As with the RTS frame, all other participants in the range are informed that the transmission is currently occupied and the transmitter is enabled for transmission. Only then does the original device start transmitting the data. Now it is not possible for the participants in a wireless network to detect collisions or other interference during transmission.
For this reason, the receiving station needs to send an acknowledgement ACK when the data packet has arrived correctly. The three frame types each consist of several fields. However, the sender field is only of interest when first contacting the recipient, so that the recipient knows which node he is now communicating with. It is still likely that two participants will send a request to send to the same station at the same time. In cases like this, however, the receiver does not send a CTS frame because the RTS frames have not arrived correctly.
The situation is initially the same as with the hidden station problem: one station is in the middle of two other stations so that they cannot reach each other. One of the two devices now wants to send data to the station in the middle. All accessible nodes receive the CTS frame that stops them from broadcasting. This avoids the hidden station problem, but creates a new one. A third station is now prevented from transmitting, even if a completely different, fourth station would have been the destination of the transmission.
This transmission would not cause a collision, but it still has to be prevented, which leads to the entire network slowing down. Before a device in the network starts a transmission, it first sends information in the duration field of the RTS frame to all other participants.
The station reveals how long the network will be occupied by the transmission. Every other device enters this information in its very personal network allocation vector which is not really a vector from a mathematical point of view.
This is managed internally and specifies the time when a delivery attempt is possible again. The network allocation vector NAV counts down continuously and is only replenished by new information from other stations. A NAV can increase the timer by a maximum of 33ms This is the maximum duration for which a transmitter may block the medium. The devices in the network are inactive while the network allocation vector has not yet expired. This saves energy. Only when the counter is set to 0 does the subscriber become active again and check the network.
The latter is the signal for all participants to reset the NVA to 0, when the medium is free again. If participants in a wireless network follow Carrier Sense Multiple Access with Collision Avoidance, certain steps must be adhered to. First, the stations monitor the transmission medium. When it comes to WLAN, this means that carrier sense monitors the radio channel and checks whether other network participants — as long as they are visible to the respective device — are currently transmitting.
If it turns out that the transmission medium is currently occupied, a random backoff is initiated: the station waits a random amount of time until a new check starts. All other stations, which are not busy with sending or receiving, experience the same. The random waiting time ensures that the participants do not start to check the network at the same time and cannot start transmitting data at the same time.
However, this only happens if the station is not already aware that the medium is occupied due to the network allocation vector NAV. If the network is free, the station initiates DCF. First, the channel is checked more thoroughly for the duration of DIFS. If the request to send has successfully arrived at the receiver and no collision has occurred, the sender receives permission from the CTS frame to occupy the transmission medium. As this happens, all other participants are informed that the network is currently in use.
This causes them to raise their network allocation vector again and wait to try again to see if the channel is free. Then the station starts the transmission. Python Pillow.
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Discrete Mathematics. Ethical Hacking. Computer Graphics. Software Engineering. Sign up. Term of the Day. Best of Techopedia weekly. News and Special Offers occasional. This is to listen for any collision scenarios in the network and to inform other devices not to transmit. If the broadcast channel is detected as "idle," the node can then start transmitting the data packet. If the broadcast channel is detected as "busy," the node holds the transmission, waits for a random time frame and then checks all over again to find out whether the channel is free.
This time frame is referred to as the backoff factor. The backoff factor is counted down using a backoff counter. If the channel is free when the backoff counter gets to zero, the node sends the data packet.
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