2. COE 341: Data & Computer Communications (T061) Dr. Marwan Abu-Amara Chapter 7: Data Link Control Protocols

3. COE 341 (T061) – Dr. Marwan Abu-Amara 2 Contents 1. Error Control a. Stop-and-Wait ARQ b. Go-Back-N ARQ c. Selective-Reject ARQ 2. Flow Control a. Stop-and-Wait flow control b. Sliding-Window flow control 3. High-Level Data Link (HDLC)

4. COE 341 (T061) – Dr. Marwan Abu-Amara 3 What is Data Link Control The logic or procedures used to convert the raw stream of bits provided by the physical layer into a “reliable” connection Requirements and Objectives:  Frame synchronization  Error control  Flow control  Addressing  Multiplexing data and control on connection  Link management

5. COE 341 (T061) – Dr. Marwan Abu-Amara 4 Error & Flow Control We now study two main functions of the data- link layer  Error control  Flow control Why Error & Flow Control together?  We usually lump Error Control and Flow Control discussions together because the data-link protocols used for error control are also used for flow control

6. COE 341 (T061) – Dr. Marwan Abu-Amara 5 Error Control Definition Error Control is the second type of methods used to “handle” errors in frames  First type is FEC Error Control methods are those that use retransmission in the case an error occurs in a frame  This process is called Automatic Repeat Request (ARQ) In general, a retransmission is needed if a:  Frame is erroneous  Frame is lost (i.e. does not arrive or arrives too late) There are several reasons for this to happen (e.g. existence of noise, network congestion…)

7. COE 341 (T061) – Dr. Marwan Abu-Amara 6 Flow Control Definition We define Flow Control as: “The set of procedures used to restrict the amount of data that a TX can send before waiting for an acknowledgement from the RX” (per Forouzan)  This is to avoid overwhelming the RX by the flow of data from TX RX does not ‘absorb’ the received data instantly It buffers (temporarily stores) the data it receives to do some processing before sending it upward to higher layers Without flow control, the RX buffer may overflow and data gets lost

8. COE 341 (T061) – Dr. Marwan Abu-Amara 7 Flow Control Assume:  No frames lost  No frames arrive in error  Frames arrive in the same order they were sent, following a variable propagation delay

9. COE 341 (T061) – Dr. Marwan Abu-Amara 8 A Model of Frame Transmission Loss Error Only one frame traveling on the link at any given time

10. COE 341 (T061) – Dr. Marwan Abu-Amara 9 Error & Flow Control Protocols There are three main types of error control protocols (ARQ-based):  Stop-and-Wait ARQ  Sliding Window ARQ Go-back-N ARQ Selective-Reject ARQ There are two main types of flow control protocols:  Stop-and-Wait  Sliding Window

11. COE 341 (T061) – Dr. Marwan Abu-Amara 10 Categories of Error Control

12. COE 341 (T061) – Dr. Marwan Abu-Amara 11 Categories of Flow Control

13. COE 341 (T061) – Dr. Marwan Abu-Amara 12 Stop-and-Wait ARQ Stop-and-Wait ARQ is a simple protocol:  TX keeps a copy of the last frame sent  After receiving the frame, RX sends back an ACK  After receiving this ACK, TX sends another frame and so on… Both Data & ACK frames are alternately numbered with “0” or “1”  Data Frame “0” is acknowledged by ACK “1”  Data Frame “1” is acknowledged by ACK “0”

14. COE 341 (T061) – Dr. Marwan Abu-Amara 13 Stop-and-Wait ARQ: Normal Operation

15. COE 341 (T061) – Dr. Marwan Abu-Amara 14 Stop-and-Wait ARQ: Abnormal Operation Stop-and-Wait ARQ deals with encountered anomalies as follows:  Lost or Damaged Frames: RX discards them silently i.e. without sending Negative-ACK (NACK) back to TX RX keeps its current value for R (R var. defined on prev. slide)  Lost or Damaged ACK: TX discards damaged ACK TX keeps a timer, after sending a frame, within which ACK must be received. Otherwise, ACK is considered lost. In both situations, Lost and Damaged ACK, the TX sends the frame again

