itec647-15s1.pdf Done Data Link Layer a) [1 mark] Why must data-link layer frames be framed? b) [1 mark] When using flag bytes or bits, what solution is used when the flag byte or bits occur in the data stream itself? c) [2 marks] What are some reasons to use checksums and CRCs rather than parity d) [4 marks] Describe roll-call polling. Why is this not used in Ethernet? In what e) B marks] What is the main technique used for error correction at the data-link checks? situations might roll-call polling be used? layer? 1