Protocol Basics

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Protocol Basics
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Introduction

• Best-try transmission
• Connectionless mode transmission
• When a transmission error is detected, even if it
  is only a single (unknown) bit, then the complete
  data block must be discarded

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Introduction (contd)

• Reliable transmission
• Connection-oriented transmission
• Detecting when errors are present
  retransmission
• A defined set of rules or control procedures must
  be adopted by both communicating parties to
  ensure the reliable
• Error control
• Error detection/correction cycle
• In Data link protocol

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Error control

• Manual error control
• processes, and then reads and stores the received
  character and initiates its output to the display
  screen
• If the displayed character is different from what
  was entered or intended, then simply enter a
  suitable control character
• When the control is received, the control program
  discards the previously entered and removes it
  from the screen

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Error control (contd)

• Echo checking
• First, transmitted to the remote computer
• Reads and stores the character and retransmits it
  back to the terminal which displays it
• If this is different from what was keyed in or
  intended, the user can again initiate the
  transmission of a suitable delete character

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Error control (contd)

• ARQ
• Automatic repeat request
• The receiving computer checks the received frame
  for possible transmission errors
• And then returns a short control message either
  to acknowledge its correct receipt or to request
  that another copy of the frame

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ARQ

• Idle RQ
• Be used with character-oriented data transmission
• Continuous RQ
• Bit-oriented transmission
• Selective repeat
• Go-back-N retransmission

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Idle RQ

• Be submitted over a serial data link between a
  source DTE and a destination DTE
• Implicit retransmission
• Secondary S acknowledges only correctly received
  frames and Primary P interprets the absence of an
  acknowledgement as an indication that the
  previous frame was corrupted

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Idle RQ (contd)

• Explicit retransmission
• When S detects that a frame has been corrupted,
  it returns a negative acknowledgement to request
  that another copy of the frame is transmitted

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Idle RQ (contd)

• Implicit retransmission

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Idle RQ (contd)

• Explicit Request

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Idle RQ (contd)

• N(S)
• Send sequence number
• The sequence number carried in each I-frame
• N(R)
• Receive sequence number
• The sequence number in each ACK and NAK frame

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Layered architecture

• Layering
• Decoupling the combined application and
  communication tasks to create two well-defined
  subtasks or layers with a formal interface
  between them
• Idle RQ protocol layer
• To transfer a series of blocks of information
  each of a defined maximum length, to a similar AP
  in the destination computer

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Layered architecture (contd)

• Relating to error detection
• The generation and return of acknowledgement
  frames, timeouts and the delivery of blocks of
  information in the same sequence as they were
  submitted
• AP layer
• Be concerned only with the procedure to transfer
  the file of data using the service provided the
  communications layer

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Layered architecture (contd)

• The name of the file, its length, the
  segmentation of the file contents into smaller
  blocks, and the reassembly of the blocks

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Layered architecture (contd)

• The service provided by a communications layer is
  expressed in the form of a service primitive
• ECB
• Event control block
• The user service primitive associated with a
  layer is passed between layers using a data
  structure

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Layered architecture (contd)

• Operations
• Source AP
• Free ECB? ?? user data ??
• Primitive type field? L_DATA.request? set
• Link layer? input queue? ??
• Idle RQ protocol entity
• Link service input queue?? ECB? detect
• Queue? head? ?? entry? ??
• I-frame ??

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Layered architecture (contd)

• Secondary protocol entity ? frame ??
• Secondary protocol entity
• Header? tailer character ??
• Link service primitive type? L_DATA.indication??
  set? output queue? ???? ECB ??? dest. AP? frame
  ??
• Destination AP
• ECB? LS_provider queue? ?? ???? ??
• ??? AP-to-AP protocol? ?? msg. block ??

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Layered architecture (contd)

• Sending side?? ACK-frame? ???? LS-user input
  queue? ???? waiting ECB? ??? ?? ??

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Protocol Specification

• More sophisticated protocols
• It is often not practical to describe fully the
  operation of the protocol
• Three most common methods
• State transition diagrams
• Extended event-state tables
• High-level structured programs
• In many case, we define a protocol as a
  combination of these coupled with time sequence
  diagrams to illustrate the user service
  primitives associated with the protocol

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Protocol Specification (contd)

• Finite state machine (automation)
• The protocol can be in just one if a finite
  number of defined states at any instant
• Transitions between states take place as a result
  of an incoming event
• As a result of an incoming event, an associated
  outgoing event is normally generated

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Protocol Specification (contd)

• State
• Idle waiting for message to send
• Idle waiting to receive an acknowledgement
• Incoming event
• A msg. becomes ready to send
• An ACK-frame is received
• Outgoing event
• On receipt of a message, send the created I-frame
  on the link

