Chapter 5 discusses data link control and protocol concepts, focusing on digital data transfer and the mechanisms required for error and flow control. It explains two primary flow control techniques: stop and wait and sliding window, along with error control methods like automatic repeat request (ARQ). The chapter outlines various protocols, including asynchronous and synchronous protocols, along with detailed structures for high-level data link control (HDLC) and its operational modes.

1. Data Link Control & Protocol
Concepts
Chapter 5

2. Introduction
 Characters are defined as binary
character codes for the purpose of Digital
Transfer.
 Data Link Layer is Concerned with digital
transfer over a link.
 Link – P to P circuit or Radio Based
Channel.
 Effective Digital Data communication
need  Control & Manage the
Exchange

3. Functions
 To Establish and maintain Effective
Communication.
 Error Control ->
Defines how a station check frames for
errors & What it does if it finds.
 Flow Control ->
Defines the way in which many frames
sent and how the stations do error
control.

4. Flow Control
 Tells the sender how much data it can
transmit before it must wait for an
acknowledgement from the receiver.
 Receiving device has
 Limited speed
 Limited amount of Memory
 So the sending device must not send
frames at a rate faster than a receiving
station can absorb them.

5. Techniques
 To control the flow of data across the
communication links:
Stop and Wait flow Control
Sliding Window flowControl.

6. Stop and Wait flow Control

7. Stop and Wait flow Control
 Source transmits frame
 Destination receives frame and replies
with acknowledgement (ACK)
 Source waits for ACK before sending
next frame
 Destination can stop flow by not
sending ACK
Adv ---- Simplicity
Disadv ---- Inefficiency

8. Sliding Window flow Control.
 Allows multiple frames to be in transit
 Receiver sends acknowledgement with
sequence number of anticipated frame
 Sender maintains list of sequence
numbers it can send, receiver maintains
list of sequence numbers it can receive
 ACK (acknowledgement, or RR –
Receiver Ready) supplemented with
RNR (receiver not ready).

10.  The problem of “Stop and Wait” is not be
able to send multiple packets
 Receiver has buffer of W (called window size)
frames
 Transmitter can send up to W frames without
ACK
 Each frame is numbered
 frames are numbered modulo-n (0 … n-1)
 ACK includes number of next frame expected

12. ERROR CONTROL:
 Error control in the data link layers is
based on automatic repeat
request(ARQ)which means
retransmission of data in three cases:
 1. Damaged frame
 2. Lost frame
 3. Lost acknowledgement

13. DAMAGED FRAME:
 A recognizable frame does arrive, but
some of the bits are in error.

14. LOST FRAME:
 A frame fails to arrive at the other side.

15. LOST ACKNOWLEDGEMENT:
 An acknowledgement fails to arrive at
the source.
 The sender is not aware that
acknowledgement has been transmitted
from the receiver

16. Automatic repeat request
 The purpose of ARQ is to turn an
unreliable data link into a reliable one.
 Three versions of ARQ have been
standardized.
 1. stop-and-wait ARQ
 2. Go-back-N ARQ
 3. selective-reject ARQ

17. STOP-AND-WAIT ARQ:
 Stop-and-wait is the simplest of the error
control protocols.
 A transmitter sends a frame then stops and
waits for an acknowledgment.
 If a positive acknowledgment (ACK) is
received, the next frame is sent.
 If a negative acknowledgment (NAK) is
received, the same frame is transmitted
again.

18.  Source transmits single frame and Wait
for ACK
 If received frame is damaged, discard it
 If transmitter receives no ACK within
timeout, retransmits
 If ACK damaged,transmitter will not
recognize it
 Transmitter will retransmit after timeout
 Receiver gets two copies of frame, but
disregards one of them
 Use ACK0 and ACK1
 ACKi means “I am ready to receive frame i”

19.  One frame received and handled at a
time
 If frame is damaged, receiver discards
it and sends no acknowledgment
 Sender uses timer to determine
whether or not to retransmit
 Sender must keep a copy of
transmitted frame until
acknowledgment is received
 If acknowledgment is damaged, sender
will know it because of numbering

22. Go-Back-N ARQ
 Uses sliding-window flow control.
 When receiver detects error, it sends
negative acknowledgment (REJ).
 Sender must begin transmitting again
from rejected frame.
 Transmitter must keep a copy of all
transmitted frames.

