Q1.
Two 802.11 stations, Station 1 and Station 2, get frames to transmit at time t=0. The 3rd station (AP) has just finished transmitting a long frame at t=0 to Station 1. The transmission parameters are: Slot time=1 and SIFS=1. Assume that the pseudo-random number generated are 1 (Station 1) and 3 (Station 2). The data size for both stations is 3 slots and each ACK, RTS, and CTS transmission takes a single slot to complete. At what times Station 1 and Station 2 will receive their acknowledgments assuming no new arrivals? You may want to draw a transmission diagram to accurately trace the timing of all transmissions in the network.
Q2.
Although a total of 14 22-MHz channels are defined for 2.4 GHz DSSS WLANs, the 14th channel is not always available. The first 13 channels follow the 5 MHz channel spacing for the centre frequency (starting from 2412) with 11 MHz assigned on both sides of the centre frequency. If we consider the first 13 channels, a maximum of three nonoverlapping channels exist. (1, 6, 11) is an example of a set of three non-overlapping channels. Can you identify another set of three non-overlapping channels among the first 13 channels? How many combinations of three non-overlapping channels are possible among the first 13 channels?
Q3.
How many successive unsuccessful transmission attempts are required for the Congestion Window (CW) variable to reach its maximum value in an 802.11n WLAN operating in the 5 GHz band? Assume the initial value CW = CWmin.
Q4.
Consider an 802.11a WLAN. A station estimates the transmission times of RTS, CTS, and
ACK as 16 s, 16 s, and 25 s, respectively. After receiving the RTS, the AP generates a CTS. What would be the value of the Duration field in the CTS header if the station wanted to send a 250 s long data frame?
Q5
Consider the example WLAN shown in Figure 1 where two BSSs are connected via a distribution system. What are the contents of the four address fields when AP X wants to forward a frame, which it received earlier from Station A with Station B as its intended destination, to AP Y?
Mahbub Hassan 2017
Figure 1 An WLAN distribution system connecting two BSSs (figure for Q5)
Q6
A WLAN standard is employing a spread spectrum coding with only 1/2 rate, which produces chips at a rate of 1/2 chips per Hz. It uses 8 chips to code a symbol. To achieve a data rate of 11 Mbps for a 22 MHz channel, what level of QAM is needed to modulate the signal?
Q7
The original OFDM for 802.11a-1999 has a 3200ns data pulse, but the symbol length is extended by another 800 ns guard interval to cater for multi-path delay spread. If the guard interval is reduced by half for a low-spread environment, what will be the increase in symbol rate?
Q8
802.11a-1999 supports 8 data rates, ranging from 6 Mbps to 54 Mbps. What data rate could be achieved if 256 QAM was used with a coding rate of 5/6?
Q9
To improve the system capacity of a WLAN installment, an access point is fitted with 4 antennas. There are a total of four UEs that connect to this WLAN. Now consider the following two scenarios. In Scenario 1, each UE has four antennas. In Scenario 2, each UE has 2 antennas. For which scenario multi-user MIMO will be capable of achieving higher capacity compared to the single-user MIMO and why?
Q10
Channel bonding helps achieve wider bandwidth by combining adjacent channels into a single channel, which in turn helps increase the data rate. Can you think of any drawbacks for channel bonding (Hint: why do we have channels in the first place?)
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