# Go-Back-N ARQ

Go-Back-N Automatic Repeat reQuest (Go-Back-N ARQ), is a data link layer protocol that uses a sliding window method for reliable and sequential delivery of data frames. It is a case of sliding window protocol having to send window size of N and receiving window size of 1.

## Working Principle

Go – Back – N ARQ uses the concept of protocol pipelining, i.e. sending multiple frames before receiving the acknowledgment for the first frame. The frames are sequentially numbered and a finite number of frames. The maximum number of frames that can be sent depends upon the size of the sending window. If the acknowledgment of a frame is not received within an agreed upon time period, all frames starting from that frame are retransmitted.

The size of the sending window determines the sequence number of the outbound frames. If the sequence number of the frames is an n-bit field, then the range of sequence numbers that can be assigned is 0 to 2n−1. Consequently, the size of the sending window is 2n−1. Thus in order to accommodate a sending window size of 2n−1, an n-bit sequence number is chosen.

The sequence numbers are numbered as modulo-n. For example, if the sending window size is 4, then the sequence numbers will be 0, 1, 2, 3, 0, 1, 2, 3, 0, 1, and so on. The number of bits in the sequence number is 2 to generate the binary sequence 00, 01, 10, 11.

The size of the receiving window is 1.

## Sender Site Algorithm of Go-Back-N 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
if ( AckNo > SeqFirst && AckNo < SeqN ) then
while ( SeqFirst <= AckNo )
Remove_copy_frame(s.seq(SeqFirst));
SeqFirst = SeqFirst + 1;
end while
Stop_Timer(s);
end if
end if
// Resend all frames if acknowledgement havn’t been received
if ( Event(Time_Out)) then
TempSeq = SeqFirst;
while ( TempSeq < SeqN )
t = Retrieve_Copy_Frame(s.seq(SeqFirst));
Send_Frame(t);
Start_Timer(t);
TempSeq = TempSeq + 1;
end while
end if
end

## Receiver Site Algorithm of Go-Back-N Protocol

Begin
frame f;
RSeqNo = 0; // Initialise sequence number of expected frame
while (true) //check repeatedly
do
Wait_For_Event(); //wait for arrival of frame
if ( Event(Frame_Arrival) then
if ( Corrupted ( f.SeqNo )
doNothing();
else if ( f.SeqNo = RSeqNo ) then
Extract_Data();
Deliver_Data_To_Network_Layer();
RSeqNo = RSeqNo + 1;
Send_ACK(RSeqNo);
end if
end if
end while
end
karthikeya Boyini

I love programming (: That's all I know