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Authors:
James F. Kurose, Keith W. Ross
Chapter:
Wireless And Mobile Networks
Exercise:
Problems
Question:5 | ISBN:9780132856201 | Edition: 6

Question

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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Answer

a) 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.
b) 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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