What is CSMA/CA protocol? 21.5




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21.4
What is CSMA/CA protocol?
21.5
What are the hidden and exposed node problems in a WLAN? Discuss 
them briefl y.
21.6
What is a Wi-Fi? Discuss briefl y.
21.7
What is IEEE 802.11n? Discuss briefl y.
21.8
Discuss the exponential backoff algorithm used in a WLAN.
21.9
What is the HiSWAN? Discuss briefl y.
21.10
In the HIPERLAN/2 PHY layer, 64-subchannel implementation is used. 
48 subcarriers are used for information transmission, 4 subcarriers for 
pilot tones used for synchronization, and 12 subcarriers reserved for 
other purposes. The HIPERLAN/2 standard uses 16-QAM with 
R
9/16 
coding for an actual payload data transmission rate of 27 Mbps. Calcu-
late the coded symbol transmission rate per subcarrier. What is the bit 
transmission rate per subcarrier?
21.11
In a DSSS WLAN how many simultaneous data users at 38.4 kbps can 
be supported by an AP so that an average bit error data rate of 10
3
is maintained for each user? Assume the following data: spreading 
rate 
3.84 Mcps, power control effi ciency 
0.8, interference factor from 
other terminals 
0.67, 3-sector antenna gain 
2.55, and cell loading 
70%. Assuming that each user transmits on average 8 packets per second, 
what is the average packet length that can be used in the system?
21.12
An FHSS WLAN system uses a 50 kHz channel over a continuous 
20 MHz spectrum. Fast frequency hopping is used, in which two hops 
occur for each bit. If binary frequency shift keying (BFSK) is the modula-
tion scheme used in the system, determine (a) number of hops per second 
if each user transmits 25 kbps data, and (b) probability of error for a 
single user operating at 
E
b
/
N
0
equal to 10 dB.
21.13
If we switch from 36 Mbps mode to 9 Mbps mode in WLAN, how much 
more (in dB) of the path loss can be afforded? If the system covers 50 m 
with 36 Mbps, what would be the coverage with 9 Mbps?
21.14
Consider the Bluetooth and IEEE 802.11 FH interference scenario (refer to 
Figure 21.20). If the acceptable error probability for the mobile terminal 
Ch21-P373580.indd 774
5/3/07 10:58:53 PM

is 10
4
, fi nd (a) (SIR)
min
that supports this error rate, and (b) calculate 
r
max
for 
d
12 m, 
4.3, 
P
BT
23 dBm and 
P
AP
43 dBm. What is 
r
max
if the IEEE 802.11 FH device is replaced by the IEEE 802.11 DS 
device?
21.15
Find the collision probability of 1200 bytes IEEE 802.11 FH packet at 
2 Mbps and the Bluetooth. Assume dwell period of an 802.11 transmis-
sion to be 3 ms and Bluetooth packet duration 0.625 ms. What is PER 
with IEEE 802.11 DS?

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