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Data link control protocol(4)


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Lecture Slides on Data Link Protocols from Chapter 23 of the book Electronic Communications Systems by Tomasi

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Data link control protocol(4)

  1. 1. Lecture 04 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. SDLC LOOP OPERATION Tx 1 1 1 SDLC Loop controller Primary Station Rx A B N TA A’ B’ GA Idle 1’s N’ TA Go Ahead (Consecutive Logic 1’s) GA N Turnaround (8 Logic 0’s) B Primary Data Frames A Copies Frame N Secondary Data Frames Line Control Unit Station N Rx A Rx Line Control Unit Station A Tx Tx B Copies Frame A N TA 1 1 1 1 GA Copies Frame B A’ TA N B A Rx Line Control Unit Station B ECE @Saint Louis University, Baguio City A’ B’ Tx GA 2
  3. 3. SDLC LOOP OPERATION 1. Primary transmits sequential frames to one or more secondary stations. 2. Each transmitted frame contains a secondary station address. 3. After a primary has completed transmitting, it follows the last flag of the last frame with eight consecutive logic 0’s (turnaround sequence) followed by continuous logic 1’s (goahead sequence. 4. The turnaround sequence alerts secondary stations of the end of the primary’s transmissions. 5. Each secondary, in turn, decodes the address field of each frame and removes frames addressed to them. 6. Secondary stations serve as repeaters for any down-line secondary stations. ECE @Saint Louis University, Baguio City 3
  4. 4. SDLC LOOP OPERATION 7. Secondary stations cannot transmit frames of their own unless they receive a frame with the P bit set. 8. The first secondary station that receives a frame addressed to it with the P bit set changes the seventh logic 1 in the goahead sequence to a logic 0, thus creating a flag. 9. The next down-loop secondary station that receives a frame addressed to it with the P bit set detects the turnaround sequence, any frames transmitted by other uploop secondary stations, and then the go-ahead sequence. 10. Each secondary station’s response frames are inserted immediately after the turnaround sequence or after a secondary response from an up-loop station. 11. The cycle is completed when the primary receives its own turnaround sequence, a series of response frames, and the go-ahead sequence ECE @Saint Louis University, Baguio City 4
  5. 5. SDLC LOOP CONFIGURE COMMAND/RESPONSE • • • • An unnumbered command/response (CFGR) that is used only in SDLC loop configurations. Contains a one byte descriptor in the information field. A CFGR command is acknowledged by a CFGR response. If the low order bit of the function descriptor is set, a specified command is initiated, if reset the command is cleared CFGR Subcommands Clear Beacon Test Monitor Mode Wrap Self-test Modified Link Test 00000000 0000000X 0000010X 0000100X 0000101X 0000110X ECE @Saint Louis University, Baguio City 5
  6. 6. SDLC LOOP CONFIGURE COMMAND/RESPONSE The following CFGR subcommands from the primary to the secondary will cause the secondary to perform the following: • CLEAR. Clear all previously set functions • BEACON TEST. Turn on or turn off its carrier • MONITOR MODE. Place itself into the monitor (receive only mode) • WRAP. Loop its transmissions directly to its receiver input • SELF-TEST. Initiate a series of internal diagnostic tests • MODIFIED LINK TEST. Respond to a TEST command with a TEST response that has an information field containing the first byte of the TEST command in the information field n times where the value of n is specified by the system ECE @Saint Louis University, Baguio City 6
  7. 7. SDLC TRANSPARENCY • Implemented such that a receiver does not identify a flag sequence where it is not supposed to be. • Prevents a transmitted bit sequence to accumulate a sequence of 01111110 (a flag sequence) which can possibly occur with any combination of control or alphanumeric characters • Implemented using zero-bit insertion or zero stuffing. • In zero stuffing a zero bit is automatically inserted after the occurrence of five consecutive logic 1’s except in a designated flag sequence. • When five consecutive 1s are received at the receiver and the next bit is a zero, the zero is automatically deleted or removed. ECE @Saint Louis University, Baguio City 7
