HDLC(high level data link control)
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The document describes High-Level Data Link Control (HDLC), a bit-oriented protocol developed by ISO for point-to-point and multipoint data links. HDLC supports full-duplex communication and was modified by ITU as Balanced Link Access Protocol for use in X.25 networks. It defines three types of stations (primary, secondary, combined), three modes of data transfer (normal response, asynchronous response, asynchronous balanced), and three types of frames (unnumbered, information, supervisory). HDLC provides reliable communication, flow control, and uses physical layer synchronization.
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HDLC(high level data link control)
- 1. Anand Bapuraya Biradar VII th EC SJCE 12/21/2013
- 2. High-Level Data Link Control (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
- 3. HDLC Overview Broadly HDLC features 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
- 4. HDLC Overview Defines three 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
- 5. HDLC Defines three types of stations STATIONS PRIMARY STATION 12/21/2013 SECONDARY STATION COMBINED STATION
- 6. HDLC The three stations 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
- 8. HDLC Defines three types of data transfer mode DATA TRANSFER MODES NORMAL RESPONSE MODE(NRM) 12/21/2013 ASYNCHRONOUS RESPONSE MODE(ARM) ASYNCHRONOUS BALANCE MODE (ABM)
- 9. HDLC The three modes 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
- 10. HDLC Defines three types of frames HDLC FRAMES U-frame 12/21/2013 I-frame S-frame
- 11. HDLC frame structure (a) Frame Format (b) Control field format 12/21/2013
- 12. 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
- 13. HDLC There are three 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
- 14. HDLC There are four 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
- 15. HDLC The unnumbered frames can be grouped into the following categories: Mode-setting commands and responses Recovery commends and responses Miscellaneous commands and responses 12/21/2013
- 16. Review of Link Layer Services Framing Error control Reliability Connection management Medium access control Switching Protocols PPP HDLC 12/21/2013