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Error Detection Algorithms, Compression, Multiplexing, (ch. 13)

Error Detection Algorithms, Compression, Multiplexing, (ch. 13). ENGR 475 – Telecommunications September 12, 2006 Harding University Jonathan White. Error Detection. Hopefully built into the physical layer encoding

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Error Detection Algorithms, Compression, Multiplexing, (ch. 13)

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  1. Error Detection Algorithms, Compression, Multiplexing,(ch. 13) ENGR 475 – Telecommunications September 12, 2006 Harding University Jonathan White

  2. Error Detection • Hopefully built into the physical layer encoding • For example, Differential Manchester physical encoding can detect errors in 1 bit • Error Detection is also employed in other layers further up the protocol stack. • We are going to be looking at transport/network layer error detection. • Your application will also hopefully do error detection.

  3. Error Detection • 3 main low-level methods: • Parity Bit (even or odd) • VRC, often used in trusted mediums • Detects approximately 65% of errors • Longitudinal redundancy check • LRC, rarely used • Detects 85% of errors • Cyclic redundancy check • CRC, very awesome, will use all your math skills • Detects upwards of 99.99995% of all errors

  4. Parity Bits • Add a single parity bit to the end of each byte sent. • Can be even or odd parity. • For odd parity, the number of 1’s must add to an odd number, including the parity. • For even parity, the number of 1’s must add to an even number, including the parity. • Used inside of computers. • What’s the overhead for parity bits? • What type of errors aren’t found?

  5. LRC • Adds horizontal and vertical checking. • Works on an n-1 X n grid. • In our example, 8 vertical 7 bit words. • Better at detecting errors, but only slightly. • What’s the overhead in our example? • Can you make an error that’s not detected?

  6. CRC • Type of hash function • Works on a block of data • Adds extra bits to the end of the data using binary division by relatively prime divisors. • Similar to previous • The receiver then computes the same hash function on the data. • If the CRCs match, then the data is trusted to be what was sent • Note: Some messages can get the same hash value, but this is a very low percentage.

  7. CRC good points • CRCs are one way • Can be very many bits long • Can be very fast • Examples of common hash functions: • MD5 • SHA1 • RIPEMD • Used on almost every digital communication link • http://en.wikipedia.org/wiki/Cyclic_Redundancy_Check

  8. CRC Example • RFID

  9. Compression • Lossless vs Lossy • Where would each work? • Video (VGA SVGA), voice, movies, source code, mathematical formulas • Digital voice is compressed only rarely. • Takes a lot of effort to compress and voice is very time sensitive. • WinZip and File Structures class.

  10. Multiplexing • Definition: • The process of combining two or more communication paths into one path. • Why use multiplexing: • Saves communication paths, which saves money. • 3 main types: • Time division • Frequency division • Space Division (not really multiplexing)

  11. Time Division Multiplexing • Only digital • The multiplexing unit slots time. • Each of the sends is assigned one slot to send in. • If the multiplexing unit can slot time fast enough, the senders will each get a guaranteed certain rate of speed. • The limit is on the bandwidth of the downward pipe. • That’s why we use fiber. • Slotting time is a big job. This is actually a slow process. • Statistical multiplexing.

  12. TDM • Invented in WW2. • Used in T1s by the 1960s. • Used in: • T1 architecture of telephones • WAV format • GSM cell phone network (TDMA). • Negatives: • Too slow

  13. Frequency Division Multiplexing • An analog technology. • Combines several signals by onto 1 medium be sending signals in defined frequency ranges. • The receiver can then “tune” into that frequency. • Very easy to do. • However, it takes up more bandwidth than TDM.

  14. FDM • Used in cable TV, FM/AM radio, portable telephones. • http://en.wikipedia.org/wiki/Frequency_division_multiplexing

  15. Space Division Multiplexing • Not really multiplexing • Each signal has its own wire that is just bundled together. • This is what the telephone company does at a junction box.

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