25. Selective Reject
 Also called selective retransmission
 Only rejected frames are retransmitted
 Subsequent frames are accepted by the
receiver and buffered
 Minimizes retransmissions
 Receiver must maintain large enough
buffer
 Complex system

27. Protocols
 set of rules that governs the operation
of functional units to achieve
communication .
 data link protocol.
 The type of protocol used to establish a
link between two stations in accordance
with the second layer of OSI model

28. Data link protocol subgroups
 Asynchronous protocols
 Synchronous protocols
 Asynchronous protocols treat each
character in a bit stream independently.
 Synchronous protocols take the whole
bit stream and chop it into characters of
equal size.

29.  Asynchronous protocols-used primarily
in modems
 feature start and stop bits and variable-length
gaps between characters.
 A variety of Asynchronous protocols :-
 1. X-modem
 2. Y-modem
 3. Z-modem
 4. Blocked Asynchronous transmission
(BLAST)

30. X-MODEM
 File transfer communication for
telephone line communication between
PC‟s.
 designed by Ward Christiansen in 1979,
X-modem.
 It is a half duplex stop-and-wait ARQ
protocol.

31.  In this protocol, transmission begins
with the sending of a NACK frame from
the receiver to the sender.
 each time the sender sends a frame, it
must wait for an acknowledgement
before the next frame can be sent.
 A frame can be resend either if
response is not received by the sender
after a specified period of time
 or if NACK is received by the sender.

32. Y-MODEM:
 1. The data unit is 1024 bytes.
 2. ITU-T CRC-16 is used for error
checking
 3. Multiple files can be sent
simultaneously

33. Z-MODEM:
 Z-modem is a protocol which combines
features of both X-modem and Y-modem.
 BLAST:
 BLAST is more powerful than X-modem.
It is full duplex with sliding window flow
control.
 It allows the transfer of data and binary
files.

35. 35
Synchronous Transmission
With Synchronous Transmission all the
letters or data in one group of data is
transmitted at one time as a block of
data called a frame or packet.
The start and end of each packet
sometimes is marked by adding
synchronization characters (SYN) at the
start/end of each packet.

36. Message Format
 Start of header (SOH)
 Header (Address, sequence, date and
time)
 Start of text (STX)
 Text
 End of text (ETX) or text length
 Trailer
 Block check character (BCC)
 Synchronization character (SYN)

37. Synchronous Data Link Protocols -
Classification
 Character-oriented protocols
 Special character for start and end of message
 Binary Synchronous Communication Protocol (BSC
or BISYNC)
 Byte-count-oriented protocols
 Special character for start of the header, count
field, message, block check character (BCC)
 DEC’s Digital Data Communication Message
Protocol (DDCMP)
 Bit-oriented protocols
 Use flag character for start and end of message
 IBM’s Synchronous Data Link Control (SDLC)
 ISO’s High-Level Data Link control (HDLC)

38. Binary Synchronous
Communications
 By IBM
 For 6-bit transcode (SBT), ASCII, EBCDIC
 SYN at start and middle of transmission
 Point to point and multipoint (polling)
 ARQ approach for error checking (ACK1, ACK0, NAK)
 Pros:
 Transparency and non-transparency modes
 Efficient, understandable, and widely used
 Point-to-point & multipoint operations
 Cons:
 Code dependent
 Half-duplex protocol
 Cumbersome for transparency mode

39. High Level Data Link Control
 HDLC
 ISO Standard
 Basis for some other DLL protocols

40. 40
High-Level Data Link Control
(HDLC)
 The OSI’s data link protocol
 A bit-oriented protocol
 On transmitting side, HDLC receives data from an
application, and delivers it to the receiver on the
other side of the link
 On the receiving side, HDLC accepts the data and
delivers it to the higher level application layer
 Both modules exchange control information, encoded
into a frame
(IBM’s SDLC has similar features and the format of HDLC)

41. 41
HDLC Frame Structure
 Flag: 01111110, at start
and end
 Address: secondary
station (for multidrop
configurations)
 Information: the data to
be transmitted
 Frame check sequence:
16- or 32-bit CRC
 Control: purpose or
function of frame
 Information frames:
contain user data
 Supervisory frames:
flow/error control
(ACK/ARQ)
 Unnumbered frames:
variety of control
functions (see p.131)

42. 42
Transparency Problem
 The problem of transparency
 Because HDLC uses 01111110 to mark the
start and end of a packet, the character
“01111110” will confuse the receiver.
 Solution - Bit Stuffing
 Add a 0 after every five 1s at sender side
and delete the 0 at receiver side.

