1. Limited time only! Sign up for a free 30min personal tutor trial with Chegg Tutors
    Dismiss Notice
Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Homework Help: Decode the given message manually or using a table only

  1. Nov 28, 2012 #1
    1. The problem statement, all variables and given/known data
    We have captured some transmission from some transmission media and it is not a clear readable text. We know that the message part of the frame is using ASCII-coding to code the message. The transmission is in several parts. We know that the start of frame is 1010101010101011 and we know that the message is always 70 bits long. What is the message in clear clear text if the message is the following.


    2. Relevant equations

    3. The attempt at a solution

    This homework was given to us by our professor after giving brief introduction about ASCII codes (nothing in detail) as a part of digital circuits course. I tried using the tables and some tools online to convert it to readable text but failed. Can you please give me a hint ?
  2. jcsd
  3. Nov 28, 2012 #2
    the binary 1010-1010-1011 is 10-10-11 in decimal

    The hex starts A-A-B which is, you guessed it, 10-10-11 in decimal

    I'd assume AAAB is the header, and the message starts with the 54...

    Not sure how far I should go with a homework question
  4. Nov 28, 2012 #3

    Thanks for the hint. It got me started. But I got stuck again. I am not sure if I should use the ASCII value of 5(hex) at a time or 54(hex) at a time. And I am sorry because homework was totally out of what was taught in class and the professor wanted us to find the solutions using online resources.
  5. Nov 28, 2012 #4
    By outside resources he means that you should find and read articles on the subject, not that you should crowd source the answer. It's a mathy class not a social engineering class.
  6. Nov 28, 2012 #5
    Ok i got your point. But I did that beforehand it worked for all other problems I had but not this one and also i compared the message part with ASCII characters in the table but i still cannot get a readable text. And there are 70 bits in the message part and each bit when decoded using the table give out something stupid and very long.
  7. Nov 28, 2012 #6


    User Avatar

    Staff: Mentor

    What does 54 in hex represent in ASCII? What does hex 52 represent in ASCII? Keep going...
  8. Nov 28, 2012 #7

    Thank you. I tried doing with a single bit value ie fot 5,2 and so on. So i did not get readable message. Thank you once again. But i still dont understand why the we use twot bits at a time but not 1 or 2. thanks once again
  9. Nov 28, 2012 #8


    User Avatar
    Homework Helper
    Gold Member

    Each hexadecimal number in the message is 4 bits. Ascii characters are represented by 8 bits (one byte).

    Think of it another way. The hexidecimal numbers in the message range from 0 to F (0, 1, 2, 3, 4, 5, 6, 7, 8, 9, A, B, C, D, E, F). That's a total of 16 values for each hex digit. (24 = 16). If only one hexadecimal numeral mapped to an Ascii character, there would be only 16 Ascii characters, maximum. One couldn't even represent the alphabet that way, not to mention numbers and special characters.

    With two hexidecimal numerals per Ascii character, there's 8 bits (one byte). That allows for a total of 28 = 256 possible representations. (And, if you haven't guessed, the Ascii table plus the extended Ascii codes total to 256 different characters.)
  10. Nov 28, 2012 #9

    Thanks a bunch for a crystal clear explanation. :smile:
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook