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Data link control protocol(3)

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Lecture slides on data link control protocols from Chapter 23 of the book Electronic Communications Systems by Wayne Tomasi

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Data link control protocol(3)

  1. 1. Lecture 03 Data-Link Protocols Prepared by: Engr. Jeffrey Des B. Binwag Chapter 23 Electronic Communications Sytems , Fifth Editon By: Wayne Tomasi ECE @Saint Louis University, Baguio City 1
  2. 2. SYNCHRONOUS DLCP • SYNCHRONOUS DATA-LINK CONTROL (SDLC) – A synchronous bit-oriented protocol (BOP) developed in the 1970s by IBM for use in system network architecture (SNA) environments – The first link-layer protocol based on synchronous, bitoriented operation – Can transfer data simplex, half-duplex, or full duplex and can support a variety of link types and topologies – Can be used on point-to-point and multipoint networks over both circuit and packet-switched networks – Defines two types of nodes: primary stations and secondary stations ECE @Saint Louis University, Baguio City 2
  3. 3. SDLC TRANSMISSION STATES – TRANSIENT STATE. Exists before and after and after an initial transmission and after each line turnaround – IDLE STATE. A circuit status assumed by the secondary after receiving 15 or more consecutive logic 1s – ACTIVE STATE. Exists whenever either the primary or one of the secondary stations is transmitting information or control signals ECE @Saint Louis University, Baguio City 3
  4. 4. SDLC FRAME FORMAT COMPONENTS 1. 2. 3. 4. 5. Flag Field (beginning and ending) Address Field Control Field Information (or text) Field Frame Check Sequence Field ECE @Saint Louis University, Baguio City 4
  5. 5. SDLC FRAME FORMAT F Flag Field 8 bits 7EH 01111110 A C I Address Control Information Field Field Field 8 bits 8 bits Variable length Span of CRC accumulation FCS F Frame Check Sequence (CRC-16) Flag Field 8 bits 7EH 01111110 Span of zero insertion ECE @Saint Louis University, Baguio City 5
  6. 6. SDLC FLAG FIELD VARIATIONS 1. One beginning and one ending flag per frame F L A G F C C I F L A G A C F L A G F C C I F L A G A C I 2. Ending flag for one frame is used as the beginning flag of the next frame (Shared flag) I F C C F L A G A C I F C C F L A G A C ECE @Saint Louis University, Baguio City I F C C F L A G A 6
  7. 7. SDLC FLAG FIELD VARIATIONS 3. The last zero of an ending flag can be the first zero of the beginning flag of the next frame(Shared Zero) I F C C Ending Flag & Starting Flag with shared zero A C I 011111101111110 F C C Ending Flag & Starting Flag with shared zero A C 011111101111110 4. Flags are transmitted continuously during the time between frames in lieu of idle ones F L A G F L A G F L A G A C I F C C F L A G ECE @Saint Louis University, Baguio City F L A G F L A G F L A G 7
  8. 8. SDLC ADDRESS FIELD • 8 bits in length • 256 possible addresses • 00000000 is designated as the null address and is never assigned to a secondary • 11111111 is designated as the broadcast address and used by the primary station to send a message to all secondary stations • The rest of the 254 addresses are utilized as a unique station address intended for one secondary station only or as a group address intended for several secondary stations. ECE @Saint Louis University, Baguio City 8
  9. 9. SDLC CONTROL FIELD FRAME FORMATS • INFORMATION FRAME – Used to transmit sequenced information that must be acknowledged by the destination station • SUPERVISORY FRAME – Used to confirm previously received information frames, convey ready or busy conditions, and for a primary to poll a secondary when the primary does not have any numbered information to send to the secondary • UNNUMBERED FRAME – Used to send network control and status information ECE @Saint Louis University, Baguio City 9