  8. 8. SDLC TRANSPARENCY • ZERO BIT INSERTION Possible Inadvertent Flag 01111110 01101111 11010011 1110001100110101 01111110 Beginning Flag Address Control Frame Check Character Ending Flag After zero bit insertion: 01111110 01101111 101010011 11100001100110101 01111110 Beginning Flag Address Control Frame Check Character Ending Flag Zero Bits Inserted ECE @Saint Louis University, Baguio City 8
  9. 9. MESSAGE ABORT • Used to prematurely terminate an SDLC frame • Done only to accommodate high-priority messages, such as emergency link recovery procedures • Implemented by sending and detecting exactly 14 consecutive logic 1’s • Zeros are not inserted in an abort sequence • Terminates an existing frame and immediately begins the higher priority frame ECE @Saint Louis University, Baguio City 9
  10. 10. INVERT-ON-ZERO-ENCODING (NRZI) • Line encoding where a logic zero causes the encoded transmission level to invert from its previous state. • Used to assure clock recovery from a received signal at the receiver • Originally intended fro asynchronous modems that do not have clock recovery capabilities • The NRZI encoder/ decoder is placed in between the DTE and DCE ECE @Saint Louis University, Baguio City 10
  11. 11. HIGH-LEVEL DATA LINK CONTROL (HDLC) • The name given to several sets of substandards defined by the ISO in 1975 as a superset of SDLC including additional capabilities • Comprises of three standards for bit-oriented Data Link Control: – ISO 3309 – ISO 4335 – ISO 7809 ECE @Saint Louis University, Baguio City 11
  12. 12. HIGH-LEVEL DATA LINK CONTROL (HDLC) • ISO 3309 SPECIFICATIONS – Defines the frame structure, delimiting sequence, transparency mechanism, and error detection method used with HDLC – Uses the same Flag field as SDLC – Uses the same CRC-16 generating sequence as SDLC as specified by CCITT V.41 although its remainder for an errorless transmission at the receiver side is F0B8 instead of zero (checksum) – Offers an optional 32-bit CRC checksum – Has an option for a virtually limitless extended addressing format in addition to the standard 8-bit SDLC address ECE @Saint Louis University, Baguio City 12
  13. 13. HIGH-LEVEL DATA LINK CONTROL (HDLC) • HDLC Extended Addressing Field b0 b0 b0 01111110 0XXXXXXX 0XXXXXXX 1XXXXXXX st nd rd Beginning Flag 1 byte 2 byte 3 byte Three-byte address field • Unlike ISO, SDLC and HDLC assign the highest order bit as b0 • If b0 is a logic zero, another address byte is expected to follow. If b0 is a logic 1 it indicates the last address byte ECE @Saint Louis University, Baguio City 13
  14. 14. HIGH-LEVEL DATA LINK CONTROL (HDLC) • INFORMATION FIELD – HDLC Permits any number of bits in the information and also permits the bits to be in multiples other than 8 bits as long as it is consistent throughout the HDLC frame ECE @Saint Louis University, Baguio City 14
  15. 15. HIGH-LEVEL DATA LINK CONTROL (HDLC) • CONTROL FIELD – The HDLC control field can be extended to 16 bits, 7 bits for ns and 7 bits for nr therefore allowing up to 127 consecutive unconfirmed frames before an acknowledgement. – The supervisory format includes a fourth status condition which is selective reject (SREJ) for the combination of 11 on bit positions b4 and b5 which is a combination previously unused in the SDLC Supervisory frame control field. – Selective reject allows the retransmission of only the frames with errors one frame at a time. ECE @Saint Louis University, Baguio City 15
  16. 16. HIGH-LEVEL DATA LINK CONTROL (HDLC) • ADDITIONAL OPERATIONAL MODES – Asynchronous Response Mode (ARM) • Allows secondary stations to send unsolicited responses to the secondary – Asynchronous Balanced Mode (ABM) • Allows network operation in a peer-to-peer network environment where each station connected has equal data responsibilities and can initiate data transmission without receiving permission from any other station • Accomplished through connection on a two-wire line using ARM for half duplex, or on a four-wire line for full duplex . – Asynchronous Disconnect Mode (ADM) • Identical to the normal disconnect mode except that the secondary can initiate a DM or RIM response at any time ECE @Saint Louis University, Baguio City 16