Hidden Problems with the Hidden Node Problem Ashikur Rahman Department of Computing Science University of Alberta Edmonton, Alberta, Canada, T6G 2E8 Email: ashikur@cs.ualberta.ca Pawel Gburzynski Department of Computing Science University of Alberta Edmonton, Alberta, Canada, T6G 2E8 Email: pawel@cs.ualberta.ca Abstract— We discuss a few problems introduced by the RTS/CTS mechanism of collision avoidance and focus on the virtual jamming problem, which allows a malicious node to effectively jam a large fragment of a wireless network at a minimum expense of power. We propose a solution to this problem and provide experimental data illustrating the impact of virtual jamming and the effectiveness of our proposed solution. I. THE HIDDEN NODE PROBLEM The notorious hidden node problem deals with a configu- ration of three nodes, like A, B, and C in Figure 1, whereby B is within the transmission range of A and C, while C is outside the range of A. In a situation like this, C will not be able to detect the ongoing transmission of A to B by carrier sensing and, consequently, it can inadvertently interfere with B’s reception of A’s packet. The transmission range of a node A is defined as the area inside which other nodes are able to correctly receive A’s packets. On the other hand, the carrier sense range of A is the area encompassing those nodes whose transmission A can perceive (carrier sense) while not necessarily being able to receive the transmitted packets. Generally, it is un- reasonable to assume that the two areas are always the same, e.g., the carrier sense range can be twice the transmission range [7]. Suppose that every node in Figure 1 has the same trans- mission range (represented by a solid circle). Node C is out of the transmission range of node A and thus would appear as a hidden node to A. However, if the carrier sense range of C is larger than the transmission range of A (see the dashed circle), C is hidden no more because it can sense the transmission of A and thus avoid interfering with it. This mechanism for eliminating the hidden node problem has been described in [7]. Carrier detection is usually controlled by thresholds ap- plied to the level of (actualy or apparently) perceived signal. Low thresholds tend to be sensitive to many factors involving more than the distance between the nodes, e.g., the natural noise level in the neighborhood. While formally, increasing the carrier sense range is possible, low thresholds may trigger many false indications, which will result in unneces- Carrier sense range A B C Transmission range Fig. 1. A hidden node scenario. sary back-offs and reduced throughput, possibly below one that could be achieved by simply ignoring the hidden node problem altogether. To alleviate the hidden node problem, Karn [5] proposed a two-way handshake involving short packets whose exchange should precede the actual transmission. The sender starts by transmitting a Request-To-Send (RTS) packet. After receiving RTS, the intended recipient sends a Clear-To-Send (CTS) packet to the sender. Both packets specify the length of time needed to transmit the actual data packet. Any third party node receiving any of the two packets will know for how long it should refrain from transmission as to avoid interfering with the exchange in progress. This protocol has been standardized into the popular IEEE 802.11 family of access schemes. The complete exchange involves four packets: RTS/CTS/DATA/ACK, with the first pair taking care of the hidden nodes, and the final ACK providing for reliable delivery (triggering retransmissions on failures). II. PROBLEMS WITH THE RTS-CTS HANDSHAKE Consider Figure 2. Suppose that node A is the sender and node B is the receiver. Let R A and R B denote the respective transmission ranges of A and B, and P A and P B denote the corresponding transmission areas, i.e., the sets of points of the rectangle U covered by the circles with the radii R A and R B drawn around the nodes. Assume that any node v in U is painted with one of the following colors: green, if v ∈ P A - P B ; red, if v ∈ P B - P A ; yellow, if v ∈ P A ∩ P B ; and white, if v ∈ (P A ∪ P B ) ′ .