16. COE 341 (T061) – Dr. Marwan Abu-Amara 15 Stop-and-Wait ARQ: Lost Frame

17. COE 341 (T061) – Dr. Marwan Abu-Amara 16 Stop-and-Wait ARQ: Lost ACK

18. COE 341 (T061) – Dr. Marwan Abu-Amara 17 Stop-and-Wait ARQ: Delayed ACK Another abnormality: TX receiving a late ACK  Timer has already expired The ACK was considered lost  Frame is re-sent again This frame will be duplicate at RX and discarded Also its ACK will be discarded when received back at TX  Then the late ACK arrives Now TX can send the next frame

19. COE 341 (T061) – Dr. Marwan Abu-Amara 18 Stop-and-Wait ARQ: Delayed ACK Lost

20. COE 341 (T061) – Dr. Marwan Abu-Amara 19 Stop-and-Wait ARQ Piggybacking Piggybacking is a method that combines the data and the ACK in one frame It is useful in bidirectional communications Stations can send their data along with ACK to data previously received Piggybacking is faster and saves bandwidth

21. COE 341 (T061) – Dr. Marwan Abu-Amara 20 Stop-and-Wait ARQ Drawback In Stop-and-Wait ARQ, the line is not efficiently utilized because only one frame is sent at a time  Its ACK must be awaited for  During this waiting period no frames are sent Inefficiency gets even worse when  TX speed is high TX quickly sends frame then sits idle  Propagation distance is high It takes longer for the frame and its ACK to reach destination  Both cases leave the TX waiting idle for longer times

22. COE 341 (T061) – Dr. Marwan Abu-Amara 21 Stop-and-Wait Efficiency Let:  T f : Time to transmit a frame = L f / R  T ACK : Time to transmit an ACK = L ACK / R  T Prop : Propagation time = (link distance) / V  T Proc : Processing time We define the frame total time as (see next slide):  T total = T f + 2 T Prop + T Proc + T ACK The utilization of the link is defined as:  U = (T f )/(T total )

23. COE 341 (T061) – Dr. Marwan Abu-Amara 22 Stop-and-Wait Efficiency It is common to ignore T Proc and T ACK because they are negligible compared to other times In which case the utilization becomes:  U = (T f )/(T f +2T prop ) = 1/(1 + 2a)  Where: a = T prop /T f  a is called the “length of the link in bits”: length of medium in bits compared to frame length (L f ) Notice that for very small a (i.e. when 1 st transmitted bit reaches RX, source will still be transmitting),U  100% Notice that for very big a (i.e. frame transmission is completed before 1st bit reaches destination),U  0% Efficient for links where a << 1 (long frames on a short link, i.e. fills link) TXRX TfTf T prop T proc T ACK

24. COE 341 (T061) – Dr. Marwan Abu-Amara 23 Stop and Wait Efficiency: Example Compare the efficiency of stop-and-wait error control for two links using the parameter ‘a’:  Fame size, L = 1000 characters of 8 bits each, = 8000 bits 200-m optical fiber link Data rate, R = 1 Gbps Typical wave velocity, V = 2 x 10 8 m/s Frame TX time, t f = L/R = 8000/(1x10 9 ) = 8  s Propagation time, t prop = d/V = 200/(2x10 8 ) = 1  s End of first frame reaches RX after 8+1 = 9  s from start ACK takes 1  s more to reach TX, which sends 2 nd frame after 10  s Utilization = 8/10 = 80% (=1/(2a+1)) Satellite link between 2 ground stations d = 2 x 36,000 km, Data rate, R = 1 Mbps Typical wave velocity, V = 3 x 10 8 m/s Frame TX time, t f = L/R = 8000/(1x10 6 ) = 8 ms Propagation time, t prop = d/V = 2x36x10 6 /(3x10 8 ) = 240 ms End of first frame reaches RX after 8+240 = 248  s from start ACK takes 240 ms more to reach TX, which sends 2 nd frame after 488 ms Utilization = 8/488 = 1.6% (=1/(2a+1))

25. COE 341 (T061) – Dr. Marwan Abu-Amara 24 Sliding Window Protocol Stop-and-Wait can be very inefficient when a > 1 Protocol:  Assumes full duplex line  Source A and Destination B have buffers each of size W frames  For k-bit sequence numbers:  Frames are numbered: 0, 1, 2, …, 2 k -1, 0, 1, … (modulo 2 k )  ACKs (RRs) are numbered: 0, 1, 2, …, 2 k -1, 0, 1, … (modulo 2 k )  A is allowed to transmit up to W frames without waiting for an ACK  B can receive up to W consecutive frames  ACK J (or RR J), where 0  J  2 k -1, sent by B, means B has received frames up to frame J-1 and is ready to receive frame J  B can also send RNR J: B has received all frames up to J-1 and is not ready to receive any more Window size W, can be less or equal to 2 k -1

