Question

Using RTS/CTS mechanism suppose there are two ISPs providing WiFi access in the same café, with each ISP operating its own AP.

a. If each ISP has configured its AP to operate over the same channel, channel 11. Will the 802.11 protocol completely break down in this situation? Discuss what occurs when two devices, each associated with a different AP, attempt to transmit at the same time.

b. Suppose now one AP changes its channel to channel 1, and the other AP still operates over channel 11. How do you answer the change?

Answer 1

CONSIDERING THE CONDITIONS AND REQUIREMENTS FROM THE QUESTION.

A)

• If it is wifi area, AP's that are overlap on certain channels. Basically the network traffic contains the destination MAC sent from your computer, so only the appropriate router will respond. It will still work, although you might experience some performance degradation, depending on how many AP's are using that channel.
• The two APs will typically have different SSIDs and MAC addresses. A wireless station arriving to the café will associate with one of the SSIDs. After association, there is a virtual link between the new station and the AP.
• Label the APs AP1 and AP2. Suppose the new station associates with AP1. When the new station sends a frame, it will be addressed to AP1. Although AP2 will also receive the frame, it will not process the frame because the frame is not addressed to it.
• Thus, the two ISPs can work in parallel over the same channel. However, the two ISPs will be sharing the same wireless bandwidth.
• If wireless stations in different ISPs transmit at the same time, there will be a collision.
• For 802.11b, the maximum aggregate transmission rate for the two ISPs is 11 Mbps.

B)

• If you separate the channels of the AP's, performance and reliability will improve slightly, as there's no additional data to parse for the router on the same channel.
• Now if two wireless stations in different ISPs transmit at the same time, there will not be a collision.

Thus, the maximum aggregate transmission rate for the two ISPs is 22 Mbps for 802.11b.

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