Hdlc

Hdlc

• 2.
  Definition • HDLC (high-level data link control) • HDLC Is a group of protocols and rules for transmitting data between network points (called nodes). • Developed by ISO.
• 3.
  description • It worksof layer 2, (data link) that encapsulate layer 3 frame • Data is organized in unit called frame and sent across a network to the destination that verify its successful arrival. • It also manage the flow at which data is sent • HDLC use synchronous transmission.
• 4.
  DATA LINK CONFIGURATION •UNBALANCED: consist of one primary and one(point to point) or more(multipoint) secondary stations and support both full duplex and half duplex transmission. • BALANCED: consist of two combined station supports both full duplex and half duplex communication
• 5.
  Data transfer modes/Responsemodes • NORMAL RESPOND MODE (NRM): Use unbalanced configuration. The primary may initiate data transfer to secondary, but the secondary may only transmit data in response to the command from the primary. • ASYNCHRONOUS RESPOND MODE(ARM): Use with an unbalanced mode. Secondary may initiate transmission without permission of primary. • ASYNCHRONOUS BALANCE MODE(ABM): Used with balanced configuration. Either combined station may initiate the data without any permission of other combined data.(can only send information not command and respond to command) •
• 6.
  STATIONS TYPES • PRIMARY:responsibility for controlling the operation of the link, • handles error recovery • Frame issued by the primary called command • SECONDARY: operate under the control of primary • Frame issued by the secondary called response • Primary station maintain the logical link with secondary. • • COMBINED: Each station can act as both primary and secondary
• 7.
  FRAMING MODE • Thereare three different classes of frames used in HDLC • (1) I-frame (information frames): which carry actual information. Such frame can also send ACK. • • (2) S-frame ( Supervisory frames): used when sender and receiver want to sent ACK only, contain no information. It can also be used for error and flow control. • • (3) U-frame (Unnumbered frames): use to link setup, responsible to maintain terms and argument between the sender and receiver.
• 8.
  FRAME STRUCTUR • I-FRAME: •U-FRAME: • S-FRAME: •
• 9.
  FLAG FIELD • Indicatestart and ending frame (closing and opening flag) • It indicate the start and ending of frame with a unique pattern 011111110 which could be found inside the frame • To destruct the inner structure a method is used called bit stuffing • BIT STUFFING: The sender will insert 0 after occurrence of 5
• 10.
  ADDRESS FIELD • Theaddress field identify the secondary station that transmit or receive the frame • Can be of 1 byte or multiple byte 1 is used to indicate the end of byte 0 is sued to indicate the continuity of byte ADDRESS of 1 byte 1 1st byte 0 2nd byte 0 3rd byte 1
• 11.
  INFORMATION FIELD • Presentonly in I-frame and some in U- frame(management information does nit contain DATA) • MANAGEMENT INFORMATION: to maintain the link, response mode, terms and arguments between sender and receiver.
• 12.
  FCS FIELD • Theframe check sequence (FCS) is an error detecting code calculated from remaining bit of frame. • Exclusive of flag the normal code is 16 bit
• 13.
  CONTROL FIELD • Thecontrol field is different in different frame as shown below: • I-FRAME: • S-FRAME: • U-FRAME: 0 SEQUENCING P/F ACKNOWLEDGMENT 1 0 SUPERVISORY FUNCTION BITS P/F ACKNOWLEDGMENT 1 1 UNNUMBERED FUNCTION P/F UNNUMBERED FUNCTION
• 14.
  • POLL/FINAL (P/F):tells the direction of frame • Poll => primary to secondary • Final => secondary to primary • Supervisory function: 2 bit code • 00 = The Receive-ready • 01 = Receive-not-ready • 10 = REJ initiates • 11 = selective reject • Unnumbered function bit: usually of 32 bits sends large number of information. • Sequencing : the informative data that is to be send