26. COE 341 (T061) – Dr. Marwan Abu-Amara 25 Sliding Window Protocol Example of Sliding-Window-Protocol: k = 3 bits, W = 7 Observations: A may Tx W = 7 frames (F0, F1, …, F6) After F0, F1, & F2 are Tx- ed, window is shrunk (i.e. can not transmit except F3, F4, …, F6) When B sends RR3, A knows F0, F1 & F2 have been received and B is ready to receive F3 Window is advanced to cover 7 frames (starting with F3 up to F1) A sends F3, F4, F5, & F6 B responds with RR4 when F3 is received – A advances the window by one position to include F2 W W W W W = distance between first unacknowledged frame and last frame that can be sent

27. COE 341 (T061) – Dr. Marwan Abu-Amara 26 Go-back-N ARQ Go-back-N ARQ improves the efficiency of the line by sending up to W frames before worrying about ACK  This is called Pipelining (several tasks are started before the 1 st is finished) For this, the frames must be sequentially numbered Sequence number must be included in the header of the frame  If m bits are reserved for sequence number then the sequence numbers range from 0 to 2 m – 1 Example:  m = 3 bits, then seq. numbers range is 0-7 and could be used as: 0, 1, 2, 3, 4, 5, 6, 7, 0, 1, 2, 3, 4, 5, 6, 7, 0, 1,…

28. COE 341 (T061) – Dr. Marwan Abu-Amara 27 Go-back-N ARQ Sliding Window At the TX side, to hold the outstanding frames until they are individually ACK, we use a window  The window size W cannot exceed 2 m – 1  W is fixed in this protocol but may be variable in others (e.g. TCP) Each time a proper ACK is received for a frame the window slides past this frame, hence the name Sliding Window  The acknowledged frames can be purged out of the TX memory As an example, next slide shows that frame 0 and 1 have been acknowledged  So the sliding window slides just past them in (b)

29. COE 341 (T061) – Dr. Marwan Abu-Amara 28 TX Sliding Window In (b), frame 0 and frame 1 were properly acknowledged

30. COE 341 (T061) – Dr. Marwan Abu-Amara 29 RX Sliding Window At the RX side, the size of the window is always 1 The RX window is centered on the next expected frame number If any other frame arrives (i.e. out of sequence arrival), it is immediately discarded If the right frame arrives, the window slides past it to zoom on the next expected frame

31. COE 341 (T061) – Dr. Marwan Abu-Amara 30 RX Sliding Window

32. COE 341 (T061) – Dr. Marwan Abu-Amara 31 Go-back-N ARQ Control Variables TX keeps track of three variables  S: Sequence number of the recently sent frame  S F : Sequence number of the first frame in the window  S L : Sequence number of the last frame in the window The frame size W is related to these variables by:  W = S L – S F + 1 On the other hand the RX has only one variable:  R: Expected frame sequence number

33. COE 341 (T061) – Dr. Marwan Abu-Amara 32 Go-back-N ARQ: TX & RX Control Variables

34. COE 341 (T061) – Dr. Marwan Abu-Amara 33 Timers & Acknowledgments TX sets a timer for each frame sent RX has no timers RX sends an ACK if a frame arrives with no errors and in order If RX receives damaged or out of sequence frame, it silently discards them until it receives the expected frame  The silence of the RX causes the TX timer to expire  This in turn causes the TX to go back and send all frames starting from the non-acknowledged frame  This is why it is called Go-back-N ARQ Example:  If TX has already sent frame-6 but the timer for frame-3 expires without receiving an ACK for it, the TX goes back and send frames 3, 4, 5, 6

35. COE 341 (T061) – Dr. Marwan Abu-Amara 34 Go-back-N ARQ Normal Operation

36. COE 341 (T061) – Dr. Marwan Abu-Amara 35 Go-back-N ARQ Lost Frame Operation Here, ACK2 acknowledges frames 0 & 1 at the same time

37. COE 341 (T061) – Dr. Marwan Abu-Amara 36 Quiz: Window Size Condition for Go-back-N ARQ Earlier, we mentioned that the size of the window must be W  2 m – 1 i.e. W < 2 m where “m” is the number of bits reserved (in the frame overhead) for the sequence number By comparing the figures across explain the need for this condition

