Question $6$

$($a$)$ In a network using Go$-$Back$-$N ARQ protocol with $m=6,$ the maximum window size is used. The values of the variables are $S_f=30,S_n=38,$ and $R_n=35.$ Assume that the network does not duplicate or reorder the packets.

$($i$)$ Determine the maximum the sizes of the sender and receiver windows.

$(2$ marks$)$

$($ii$)$ Determine the sequence numbers of the packets still in transit.

$(2$ marks$)$

$($iii$)$ Determine the acknowledgment numbers of the ACKs still in transit.

$(2$ marks$)$

$($iv$)$ Determine the sequence number of the next packet to be sent at the sender$.$

$(1$ mark$)$

$($v$)$ An ACK with acknowledgment number $33$ has just arrived at the sender$.$ Determine the value of $S_f.$

$(1$ mark$)$

$($vi$)$ From $($v$),$ determine the reliabilities of the forward and reverse channels. Explain briefly.

$(3$ marks$)$

$($b$)$ Consider a network using Selective Repeat ARQ protocol with $m=6.$ The maximum window size is used. The values of the variables are $S_f=30$ and $S_n=38.$ Packets with sequence numbers $32,34,35$ and $36$ have been received by the receiver and the respective ACKs received by the sender$.$ Further assume that all acknowledgments can be received immediately after they are sent and there is no loss in acknowledgments.

$($i$)$ What are the sizes of the sender and receiver windows?

$(1$ mark$)$

$($ii$)$ What is the value of $R_n?$

$(1$ mark$)$

$($iii$)$ What are the sequence numbers of the packets still in transit?

$(2$ marks$)$

$($iv$)$ What is the sequence number of the next packet to be sent at the sender$?$

$(1$ mark$)$

$($v$)$ Now, the packets with sequence number $30$ and $31$ arrive at the receiver. What is the receiver's response to this event? What is the next value of $R_n?$

$(4$ marks$)$