COE 341: Data

1
COE 341 Data Computer CommunicationsDr.
Marwan Abu-Amara

• Chapter 7
• Data Link Control Protocols

2
Contents

• Error Control
• Stop-and-Wait ARQ
• Go-Back-N ARQ
• Selective-Reject ARQ
• Flow Control
• Stop-and-Wait flow control
• Sliding-Window flow control
• High-Level Data Link (HDLC)

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

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

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)

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

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

8
A Model of Frame Transmission
Loss
Only one frame traveling on the link at any
given time
Error
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

10
Categories of Error Control
11
Categories of Flow Control
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

13
Stop-and-Wait ARQ Normal Operation
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

15
Stop-and-Wait ARQ Lost Frame
16
Stop-and-Wait ARQ Lost ACK
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

18
Stop-and-Wait ARQ Delayed ACK
Lost
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

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

21
Stop-and-Wait Efficiency

• Let
• Tf Time to transmit a frame Lf / R
• TACK Time to transmit an ACK LACK / R
• TProp Propagation time (link distance) / V
• TProc Processing time
• We define the frame total time as (see next
  slide)
• Ttotal Tf 2 TProp TProc TACK
• The utilization of the link is defined as
• U (Tf)/(Ttotal)

22
Stop-and-Wait Efficiency

• It is common to ignore TProc and TACK because
  they are negligible compared to other times
• In which case the utilization becomes
• U (Tf)/(Tf 2Tprop) 1/(1 2a)
• Where a Tprop/Tf
• a is called the length of the link in bits
  length of medium in bits compared to frame length
  (Lf)
• Notice that for very small a (i.e. when 1st
  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 ltlt 1 (long frames on
  a short link, i.e. fills link)

23
Stop and Wait Efficiency Example

• Compare the efficiency of stop-and-wait error
  control for two links using the parameter a
• Frame 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 108 m/s
• Frame TX time, tf L/R 8000/(1x109) 8 ms
• Propagation time, tprop d/V
• 200/(2x108) 1 ms
• End of first frame reaches RX after 81 9 ms
  from start
• ACK takes 1 ms more to reach TX, which sends 2nd
  frame after 10 ms
• Utilization 8/10 80 (1/(2a1))

• Satellite link between 2 ground stations
• d 2 x 36,000 km, Data rate, R 1 Mbps
• Typical wave velocity, V 3 x 108 m/s
• Frame TX time, tf L/R 8000/(1x106) 8 ms
• Propagation time, tprop d/V
• 2x36x106/(3x108) 240 ms
• End of first frame reaches RX after 8240 248
  ms from start
• ACK takes 240 ms more to reach TX, which sends
  2nd frame after 488 ms
• Utilization 8/488 1.6 (1/(2a1))

24
Sliding Window Protocol

• Stop-and-Wait can be very inefficient when a gt 1
• Protocol
• Assumes full duplex line
• Source A and Destination B have buffers each of
  size W frames
• For m-bit sequence numbers
• Frames are numbered 0, 1, 2, , 2m-1, 0, 1,
  (modulo 2m)
• ACKs (RRs) are numbered 0, 1, 2, , 2m-1, 0, 1,
  (modulo 2m)
• 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 ? 2m-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 2m-1

25
Sliding Window Protocol

• Example of Sliding-Window-Protocol m 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 distance between first unacknowledged
frame and last frame that can be sent
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 1st 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 2m 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,

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 2m 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)

28
TX Sliding Window
In (b), frame 0 and frame 1 were properly
acknowledged
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

30
RX Sliding Window
31
Go-back-N ARQ Control Variables

• TX keeps track of three variables
• S Sequence number of the recently sent frame
• SF Sequence number of the first frame in the
  window
• SL Sequence number of the last frame in the
  window
• The frame size W is related to these variables
  by
• W SL SF 1
• On the other hand the RX has only one variable
• R Expected frame sequence number