  10. 10. SDLC CONTROL FIELD INFORMATION FRAME • INFORMATION FRAME (Primary to Secondary) b0 b1 b2 b3 b4 _ P/P nr b5 b6 ns b7 0 • INFORMATION FRAME (Secondary to Primary) b0 b1 nr b2 b3 _ F/F b4 b5 ns ECE @Saint Louis University, Baguio City b6 b7 0 10
  11. 11. SDLC CONTROL FIELD INFORMATION FRAME • nr (Number received). Indicates the number of the next frame expected or the frame number of the frame containing an error for Go-back-nframes sliding window ARQ _ • P/P (Poll or not poll). Indicates if the primary is sending a poll to the secondary or not • ns (Number sent). Indicates the frame number of the frame currently being sent _ • F/F (Final or not final frame). Indicates if the secondary is sending its final frame or not • b7 = 0, identifies the control field as an information frame ECE @Saint Louis University, Baguio City 11
  12. 12. SDLC EXCHANGE OF INFORMATION FIELDS
  13. 13. SDLC CONTROL FIELD SUPERVISORY FRAME b0 b1 nr b2 b3 b4 b5 b6 b7 _ P/P _ F/F X X 0 1 Function Code Indicates supervisory frame Supervisory Frame Functions b4 b5 Receive Status 0 0 Ready to receive (RR) 0 1 Ready not to receive (RNR) 1 0 Reject (REJ) 1 1 Not used with SDLC (Used in HDLC for Selective Reject SREJ) ECE @Saint Louis University, Baguio City 13
  14. 14. SDLC CONTROL FIELD UNNUMBERED FRAME b0 X b1 b2 b3 b4 b5 b6 b7 X _ P/P _ F/F X X 1 1 X Function Code Function Code Indicates unnumbered frame Unnumbered Frame Functions [000(P/F)0011] Unnumbered Information (UI) [010(F)0011] Request Disconnect (RD) [000(P)0111] Set Initialization Mode (SIM) [010(P)0011] Disconnect (DISC) [000(F)0111] Request Initialization Mode (RIM) [011(F)0011] Unnumbered Acknowledgment (UA) [100(P)0011] Set Normal Response Mode (SNRM) [100(F)0111] Frame Reject (FRMR) [000(F)1111] Disconnect Mode (DM) [111(P/F)0011] TEST [101(P/F)0011] Exchange Station ID (XID) [110(P/F)0111] Configure (CFGR) [111(F)1111] Beacon (BCN) ECE @Saint Louis University, Baguio City 14
  15. 15. SDLC CONTROL FIELD UNNUMBERED FRAME • UI. Used to send unnumbered information • SIM. Places a secondary station into the initialization mode • RIM. Sent by a secondary station to request the primary to send a SIM command • SNRM. Places a secondary into the normal response mode • DM. Sent by the secondary if the primary sends numbered information while the secondary is in the normal disconnect mode • RD. Sent by the secondary when it wants the primary to place it into disconnect mode • DISC. Places the secondary in disconnect mode • UA. An affirmative response that indicates compliance to SIM, SNRM, or DISC commands and acknowledge unnumbered information frames • FRMR. Used by the secondary to report any of four possible procedural errors ECE @Saint Louis University, Baguio City 15
  16. 16. SDLC CONTROL FIELD UNNUMBERED FRAME • PROCEDURAL ERRORS – Invalid command – Prohibited information field – Buffer overrun – Received nr disagrees with ns ECE @Saint Louis University, Baguio City 16
  17. 17. SDLC INFORMATION FIELD • • • • • • • The number of bits in the SDLC information field is variable but must be a multiple of 8 bits (1 byte) Not all SDLC frames allow an information field The data in an information field may either be user information or control information SDLC Frame Check Character FIELD Contains the error detection mechanism for SDLC Equivalent with the BCC used in BSC Uses CRC-16 with the generating polynomial applied on the control in the address and information fields Uses the CCITT V.41 specified generating polynomial: x16 + x12 + x5 + x1 ECE @Saint Louis University, Baguio City 17

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