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in Computer Networks by (user.guest)

Suppose there are two ISPs providing WiFi access in a particular café, with each ISP operating its own AP and having its own IP address block. 

a. Further suppose that by accident, each ISP has configured its AP to operate over channel 11. Will the 802.11 protocol completely break down in this situation? Discuss what happens when two stations, each associated with a different ISP, attempt to transmit at the same time. 

b. Now suppose that one AP operates over channel 1 and the other over channel 11. How do your answers change?

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Best answer

a.

The 802.11 protocol does not break down completely even though the two ISPs configured their APs to operate over the same channel.

• The two APs will have different SSIDs and MAC addresses and one of the SSISs will be assigned to the wireless station that connects cafe.

• A virtual link will be maintained between the new station and the AP.

• Consider that the two Aps be AP1 and AP2. Also, consider that the new station is associated with AP1.

• A frame that is sent by the new station will be addressed to AP1.

• The frame will also be received by AP2. As the frame is not addressed to AP2, AP2 will not process the frame.

• The two APs can work in parallel over the same channel.

• The transmission rate may be degraded as the two stations are transmitting over the same channel.

When the two stations, each associated with a different ISP, attempt to transmit at the same time, collision (due to interference) will occur, because both stations transmitting over the same channel and at the same time.

The two stations, each associated with a different ISP, can transmit data at the same time without collisions.

• The performance will be upgraded when one AP operates over channel 1 and the other over channel 11.

• Both APs are using non-overlapping channels.

• The two stations are transmitting over different channels.

• Therefore, the aggregate transmission rate will be 22 Mbps.


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