Bit-Oriented Protocols-Cont

o    Information  field  contains  the  user's  data  in  an  I-Frame  and  Network  Management information in a U-Frame o    An S-Frame has no information field o    Its  length  can  vary  from  one  network  to  another  but  remains  fixed  within  each network o    It is possible to send Control information in the information field of the I-Frame along with data. o    This process is called Piggybacking o    The FCS is HDLC's error detection field o    It can contain a two- or four byte CRC

Bit-Oriented Protocols-Cont

o

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

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

Bit-Oriented Protocols-Cont

Configuration (4) Balanced Configuration o    Both stations in a point-to-point topology are of combined type o    HDLC does not support balanced multipoint Station Types & Configurations

Modes o    A mode in HDLC is the relationship b/w two devices involved in an exchange o    The mode describes who controls the link o    HDLC supports 3 modes of communication b/w stations: -Normal Response Mode (NRM) -Asynchronous Response Mode (ARM) -Asynchronous Balanced Mode (ABM)

Normal Response Mode (NRM) o    Refers to the standard primary-secondary relationship o    Secondary device must have permission from primary device before transmitting o    Once permission has been granted, the secondary may initiate a response transmission of one or more frames containing data

Bit-Oriented Protocols

Bit-Oriented Protocols  o    In  character-oriented  protocols,  bits  are  grouped  into  predefined  patterns  forming characters o    By comparison, bit-oriented protocols can pack more information into shorter frames A lot of bit-oriented protocols have been developed over the years: o    One  of  these  HDLC  is  the  design  of  the  ISO  and  has  become  the  basis  for  all  bit- oriented protocols in use today •In 1975, IBM gave Synchronous Data Link Control (SDLC) •In 1979, ISO answered with High Level Data Link Control (HDLC) o    Since 1981, ITU-T has developed a series of protocols called Link Access Protocols o    LAPs: LAPB, LAPD, LAPM, LAPZ etc. all based on HDLC HDLC is basis for all protocols, so we will study it in detail: High Level Data Link Control (HDLC) o    Bit-oriented data link protocol designed for: -    Full Duplex and Half Duplex -    Point-to-point And Multipoint Links Characterization of HDLC HDLC can be characterized by: -Station Types -Configurations -Response Modes STATION TYPES (1) HDLC differentiates b/w 3 types of stations: -    Primary Station -    Secondary Station -    Combined Station STATION TYPES (2) Primary Station o    Primary station works in the same way as primary devices in the discussion of flow control o    The  primary  is  a  device  in  point-to-point  or  multipoint  line  configuration  that  has complete control of the link STATION TYPES (3) Secondary Station o    The primary sends commands to the secondary stations o    A primary issues commands and a secondary issues responses STATION TYPES (4) Combined Station o    A combined station can both command and respond o    A combined station is one of a set of connected peer devices programmed to behave either as a primary or as a secondary depending on the nature and the direction of the transmission Configuration (1) o    Configuration refers to the relationship of the hardware devices on a link o    Primary , secondary and combined stations can be configured in three ways: ¾   Unbalanced Configuration ¾   Symmetrical Configuration ¾   Balanced Configuration Configuration (2) Unbalanced Configuration o    Also called Master/Slave Configuration o    One device is a primary and others are secondary o    Unbalanced  configuration  can  be  point  to  point  if  only  two  devices  are involved o    Most   of   the   times   it   is   multipoint   with   one   primary   controlling   several secondaries Configuration (3) Symmetrical Configuration o    Each physical station on a link consists of two logical stations, one a primary and the other a secondary o    Separate lines link the primary aspect of one physical station to the secondary aspect of another physical station