Chapter Three 1

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Business Data Communications

• Chapter Three
• Data Link Layer Fundamentals

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Primary Learning Objectives

• Understand the function of the data link layer
• Distinguish Logical Link Control from Media
  Access Control
• Describe the two types of flow control
• Explain line discipline
• Define the components of error control
• Recognize two methods of delineating data in a
  bit stream
• Identify devices and components associated with
  the data link layer

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The Data Link Layer Its Function

• Sits above the physical and below the Network
  Layers
• Formats data bits into frames
• Has two components
• Logical Link Control 802.2
• Media Access Control 802.3 for Ethernet
• Is responsible for
• Line discipline
• Flow control
• Error control

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Components of the Data Link Layer
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Logical Link Control (LLC)

• Designated by the IEEE as 802.2 and sits above
  the Media Access Control
• Provides three types of frame delivery service
  using protocol data units
• Type 1 connectionless without acknowledgement,
  the most used delivery service
• Type 2 connection-oriented with acknowledgement
• Type 3 connectionless with acknowledgement

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Logical Link Control (LLC)

• LLC supports three types of frames
• I Information
• is connection-oriented
• S Supervisory
• manages the Information frames
• U Unnumbered
• used by connectionless services and terminates
  connection-oriented services
• Only Type 2 delivery service uses all three types
  of frames

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Logical Link Control (LLC)
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Media Access Control (MAC)

• Has various IEEE designations, with the most
  common being 802.3 for Ethernet
• Determines how devices share a common circuit
• Falls into one of two categories
• Contention (802.3, Ethernet, for example)
• Controlled access (802.5, Token Ring, for
  example)
• FDDI, another form of controlled access, is an
  ANSI/ITU-T standard

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Media Access Control 802.3
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Media Access Control 802.5
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Media Access Control (MAC)

• Standard 802.3 networks using hubs and bridges
  can suffer from significant collision impairment
  under high traffic
• Modern 802.3 networks using switching technology
  have greatly eased this problem
• Switches do not change the underlying 802.3
  architecture

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Media Access Control (MAC)
Format of a MAC Protocol Data Unit (PDU)
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Flow Control

• Prevents a sender from overwhelming a receiver
  with traffic
• A sender and receiver each have a memory area in
  which they can store frames
• This memory is sometimes referred to as a buffer
• A sender can overwhelm, or overflow, a receivers
  memory buffer without proper flow control
• If an overflow occurs, data would likely be lost

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Flow Control

• Two common forms of flow control are
• Stop-and-wait
• Each single frame sent requires receipt of one
  acknowledgement
• Sliding windows
• The sending of multiple frames requires a single
  acknowledgement returned

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Flow Control

• Stop-and-wait
• Most efficient for messages containing a few
  large frames that traverse short links
• Requires one acknowledgement for each frame sent
• Sliding windows
• Most efficient for messages containing many small
  frames that traverse long links
• Allows for one acknowledgement for multiple
  frames

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Stop-and-Wait Flow Control
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Sliding Windows Flow Control
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Line Discipline

• Can be viewed as a polite means of controlling
  a conversation between communicating devices
• Associated with two types of network
  environments
• Point-to-point between communicating devices
  using half- or full-duplex circuits
• Multipoint with communicating devices going
  through a central controlling device
• The central control device is often a mainframe
  with connected terminals

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Error Control

• No system is perfect errors should be expected
• Errors can result when data is lost, corrupted,
  or damaged, making error control critical
• Error control has two components
• Error Detection
• Error Correction
• The two components are equally important

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Error Detection

• Common error-detection methods include
• Parity checking
• 50 probability of detection
• Longitudinal redundancy checking
• 98 probability of detection
• Checksum checking
• 99.6 probability of detection
• Cyclical redundancy checking
• 99.9 probability of detection

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Parity Checking

• An extra parity bit is added to the byte
• Assuming even parity
• 10000010 data sent
• 10000110 - data received
• Error detected on receiver side (single bit)
• 10000010 data sent
• 10011010 data received
• No error detected on receiver side (multiple bit)
• Simple parity detects only single bit errors

