How 802.11 LAN will avoid packet collision in a wireless medium?

Question

Say we have a router (capable of the 802.11 protocol standards) and some number of devices that are configured to communicate with this router. I have learnt about how devices/nodes use CSMA/CA to transmit data, and other mechanisms such as RTS/CTS & NAV, to help prevent collisions in the wireless medium.

However,Since it's a wireless medium each packet will reach all other surrounding devices. So if my laptop is one of those devises, then will it intercept each and every packet that floats through the channel? or device to device communication will happen through different frequencies, so that other devices cant receive these signals?

If all devices use same frequency, then again there is a posibility of superimposition of these waves on each other and leading to packet distorion. How will it overcome this problem?

Answer 1

Physical carrier sensing is pretty much the only way to avoid collisions. Stations "listen" (sense the energy in the channel) before transmitting any signals. This is called clear channel assessment (CCA). When CCA fails i.e. when a transmission is made (such as in the case of hidden/exposed nodes) during a concurrent and existing transmission, a collision occurs. The CCA threshold is an important metric that can be fine-tuned in devices to adjust the degree up to which interference can be tolerated. In other words, even if a collision occurs, it is possible that the signal-to-interference-plus-noise ratio is sufficient to operate at lower data rates.

As you mentioned, the process of physical carrier sensing is eased via virtual carrier sensing (NAV). Using the NAV counter, a station is aware of the duration for which a transmission between some other stations will occur, and can 'back-off' without introducing the chance of a collision.

There is a large body of literature dealing with MIMO communications, wherein the spatial domain is exploited to transmit concurrent transmissions over the same frequency at the same time. This is a large topic in itself, and too broad to cover here.

Answer 2

Wi-Fi/802.11 uses CSMA/CA to avoid collisions as good as possible. RTS/CTS is an optional, additional method.

Note that not all frames reach all stations unless they're close together. Two stations spaced far enough apart can both communicate with the WAP in the middle, yet they can't hear each other (hidden station problem) - that's why having the WAP coordinate transmissions with RTS/CTS might be beneficial.

Generally, each Wi-Fi channel (bundle) is a half-duplex medium where only one station at a time may transmit, multiple simultaneous transmission attempts cause a collision. However, using beamforming - MU/MIMO - the WAP may be able to transmit multiple spatially separate beams to different stations at the same time.

Answer 3

collision cannot be detect in hide 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

Answer 4

packet collision is not possible 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