Past exam of the computer science course of the University of Oxford / 2015 / Computer networks / 1 / d / Solution

A physical link can deliver a continuous stream of bits; the data link layer needs to identify where each separately meaningful unit begins and ends. Framing supplies those boundaries, so the receiver can separate the data-link frame header, data and checking information, associate an error detection and correction result with a particular data-link frame, and regain alignment after damaged input.

Timing the individual bits is insufficient: knowing where each bit begins does not tell the receiver which bit starts a new data-link frame. Thus