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Chapter 3: The Data Link Layer

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... Link Control (SDLC) ... SDLC is capable of supporting both half-duplex and full-duplex ... Extended address: SDLC allows for an 8-bit address, whereas HDLC ... – PowerPoint PPT presentation

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Title: Chapter 3: The Data Link Layer


1
Chapter 3 The Data Link Layer
2
Learning Objectives
  • Frame
  • Parity
  • Arithmetic Checksum
  • Error Correction
  • Hamming Distance
  • Hamming Code

continued
3
Learning Objectives
  • Asynchronous Transmission
  • Synchronous Transmission
  • BISYNC Transmission
  • Data Link Escape
  • Byte Count Synchronous Transmission
  • Synchronous Data Link Control
  • High-level Data Link Control

continued
4
Introduction
  • The second layer of the OSI model is the data
    link layer and resides immediately above the
    physical layer.
  • The responsibilities involve
  • Transforming the physical layer into a
    transmission line that appears free of
    transmission errors.

5
Frame
  • The data link layer creates a unit called a
    frame.
  • A frame typically contains one or more fields
    that are involved in transforming the physical
    layer into an error-free transmission line.
  • The frame is passed to the senders physical
    layer which then transmits the frame over a
    medium to the physical layer of the receiver.

continued
6
Frame
  • The frame is placed the information necessary to
  • Perform error checking of the transmitted data.
  • Control lost and duplicate frames.
  • Provides the means to prevent a transmitter from
    overrunning a receiver with data faster than the
    receiver can process it.

continued
7
Error DetectionMurphys Law
  • The transmission of data over an appropriate
    medium is much likely Murphys Law if something
    can go wrong, it will.

8
Error Detection Techniques
  • Parity
  • Arithmetic Checksum
  • Cyclic Redundancy Checksum
  • Simple parity is the easiest error detection
    method to incorporate and comes in two basic
    forms even parity and odd parity
  • Double parity tries to solve the problem of
    detecting even numbers of errors by adding
    additional parity check bits.

9
Error Detection
  • A more advanced technique of error detection is
    arithmetic checksum. While this technique does
    not have widespread use, it is relatively simple
    to program and reasonably effective.
  • The checksum value is created by adding together
    the ordinal values of each character in a
    transmitted packet. This checksum is then
    appended to the packet of transmitted data in the
    form of one or two characters.

10
Error Detection
  • The cyclic redundancy checksum (CRC) is one of
    the few cases in computer science where you get
    more than you paid for. The CRC methods usually
    adds 16 or 32 check bits to potentially large
    data packets while approaching near 100 percent
    error detection.

11
Error Correction
  • The concept of error correction is often
    interpreted in two different ways
  • Error Correction
  • Forward Error Correction
  • In error correction the data is transmitted to a
    receiver the receiver performs an error
    detection procedure and discovers the data is
    corrupted.

12
Error Correction
  • The forward error correction requires the
    transmitter to transmit a great deal of
    additional information along with the original
    data.

13
Hamming Distanceand Hamming Code
  • Hamming distance is the smallest number of bits
    by which character codes differ.
  • A Hamming code is a data code that includes the
    redundant bits necessary to perform forward error
    correction.

14
Asynchronous Transmission Synchronous
Transmission
  • Asynchronous transmission is one of the simplest
    examples of data link protocol and is found
    primarily on point-to-point connections.
  • Synchronous transmission is markedly different
    from the asynchronous transmission in that the
    unit of transmission is not a single character
    but a sequence of characters.

15
BISNYC Transmission
  • BISYNC protocol was designed to provide a
    general-purpose data link protocol for
    point-to-point and multipoint connection.
  • BISYNC (or BSC) is a half-duplex protocol in that
    it allows data to be transmitted in both
    directions, but not at the same moment.

16
Transparency
  • Transparency is a system that allows the binary
    equivalent of the ETX (End of TeXt) character to
    occur in the text but not be recognized as the
    control character ETX.

17
Synchronous Data Link Control (SDLC)
  • SDLC (Synchronous Data Link Control) is a bit
    synchronous protocol with it, the receiver
    examines individual bits looking for control
    information.

18
Comparison of SDLC and BISNYC
  • SDLC is capable of supporting both half-duplex
    and full-duplex connections where BISYNC is
    essentially half-duplex, since the primary sends
    a message, the secondary responds, the primary
    sends another message, the secondary responds,
    and so on.

19
Comparison of SDLC and BISNYC
  • SDLC is a code independent whereas BISYNC is code
    dependent. BISYNC requires a data code such as
    ASCII or EBCDIC that includes control codes such
    as SOH (Start Of Header), STX (Start of TeXt), or
    ETX (End of TeXt) in the code set.SDLC does not
    rely on character codes to control execution.

20
High-level Data Link ControlHDLC
  • HDLC (High-level Data Link Control is the data
    link standard created by ISO that closely
    resembles SDLC.

21
Comparison of HDLC and SDLC
  • Extended address SDLC allows for an 8-bit
    address, whereas HDLC can be extended to have an
    address size that is multiple of 8 bits.
  • Extended control field The control field in HDLC
    may be 8 bits (as in SDLC) or 16 bits in length.

22
Comparison of HDLC and SDLC
  • Extended checksum the cyclic checksum field may
    be either 16 bits (as in SDLC) or an extended
    32-bit checksum.
  • Selective reject Supervisory frames in SDLC can
    be Receive Ready, Receive Not Ready, and Reject.
    HDLC adds Selective Reject (SREJ), which tells the

23
Comparison of HDLC and SDLC
  • sender that a message was in error and to go
    back to than one message and retransmit it but
    not all the messages that followed it.

24
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