RTS/CTS-Induced Congestion in Ad Hoc Wireless LANs Saikat Ray,Jeffrey B. Carruthers and David Starobinski WCNC 2003

Outline  Introduction  RTS/CTS-Induced Congestion  Solution to False Blocking ： RTS Validation  Simulation  Summary

Introduction  The performance of a wireless network critically depends upon the MAC protocol used CSMA Hidden Terminal Hidden Terminal RTS/CTS

Introduction  In ad hoc network RTS/CTS mechanism leads to network-level congestion The blocking problem

RTS/CTS-Induced Congestion RTS CTS DATA ACK IEEE A B C D

RTS/CTS-Induced Congestion The Blocking Problem AB C D RTS Blocked

RTS/CTS-Induced Congestion The False Blocking and its Propagation AB C D RTS Blocked E F G

RTS/CTS-Induced Congestion Data Blocked by A Blocked by c A B CD E F G RTS Blocked by E RTS In the cycle ： {A, B, C, D, E, F, A} Sender ： {A, C, E} Receiver ： {B, D, F} In the cycle ： {A, B, C, D, E, F, A} Sender ： {A, C, E} Receiver ： {B, D, F} Pseudo-Deadlock

Solution to False Blocking ： RTS Validation  RTS Validation Upon overhearing an RTS packet Deferring until the corresponding DATA packet transmission is expected to begin (RTS_Defer_Time) And then assesses the sate of the channel (Clear-Channel Assessment Time)

RTS Validation  The nod senses busy channel following RTS_Defer_Time and therefore Continues deferral (Requested_Defer_Time) RTSCTS SIFS Data Carrier Sense Defer

RTS Validation  The node assesses idle channel following RTS_Defer_Time and therefore defer no longer RTSCTS SIFS Carrier Sense Defer

Solution to False Blocking ： RTS Validation  RTS Validation is a backward-compatible approach the intelligent nodes (using RTS Validation nodes) may be able to transmit more packet Since they defer for a much smaller time in the case of false blocking

Simulation

Conclusion  RTS/CTS mechanism may congest a network instead of stabilizing it  Using of RTS Validation improves the network performance in three aspects It eliminates congestion by stabilizing the network throughout at high load It increase the peak throughout by 60% It significantly reduce the average delay