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Protocol Specification (contd)

• On receipt of a NAK-frame,, retransmit the
  waiting I-frame
• Local or specific actions
• Start a timer
• Increment the send sequence variable

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Idle RQ specification

• Operation
• An incoming event has started to be processed
• All processing functions associated with event
• The generation of any outgoing events
• Local actions
• A possible change in state

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Idle RQ specification(contd)

• Communications subsystem architecture and
  protocol entity interfaces

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Idle RQ specification(contd)
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Idle RQ specification(contd)

• Specification
• Incoming (and outgoing) event interface? queue?
  ???? protocol entity? ??
• Transmission? ???? ?? protocol entity?
  transmit-receive procedure ??? ???? queue ??
• Protocol entity? timer procedure???? queue ??
• Timer? ??? ?? ?? queue? ?? protocol entity?
  message ??

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Idle RQ specification(contd)

• Transmit-receive procedure? ??
• Frame? ?? ??? protocol entity? ?? ??? ?? queue?
  ??
• Protocol entity? sequential system??? ?? incoming
  event? ??? ? ??? ???? ?? ??

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Idle RQ specification(contd)
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Idle RQ specification(contd)

• State diagram? ?.??
• Protocol? ??? ????? ??
• Space limitation??? ?? ??? incoming event? error
  condition? ???? ?? ???
• Event-state table method
• ?? ??? ??? incoming event ??? ??? outgoing event?
  specific action, ??? ?? ??
• Predicate? ??? ???? ??? ??

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Link Utilization
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Continuous RQ

• Link utilization is much improved at the expense
  of increased buffer storage requirements

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Continuous RQ (contd)

• When an error does occur
• Selective Repeat
• S selects and requests the retransmission of just
  those frames in the sequence that are corrupted
• Go-back-N
• S detects the receipt of an out-of-sequence
  I-frame and requests P to retransmit all
  outstanding unacknowledged I-frames from the last
  correctly received, and hence acknowledged,
  I-frame

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Selective repeat

• Two implementation types
• Implicit retransmission
• S acknowledges correctly received frames and P
  determines from the sequence of ACK-frames
  received that a frame has been lost
• Explicit request
• S returns a specific negative acknowledgement for
  a frame that is missing from the sequence

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Selective repeat (contd)

• In the event of frames being received out of
  sequence, S retains these in the link receive
  list until the next in-sequence frame is received

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Selective repeat (contd)
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Selective repeat (contd)
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Go-back-N

• With go-back-N
• When the secondary detects an out-of-sequence
  frame, it informs the primary to start to
  retransmit frames from a specified frame number
• Frame ??? ???
• ??? selective repeat?? ??

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Go-back-N (contd)
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Go-back-N (contd)

• Timeout mechanism

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Flow Control

• Flow control
• Be concerned with controlling the rate of
  transmission of characters or frames on a link so
  that the receiver always has sufficient buffer
  storage resources to accept them prior to
  processing

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X-ON/X-OFF

• A character-oriented terminal-to-computer link
• X-OFF
• To the controlling device within the terminal,
  instructing it to cease transmission
• On receipt of the X-OFF character, the terminal
  either ignores and further characters entered at
  the keyboard or buffers them in a local buffer
  until the overload has been cleared

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X-ON/X-OFF (contd)

• X-ON
• When the overload condition decays and the
  computer is able to accept further characters, it
  returns a companion control character X-ON to
  inform the terminal control device that it may
  restart sending characters

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Sliding window

• Sliding window
• It sets a limit on the number of I-frames that P
  may send before receiving an ack.
• P monitors the of unacknowledged I-frames
  currently held in the retransmission list
• If the dest side of the link is unable to pass on
  the frames sent to it
• S stops returning ack frames
• The retransmission list at P builds up and this
  in turn can be interpreted as a signal for P to
  stop transmitting further frames until ack start
  to flow again

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Sliding window (contd)

• Send window
• the max limit is set on the number of I-frames
  that can be awaiting ack
• If set to 1, it reverts to idle RQ
• Receive window
• the max of frame buffers required at S

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Sliding window (contd)
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Sequence numbers

• Until now
• Be assumed that the sequence inserted into each
  frame by P is imply the previous sequence plus
  one and that the range of available is infinite
• It is possible to limit the range of sequence
  required to identify each transmitted frame
  uniquely

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Sequence numbers (contd)
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Sequence numbers (contd)
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Protocol specification
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Link management

• Link management
• Not be concerned with the actual transfer of user
  data
• Initialization or link set-up phase
• Link disconnection phase