43. 43
HDLC Operation
 Initialization: S-frames specify mode
and sequence numbers, U-frames
acknowledge
 Data Transfer: I-frames exchange user
data, S-frames acknowledge and
provide flow/error control
 Disconnect: U-frames initiate and
acknowledge

44. High-Level Data Link Control (HDLC)
 HDLC was defined by ISO for use on both
point-to-point and multipoint data links.
 It supports full-duplex communication
 Other similar protocols are
 Synchronous Data Link Control (SDLC) by IBM
 Advanced Data Communication Control Procedure
(ADCCP) by ANSI
 Link Access Procedure, Balanced (LAP-B) by
CCITT, as part of its X.25 packet-switched
network standard

45. HDLC Overview
Broadly HDLC features are as follows:
 Reliable protocol
 selective repeat or go-back-N
 Full-duplex communication
 receive and transmit at the same time
 Bit-oriented protocol
 use bits to stuff flags occurring in data
 Flow control
 adjust window size based on receiver capability
 Uses physical layer clocking and
synchronization to send and receive frames

46. HDLC Overview
 Defines three types of stations
 Primary
 Secondary
 Combined
 Defines three types of data transfer mode
 Normal Response mode
 Asynchronous Response mode
 Asynchronous Balanced mode
 Three types of frames
 Unnumbered
 information
 Supervisory

47. HDLC
 The three stations are :
 Primary station
 Has the responsibility of controlling the operation of data
flow the link.
 Handles error recovery
 Frames issued by the primary station are called
commands.
 Secondary station,
 Operates under the control of the primary station.
 Frames issued by a secondary station are called
responses.
 The primary station maintains a separate logical link with
each secondary station.
 Combined station,
 Acts as both as primary and secondary station.
 Does not rely on other for sending data

48. HDLC
Primary
Commands
Responses
Unbalanced Mode
Secondary Secondary
Balanced mode
Combined Combined
commands/Responses

49. HDLC
 The three modes of data transfer operations are
 Normal Response Mode (NRM)
 Mainly used in terminal-mainframe networks. In this case,
 Secondaries (terminals) can only transmit when specifically
instructed by the primary station in response to a polling
 Unbalanced configuration, good for multi-point links
 Asynchronous Response Mode (ARM)
 Same as NRM except that the secondaries can initiate
transmissions without direct polling from the primary station
 Reduces overhead as no frames need to be sent to allow
secondary nodes to transmit
 Transmission proceeds when channel is detected idle , used
mostly in point-to-point-links
 Asynchronous Balanced Mode (ABM)
 Mainly used in point-to-point links, for communication between
combined stations

50. Non-operational Modes
 Normal Disconnected Mode
 Asynchronous Disconnected Mode
Both the above modes mean that the secondary node is
logically disconnected from the primary node
 Initialization Mode
 A node negotiates transmission parameters with the other
node E.g., flow control information
 Parameters negotiated in this mode are used during any of
the data transfer modes

51. Data Link Control HDLC frame
structure
(a) Frame
Format
(b) Control
field
format

52. Data Link Control
HDLC frame structure
(c) Extended address field (d) Extended control field

53. HDLC
 Flag: 01111110- start and ending delimiter. Bits are stuffed for flags in
data frames
 FCS: 16-bit CRC using generating polynomial
G(x) = x16 + x12 + x5 + 1
 Address field:
 mainly used in multidrop link configuration, and not used in point-to-point
 In unbalanced configuration, every secondary is assigned a unique address.
Contains address of secondary station in both command and response
frames
 In balanced mode, command frame has destination address and response
frame has sending node’s address
 Group addresses are also possible. E.g., One command sent to all the
secondaries
 In I-frames, N(s) is the sequence number of the frame being sent, and
R(s) is the sequence number of the frame being expected.
 The P/F bit, known as the poll/final bit, is used with different meaning
in different contexts.
 It is used to indicate polling, to indicate the final I-frame, etc

54. HDLC
 There are three different classes of frames
used in HDLC
 Unnumbered frames, used in link setup and
disconnection, and hence do not contain ACK.
 Information frames, which carry actual
information. Such frames can piggyback ACK in
case of ABM
 Supervisory frames, which are used for error and
flow control purposes and hence contain send and
receive sequence numbers

55. HDLC
 There are four different supervisory frames
 SS=00, Receiver Ready (RR), and N(R) ACKs all
frames received up to and including the one with
sequence number N(R) - 1
 SS=10, Receiver Not Ready (RNR), and N(R) has
the same meaning as above
 SS=01, Reject; all frames with sequence number
N(R) or higher are rejected, which in turns ACKs
frames with sequence number N(R) -1 or lower.
 SS=11, Selective Reject; the receive rejects the
frame with sequence number N(R)

