Error Detection And Correction Methods-Cont

Line Discipline  Enquiry / Acknowledgement (ENQ/ACK) o    Used primarily in systems where there is no question of wrong receiver getting the transmission o    In  other  words  when  there  is  a  Dedicated  Link  b/w  the  two  devices  so  that  the only device that can receive data is the intended one o    Enquiry / Acknowledgement (ENQ/ACK) o    ENQ/ACK  coordinates  who  may  start  a  transmission  and  whether  or  not  the intended recipient is ready and enabled o    Using ENQ/ACK, a session can be initiated by either station on a link as long as both are of equal rank- a printer for example cannot initiate communication with a CPU o    Enquiry / Acknowledgement (ENQ/ACK) o    In  both  half  duplex  and  full  duplex  TX,  the  initiating  device  establishes  the session o    In half-duplex, the initiator then sends its data while the responder waits o    The  respondent  may  take  over  the  link  when  the  initiator  is  finished  or  has requested a response o    In  full  duplex,  both  the  devices  can  TX  simultaneously  once  the  link  has  been established How It Works? o    The  receiver  must  answer  either  with  an  acknowledgement  (ACK)  frame  if  it  is ready to receive or with a negative acknowledgement ( NAK), if it is not o    By requiring a response, even if the answer is negative, the initiator knows that his enquiry was in fact received even if the receiver is currently unable to receive o    If neither an ACK or a NAK is received within a specified time limit, the initiator assumes  that  an  ENQ  frame  was  lost  in  transit,  it  disconnects  and  sends  a replacement o    An initiating system ordinarily makes 3 such attempts before giving up o    If the response to the ENQ is negative for 3 attempts, the initiator disconnects and begins the process again at another time ŠIf the response is positive the initiator is free to send its data o    The initiator first transmits a frame called an enquiry (ENQ) asking if the receiver is available to receive data o    Once  all  of  its  data  have  been  transmitted,  the  sending  system  finishes  with  an End of Transmission (EOT) frame ™  Poll / Select Primary-Secondary communication o    This method works with topologies where one device is designed as a Primary station and the other devices are Secondary stations o    Multipoint systems must coordinate several nodes, not just two o    The questions are not only Are you Ready? But also Which of the device has the right to use the channel o    Whenever  a  multipoint  link  consists  of  a  primary  device  and  multiple  secondary devices  using  a  single  TX  line  ,  all  exchanges  must  be  made  through  the  primary device even when the ultimate destination is a secondary device o    The primary device controls the link and the secondary device follow sits instruction o    It  is  up  to  the  primary  to  determine  which  device  is  allowed  to  use  the  channel  ata given time o    The primary therefore is always the initiator of the a session ŠIf  the  primary  wants  to  receive  data,  it  asks  the  second-arise  if  they  have  anything  to send, This is called POLLING ŠIf  the  primary  wants  to  send  data,  it  tells  the  target  secondary  to  get  ready  to  receive, This function is called SELECTING