Bit-Oriented Protocols-Cont

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The second field of HDLC frame contains the address of the secondary station that is either the originator or the destination of the frame

o    If  a  primary  station  creates  Frame  it  includes  a  'To'  address  and  if  a  secondary creates the frame, it contains a 'From' address o    Can be of one byte or several bytes depending upon the network o    If the address field is only 1 byte, the last bit is always a 1 o    If the address is of several bytes, all bytes but the last one will end with 0 , and the last will end with a 1 o    Ending each intermediate byte with 0 indicates to the receiver that there are more address bytes to come o    The control field is a one o two byte segment of the frame used for flow management o    The two byte case is called the Extended Mode Control fields differ depending on the frame type: -    If the control field is a 0, the frame is an I-Frame -    If the first bit is 1 and the second bit is a 0 , it is S-Frame -    If both first and second bits are 1's, it is U-Frame o    P/F bit is a single bit with dual purpose o    It has meaning only when it is '1' and it can mean Poll or Final o    When the frame is sent by a primary to secondary, it means POLL o    When the frame is sent by a secondary to a primary, it is FINAL HDLC Control Field -EXTENDED o    Control  field  in  the  I-Frame  and  S-Frame  is  two  bytes  long  to  allow  seven  bits  of sending and receiving sequence o    However the control field in the U-Frame is still one byte