Suppose there are two ISPs providing WiFi access in a particular coffee shop, wi

Question

Suppose there are two ISPs providing WiFi access in a particular coffee shop, with each ISP operating its own AP and having its own IP address block. Also suppose that all wireless stations are connected to the APs using 802.11b.

a) Suppose that by accident, each ISP has configured its AP to operate over channel 11. 1. Will the 802.11 protocol completely break down in this situation? 2. Discuss what happens when two stations, each associated with a different ISP, attempt to transmit at the same time? What is the aggregate transmission rate of both stations in this case?

b) Now suppose that one AP operates over channel 1 and the other over channel 11. Discuss what happens when two stations, each associated with a different ISP, attempt to transmit at the same time? What is the aggregate transmission rate of both stations in this case?

Explanation / Answer

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 (that is, one of the APs). 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.
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Now if two wireless stations in different ISPs (and hence different channels) 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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