Bit-Oriented Protocols-Cont

Asynchronous Response Mode (ARM)

o    A  secondary  may  initiate  a  TX  w/o  permission  from  the  primary  whenever  the channel is idle o    ARM does not alter the primary secondary relationship in any other way o    All  transmissions  from  the  primary  still  go  to  the  secondary  and  are  then  relayed  to the other devices Asynchronous Balanced Mode (ABM) o    All  stations  are  equal  and  therefore  only  combined  stations  connected  in  point-to- point are used o    Either   combined   station   may   initiate   TX   with   the   other   combined   station   w/o permission HDLC Frames HDLC defines 3 types of Frames: -    Information Frames (I-Frames) -    Supervisory Frames (S-Frames) -    Unnumbered Frames(U-Frames) I-Frames are used to transport user data and control information relating to user data S-Frames are used only to transport control information U-Frame are reserved for System Management Each frame in HDLC may contain up to six fields -A beginning Flag Field -An address field -A control field -An information Field -A frame check sequence (FCS) -An ending Flag Field

•The  flag  field  of  an  HDLC  frame  is  an  8-bit  sequence  with  a  bit  patter  01111110  that identifies both the beginning and the ending of the of a frame •It serves as a Synchronization pattern for the receiver •Fig. shows placement of 2 flag fields in an I-Frame HDLC Address Field