Question $3$

A link layer protocol carries information in both directions on a full$-$duplex channel. In each

direction, the physical layer bit rate is $50$ Mbit$/$s$,$ the propagation delay is $1$ ms and errors occur

in bursts, with an average rate of $6\mathrm{.}5$ burst$/$s$.$ The link$-$layer frame is structured in bytes, as

shown below. The SIZE byte gives the number of bytes in the data block, which can be zero.

Many frames will carry the maximum number of data bytes, but the average number of data

bytes in a frame is $80\%$ of the maximum.

START SIZE SEQ EXPECT DATA $----$ DATA CKS$1$ CKS$2$ END

The protocol is sliding window with selective reject, with an acknowledgement for every block

of data received. There are no separate acknowledgement frames: the EXPECT byte contains

the sequence number of the next block of data expected.

a$)$ What range of sequence numbers is needed to allow this protocol to operate efficiently?

Show how you arrive at your answer. $(30\%)$

b$)$ Calculate the overall throughput of this system $($data delivered in both directions$).$ State

clearly any assumptions or approximations that you make. $(50\%)$

c$)$ Suggest one change to the link$-$layer protocol that would improve the throughput, and give

a reason for your suggestion. Calculation of the new throughput is not required. $(20\%)$