The error rates in NetSim wired links are based on a standard error measurement unit called BER or Bit Error Rate.
BER represents the ratio of errored bits to total bits.
The BER value can be set by the user. A typical value of BER, say 1 * 10 Exp (-6), which equals 0.000001, means that 1 bit is in error for every one million bits transmitted (one million is equal to 10 power 6).
It is important to note that Bit Error Rate is NOT equal to Packet error rate. Please see https://en.wikipedia.org/wiki/Bit_error_rate to see the governing equation between BER and PER.