1 / 53

Understanding Data Link Control: Flow Control, Error Detection, and Automatic Repeat Request

This chapter delves into Data Link Control (DLC), focusing on key aspects such as flow control, error detection, and correction techniques essential for reliable data transmission. Flow control ensures that data flow does not overwhelm the receiver's buffer, while error detection mechanisms, including cyclic redundancy checks (CRC) and parity bits, identify transmission errors. The chapter also discusses Automatic Repeat Request (ARQ) methods, including Stop-and-Wait and Go-Back-N, which enable efficient data retransmission and ensure reliable communications over data links.

tyme
Download Presentation

Understanding Data Link Control: Flow Control, Error Detection, and Automatic Repeat Request

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. William StallingsData and Computer Communications Chapter 7 Data Link Control

  2. Data Link Control • Flow Control • Regular the flow of data from a sender so receiver’s buffer won’t overflow. • Error Detection • Performed error-detecting code • Appends to the transmission bits • Receiver compares the code based on the receiving bits • Error Control • Retransmission of lost data

  3. Why Data Link Control • Frame synchronization • Flow control • Error control • Addressing • Control and data on same link • Link management

  4. Flow Control • Ensuring the transmitter does not overflow the receiver’s buffer • Transmission time • Time taken to transmit all bits into medium • Proportional to frame size • Propagation time • Time for a bit to traverse the link

  5. Model of Frame Transmission

  6. Stop and Wait • Source transmits frame and wait for an ACK • Destination receives frame and replies with acknowledgement • Source receives ACK and then sends next frame • Destination can stop flow by not sending ACK • Works well for a few large frames • Inefficient link utilization

  7. Fragmentation • Large block of data may be split into small frames due to • Limited buffer size • Errors detected sooner (transmission time is less) • If errors, retransmission of smaller data • Prevents one station occupying medium for long periods • Stop and wait becomes inadequate

  8. Stop and Wait Link Utilization

  9. Sliding Windows Flow Control • Allow multiple frames to be sent • Receiver has buffer W long • Transmitter can send up to W frames without ACK • Each frame is numbered • ACK includes number of next frame expected • Sequence number bounded by size of field (k) • Sequence number is between 0 .. 2k - 1 • Frames are numbered modulo 2k

  10. Sliding Window Diagram

  11. Example Sliding Window

  12. Sliding Window Enhancements • Receiver can acknowledge frames without permitting further transmission (Receive Not Ready) • Must send a normal acknowledge to resume • If duplex, use piggybacking • Able to send both data and ACK in the same frame • If no data to send, use acknowledgement frame • If data but no ACK to send, send both data and last ACK number again. Receivers ignores the duplicate ACKs.

  13. Error Detection • Additional bits added by transmitter for error detection code based on the data • Receivers performs the same and compare the results. If different, an error is detected. • Example - Parity • Parity bit is added to the code • Even parity (number of ones = even) Synch. transmission • Odd parity (number of ones = odd) Asynch. transmission • Odd parity - even number of bit errors goes undetected

  14. Cyclic Redundancy Check (CRC) • For a block of k-bit, transmitter generates n-bit frame check sequence (FCS) • Transmit k+n bits which is exactly divisible by some number, P • Receiver divides frame by that number • If no remainder, assume no error • See Stallings chapter 7, page 203 for example (Will prove and work on the example on page 204 in class. • T = (k + n)-bit transmitted frame, n < kM = k-bit messageF = n-bit FCSP = pattern of n+1 bits, this will be given.T = 2nM + F = = 2nM + RT/P = (2nM + R)/PTo find F where F = R, we do 2nM ÷ P and then take remainder. We also use mod-2 with no carry or exclusive-OR operation for this division.

  15. Cyclic Redundancy Check (CRC) Given Message M = 1010001101 (10 bits) Pattern P = 110101 (6 bits) Find FCS or CRC or R = ? (5 bits) on Page 204 • 2nM ÷ P =1010001101x 25 = 101000110100000 • Make sure use exclusive-OR operation. • After the division, Q = quotient = 1101010110R = FCS = 01110 • Receiver perform T/P. If remainder R = 0, no error. Otherwise error occurs.Xmitted Frame = T = M2n + R = 101000110101110

  16. Error Control • Detection and correction of errors in the frame • Types of errors • Lost frames • Damaged frames • Error control techniques – automatic repeat request (ARQ) • Error detection • Positive acknowledgment • Retransmission after timeout • Negative acknowledgement and retransmission

  17. Automatic Repeat Request (ARQ) • To make the link transmit reliable. • Based on flow control mechanism • Types of ARQ • Stop and wait • Go back N • Selective reject (selective retransmission)

  18. Stop and Wait ARQ • Source transmits single frame, start timer • Wait for ACK before next send • If no ACK within timeout, re-send the same frame. • If received frame damaged, discard it • If ACK damaged,transmitter will not recognize it • Transmitter will retransmit • Receive gets two copies of frame • Use ACK0 and ACK1

  19. Stop and Wait -Diagram

  20. Stop and Wait - Pros and Cons • Simple • Inefficient

  21. Go Back N ARQ (1) • Based on sliding window • If no error, use RR (receive ready or piggybacked ACK) as usual with next frame expected • Use window to control number of outstanding frames • If error, reply with REJ (reject) • Discard that frame and all future frames until error frame received correctly • Transmitter must go back and retransmit that frame and all subsequent frames

  22. Go Back N - Damaged Frame • Receiver B detects error in frame i, discards frames and takes no action. • Transmitter A sends i+1 frame. B reject with REJ i A gets REJ i A retransmits frame i and all subsequent frames • A does not send more frames. A times out. Sends RR (p bit = 1).B replies with RR with next frame, i , expected A re-sends frame i .