32
Go-back-N ARQ TX RX Control Variables
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

34
Go-back-N ARQ Normal Operation
35
Go-back-N ARQ Lost Frame Operation
Here, ACK2 acknowledges frames 0 1 at the same
time
36
Quiz Window Size Condition for Go-back-N ARQ

• Earlier, we mentioned that the size of the window
  must be W ? (2m 1) i.e. W lt 2m 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

37
Sliding Window Protocol

• Animation for Sliding Window protocol
• Sliding Window Protocol Simulation
  (http//www.cs.stir.ac.uk/kjt/software/comms/jasp
  er/SWP3.html)

38
Example Problem 7-9

• 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
• Before A sends any frames
• After A sends frame 0, 1, 2 and B acknowledges 0,
  1 and the ACKs are received by A
• After A sends frames 3, 4, and 5 and B
  acknowledges 4 and the ACK is received by A

39
Example Problem 7-9 - Solution

• a)

b)
c)
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

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

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 ? 2m)
  i.e. W ? 2(m-1)
• RX window must have the same size as the TX
• This is 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

43
Selective-Repeat ARQ TX RX Control Variables
Here, S SF
44
Selective-Repeat ARQ Operation Lost Frame Example
Here, ACK2 acknowledges frames 0 1 at the same
time
0
45
Quiz Window Size Condition for Selective-Repeat
ARQ

• Earlier, we mentioned that the size of the window
  must be W ? 2m 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

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 Tprop/Tf (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 !!

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 108 m/s
• Frame TX time, tf L/R 8 ms
• Propagation time, tprop d/V 1 ms
• a tprop / tf 0.125
• 100 link utilization is achieved with window
  size W
• W ? (2 a1) ? (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 108 m/s
• Frame TX time, tf L/R 8 ms
• Propagation time, tprop d/V 240 ms
• a tprop / tf 30
• 100 link utilization is achieved with
  window size W
• W ? (2 a1) ? (2 x 30 1) ? 61
• W 61, k 6 bit
  (Large window and buffer sizes)
• For k 3 bits, W 7
• Utilization U W/(2a1) 7/(61)
  11.5 gt 1.6 for Stop and wait.

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

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), and
• HDLC Link Access Procedure Balanced (LAPB)
• Usually when referring to HDLC people mean LAPB
  or some variation

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

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 HDLC

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

53
HDLC NRM
54
HDLC ABM
55
HDLC Frame Structure
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

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
  6th/7th 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

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

59
HDLC Bit Stuffing Removal
60
HDLC Bit Stuffing

• Problems With bit stuffing, Single-bit errors
  could split a frame into 2 or merge two frames
  into 1.

01111100
Frame Splitting
(Bit stuffed)
01111110
01111110
Frame Merging
(Bit stuffed)
01111100
61
HDLC Frame Structure Address Field
Extended 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

62
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)
• 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

63
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

64
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
  X16X12X51), or
• 32-bit optional FCS using CRC-32

65
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

66
HDLC Generic Frame Format
Indicates that the first transmitted field is the
Header flag, then the address, then the control
Trailer
Header
67
HDLC I, S U Frames Format
68
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

69
HDLC I-Frames
70
HDLC S-Frames
71
HDLC U-Frames
72
U-frame control command and response
Command/response Meaning
SNRM Set normal response mode
SNRME Set normal response mode (extended)
SABM Set asynchronous balanced mode
SABME Set asynchronous balanced mode (extended)
UP Unnumbered poll
UI Unnumbered information
UA Unnumbered acknowledgment
RD Request disconnect
DISC Disconnect
DM Disconnect mode
RIM Request information mode
SIM Set initialization mode
RSET Reset
XID Exchange ID
FRMR Frame reject
73
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

74
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
mode7-bit sequence
75
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

76
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 F1

77
HDLC Operation

• 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 As solicitation

78
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 Bs first I-frame is also numbered 0
  N(S) field and contains a 2 in its N(R) field,
  acknowledging the receipt of As 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 As frame 2 to arrive next

79
HDLC Example Piggybacking w/o Errors
80
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

81
HDLC Example Piggybacking with Errors