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Longitudinal Redundancy Checking -- LRC

• Longitudinal literally means lengthwise
• The sender, for each byte in the message,
  calculates a parity value, creating an additional
  block check character or BCC
• As with parity checking, the parity value is odd
  or even
• The BCC is added to the end of the message block
• The receiver performs the same lengthwise LRC
  computation
• If the receivers calculated BCC does not equal
  the senders calculated BCC, the receiver assumes
  a transmission error

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Longitudinal Redundancy Checking
01000010 01011001 01010100 01000101 Before BCC
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Longitudinal Redundancy Checking
01000010 01011001 01010100 01000101 00001010
After BCC The BCC added to the end of the data
block.
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Checksum Checking CC

• The message sender
• Evaluates each binary byte in the message to its
  decimal value
• Totals the decimal values of all bytes
• Divides the total by 255, creating a remainder
• Using the remainder for the CC, adds the CC to
  the end of the message block
• The message receiver
• Performs the same byte-by-byte calculation and
  creates his own CC
• Compares his calculated CC to the senders
• Assumes a transmission error if the two CC values
  differ

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Checksum Checking CC
308 / 255 1.21 CC 21
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Error Control

• The most common error correction technique is to
  simply retransmit the data in error
• Easy, but requires time for the retransmission
• A second error correction technique is called
  forward error correction
• The core message is sent along with redundant
  data bits
• The redundant data bits can, if necessary, be
  used by the receiving device to correct errors on
  site without retransmission
• However, forward error correction results in
  inefficient use of a circuit if too many
  redundant data bits are sent and not used

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Data Delineation

• A transmitted bit stream contains not only the
  core message but control information as well
• Control information could include
• Source address
• Destination address
• Length of message field
• Error control data
• Other non-core information
• Data delineation differentiates between core and
  other data

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Data Delineation

• Two key methods providing data delineation are
• Asynchronous data link protocols
• Synchronous data link protocols
• Asynchronous protocols
• Are used mostly by mainframes and their connected
  terminals
• Provide byte-by-byte delineation
• Synchronous protocols
• Are used in LANs, BNs, MANs, and WANs
• Provide delineation for groups of bytes

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Data Delineation

• Popular asynchronous protocols include
• XModem
• YModem
• ZModem
• Kermit
• Asynchronous protocols require that every data
  byte have a start and stop bit before and after
  it
• Generally less efficient than synchronous
  protocols

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Asynchronous Transmission
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Data Delineation

• Synchronous protocols are either bit- or
  byte-oriented
• Bit-oriented protocols are more flexible
• They do not require a predetermined byte
  character format, such as EBCDIC or ASCII
• They are more complicated
• Bit stuffing may be required
• HDLC is a formalized bit-based protocol
• Byte-oriented protocols
• Are based upon known byte based data
• Ethernet is a very common byte-based protocol

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Simple Synchronous Transmissions
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Data Link Layer Devices and Components

• A Network Interface Card is
• A component rather than device
• Essential to connect a device to a network
• Bridges
• Link segments of the same logical network
• Filter traffic, and so can improve network
  performance
• Switches
• Offer more functionality than hubs and bridges
• Provide point-to-point connections to devices
  plugged into them
• Have transformed how standard Ethernet is
  configured

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A 3Com Network Interface Card (NIC)

• NICs have a physical address
• NIC addresses must be unique
• NIC addresses can be bypassed, or overridden, by
  software, but care must be taken when doing this
  to avoid address duplication

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A Linksys Wireless Bridge

• Bridges filter network traffic

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A Bridge Filtering Traffic
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A standard Cisco Switch

• Switches have mostly replaced hubs in modern
  Ethernet networks

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In Summary

• The data link layer
• Is stacked above the physical and below the
  network layers
• Formats data bits into units called frames
• Is composed of two stacks, the logical link and
  the media access controls
• Performs error control
• Has devices such as bridges and switches with
  which it is particularly associated