Question

1. (Important: Assume RTS/CTS mechanism is used) 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?

2. In IEEE 802.15.4, when BI=SD in superframe, what is the duty cycle? Can you achieve a duty cycle of 1%? And how?

Answer 1

Solution:
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.

2)duty cycle =BI/SD

So we achieve1 %