$1.$ List the main services that Data Link Layer provides.

$2.$ Answer the following questions

$(1)$ What are the drawbacks of the simplex stop$-$and$-$wait Protocol?

$(2)$ Does adding a timer in the sending end make the stop$-$andwait protocol reliable for a noisy channel?

$(3)$ What can the piggybacking mechanism improve?

$(4)$ Draw PPP frame format and point out functions of fields

$3.$ The following character encoding is used in a data link protocol: A: $01000111$; B: $11100011$; FLAG: $01111110$; ESC: $11100000$ Show the bit sequence transmitted $($in binary$)$ for the fourcharacter frame: A B ESC FLAG when each of the following methods is used:

$($a$)$Character count

$($b$)$Flag bytes with byte stuffing