56. HDLC
 The unnumbered frames can be
grouped into the following categories:
 Mode-setting commands and responses
 Recovery commends and responses
 Miscellaneous commands and responses

57. Review of Link Layer
 Services
 Framing
 Error control
 Reliability
 Connection management
 Medium access control
 Switching
 Protocols, Standards
 Ethernet
 Token Ring
 FDDI
 Wireless
 PPP
 HDLC

58. HDLC Station Types
 Primary station
 Controls operation of link
 Frames issued are called commands
 Secondary station
 Under control of primary station
 Frames issued called responses
 Combined station
 May issue commands and responses

59. 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

60. HDLC Transfer Modes (1)
 Normal Response Mode (NRM)
 Unbalanced configuration
 Primary initiates transfer to secondary
 Secondary may only transmit data in
response to command from primary
 Terminal-host communication
 Host computer as primary
 Terminals as secondary
 not so common nowadays

61. HDLC Transfer Modes (2)
 Asynchronous Balanced Mode (ABM)
 Balanced configuration
 Either station may initiate transmission
without receiving permission
 Most widely used

62. Frame Structure
 All transmissions in frames
 Single frame format for all data and
control exchanges

63. Frame Structure Diagram

64. Flag Fields
 Delimit frame at both ends
 01111110
 Receiver hunts 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

65. Bit Stuffing Example

66. Address Field
 Identifies secondary station that sent or
will receive frame
 Usually 8 bits long
 May be extended to multiples of 7 bits
with prior agreement
 leftmost bit of each octet indicates that it is
the last octet (1) or not (0)

67. Frame Types
 Information - data to be transmitted to
user
 Acknowledgment is piggybacked on
information frames
 Supervisory – ARQ messages
(RR/RNR/REJ/SREJ) when piggyback
not used
 Unnumbered – supplementary link
control functions. For examples,
 setting the modes

68. Control Field Diagram

69. Poll/Final Bit
 Use depends on context. A typical use
is below.
 Command frame
 P bit set to 1 to solicit (poll) supervisory
frame from peer
 Response frame
 F bit set to 1 to indicate response to
soliciting command

70. Information Field
 Only in information and some
unnumbered frames
 Must contain integral number of octets
 Variable length

71. Frame Check Sequence Field
 FCS
 Error detection
 16 bit CRC
 Optional 32 bit CRC

72. HDLC Operation
 Exchange of information, supervisory
and unnumbered frames
 Three phases
 Initialization
 Data transfer
 Disconnect

73. Initialization
 Issue one of six set-mode commands
 Signals other side that initialization is
requested
 Specifies mode (NRM, ABM, ARM)
 Specifies 3- or 7-bit sequence numbers
 If request accepted HDLC module on
other side transmits unnumbered
acknowledged (UA) frame
 If request rejected, disconnected mode
(DM) sent

74. Data Transfer
 Both sides may begin to send user data in I-frames
 N(S): sequence number of outgoing I-frames
 modulo 8 or 128, (3- or 7-bit)
 N(R) acknowledgment for I-frames received
 seq. number of I-frame expected next
 S-frames are also used for flow and error control
 Receive ready (RR) frame acknowledges last I-frame received
 Indicating next I-frame expected
 Used when there is no reverse data
 Receive not ready (RNR) acknowledges, but also asks peer to
suspend transmission of I-frames
 When ready, send RR to restart
 REJ initiates go-back-N ARQ
 Indicates last I-frame received has been rejected
 Retransmission is requested beginning with N(R)
 Selective reject (SREJ) requests retransmission of single frame

75. Disconnect
 Send disconnect (DISC) frame
 Remote entity must accept by replying
with UA
 Informs layer 3 user about the termination
of connection

76. Other DLC Protocols
(LAPB,LAPD)
 Link Access Procedure, Balanced (LAPB)
 Part of X.25 (ITU-T)
 Subset of HDLC - ABM (Async. Balanced
Mode)
 Point to point link between user and packet
switching network node
 HDLC frame format
 Link Access Procedure, D-Channel (LAPD)
 Part of ISDN (ITU-T)
 ABM
 Always 7-bit sequence numbers (no 3-bit)
 always 16-bit CRC