38. COE 341 (T061) – Dr. Marwan Abu-Amara 37 Sliding Window Protocol Animation for Sliding Window protocol  Sliding Window Protocol Simulation (http://www.cs.stir.ac.uk/~kjt/software/comms/jasper/S WP3.html) Sliding Window Protocol Simulation (http://www.cs.stir.ac.uk/~kjt/software/comms/jasper/S WP3.html)

39. COE 341 (T061) – Dr. Marwan Abu-Amara 38 Two neighboring nodes (A and B) use a sliding- window protocol with a 3-bit sequence number. As the ARQ mechanism, go-back-N is used with a window size of 4. Assuming A is transmitting and B is receiving, show the window positions for the following succession of events: a) Before A sends any frames b) After A sends frame 0, 1, 2 and B acknowledges 0, 1 and the ACKs are received by A c) After A sends frames 3, 4, and 5 and B acknowledges 4 and the ACK is received by A Example: Problem 7-9

40. COE 341 (T061) – Dr. Marwan Abu-Amara 39 Example: Problem 7-9 - Solution a) b) c) WWW

41. COE 341 (T061) – Dr. Marwan Abu-Amara 40 Sliding Window Protocol - Piggybacking When using sliding window protocol in full duplex connections:  Node A maintains its own transmit window  Node B maintains its own transmit window  A frame contains: data field + ACK field  There is a sequence number for the data field, and a sequence number for the ACK field

42. COE 341 (T061) – Dr. Marwan Abu-Amara 41 Selective-Repeat ARQ Go-back-N ARQ simplifies the job of the RX  RX keeps track of one control variable R only  Out-of-sequence frames are simply dropped, not buffered However, this protocol is inefficient for noisy links where the probability of damaged frames is high  This means many retransmissions will occur which uses up the bandwidth of the link For noisy links, a more efficient mechanism is used in which only the damaged frame is retransmitted (not all N frames)  It is called Selective-Repeat ARQ

43. COE 341 (T061) – Dr. Marwan Abu-Amara 42 Selective-Repeat ARQ TX & RX Windows Window configuration for the TX is the same as for Go-back-N ARQ  However, the window size should be W  ( ½  2 m ) i.e. W  2 (m-1) RX window must have the same size as the TX  This a major difference with Go-back-N ARQ where window W = 1 for the RX RX window specifies a range of a acceptable frame sequence numbers Also, in selective-repeat ARQ the RX uses negative ACK (NACK) to report the sequence of a damaged frame to the TX

44. COE 341 (T061) – Dr. Marwan Abu-Amara 43 Selective-Repeat ARQ: TX & RX Control Variables Here, S = S F

45. COE 341 (T061) – Dr. Marwan Abu-Amara 44 Selective-Repeat ARQ Operation: Lost Frame Example Here, ACK2 acknowledges frames 0 & 1 at the same time 0

46. COE 341 (T061) – Dr. Marwan Abu-Amara 45 Quiz: Window Size Condition for Selective- Repeat ARQ Earlier, we mentioned that the size of the window must be W  2 m – 1 where “m” is the number of bits reserved (in the frame overhead) for the sequence number By comparing the figures across explain the need for this condition

47. COE 341 (T061) – Dr. Marwan Abu-Amara 46 Sliding Window Protocol - Efficiency Refer to Appendix A When window size is W (for error free), link utilization, U, is given by where a = T prop /T f (i.e. length of link in bits) Sliding window protocol can achieve 100% utilization if W  (2a + 1) The smaller the W needed the better! (Why?)  To get high value for U, small value for a is needed as well !!

48. COE 341 (T061) – Dr. Marwan Abu-Amara 47 Sliding Window Efficiency: Example Compare the efficiency of Sliding Window flow control for two links using the parameter ‘a’:  Fame size, L = 1000 characters of 8 bits each, = 8000 bits 200-m optical fiber link Data rate, R = 1 Gbps Typical wave velocity, V = 2 x 10 8 m/s Frame TX time, t f = L/R = 8  s Propagation time, t prop = d/V = 1  s a = t prop / t f = 0.125 100 % link utilization is achieved with window size W: W  (2 a+1)  (2 x 0.125 +1)  1.25 i.e. W = 2 (A window of just 2 frames) Satellite link between 2 ground stations d = 2 x 36,000 km, Data rate, R = 1 Mbps Typical wave velocity, V = 3 x 10 8 m/s Frame TX time, t f = L/R = 8 ms Propagation time, t prop = d/V = 240 ms a = t prop / t f = 30 100 % link utilization is achieved with window size W: W  (2 a+1)  (2 x 30 +1)  61 W = 61, k = 6 bit (Large window and buffer sizes) For k = 3 bits, W = 7: Utilization U = W/(2a+1) = 7/(61) = 11.5% > 1.6% for Stop and wait.

