2) CWNPCert: Wireless LAN Administration

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Objective: 802.11 Regulations and Standards
SubObjective: Spread Spectrum Technologies

 

Single Answer Multiple Choice

 

Given: 802.11a WLANs use Orthogonal Frequency Division Multiplexing (OFDM) encoding. Which of the following statements regarding OFDM operation in 802.11a WLANS is NOT true?

 

A. 24 sub-carriers are used as parallel symbol transmission paths.
B. OFDM exhibits better tolerance for delay spread than Direct Sequence Spread Spectrum (DSSS) encoding.
C. OFDM transmissions in UNI-1 band are limited to 40 mW.
D. Four pilot or sub-carriers are used to monitor the channel.
E. 64QAM modulation is used at the 54 mbps data rate.

 

Answer:
A. 24 sub-carriers are used as parallel symbol transmission paths.

 

Tutorial:
There are 48 sub-carriers, not 24, used as parallel symbol transmission paths. This is how OFDM achieves the better data rates.

 

All other statements are true.

 

OFDM exhibits better tolerance for delay spread than DSSS encoding.

 

According to the United States Federal Communication Commission (FCC) regulations, OFDM transmissions in the UNI-1 band are limited to 40 mW.

 

Of 52 sub-carriers, four are used as a reference to monitor the channel.

 

OFDM uses DBPSK at speeds of 6 and 9 Mbps, DQPSK at speeds of 12 and 18 Mbps, 16QAM at speeds of 24 and 36 Mbps, and 64QAM at speeds of 48 and 54 Mbps.

 

Reference: CWNA Study Guide Third Edition, pp. 199-202.

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