CIS 321
Data Communications &
Networking
Chapter 11 – Data Link Control and
Protocols
Introduction
 Protocol – set of rules governing communication
specific to one or more layers of the OSI model
 Data link protocols define the rules devices use to
implement data link layer functions
 Contain rules for line discipline, flow control, and
error control
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Data Link Layer Functions
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Flow Control
 Coordinates amount of data sent before receiving
acknowledgement
 Purpose: prevent overwhelming receiver

Buffer overflow
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Error Control
 Error detection and error correction
 Receiver informs sender of any frames lost or
damaged and coordinates retransmission of
those frames by the sender

Usually handled via automatic repeat request (ARQ)
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Flow and Error Control Mechanisms
 Stop-and-Wait ARQ
 Go-back-N ARQ
 Selective-Repeat ARQ
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11.2 Stop-and-Wait ARQ
 Sender keeps copy of last frame sent and waits
for ACK for that frame
 Next frame cannot be sent until ACK has been
received
 Frames are numbered alternately 0 and 1
 Damaged or lost frames are resent
 Repeats until EOT is sent
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Stop-and-Wait Normal Operation
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Lost or Damaged Frame
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Lost Acknowledgement
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Delayed Acknowledgement
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Stop-and-Wait
 Advantage: simplicity; each frame is checked and
ACK’d before next frame is sent

Numbering of frames prevents duplication
 Disadvantage: inefficiency; slow
 Frame and ACKs use entire bandwidth
 If distance is long between devices, time spent
waiting can be significant
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Bidirectional Transmission
 Possible if two parties have separate channels for
full-duplex transmission or share same channel
for half-duplex transmission
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Piggybacking
 Method of combining data frame and
acknowledgement
 Saves bandwidth due to less overhead from
separate data frame and ACK frame into one
frame
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Piggybacking
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Sliding Window
 Sender may transmit several frames before
needing an ACK
 Much more efficient; receiver may use a single
ACK to confirm multiple frames
 Sliding window refers to upper and lower limit on
number of frames that may be transmitted before
ACK is required
 Frames must be numbered to allow receiver to
identify which frame is acknowledged
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11.3 Go-Back-N ARQ
 Allows for more efficient transmission – send
multiple frames before requiring an ACK
 Specify a window or range of sequence numbers
of frames that may be received
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Sliding Window
 Receiver includes number of next frame it
expects to receive in ACK
 Sender then knows all previous frames through
that number have been received
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Sender Window
 Window contains 0 to
2m -1 frames
 Window shrinks as
frames are sent out
 Once ACK arrives,
window expands
equal to number of
frames acknowledged
by ACK
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Receiver Window
 Size of window is always 1
 Receiver expects the next
ordered frame (must always be
in order)
 Any frame arriving out of order
is discarded
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Control Variables
 Sender : S – sequence number of recently sent frame; SF
- sequence number of first frame in window; SL –
sequence number of last frame in window
 Window size is W = SL - SF + 1
 Receiver : R – sequence number of next frame expected
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Go-Back-N Operation
 Timers: sender sets a timer for each frame (none
for receiver)
 Acknowledgement – receiver sends positive ACK;
silent on damaged or out of order frames
 Resending Frames – sender sends set of frames
from damaged up to last one sent and ACK’d
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11.4 Selective Repeat ARQ
 Go-Back-N less efficient since all out of order or
damaged frames must be resent in order
 Selective Repeat is a more efficient method, yet
required more processing
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Sender and Receiver Windows
 Sender window and control variables are same
as Go-Back-N
 Receiver window is same size; looks for range of
sequence numbers

Requires two control variables to define window
boundaries: RF and RL
 Also defines a negative acknowledgement (NAK)
to report sequence number of damaged frame
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Selective Repeat ARQ
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Selective Repeat ARQ, Lost Frame
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Comparisons between Go-back-n
and Selective-Reject
 Sending only specific damaged or lost frames
requires complexity of sorting and more storage
is required in select-reject
 Go-back-n is typically used due to simplicity
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11.5 HDLC
 High-level Data Link Control – protocol
supporting half-duplex and full-duplex
communication over point-to-point and multipoint
links
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HDLC Modes of Communication
 Relationship between two devices involved in an
exchange
 Defines who controls the link
 Two modes:


Normal response mode (NRM)
Asynchronous balanced mode (ABM)
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Normal Response Mode
 Refers to standard primary-secondary
relationships
 Used for all exchanges in unbalanced
configurations
 Primary can issues commands
 Secondary must have permission from primary
before responding or sending data
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NRM
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Asynchronous Balanced Mode
 All stations are equal
 Stations in point-to-point configurations act as both
primary and secondary
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HDLC Frames
 Three types; each functions as an envelope to
transmit a specific type of message
 Information frames (I-frames) – transports user
data and control info relating to user data
 Supervisory frames (S-frames) – used to
transport control info for data link layer flow and
error controls
 Unnumbered frames (U-frames) – used for
system mgmt and link mgmt
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HDLC Frame
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Flag Field
 Marks the beginning and end of frame and
provides synchronization with 01111110
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Address Field
 Contains address of secondary station that is
originator or destination of the frame
 If created by primary, contains a to address
 If created by secondary, contains a from address
 May be one byte or several bytes long
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Control Field
 One- or two-byte segment for flow management
 Fields differ depending on frame type
 First or first and second bits identify type of frame
 All three frame types contain a poll/final (P/F) bit;
used to identify whether frame was sent by
primary to a secondary or from secondary to
primary
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Information Field
 Contains user’s data in an I-frame; network
management in a U-frame; not included in an Sframe
 Often used in a data frame to acknowledge
receipt of another separate frame – called
piggybacking
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FCS Field
 Error detection field
 Stores either two- or four-byte CRC
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Different Control Frames
 I-frame – used for user information and transport
and piggyback acknowledgements
 S-frame – used for acknowledgement, flow
control, and error control when piggybacking is
not appropriate
 U-frame – used to exchange session
management and control information between
devices
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Data Transparency
 To address the possibility that a bit pattern may
match flag field indicator and be misinterpreted,
bit stuffing may be used
 Anytime 5 consecutive 1s are encountered, a
redundant 0 is inserted
 Identifies that the current sequence is not a flag
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Bit Stuffing and Removal
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Coming Up… Ch 12
 Point-to-Point Access: PPP (brief)
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Credits
 All figures obtained from publisher-provided
instructor downloads
Data Communications and Networking, 3rd edition by
Behrouz A. Forouzan. McGraw Hill Publishing, 2004
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