RTS-CTS Mechanism in 802.11 Wireless Networks - 11/01/2002

Saikat Ray

In wireless networks, packet collisions due to hidden nodes is a primary issue; especially in indoor environments where hidden nodes can form easily due to the presence of obstacles. Therefore, in random medium access schemes, a mechanism called RTS-CTS handshake is considered to be essential for mitigating the effects of hidden nodes. RTS-CTS mechanism attempts to prevent packet collisions by prohibiting nodes from transmitting. Due to this, however, RTS-CTS mechanism creates interesting interdependencies. In this talk, we will describe such interdependencies and their effects. In particular, we will show that RTS-CTS mechanism may unnecessarily "block" nodes from transmitting. This effect of false blocking can in fact propagate through the entire network creating a big chain of blocked nodes. If this "chain" is closed due to any reason, we would observe a lock. Although such locks are not necesarily permament, it may take long for the nodes to come out of the lock. All these effects together affects network efficiency significantly. In particular, my simulation results show that this may create congestion in the network, where the throughput decreases with increasing load after a certain point. We will also present a simple backward-compatible modification to the RTS-CTS mechanism that increases the peak throughput of the network by 60% in addition to removing congestion. We will conclude the talk by discussing some of the future directions of this work.