77. Other DLC Protocols (LLC)
 Logical Link Control (LLC)
 IEEE 802
 For LANs (Local Area Networks)
 Link control split between medium access control layer (MAC) and
LLC (on top of MAC)
 Different frame format
 Two addresses needed (sender and receiver) – actually at MAC layer
 Sender and receiver SAP addresses
 Control field is same as HDLC (16-bit version for I and S frames; 8-
bit for U frames)
 No primary and secondary - all stations are peers
 Error detection at MAC layer
 32 bit CRC

78. Other DLC Protocols (LLC)
 LLC Services
 3 alternatives
 Connection Mode Services
 Similar to HDLC ABM
 Unacknowledged connectionless services
 no connection setup
 No flow-control, no error control, no acks (thus
not reliable)
 good to be used with TCP/IP. Why?
 Acknowledged Connectionless Service
 No connection setup
 reliable communication

79. High-level Data Link Control (HDLC)
- I
 Operating modes
 Normal response mode (NRM) for a primary node
and one or more secondary modes on a circuit
(polling)
 Asynchronous balanced mode (ABM) for nodes
with peers (most used for efficiency with no
polling in full duplex)
 Asynchronous response mode (ARM) for primary
and secondary nodes with same transmission right
(rarely used)
 Frames
 Supervisory or S format: control acknowledge
 Unnumbered or U format: (operation mode, start,
termination)
 Information or I format: (data)

80. High-level Data Link Control (HDLC)
- I
 Flag
 01111110
 Bit stuffing 0 after five 1s by hardware
 For synchronization
 Other fields
 Address field
 Control field: type of frame, sequence number for
information frames
 Information field: multiple of 8 bits
 Frame check sequence (FCS) field: error checking
 Frame flow
 Initialization phase, data transfer phase, and
disconnect phase

81. Other Data Link Control Protocols
 Link access procedure, balanced (LAPB)
 Full-duplex, point-to-point mode
 For X.25 DTE and packet switching network
 Link access procedure, D-channel (LAPD)
 ISDN network
 Link access procedure for frame –mode
bearer service (LAPF)
 High-speed packet switching network on low error
rate digital circuits
 Asynchronous transfer mode
 Data transfer on high s-speed, digital, error free
network

82. High Level Data Link Control
 One of the more popular data link
control protocols
 Similar to IBM’s SDLC but more flexible
 Many data link protocols are based on
HDLC – thus if you learn HDLC, you will
understand many others, such as all the
LAP standards
11.82

83. HDLC Station Types
 Primary station
11.83
 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

84. HDLC Link Configurations
 Unbalanced
11.84
 One primary and one or more secondary
stations
 Supports full duplex and half duplex
 Balanced
 Two combined stations
 Supports full duplex and half duplex

85. HDLC Transfer Modes (1)
 Normal Response Mode (NRM)
11.85
 Unbalanced configuration
 Primary initiates transfer to secondary
 Secondary may only transmit data in
response to command from primary
 Used on multi-drop lines
 Host computer as primary
 Terminals as secondary

86. HDLC Transfer Modes (2)
 Asynchronous Balanced Mode (ABM)
11.86
 Balanced configuration
 Either station may initiate transmission
without receiving permission
 Most widely used
 No polling overhead

87. HDLC Transfer Modes (3)
 Asynchronous Response Mode (ARM)
11.87
 Unbalanced configuration
 Secondary may initiate transmission
without permission from primary
 Primary responsible for line
 Rarely used

88. Frame Structure
 Synchronous transmission
 All transmissions in frames
 Single frame format for all data and
control exchanges
11.88

89. Frame Structure Diagram
11.89

90. 11.90
HDLC frame types

91. 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
11.91

92. Control Field
 Different for different frame type
11.92
 Information - data to be transmitted to
user (next layer up)
 Flow and error control piggybacked on
information frames
 Supervisory - ARQ when piggyback not
used
 Unnumbered - supplementary link control
 First one or two bits of control field
identify frame type

93. Control Field Diagram
11.93

94. Poll/Final Bit
 Use depends on context
 Command frame
11.94
 P bit
 1 to solicit (poll) response from peer
 Response frame
 F bit
 1 indicates response to soliciting command

95. Information Field
 Only in information and some
unnumbered frames
 Must contain integral number of octets
 Variable length
11.95

96. Frame Check Sequence Field
 FCS
 Error detection
 16 bit CRC
 Optional 32 bit CRC
11.96

97. HDLC Operation
 Exchange of information, supervisory
and unnumbered frames
 Three phases
11.97
 Initialization
 Data transfer
 Disconnect

98. HDLC Bit-Oriented Frame
Format
Information
Frame
Supervisory
Frame
Unnumbered
Frame
Frame Sequence # Poll/Final Next Frame Expected