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Quiz on IEEE 802.11a specifications

by Krishna Sankar on October 7, 2009

The IEEE 802.11a specifications are used by many to understand a wireless communication link built using OFDM. In this post, I have put together a set of 10 multiple choice questions based on 802.11a specifications. The questions are on the building blocks in 802.11a specifications, preamble structure and so on. Upon completion of the quiz, you will be lead to a page showing the correct answers and their explanations.

Click here to download IEEE 802.11a specifications.

Note: The quiz might not be visible on RSS reader or over email. Please visit the site to access the quiz.

Good luck!

The spacing between subcarriers in 802.11a specifications is






The duration of an OFDM symbol in 802.11a specification is






The coding scheme used in 802.11a specifications is






The number of used subcarriers in 802.11a specification is






The modulation used in SIGNAL field in 802.11a specification is






The acronym CCA in 802.11a specification stands for





The number of short preambles used in 802.11a specification is






The duration of the long preamble in 802.11a specification is






The number of TAIL bits in 802.11a specification is






The transmitter center frequency tolerance in 802.11a specification is








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{ 10 comments… read them below or add one }

mohan November 11, 2012 at 12:54 pm

Krishna,
In OFDM, why is the CRC8 or CRC16 or CRC32 is passed through an inverter ? I mean, after the CRC is calculated, before appending it to data the CRC’s one’s complement is calculated. Why not the CRC used as is ??

Thanks
Mohan

Reply

Krishna Sankar November 12, 2012 at 6:45 am

@mohan: That was a nice question. I did some quick googling and found the following link
http://en.wikipedia.org/wiki/Computation_of_CRC#Post-invert
http://en.wikipedia.org/wiki/Mathematics_of_CRC#Variations
Quoting from wiki entry:
“Appending 0 bits to a message is equivalent to multiplying its polynomial by x, and if it was previously a multiple of the CRC polynomial, the result of that multiplication will be, as well. A similar solution can be applied at the end of the message, inverting the CRC register before it is appended to the message. Again, any non-zero change will do; inverting all the bits is simply the most common.”

I have not done the math to verify it myself. Will do that in another post.

Reply

preet August 4, 2010 at 11:22 am

please tell how to calculate all the parameters of IEEE 802.11a specification,

Reply

Krishna Sankar August 10, 2010 at 4:55 am

@preet: Plz use the IEEE specifications @ http://standards.ieee.org/getieee802/download/802.11a-1999.pdf

Reply

preet July 31, 2010 at 10:11 pm

can you sujjest any method for solving sunchronization problem in ofdm,,,,,

Reply

Krishna Sankar August 2, 2010 at 5:52 am

@preet: Synchronization is quite an involved issue. Need to fidn algorithms for frequency/phase and time synchronization. Have brief post on frequency offset estimation in OFDM
http://www.dsplog.com/2008/03/03/frequency-offset-estimation-using-80211a-short-preamble/

Reply

Sudeep October 12, 2009 at 8:11 am

Regarding frequency offset tolerance, does it mean for 802.11g, this figure needs to be only around 100 KHz ~(2.4*40)? Or does the tolerance become +/- 40ppm in that case?

Reply

Krishna Sankar October 13, 2009 at 4:58 am

@Sudeep: For the 802.11g (ERP-PHY, see clause 19.4.7.2 in IEEE 802.11-2007 specifications), the tolerance specified is +/-25ppm. So the maximum frequency offset comes around to 2.5GHz * 50ppm = 125kHz.

Reply

Sudeep October 7, 2009 at 9:10 am

Regarding the question on symbol duration, the checkbox answer shows it as 3.2 us, where as in the explanation it is mentioned as 4 us including the GI. I think the check box should also show 4 us as the correct option to avoid confusion.

Reply

Krishna Sankar October 8, 2009 at 5:36 am

@Sudeep: Thanks for noticing it. I corrected.

Reply

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