Problems with the Physical Layer
Firstly communication circuits are known to make errors occasionally, i.e. due to thermal or impulsive noise, or due to signal distortion caused by a channels limited bandwidth (see previous section)
Also, communication channels are known to have a finite data-rate and a non-zero propagation delay (i.e. the signal takes time to travel from the sender to the receiver)
All of these factor must be considered when designing algorithms for the data link layer