Computer Network | Collision Avoidance in wireless networks

We take a close look at so-called WiFi which is also known as IEEE standard 802.11

Consider the situation depicted in the figure, where each of four nodes is able to send and receive signals that reach just the nodes to its immediate left and right.
For example, B can exchange frames with A and C but it cannot reach D, while C can reach B and D but not A. (A and D’s reach is not shown in the figure.) Suppose both A and C want to communicate with B and so they each send it a frame. A and C are unaware of each other since their signals do not carry that far. These two frames collide with each other at B, but unlike an Ethernet, neither A nor C is aware of this collision. A and C are said to be hidden nodes with respect to each other.

According to Wikipedia, the hidden node problem can be defined as “In wireless networking, the hidden node problem or hidden terminal problem occurs when a node is visible to a wireless access point (AP), but not to other nodes communicating with that AP.”

Collision cannot be detected in hidden node problem

This is because the nodes A and C are out of range of each other(and so cannot detect a collision while transmitting). Thus, Carrier sense multiple access with collision detection (CSMA/CD) does not work, and collisions occur. The data received by the access point is corrupted due to the collision. To overcome the hidden node problem, RTS/CTS handshaking (IEEE 802.11 RTS/CTS) is implemented in addition to the Carrier sense multiple access with collision avoidance (CSMA/CA) scheme.



A related problem, called the exposed node problem, occurs under the following stated circumstances:

Suppose B is sending to A (as in the above Figure). Node C is aware of this communication because it hears B’s transmission. It would be a mistake for C to conclude that it cannot transmit to anyone just because it can hear B’s transmission.
For example, suppose C wants to transmit to node D. This is not a problem since C’s transmission to D will not interfere with A’s ability to receive from B.

We address these problems by an algorithm known as Multiple Access with Collision Avoidance (MACA). The sender and receiver exchange frames with each other before transmitting data. This informs all nearby nodes that a transmission is about to begin. Sender transmits Request to Send (RTS) frame to receiver . The receiver then replies with clear to send (CTS) frame back to the sender. Any node that receives CTS frame knows that it is close to the receiver, therefore, cannot transmit a frame. Any node that receives RTS frame but not the CTS frame knows that is not close to the receiver to interfere with it, So it is free to transmit data.

Reference –
Wikipedia

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