Go Back ( GBN)

Go back  N ARQ is specific  instance of the  automatic  repeat request protocols  in which  the sending  process continues to send  a number of frames specific by a window size  without receiving an ACK packet  from the  receiver. The receiver process  keeps  track  of the sequence number of the next  frame it  expects to receive and send that number  will every ACK it sends. If a frame from the send  does  not reach  the receiver the receive will  stop  acknowledging received frames. Once the sender has sent all of the  frames in its window  it will  detect that all of the  frames since the first  lost frame  are outstanding and will go back to sequence number of the last ACK  it received from  the receiver process and fill its window starting with  that frame  and continue the process over again. In simple  words  go back  n is connection oriented protocols  in which  the transmitter  has  a window of  sequence numbers  that may be transmitted  without  acknowledgment  thee receiver will only  accept the next  sequence number  it is expecting other  sequence number are silently ignored.

Figure  shows the  sender view  of the range  of sequence numbers  in a GBN  protocols, if  we define  base to  the sequence  number of the oldest unacknowledged packet and to be  the smaller unused  sequence number   then four intervals in the range of sequence numbers can be identified. Sequence numbers  in the interval  the interval  corresponds to packet that have been sent, but  not yet acknowledged  sequence  numbers in the interval can be used for packets that can  be sent  immediately  should   arrive from  the upper  layer. Finally  sequence numbers  greater than  or equal  to base + N cannot  be used until  an unacknowledged packet  currently  in the pipeline has been  acknowledged.

As  suggested by figure  the range  of permissible sequence numbers for  transmitted but  not yet  acknowledged packets  can be  viewed as a window  of size  n over the range  of sequence numbers. As  the protocols  operates this window slides forward over the sequence number space. For  this  reason N is often  referred to as the window  size and  the GBN  protocols  itself as sliding  window  protocols.

In practice a packet  sequence number  is carried  in a fixed length fields  in the  packet  header if K is the number  of bits in  the packet sequence number fields the range of  sequence numbers is thus [ 0, 2^K -1] . with  a finite range  of sequence numbers , all  arithmetic involving  sequence number space can be thought of as a ring of size 2^k arithmetic  that is the sequence number  space can be through a as a ring of size 2^k. Where  sequence number 2^k-1 is immediately followed  by sequence numbers

                                         figure sender view of sequence number in Go Back -N