49. COE 341 (T061) – Dr. Marwan Abu-Amara 48 Stop-and-Wait & Sliding Window as Flow Control Protocols Problem:  Qualitatively explain how the Stop-and-Wait and the Sliding Window protocols could be used for flow control  Hint: Think about the use of ACK and when the RX could send it back to the TX

50. COE 341 (T061) – Dr. Marwan Abu-Amara 49 High-Level Data Link Control Protocol (HDLC)  One of the most important data link control protocols and it is the basis for many data link control protocols.  The job of the HDLC layer (Link Layer, Layer 2) is to ensure that data passed up to the next layer has been received exactly as transmitted (i.e error free, without loss and in the correct order)  Another important job is flow control, which ensures that data is transmitted only as fast as the receiver can receive it.  There are two distinct HDLC implementations  HDLC NRM (see (SDLC), andSDLC  HDLC Link Access Procedure Balanced (LAPB)  Usually when referring to HDLC people mean LAPB or some variation

51. COE 341 (T061) – Dr. Marwan Abu-Amara 50 High-Level Data Link Control Protocol (HDLC)  To satisfy a variety of applications, HDLC defines three types of stations, two link configurations, and three types of data transfer modes of operation.  Station types:  Primary Station (PS):  Responsible for controlling the operation of the link  Frames issued by the PS are called commands  Secondary Station (SS): Frames issued by the SS are called responses – operates under the control of a primary station  Combined Station: issues commands and responses  Link configurations:  Unbalanced: one primary plus one or more secondary  Balanced: two combined stations

52. COE 341 (T061) – Dr. Marwan Abu-Amara 51 High-Level Data Link Control Protocol (HDLC)  Transfer modes:  Normal Response Mode (NRM) – used in unbalanced config.; secondary may only tx data in response to a command from primary  Asynchronous Response Mode (ARM) – used in unbalanced config.; Secondary may initiate data tx without explicit permission; primary still retains line control (initialization, error recovery, …)  Asynchronous Balanced Mode (ABM) – used in balanced config.; either combined station may tx data without receiving permission from other station  Animation for HDLCHDLC

53. COE 341 (T061) – Dr. Marwan Abu-Amara 52 HDLC – Applications NRM:  Point-multipoint (multi-drop line): one computer (primary) polls multiple terminals (secondary stations) number of terminals are connected to a host computer ARM: rarely used ABM: most widely used (no polling involved)  Full duplex point-to-point

54. COE 341 (T061) – Dr. Marwan Abu-Amara 53 HDLC NRM

55. COE 341 (T061) – Dr. Marwan Abu-Amara 54 HDLC ABM

56. COE 341 (T061) – Dr. Marwan Abu-Amara 55 HDLC – Frame Structure

57. COE 341 (T061) – Dr. Marwan Abu-Amara 56 HDLC – Frame Format Flag:  Size: 1 Byte  Special pattern 0 1 1 1 1 1 1 0 used as frame begin/end and synch.  Used in Header and trailer Address:  Size: 1 Byte (or more for larger networks)  If primary station created the frame, the address is that of the destination secondary station  If secondary station created the frame, the address is that of the source secondary station  Networks not using “primary/secondary” (e.g. Ethernet) use 2-Byte address (source/destination) Control:  Size: 1 or 2 Bytes  Used for error & flow control Information:  Size: Varies from network to network. Always fixed within a network  Contains user data from Network layer or Network Management information FCS:  Size: 2 or 4 Bytes  Implements ITU-T CRC for error detection

58. COE 341 (T061) – Dr. Marwan Abu-Amara 57 HDLC – Frame Structure – Flag Field  Flag Field: unique pattern 01111110  Used for synchronization  To prevent this pattern form occurring in data  bit stuffing procedure is used  Tx-er inserts a 0 after each 5 1s  Rx-er, after detecting flag, monitors incoming bits – when a pattern of 5 1s appears; the 6 th /7 th bit are checked:  If 0, it is deleted  If 10, this is a flag  If 11, this is an ABORT Pitfalls of bit stuffing: one bit errors can split one frame into two or merge two frames into one

