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Flow control is design issue at Data Link Layer. It is technique that generally observes proper flow of data from sender to receiver. It is very essential because it is possible for sender to transmit data or information at very fast rate and hence receiver can receive this information and process it. This can happen only if receiver has very high load of traffic as compared to sender, or if receiver has power of processing less as compared to sender.
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.
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.
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(); 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 Receive_Frame_From_Physical_Layer(); 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
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