A Protocol Using Selective Repeat

Computer NetworkComputer EngineeringMCA

Selective repeat protocol, also called Selective Repeat ARQ (Automatic Repeat reQuest), is a data link layer protocol that uses sliding window method for reliable delivery of data frames. Here, only the erroneous or lost frames are retransmitted, while the good frames are received and buffered.

It uses two windows of equal size: a sending window that stores the frames to be sent and a receiving window that stores the frames receive by the receiver. The size is half the maximum sequence number of the frame. For example, if the sequence number is from 0 – 15, the window size will be 8.

Working Principle

Selective Repeat protocol provides for sending multiple frames depending upon the availability of frames in the sending window, even if it does not receive acknowledgement for any frame in the interim. The maximum number of frames that can be sent depends upon the size of the sending window.

The receiver records the sequence number of the earliest incorrect or un-received frame. It then fills the receiving window with the subsequent frames that it has received. It sends the sequence number of the missing frame along with every acknowledgement frame.

The sender continues to send frames that are in its sending window. Once, it has sent all the frames in the window, it retransmits the frame whose sequence number is given by the acknowledgements. It then continues sending the other frames.

Sender Site Algorithm of Selective Repeat Protocol

begin
frame s; //s denotes frame to be sent
frame t; //t is temporary frame
S_window = power(2,m-1); //Assign maximum window size
SeqFirst = 0; // Sequence number of first frame in window
SeqN = 0; // Sequence number of Nth frame window
while (true) //check repeatedly
do
Wait_For_Event(); //wait for availability of packet
if ( Event(Request_For_Transfer)) then
//check if window is full
if (SeqN–SeqFirst >= S_window) then
doNothing();
end if;
Get_Data_From_Network_Layer();
s = Make_Frame();
s.seq = SeqN;
Store_Copy_Frame(s);
Send_Frame(s);
Start_Timer(s);
SeqN = SeqN + 1;
end if;
if ( Event(Frame_Arrival) then
r = Receive_Acknowledgement();
//Resend frame whose sequence number is with ACK
if ( r.type = NAK) then
if ( NAK_No > SeqFirst && NAK_No < SeqN ) then
Retransmit( s.seq(NAK_No));
Start_Timer(s);
end if
//Remove frames from sending window with positive ACK
else if ( r.type = ACK ) then
Remove_Frame(s.seq(SeqFirst));
Stop_Timer(s);
SeqFirst = SeqFirst + 1;
end if
end if
// Resend frame if acknowledgement haven’t been received
if ( Event(Time_Out)) then
Start_Timer(s);
Retransmit_Frame(s);
end if
end

Receiver Site Algorithm of Selective Repeat Protocol

Begin
frame f;
RSeqNo = 0; // Initialise sequence number of expected frame
NAKsent = false;
ACK = false;
For each slot in receive_window
Mark(slot)=false;
while (true) //check repeatedly
do
Wait_For_Event(); //wait for arrival of frame
if ( Event(Frame_Arrival) then
Receive_Frame_From_Physical_Layer();
if ( Corrupted ( f.SeqNo ) AND NAKsent = false) then
SendNAK(f.SeqNo);
NAKsent = true;
end if
if ( f.SeqNo != RSeqNo AND NAKsent = false ) then
SendNAK(f.SeqNo);
NAKsent = true;
if ( f.SeqNo is in receive_window ) then
if ( Mark(RSeqNo) = false ) then
Store_frame(f.SeqNo);
Mark(RSeqNo) = true;
end if
end if
else
while ( Mark(RSeqNo))
Extract_Data(RSeqNo);
Deliver_Data_To_Network_Layer();
RSeqNo = RSeqNo + 1;
Send_ACK(RSeqNo);
end while
end if
end if
end while
end
Published on 29-Mar-2019 05:34:17
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