59. COE 341 (T061) – Dr. Marwan Abu-Amara 58 HDLC Data Transparency If the data field of an HDLC frame contains a pattern identical to the flag pattern (01111110), the RX will interpret it as end-of-frame flag  Next bits will be considered part of the next frame  This is called lack of Transparency Bit Stuffing is the process of adding one extra 0 whenever there are five consecutive 1s in the data so that the receiver does not mistake the data for a flag

60. COE 341 (T061) – Dr. Marwan Abu-Amara 59 HDLC Bit Stuffing & Removal

61. COE 341 (T061) – Dr. Marwan Abu-Amara 60 HDLC Bit Stuffing Algorithm

62. COE 341 (T061) – Dr. Marwan Abu-Amara 61 HDLC Bit Stuffing Problems: With bit stuffing, Single-bit errors could split a frame into 2 or merge two frames into 1. 01111100 (Bit stuffed) 01111110 Frame Splitting Frame Merging 01111110 01111100

63. COE 341 (T061) – Dr. Marwan Abu-Amara 62 HDLC Frame Structure – Address Field  Address field identifies the secondary station that transmitted or is to receive frame  Not used (but included for uniformity) for point-to-point links  Extendable – by prior arrangement  Address = 11111111 (single octet) used by the primary to broadcast to all secondary stations Extended Address Field

64. COE 341 (T061) – Dr. Marwan Abu-Amara 63 HDLC Frame Structure – Control Field  Three types of frames: I, S, and U  Information frame (I): carry user data (upper layers) – flow and error control info is piggybacked on these frames as well  Supervisory frame (S): carry flow and error control info when piggybacking is not used  Unnumbered frame (U): provide supplementary link control  First 2 bits of the control field determine the type of frame, the remaining are organized as shown in figure 7.7.c and d (next slide)next slide  Poll/Final (P/F) bit:  In command frames (P): used to solicit response from peer entity  In response frames (F): indicate response is the result of soliciting command

65. COE 341 (T061) – Dr. Marwan Abu-Amara 64 HDLC Frame Structure – Control Field  Extension of sequence number  7-bit sequence numbers rather than 3-bit ones  “Set-mode” command  extends control field to 16 bit for S and I frames  Unnumbered frames always use 8-bit

66. COE 341 (T061) – Dr. Marwan Abu-Amara 65 HDLC Frame Structure – Information/FCS Fields  Information field:  Present ONLY in I-frames and some U-frames  Contains integer number of octets  Length is variable – up to some system defined maximum  FCS field:  CRC error detecting code  Calculated from ALL remaining bits in frame (excluding flag field)  Normally 16 bits (CRC-CCITT polynomial = X 16 +X 12 +X 5 +1), or  32-bit optional FCS using CRC-32

67. COE 341 (T061) – Dr. Marwan Abu-Amara 66 HDLC Frames HDLC uses Synchronous transmission HDLC defines three types of frames  Information frames (I-frames) Used to transport user data and user control information relating to the user data (e.g. piggybacking)  Supervisory frames (S-frames) Used to transport control information only  Unnumbered frames (U-frames) Used in the link management

68. COE 341 (T061) – Dr. Marwan Abu-Amara 67 HDLC Generic Frame Format Indicates that the first transmitted field is the Header flag, then the address, then the control… Header Trailer

69. COE 341 (T061) – Dr. Marwan Abu-Amara 68 HDLC I, S & U Frames Format

70. COE 341 (T061) – Dr. Marwan Abu-Amara 69 HDLC I-Frames I-frames are designed to carry user data from Network-layer They can include flow & error control information (piggybacking) The bits of the I-frame Control field are:  First bit is always “0”. It means it is an I-frame  Next 3-bits, called N(S), define the sequence of the frame So possible sequences are 0,1,2,3,4,5,6,7  Next bit is called P/F (Poll or Final) When a primary station polls other stations, it sets this bit to 1 When a secondary station responds to a poll, it sets this bit to 1  Next 3-bits, called N(R), define the value of the ACK when piggybacking is used

