Ch'10 Data Link Control

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Ch.10 Data Link Control

• Lecturer Tae-Hyong Kim (B201-4)
• thkim_at_cespc1.kumoh.ac.kr

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Contents

• Introduction
• Line Discipline
• Flow Control
• Error Control

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Data Link Control
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1. Line Discipline

• Line discipline answers the question, who should
  send now?
• oversee the establishment of links and the right
  of a particular device to transmit at a given
  time
• can be done in two ways

• Enquiry/acknowledgment
• peer-to-peer communication

• primary-secondary communication

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Line Discipline (contd)

• ENQ/ACK
• Session can be initiated by either station on a
  link as long as both are of equal rank.
• Ex) printer can not initiate CPU
• Initiating device establishes the session.

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Line Discipline (contd)

• How it works

Negative AcK, Timeout
or NAK
?3 ENQ trials
End Of Transmission
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Line Discipline (contd)

• Poll/Select
• For multipoint link
• Primary controls the link initiator of a
  session
• Secondaries follow instructions of primary

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Line Discipline (contd)

• How it works
• polling If the primary want to receive data, it
  asks the secondaries if they have anything to
  send.
• selecting If primary want to send data, it
  tells the target secondary to get ready to
  receive.
• Therefore, the primary is always the initiator of
  a session.

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Line Discipline (contd)

• Address
• identifies each frame
• either to or from a specific device (secondary)
  on the link
• address field or header in the frame
• Primary to Secondary
• the address indicates recipient of the data
• Secondary to Primary
• The address indicates originator of the data

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Line Discipline (contd)

• Select
• used whenever the primary device has something to
  send(select(SEL) frame)
• Primary tells the target secondary to get ready
  to receive.

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Line Discipline (contd)

• Poll
• to solicit transmissions from the secondary
  devices
• The secondaries are not allowed to transmit data
  unless asked

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2. Flow Control

• a set of procedures used to restrict the amount
  of data that the sender can send before waiting
  for acknowledgment
• categories

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Flow Control (contd)

• Stop and Wait

• Advantage simplicity
• Disadvantage inefficiency, slow

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Flow Control (contd)

• Stop and Wait link Utilization

If Tproc is negligible
Max 1/Tframe but decreased to 1/(Tframe2
Tprop Tack)
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Flow Control (contd)

• Sliding window
• several frames can be in transit at a time
• Frame numbering modulo n (0, 1, , n-1)
• Window size n-1 ? Why? For no ambiguity in the
  ACK
• ACK includes the number of the next frame it
  expects to receive.
• all frames up through the number-1 have been
  received

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Flow Control (contd)

• Sender Window
• Receiver Window

represents the no. of frames that may be still
be received before an ACK must be sent
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Flow Control (contd)

• Example

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3. Error Control

• methods of error detection and retransmission
• ARQ(Automatic Repeat Request)
• means retransmission of data in three cases
• damaged frame
• lost frame
• lost acknowledgment

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Error Control (contd)

• Categories of error control

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Error Control (contd)

• Stop-and-Wait ARQ
• For retransmission to work, four features are
  added to the basic flow control mechanism.
• sending device keeps a copy of the last frame
  transmitted for retransmission
• data frame and ACK frame are numbered alternately
  0 and 1
• NAK frame(not numbered) for retransmission of
  previous frame
• timer(sending device)

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Error Control (contd)

• Damaged Frames

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Error Control (contd)

• Lost Data Frame

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Error Control (contd)

• Lost Acknowledgment

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Error Control (contd)

• Sliding Window ARQ
• go-back-n ARQ
• If one frame is lost or damaged, all frame sent
  since the lost frame acknowledged are
  retransmitted.
• selective-reject ARQ
• Only the specific damaged or lost frame is
  retransmitted.

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Error Control (contd)

• Three features are added to the basic flow
  control mechanism (sliding window)
• sending device keeps copies of all transmitted
  frame until they have been acknowledged
• ACK
• carry the number of the next frame expected
• NAK
• carry the number of the damaged frame itself
• equipped with a timer to enable it to handle lost
  acknowledgments

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Error Control (contd)

• go-back-n ARQ Damaged Frame

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Error Control (contd)

• go-back-n ARQ Lost Data Frame

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Error Control (contd)

• go-back-n ARQ Lost Acknowledgment

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Error Control (contd)

• Selective-Reject ARQ
• Differs from a go-back-n ARQ
• Sorting logic (in the receiving device)
• Store frames received after a NAK has been sent
  (in the receiving device)
• until the damaged frame has been replaced
• Searching mechanism (in the sending device)
• to find and select only the requested frame
• ACK numbers, like NAK numbers,
• must refer to the frame received (or lost)
  instead of the next frame expected
• Recommended window size (n1)/2
• n-1 the go-back-n window size

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Error Control (contd)

• Selective-Reject ARQ

Window size recommendation for selective-reject
(n1)/2
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ARQ Performance

• Sliding Window Protocol
• Stop and Wait ARQ

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ARQ Performance

• Go-back-N ARQ
• Selective-Repeat ARQ

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ARQ Performance

• Error-free Stop-and-wait

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ARQ Performance

• Stop-and-wait ARQ with error

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ARQ Performance

• Stop-and-wait ARQ with error

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ARQ Performance

• The parameter a

Transmission time is normalized to 1
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ARQ Performance

• Find a and normalized throughput
• Ex1) consider a WAN using ATM with optical fiber,
  with the two stations 1000km apart, R155.52Mbps.
• Sol)
• Transmission Time
• (538)/(155.52106)2.710-6 s
• Propagation time
• (106m)/(3108m/s)0.3310-2 s
• a Prop./Trans. 0.3310-2 / 2.710-6 1222
• Normalized throughput 1 /(12a)0.0004

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ARQ Performance
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ARQ Performance

• Performance of stop-and-wait protocol(P10-3)

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ARQ Performance Sliding-window ARQ
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ARQ Performance Sliding-window ARQ

• W2n-1
• W1 stop and wait
• W7 many case
• W127 high speed WANs

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ARQ Performance Sliding-window ARQ

• Selective-reject ARQ

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ARQ Performance Sliding-window ARQ

• Go-back-N ARQ

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ARQ Performance Sliding-window ARQ
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ARQ Performance Sliding-window ARQ