4. (WiFi) (a) Why cannot the CSMA/CD work for the WiFi? (b) What is the hidden t

Question

4. (WiFi) (a) Why cannot the CSMA/CD work for the WiFi? (b) What is the hidden terminal problem? (c) How can we resolve the hidden terminal problem? (d) Suppose that the back-off value is always chosen from 0, 1). Let X(t) be the backoff value at time t. Draw the transition diagram of X(t) (e) Following the above question and consider three nodes are using the wireless medium. What is the steady-state distribution of X (t)? (f) Following the above question, what is the successful transmission when a packet is transmitted?

Explanation / Answer

a)Answer:

Wireless transceivers can't send and receive on the same channel at the same time, so they can't detect collisions. This is due to the fact that there's an incredible difference between send power (generally around 100mw) and receive sensitivity (commonly around 0.01 to 0.0001mw). The sending would cover up any possible chance of receiving a foreign signal, no chance of "Collision Detection". For this reason Collision Avoidance with Control Messages is necessary.

On most wired networks the (like Ethernet) the voltage is around 1 to 2.5v; both sending and receiving are roughly the same voltage. So if you're sending a 2.5v signal, and someone else collides with a -2.5v signal, the "Detection" parts will see a signal somewhere around 0v and know a collision occurred.

b)Answer:

In wireless networking, the hidden node problem orhidden terminal problem occurs when a node is visible from a wireless access point (AP), but not from other nodes communicating with that AP.

If B and D transmit to C at the same time; C will experience interference.

In wireless networking, the hidden node problem or hidden terminal problem occurs when a node is visible from a wireless access point (AP), but not from other nodes communicating with said AP. This leads to difficulties in media access control.
c)Answer:

In a wireless network, it is likely that the node at the far edge of the access point's range, which is known as A, can see the access point, but it is unlikely that the same node can see a node on the opposite end of the access point's range, B. These nodes are known as hidden. The problem is when nodes A and B start to send packets simultaneously to the access point. Since node A and B can not sense the carrier, Carrier sense multiple access with collision avoidance (CSMA/CA) does not work, and collisions occur, scrambling data. To overcome this problem, handshaking is implemented in conjunction with the CSMA/CA scheme. The same problem exists in a MANET.

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