71. COE 341 (T061) – Dr. Marwan Abu-Amara 70 HDLC I-Frames

72. COE 341 (T061) – Dr. Marwan Abu-Amara 71 HDLC S-Frames

73. COE 341 (T061) – Dr. Marwan Abu-Amara 72 HDLC U-Frames

74. COE 341 (T061) – Dr. Marwan Abu-Amara 73 U-frame control command and response Command/responseMeaning SNRMSet normal response mode SNRMESet normal response mode (extended) SABMSet asynchronous balanced mode SABMESet asynchronous balanced mode (extended) UPUnnumbered poll UIUnnumbered information UAUnnumbered acknowledgment RDRequest disconnect DISCDisconnect DMDisconnect mode RIMRequest information mode SIMSet initialization mode RSETReset XIDExchange ID FRMRFrame reject

75. COE 341 (T061) – Dr. Marwan Abu-Amara 74 HDLC – Operation  HDLC operation consists of the exchange of I- frames, S-frames, and U-frames between two stations.  Table 7.1 list types of Control/Response functions for various frame types  The operations of HDLC involve three phases:  Initialization (by either side): U-Frames Both agree on various options  Data Transfer (by the two sides): I- and S-Frames Exchange of user data and control info for flow and error control  Disconnect (by either side): U-Frames Signaling termination of operation

76. COE 341 (T061) – Dr. Marwan Abu-Amara 75 HDLC – Operation  Initialization: (Link Setup & Disconnect)  Issue SABME command and starts timer  B responds with UA (or DM if request is rejected)  A receives UA and initializes its variables  To disconnect: issue DISC command  Followed by UA SABME: Set Asynchronous balanced/extended mode;7-bit sequence

77. COE 341 (T061) – Dr. Marwan Abu-Amara 76 HDLC – Operation  Data Transfer:  Once initialization is complete a logical path is established.  Both sides start to send I-frames (Full-duplex exchange) starting with seq. number 0  N(S), N(R) both are seq. number to support flow and error control  N(R) is the ACK for the I-frame received; it enables the HDLC module to indicate which number I-frame it expect to receive next.  RR is used (S-frame) when there is no reverse user data (I-frame) traffic

78. COE 341 (T061) – Dr. Marwan Abu-Amara 77 HDLC – Operation  Disconnect:  Either side can issue a DISC frame to request disconnect  The remote entity MUST accept the request by sending UA.  Any outstanding I-frame will be lost.  Busy condition: Notice the use of P/F bit.  When A is unable to keep up with the speed of the transmitter “B” or buffer is full.  A sends RNR, to halt the transmission of B  To check the readiness of A, periodically B sends RR frame with P set.  Once the condition of being busy is cleared A responds with F=1

79. COE 341 (T061) – Dr. Marwan Abu-Amara 78 HDLC – Operation  Reject Recovery  Reject Recovery:  I-frame 4 was lost  B receives I-frame 5 (out of order) – responds with REJ 4  A resend I-frame 4 and all subsequent frames (Go-back-N)  Timeout Recovery:  A sends I-frame 3 – but it is lost  Timer expires before acknowledgement arrives  A polls Node B  B responds indicating it is still waiting for frame 3 – B set the F bit because this a response to A’s solicitation

80. COE 341 (T061) – Dr. Marwan Abu-Amara 79 HDLC Example: Piggybacking w/o Errors Next slide shows an exchange using piggybacking when there is no error Station A begins the exchange of information with an I- frame numbered 0 followed by another I-frame numbered 1 Station B piggybacks its acknowledgment of both frames onto an I-frame of its own Station B’s first I-frame is also numbered 0 [N(S) field] and contains a 2 in its N(R) field, acknowledging the receipt of A’s frames 1 and 0 and indicating that it expects frame 2 to arrive next Station B transmits its second and third I-frames (numbered 1 and 2) before accepting further frames from station A  Its N(R) information, therefore, has not changed: B frames 1 and 2 indicate that station B is still expecting A’s frame 2 to arrive next

81. COE 341 (T061) – Dr. Marwan Abu-Amara 80 HDLC Example: Piggybacking w/o Errors

82. COE 341 (T061) – Dr. Marwan Abu-Amara 81 HDLC Example: Piggybacking with Errors Suppose frame 1 sent from station B to station A has an error Station A informs station B to resend frames 1 and 2 (the system is using the Go-Back-N mechanism) Station A sends a reject supervisory frame to announce the error in frame 1 Next slide shows the exchange

83. COE 341 (T061) – Dr. Marwan Abu-Amara 82 HDLC Example: Piggybacking with Errors