Essay Example on CSMA CD shortcut for Carrier Sense Multiple Access Collision

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Detect it is the protocol for carrier transmission access in Ethernet networks Moreover how network devices respond is defined by CSMA CD basically when two devices attempt to use a data channel at the same time and come across a data collision After that each device will wait for random time and will try again until it is transmission sent successfully Furthermore the CSMA CD method is specified in IEEE 802 3 and ISO 8802 3 standard The next figure shows the process of CSMA CD method The best example for CSMA CD is to assume that there are many people in a meeting having a conversation Each one has a chance to speak while the others listen to the one whose speaking and waiting for their opportunity to speak This resembles the Carrier Sense Multiple Access CSMA which is a protocol of a network that senses or listens to the network signal over a medium before any data is transmitted Once the speaker is over a moment of silence takes place After that other or others will try to speak and if more than one person speak at the same time collision will occur As a result one is allowed to speak while others are silenced this is known as collision detection CD However CSMA CD is not suitable for Wi Fi because Wi Fi networks have hidden node problem which is basically as shown in figure 2 assuming B is the Access Point station

A is able to transmit information to station B AP and B AP can transmit information to A as well However stations A and C are hidden from each other so they cannot transmit any information between them As a result Carrier Sense Multiple Access Collision Detect CSMA CD will not be able to work in this case and collisions will happen Therefore the best solution is to use Carrier Sense Access Collision Avoidance CSMA CA CSMA CA shortcut for Carrier Sense Multiple Access Collision Avoidance it is the protocol for carrier transmission access in Wi Fi networks It checks or listen to the medium before a node transmits data the node send it s signal when the medium is not busy After that it will back off and wait for a random amount of time and try again if it detects the medium is used Furthermore CSMA CA tries to prevent collisions before they happen unlike CSMA CD which deals with transmissions after a collision has occurred While the best example to understand how Carrier Sense Access Collision Avoidance CSMA CA works is to assume that there is a lecture Once the lecturer is over the lecturer will ask the student if there are any questions regarding the lecture Where all the student getting prepared to ask some question and this known as Multiple Access MA Then one of the student will look right and left to see if anyone is going to ask question and this is knowns as Carrier Sense When there is one student wants to ask question the rest are waiting for question to be asked this is known as Collision Avoidance CA Figure 4 shows CSMA CA mechanism RTS STC is a shortcut for Request to Send Clear to Send It is an optional method in Carrier sense multiple access Collision Avoidance


CSMA CA to solve the hidden node problem It minimises the frame collisions in hidden stations figure 3 will be used again to explain the methodology of how RTS STC in CSMA CA solves the hidden node problem Whenever a node wants to send any data to another node it takes the permission from the receiver instead of acting as a sensor After that to have the permission there is a particular data known as RTS Request to Send that has to be sent For example in the scenario where node A wants to send data to node B in figure 3 it would start by sending an RTS packet to node B Node B will receive the packet and in the meantime no one is transferring any data to node B Next node B will give the permission for transmitting by sending a special packet known as CTS Clear to Send which contains the duration of this transmission The previously mentioned packet will be sent as a broadcast to both nodes A and C When node A receives it it will find out that the permission is granted and it will start transmission directly However when node C receive the broadcast it would know that a permission has been granted and there is a transmission taking place right now Node C will check the duration of that transmission between nodes B and A Consequently node C will not transmit any data Therefore instead of collision detection collision avoidance is used here and this is how this hidden terminal problem is solved Finally if there are a lot of collisions as a result of high utilisation and large frames the same principle can be applied Figure 5 gives data on the process of RTS CTS in CSMA CD 802 11 uses two of Carrier Sense CS mechanisms to estimate a clean channel One of them is Virtual Carrier Sense Mechanism it is referring to Network Allocation Vector NAV It used for maintaining the prediction of the future of traffic on the medium based on duration information that is revealed in RTS CTS frames from the duration field presented in the frames or prior to the actual exchange of data that are transmitted by STAs Figure 7 shows the Virtual carrier sense mechanism


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