ annot hear CT5 inhibited B sends rts to a (overheard by c) A sends cts to B C cannot hear As cts C assumes a is either down or out of range c does not inhibit its transmissions to D CS5602: Principles and Techniques for Sensors and Information Perception
Exposed Terminal Problem ▪ B sends RTS to A (overheard by C) ▪ A sends CTS to B ▪ C cannot hear A’s CTS ▪ C assumes A is either down or out of range ▪ C does not inhibit its transmissions to D CS5602: Principles and Techniques for Sensors and Information Perception 37 A B C D RTS RTS CTS Cannot hear CTS Tx not inhibited
Collisions Still possible -RTs packets can collide Binary exponential backoff performed by stations that experience Rts collisions RTS collisions not as bad as data collisions in CSMA (since RTS packets are typically much smaller than DATA packets for traditional wireless networks! CS5602: Principles and Techniques for Sensors and Information Perception
Collisions ▪ Still possible – RTS packets can collide! ▪ Binary exponential backoff performed by stations that experience RTS collisions ▪ RTS collisions not as bad as data collisions in CSMA (since RTS packets are typically much smaller than DATA packets) ▪ for traditional wireless networks! CS5602: Principles and Techniques for Sensors and Information Perception 38
DEWMAC-DCF CSMA/CA with RTS/CTs (Network Allocation Vector(NAV)) Both Physical Carrier Sensing and Virtual Carrier Sensing used in 802.11 If either function indicates that the medium is busy 802.11 treats the channel to be busy Virtual Carrier Sensing is provided by the nav (Network Allocation Vector) CS5602: Principles and Techniques for Sensors and Information Perception
DFWMAC-DCF CSMA/CA with RTS/CTS (Network Allocation Vector (NAV)) ▪ Both Physical Carrier Sensing and Virtual Carrier Sensing used in 802.11 ▪ If either function indicates that the medium is busy, 802.11 treats the channel to be busy ▪ Virtual Carrier Sensing is provided by the NAV (Network Allocation Vector) CS5602: Principles and Techniques for Sensors and Information Perception 39
DEWMAC-DCF CSMA/CA with RTS/CTs (Network Allocation Vector(NAV)) Most 802. 11 frames carry a duration field which is used to reserve the medium for a fixed time period Tx sets the nav to the time for which it expects to use the medium Other stations start counting down from Nav to o As long as NAv>0, the medium is busy CS5602: Principles and Techniques for Sensors and Information Perception
DFWMAC-DCF CSMA/CA with RTS/CTS (Network Allocation Vector (NAV)) ▪ Most 802.11 frames carry a duration field which is used to reserve the medium for a fixed time period ▪ Tx sets the NAV to the time for which it expects to use the medium ▪ Other stations start counting down from NAV to 0 ▪ As long as NAV > 0, the medium is busy CS5602: Principles and Techniques for Sensors and Information Perception 40