Suppose we only have three nodes (A B C) placed in a row, and each node\'s signa
ID: 3809639 • Letter: S
Question
Suppose we only have three nodes (A B C) placed in a row, and each node's signal can only transmit until its immediate neighbors (e.g., A's signal can only reach B and cannot reach C), as shown in the following figure. We assume that RTS/CTS is adopted and every RTS/CTS indicates the expected amount of transmission time. Every node restrains from sending or receiving when hearing RTS/CTS for the period of time indicated in the RTS/CTS packet. a) Can the hidden terminal problem be prevented? If yes, illustrate how it is prevented with an example. If not, give an example in which the hidden terminal problem occurs. b) If we assume that a node can send and receive at the, can the exposed terminal problem be prevented? If yes, illustrate how it is prevented with an example. If not, give an example in which the exposed terminal problem occurs.Explanation / Answer
Hidden terminal problem
Suppose A wants to send to B and at the same time C wants to send a data packet to B. Without RTS/CTS there would have been collision. But with RTS and CTS we can avoid data collision.
T1: A send RTS to B
T2: C send RTS to B
T3: B send CTS to A
(NOTE that CTS is also heard by node C, and becomes aware that C is not allowed to send data to B)
T4: A receives CTS
T5: A sends data to B without collision
NOTE: RTS/CTS reduces throughput
b)
Suppose B want to send a data packet to C and at the same time A wants to send data packet to B
T1: A sends RTS to B
T2: B sends RTS to C
(NOTE that A hears RTS that B wants to send to C, but C is unknown to A)
T3: B sends CTS to A
T4: C sends CTS to B
A sends data to B and B sends data to C simultaneously without having to wait for channel to be free
NOTE: T i happens before T i+1