HDLC(high level data link control)

HDLC(high level data link control)

• 1.
  Anand Bapuraya Biradar VIIth EC SJCE 12/21/2013
• 2.
  High-Level Data LinkControl (HDLC)  HDLC was defined by ISO for use on both pointto-point and multipoint data links.  It supports full-duplex communication  ITU modified HDLC for use in X.25 network interface and called it Balanced Link Access Protocol (LAPB)  Other similar protocols are  Synchronous Data Link Control (SDLC) by IBM  Advanced Data Communication Control Procedure (ADCCP) by ANSI 12/21/2013
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  HDLC Overview Broadly HDLCfeatures are as follows:  Reliable protocol  selective repeat or go-back-N  Full-duplex communication  receive and transmit at the same time  Bit-oriented protocol  use bits to stuff flags occurring in data . i.e, it does NOT recognize or interpret byte value  Flow control  adjust window size based on receiver capability  Uses physical layer clocking and synchronization to send and receive frames 12/21/2013
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  HDLC Overview  Definesthree types of stations  Primary  Secondary  Combined  Defines three types of data transfer mode  Normal Response mode  Asynchronous Response mode  Asynchronous Balanced mode  Three types of frames  Unnumbered  information  Supervisory 12/21/2013
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  HDLC Defines threetypes of stations STATIONS PRIMARY STATION 12/21/2013 SECONDARY STATION COMBINED STATION
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  HDLC  The threestations are :  Primary station  Has the responsibility of controlling the operation of data flow .  Handles error recovery  Frames issued by the primary station are called commands.  Secondary station,  Operates under the control of the primary station.  Frames issued by a secondary station are called responses.  The primary station maintains a separate logical link with each secondary station.  Combined station,  Acts as both as primary and secondary station. 12/21/2013
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  HDLC Unbalanced Mode Commands Primary Responses Secondary Secondary Balanced mode Combined Combined commands/Responses 12/21/2013
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  HDLC Defines threetypes of data transfer mode DATA TRANSFER MODES NORMAL RESPONSE MODE(NRM) 12/21/2013 ASYNCHRONOUS RESPONSE MODE(ARM) ASYNCHRONOUS BALANCE MODE (ABM)
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  HDLC  The threemodes of data transfer operations are  Normal Response Mode (NRM)   Secondary station can send ONLY when the primary station instruct it to do so Two common configurations - Point-to-Point link (one primary station and one secondary station) - Multipoint link (the primary station maintain different sessions with different secondary stations)  Asynchronous Response Mode (ARM)   More independent secondary station Can send data or control information without explicit permission to do so (note that it is still can not send commands)  Asynchronous Balanced Mode (ABM)   12/21/2013 Mainly used in point-to-point links, for communication between combined stations Either stations can send data, control information and commands
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  HDLC Defines threetypes of frames HDLC FRAMES U-frame 12/21/2013 I-frame S-frame
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  HDLC frame structure (a)Frame Format (b) Control field format 12/21/2013
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  HDLC  Flag: 01111110-indicates start and ending of frames  FCS: 16-bit CRC using generating polynomial G(x) = x16 + x12 + x5 + 1  Address field:  When a primary station is sending a frame, the address field contains the receiver identity  If a secondary station is sending the frame, the address field contains the sender identity  In some cases, it contains a group or broadcast address  In I-frames, N(s) is the sequence number of the frame being sent, and N(r) is the sequence number of the frame being expected.  The P/F bit, known as the poll/final bit, is used with different meaning in different contexts.  It is used to indicate polling, to indicate the final I-frame, etc 12/21/2013
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  HDLC  There arethree different classes of frames used in HDLC  Unnumbered frames, used in link setup and disconnection, and hence do not contain ACK.  Information frames, which carry actual information. Such frames can piggyback ACK in case of ABM  Supervisory frames, which are used for error and flow control purposes and hence contain send and receive sequence numbers 12/21/2013
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  HDLC  There arefour different supervisory frames  SS=00, Receiver Ready (RR), and N(R) ACKs all frames received up to and including the one with sequence number N(R) - 1  SS=10, Receiver Not Ready (RNR), and N(R) has the same meaning as above  SS=01, Reject; all frames with sequence number N(R) or higher are rejected, which in turns ACKs frames with sequence number N(R) -1 or lower.  SS=11, Selective Reject; the receive rejects the frame with sequence number N(R) 12/21/2013
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  HDLC  The unnumberedframes can be grouped into the following categories:  Mode-setting commands and responses  Recovery commends and responses  Miscellaneous commands and responses 12/21/2013
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  Review of LinkLayer  Services       Framing Error control Reliability Connection management Medium access control Switching  Protocols  PPP  HDLC 12/21/2013
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  ANY QUESTIONS……?? 12/21/2013
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  •Thank you…… 12/21/2013