  23. Go Back N - Damaged RR 2. Damaged RR • Receiver B gets frame i and send s RR (i+1) which is lost.Acknowledgements are cumulative, so next acknowledgement (i+n) may arrive before transmitter times out on frame I • If transmitter times out, it sends RR with P bit set as before.This can be repeated a number of times before a reset procedure is initiated.

  24. Go Back N - Damaged Rej • This is equivalent to Case 1b as discussed earlier.

  25. Go Back N - Diagram

  26. Selective-Reject ARQ • Also called selective retransmission • Only rejected frames are retransmitted (SREJ) • Subsequent frames are accepted by the receiver and buffered • More efficient, minimizes retransmission • Receiver must maintain large enough buffer • More complex logic in transmitter for out-of-order frames

  27. Selective Reject -Diagram

  28. High Level Data Link Control • HDLC • ISO 33009, ISO 4335

  29. HDLC Station Types • Primary station • Controls operation of link • Frames issued are called commands • Maintains separate logical link to each secondary station • Secondary station • Under control of primary station • Frames issued called responses • Combined station • May issue commands and responses

  30. HDLC Link Configurations • Unbalanced • One primary and one or more secondary stations • Supports full duplex and half duplex • Balanced • Two combined stations • Supports full duplex and half duplex

  31. HDLC Transfer Modes (1) • Normal Response Mode (NRM) • Asynchronous balanced mode (ABM) • Asynchronous response mode (ARM)

  32. HDLC Transfer Modes (2) • Normal Response Mode (NRM) • Unbalanced configuration • Primary initiates transfer to secondary • Secondary may only transmit data in response to command from primary • Used on multi-drop lines (multiple terminals connect to host) • Point-to-point links

  33. HDLC Transfer Modes (3) • Asynchronous Balanced Mode (ABM) • Balanced configuration • Either station may initiate transmission without receiving permission • Most popular • More efficient - no polling overhead

  34. HDLC Transfer Modes (4) • Asynchronous Response Mode (ARM) • Unbalanced configuration • Secondary may initiate transmission without permission form primary • Primary responsible for line • Rarely used

  35. Frame Structure • Synchronous transmission • All transmissions are in frames • Single frame format for all data and control exchanges (header + Data + trailer)

  36. Frame Structure Diagram

  37. Flag Fields • Delimit frame at both ends • 01111110 • May close one frame and open another • Receiver looks for flag sequence to synchronize • Bit stuffing used to avoid confusion with data containing 01111110 • 0 inserted after every sequence of five 1s • If receiver detects five 1s it checks next bit • If 0, it is deleted • If 1 and seventh bit is 0, accept as flag • If sixth and seventh bits 1, sender is indicating abort

  38. Bit Stuffing • Example with possible errors

  39. Address Field • Identifies secondary station that sent or will receive frame • Usually 8 bits long • May be extended to multiples of 7 bits • LSB of each octet indicates that it is the last octet (1) or not (0) • All ones (11111111) is broadcast

  40. Control Field • Frame types • Information (I-frame) - data to be transmitted to user, flow control, and error control are piggybacked in ARQ • Supervisory (S-frame) - ARQ when piggyback not used • Unnumbered (U-frame) - supplementary link control • First one or two bits of control filed identify frame type • Remaining bits explained in the following slide

  41. Control Field Diagram

  42. Poll/Final Bit • Use depends on context • Command frame • Referred as P bit • 1 to solicit (poll) response from peer • Response frame • Referred as F bit • 1 indicates response to soliciting command

  43. Information Field • Only in I-frames and some U-frames • Must contain integral number of octets • Variable length, system dependent

  44. Frame Check Sequence Field • FCS • Error detection • Normal 16-bit CRC (Section 7.2) • Optional 32-bit CRC

  45. HDLC Operation • Exchange of I-frames, S-frames, and U-frames between two stations • Three phases • Initialization • Data transfer • Disconnect

  46. Examples of Operation (1)

  47. Examples of Operation (2)

  48. Other DLC Protocols (LAPB,LAPD) • Link Access Procedure, Balanced (LAPB) • Part of X.25 (ITU-T) • Subset of HDLC – ABM, same frame format as HDLC • Point to point link between system and packet switching network node • Link Access Procedure, D-Channel • ISDN (ITU-D) • ABM • Always 7-bit sequence numbers (no 3-bit) • 16 bit address field contains two sub-addresses, One for device and one for user

  49. Other DLC Protocols (LLC) • Logical Link Control (LLC) • IEEE 802,for LAN • Different frame format • Link control split between medium access layer (MAC) and LLC (on top of MAC) • No primary and secondary - all stations are peers • MAC Layer - both sender and receiver address • Error detection at MAC layer - 32 bit CRC • LLC layer - destination and source access points (DSAP, SSAP), same format as HDLC • Services – connection-mode, unACK connectionless and ACK connectionless

  50. Other DLC Protocols (Frame Relay) (1) • Streamlined capability over high-speed packet-switched networks • Used in place of X.25 • Uses Link Access Procedure for Frame-Mode Bearer Services (LAPF) • Two protocols • Control - similar to HDLC • Core - subset of control

More Related