Selective Repeat Protocol

Selective Repeat Protocol:

Selective Repeat is part of the automatic repeat-request (ARQ). With selective repeat, the sender sends a number of frames specified by a window size even without the need to wait for individual ACK from the receiver as in Go-Back-N ARQ. The receiver may selectively reject a single frame, which may be retransmitted alone; this contrasts with other forms of ARQ, which must send every frame from that point again. The receiver accepts out-of-order frames and buffers them. The sender individually retransmits frames that have timed out.

Step 1:

This is almost similar to GBN but main advantage is receiver can store packets in its buffer

If sender sends packet 0,1,2,3 and packet 2 is lost.

Receiver will get packet 0,1,3. It will store all packets and ACK for 0,1,3 and it also shift its window from packet 2.

When sender get ACK of packet 0,1,3 it will shift its window from 2 3 4 5. But packet 2 has not ACK so sender will wait for time and send again 2 3 4 5.

When receiver will get packet 2 3 4 5 it will send ACK but found duplicate of 3 it will discard and shift window from 6 7 8 9

Step 2:

If total packet number window size 4 and packet number 0 1 2 3 then it can occurs exchange packet 0 of 1^st window with packet 0 of 2^nd window. That’s why packet number should be double than window size. Example window size 4 then packet number should be 0 1 2 3 4 5 6 7.

This is the main theme of selective repeat.

Now we can send error free data from PC1 to PC2.
But we use TCP/IP Protocol Suits. And TCP/IP Protocol Suits uses two protocols in Transport layer - TCP (Transmission Control Protocol) and UDP (User Datagram Protocol).

TCP use its own techniques to send error free data. So lets learn a little about TCP. TCP is a connection oriented protocol. What is connection oriented protocol lets see.


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