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If the Sender Window size is 128 using selective repeat ARQ. Then the sequence number of frame to be send after sending 400 th frame is ?
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In case of GBN

in GBN there is n bits sequence number

then senders window size = 2 n - 1

and receiver windows size  = 1

total sequence number = 2n - 1 + 1   = 2(0 to 2 n- 1 sequences we use)

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Here in this case available sequence number = 128 + 128 = 256 that's why (0 to 255)

for goback n Available sequence number = 128 + 1 = 129 so (0 to 128)

because sender WS = 2^k - 1 and receiver is 1.

Am i correct ?
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Na462  yes you are right

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in slective repeat : #sequence number = sender ws + receiver ws

= 128 +128 (since sws=rws)

total size of sequence number = 256(0-255)

therefore for 4oo frame , we have 400mod 256 =144 answer
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arvin  why not  be 143 ?? as seq no start from "0"

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sequence number of the frame to be send after sending 400thframes

check the question once.. @magma

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got it :p
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@magma please can you solve it for GBN and tell the answer. using same data!
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yeah sure ,

In GBN

No of sequence number = Sender's window size + 1 (receiver window size)

= 128 +1

= 129  [sequences  0 to 128]

No of sequence number consumed after sending the 400 frames  = 400 % 129 = 13

it means  0 to 12 sequences  we used already

after this we send a frame with sequence no : 13

understood ??
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Yes magma! Perfect, thank you! :)

For selective repeat ARQ, we know that sender window size=receiver window size

and total no number of sequence numbers=2N (where N is the sender window size)

Hence 0-255 would be consumed for sending 256 frames.Next 0-143 would be consumed for sending 144 frames hence total number of frames sent=256+144=400.Hence the sequence number for next frame should be 144.

the sequence no of 400th frame will be 143