1 answer

Q1. [6 points) Suppose that you have a Wi-Fi network whose MAC layers use CSMA/CA with...

Question:

Q1. [6 points) Suppose that you have a Wi-Fi network whose MAC layers use CSMA/CA with RTS/CTS. If you know that the SIFS is

Q1. [6 points) Suppose that you have a Wi-Fi network whose MAC layers use CSMA/CA with RTS/CTS. If you know that the SIFS is u, DIFS is 3 u, propagation delay is 2u, and each of RTS and CTS is 5 u, respectively, where u is a fixed period of time. Answer the following: a) Calculate the earliest time for the receiver to send the CTS message? b) If the length of the data packet is 1000 u, calculate the shortest time for the receiver to send the ACK signal? c) Can you make SIFS = 0? Justify your answer.

Answers

The communication in above system like this:

1. Sener wait for DIFS(Distributed Inter-frame Space) before sending an RTS packet.
1. The sender send a RTS (Request to Send) Packet.
2. After receiving the RTS packet, the receiver waits for SIFS(Short Inter-frame Space).
3. Then the receiver sends a CTS(Clear to Send) packet to the sender if it is ready to receive
4.

After the sender receives the CTS packet, it waits for SIFS and then sends the data to the receiver.
5. After receiving the data, the receiver waits for SIFS and then sends an ACK(acknowledgment) for the data.


Now we will answer the given questions:

a. The earliest time it will take the receiver to send a CTS message = DIFS + RTS + Time to propagate RTS + SIFS
= 3μ + 5μ + 2μ + μ = 11μ
So, after 11μ, receiver will send first CTS packet

b. Shortest time for the receiver to send ACK signal = DIFS + RTS + Time to propagate RTS + SIFS + CTS + Time to propagate CTS + SIFS + Length of Data + Time to propagate Data + SIFS

= 3μ + 5μ + 2μ + μ + 5μ + 2μ + μ + 1000μ + 2μ + μ = 1022μ
After getting the data and SIFS, the receiver sends ACK. In this case after 1022μ.


c.

SIFS cannot be zero. It is time required by the node to convert from receiving mode to sender mode. So, this time should be greater than 0.

.

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