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I recently read about CRC, a checksum to detect package corruption and drop corrupted packages. But how does a packet in an IP-based network get corrupted in the first place?

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    Your question basically boils down to "Why are systems imperfect?" There's implementation mistakes, environmental conditions... Systems don't run in perfect isolation. No matter how good a system you make, it will still be affected by random noise from the environment - cosmic rays, EM interference, "that faulty device on the network/power supply"... Even your computer's RAM/HDD has error-detection and correction mechanisms, and that's right there in your computer's box - would you expect communication over thousands of kilometers to be less faulty?
    – Luaan
    May 16, 2017 at 13:49

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Packets are long streams of binary numbers (zeros and ones).

The zeros and ones are usually changes of an electric signal, specifically voltage changes.

To simplify, suppose that 0 is 0 volts and 1 is represented alternatively as 5 volts and -5 volts.

Then a long string of zeros and ones will look like this in terms of voltage variations in the cable:

enter image description here

As you can see, the first 1 is 5 volts, but the next 1is -5 volts. This is neccesary to avoid that a long string of 1s energizes the communication line.

If the cable is exposed to electromagnetic interference, then the voltage levels can be altered. If a -5 volts value becomes -2 volts then the receiver can't identify it precisely as a zero or a one.

If your data cable runs for a long stretch parallel to power cables, then the power cable can cause interference to the data cable and induce errors.

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    This gets worse for multi level PAM encodings, like 5 level 1000BASE-T or 10GBASE-T with 16 levels. Just look at one of their eye diagrams: 0397dc3.netsolhost.com/images/wp_ChooseCAT5/wp_ChooseCAT5_2.jpg
    – PlasmaHH
    May 16, 2017 at 14:25
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    WiFi interference is maybe more obvious/likely. Most wireless systems include some level of FEC because there is often some interference and the parity overhead is worth it to avoid the massive time hit that giving a bad packet to higher layers might make as they need to retry.
    – Nick T
    May 16, 2017 at 22:06

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