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Data Link Protocols

Data Link Protocols. Primary Station (a station than can issue commands) Secondary Station (a station that issues responses to commands) Combined Stations (a station can operate as either primary or secondary, issuing either requests, or responses). HDLC Station Types.

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Data Link Protocols

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  1. Data Link Protocols ICSA 341 Data Communications & Networking

  2. Primary Station (a station than can issue commands) Secondary Station (a station that issues responses to commands) Combined Stations (a station can operate as either primary or secondary, issuing either requests, or responses) HDLC Station Types ICSA 341 Data Communications & Networking

  3. HDLC Link configurations Unbalanced (top) (one primary and multiple secondary stations over pt-to-pt or multi-point link) Symmetrical (like unbalanced but control of link can shift between machines) Balanced (bottom) (combined station over pt-to-pt link -- a peer-to-peer arrangement) ICSA 341 Data Communications & Networking

  4. HDLC Data Transfer Modes • Normal Response Mode (NRM) • Unbalanced link configuration w/single primary and multiple secondary stations • Secondary stations can only transfer data when polled by the primary station • Asynchronous balanced mode (ABM) • Balanced link configuration w/combined stations • Either station can initiate data transfer at any time • Asynchronous response mode (ARM) • Unbalanced link configuration w/single primary and multiple secondary stations • Secondary stations are allowed to transfer data without a poll from the primary station ICSA 341 Data Communications & Networking

  5. General HDLC Frame Format Sandwich the information between aheader and trailer DLC Header DLC Trailer INFORMATION ICSA 341 Data Communications & Networking

  6. Initial Breakdown 01111110 01111110 FLAG FIELD INFORMATION ADDRESS CONTROL FCS ICSA 341 Data Communications & Networking

  7. HDLC Frame Types • Information (I) - carries user data and flow/error control information • Supervisory (S) - used to provide additional flow/error control functions • Unnumbered (U) used to provide additional link control functions ICSA 341 Data Communications & Networking

  8. CONTROL FIELD • Either 8 or 16 bit field - used for flow management • different for each type of frame • If start bit = 0 it is an information frame • If first two bits = “10”, it is a supervisory frame • If first two bits = “11”, it is an unnumbered frame ICSA 341 Data Communications & Networking

  9. Specific HDLCControl Bits: I, S, U 1 2 3 4 5 6 7 8 0 N(S) P/F N(R) 1 0 S P/F N(R) 1 1 M(1) P/F M(2) N(S) = sequence number of I frame being sent N/R = sequence # of next expected I frame P/F = poll/final bit S = supervisory frame code M(1) M(2) = unnnum. code ICSA 341 Data Communications & Networking

  10. Specific HDLCControl Bits: S Selective Reject RRR P/F 1101 ICSA 341 Data Communications & Networking

  11. Specific HDLCControl Bits: U ICSA 341 Data Communications & Networking

  12. Specific HDLCControl Bits: I ICSA 341 Data Communications & Networking

  13. Information Variable length (usually multiple of 8 bytes) I-frame = User Data S-frame = does not exist U-frame = management data (network management) Frame Check Sequence Uses the standard CRC (16 bit) or CRC-32 (32 bit) Other fields ICSA 341 Data Communications & Networking

  14. Supervisory Frame Types • Receive Ready (RR) • ACK • Poll • Negative Poll • Positive Select ICSA 341 Data Communications & Networking

  15. Supervisory Frame Types • Receive Not Ready(RNR) • ACK • Select • Negative Select • Reject - Go Back n ARQ • Selective Reject - Selective Reject ARQ ICSA 341 Data Communications & Networking

  16. Data transfer example - Peers Station A Station B SABM • U-frame, SABM, P=1, establishes link in ABM • Station B returns UA, F=1 indicating agreement on mode and the P/F bits will no longer be used • Station A sends I frame, numbered 0 then another numbered 1 UA data 0 data 1 data 0 ACK 2 data 1 ACK 2 data 2 ACK 2 RR, ACK 3 ICSA 341 Data Communications & Networking

  17. Data transfer example - Peers Station A Station B SABM • Station B now sends it’s own I frame but also ACKs I frames 0 and 1 from Station A • Station B follows with I frames 1 & 2, the ACK stays the same as no other data has been sent from Station A • Station A has no other data to send, but must ACK data from Station B, so an S-frame is sent, with RR, ACK 3 UA data 0 data 1 data 0 ACK 2 data 1 ACK 2 data 2 ACK 2 RR, ACK 3 ICSA 341 Data Communications & Networking

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