Gate-2006 Go-Back-n problem :)

[Imagine]

Station A needs to send a message consisting of 9 packets to station B
using a sliding window (window size=3) and go back n error control
strategy. All packets are ready and immediately available for
transmission. If every 5th packet that A transmits gets lost(but no
ack from B is lost), then what is the number of packets that A will
transmit for sending the message to B?
a)12
b)14
c)16
d)18

What will be the ans... I have confusion with 12 ? what do you think ?
*?*? Reply...

[debopriyo banerjee]

16

[srikanta s]

yes it should be 12 only . first it will transmit 1 2 3 frames then 4 5 6 there 5th frame error .then it will retransmit  5 6 7 then 8 and 9 .here again 9th frame error because it is 5th frame. then it will retransmit 9 . so total 12 frames.

[debopriyo banerjee]

it will not be 12 it will be 16
window size is 3 so sequence numbers will be 0 1 2 3
first it will  transmit 0 1 2 3 then 0 1 2 this 0 is the fifth frame so it is error retransmit 0 1 2 again this last 2 is the fifth frame retransmit 0 1 2 then transmit 3 0 then again the last 0 is the fifth frame so retransmit 0. so the sequence of transmitting is 0 1 2 3 0 1 2 0 1 2 0 1 2 3 0 0

[maulik parekh]

but how can it sends 0123 first time bcoz window size is 3 not 4 so how its possible at first time it will send 4 packets ?? 

[Tanu Kanvar]

16 is the answer

Q doesn't say that only after 3rd packet B will send the acknowledgement for 1st packet from A
So we can say

packet1 from A - Tranmit1
       acknowledgement (ack) for packet1(pckt) from B
packet2 from A - Transmit2
       ack for pckt2 from B
packet3 from A - Transmit3
       ack for pckt3 from B
packet4 from A - Transmit4
       ack for pckt4 from B
packet5 from A - Transmit 5          (packet5 lost, so no ack from B for pckt5)
packet6 from A - Transmit 1          (go back n, so no ack for pckt6 also)
packet7 from A - Transmit 2          (go back n, so no ack for pckt7 also)
                             Timeout for packet5
packet5 from A - Transmit 3
       ack for pckt5 from B
packet6 from A - Transmit 4
       ack for pckt6 from B
packet7 from A - Transmit 5         (packet7 lost, so no ack for pckt7)
packet8 from A - Transmit 1         (go back n, so no ack for pckt8 also)
packet9 from A - Transmit 2         (go back n, so no ack for pckt9 also)
                             Timeout for packet7
packet7 from A - Transmit 3
       ack for pckt7 from B
packet8 from A - Transmit 4
       ack for pckt8 from B
packet9 from A - Transmit 5         (packet9 lost, so no ack for pckt9)
                             Timeout for packet9
packet9 from A - Transmit 1
       ack for pckt9 from B

B now receives all 9 packets.
In total 16 transmits from station A