The protocol used by the mac layer of the 802.11b standard of the wifi network is the CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) protocol.(Recommended learning:phpstorm)
Carrier Sense Multiple Access/Collision Detection (CSMA/CD): In CSMA, due to the propagation delay of the channel , conflicts can still occur when two stations send frames when they hear that there is no signal present on the bus. Since the CSMA algorithm does not have a conflict detection function, even if a conflict occurs, the damaged frames still have to be sent, which reduces the bus utilization.
An improved solution for CSMA is to make the station continue to monitor the media during transmission time. Once a conflict is detected, it immediately stops sending and sends a series of short blocking messages (Jam) to the bus to notify the bus. Conflicts have occurred on each station, so that the channel capacity will not be wasted by transmitting damaged frames in vain, and the bus utilization can be improved. This is called the Carrier Sense Multiple Access/Conflict Detection Protocol, abbreviated as CSMA/CD. This protocol has been widely used in Ethernet and IEEE802.3 standards.
At this point, the wasted bandwidth is reduced to the time spent detecting conflicts.
So, how to estimate the required conflict detection time?
For the baseband bus, the time used to detect a conflict at this time is equal to any two Twice the maximum propagation delay between stations, so for baseband CSMA/CD, it is required that the packet length should be at least twice the propagation delay, otherwise the transmission has been completed before a collision is detected, but the packet is actually destroyed by the collision.
Application: CSMA/CD uses a contention method to determine access rights to the medium. This contention protocol is generally used in bus networks.
Carrier Sense Multiple Access (CSMA) development and existing problems:
A key technical issue in the CSMA/CD bus network is the problem of conflict control or conflict resolution , that is, packets that are collided due to transmission conflicts must be retransmitted after a random delay. Typical conflict control algorithms, that is, backoff algorithms, have the following five types: binary exponential backoff algorithm BEB, polynomial backoff algorithm PB, and linear value-added backoff Algorithm LIB, fixed average backoff algorithm FMB, sequential backoff algorithm OB.
The so-called conflict means that if there are two or more workstations on the network sending data at the same time, a mixture of signals will occur on the bus. If any workstation sends data at the same time, there will be a problem on the bus. This creates a mixture of signals, and no workstation can tell what the real data is. This situation is called data conflict or collision.
In order to reduce the impact of conflicts after they occur. During the process of sending data, the workstation must constantly check whether the data it sends conflicts with the data of other workstations during the transmission process. This is collision detection.
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