Digital%20Data%20Communications%20Techniques

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Digital Data Communications Techniques

• Ir. Hary Nugroho MT.

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Data Transmission
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• The Successful transmission of data depends
  principally on two factor
• The quality of the signal being transmitted
• The characteristics of transmission medium

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Quality of the signal

• Timing Problem controlling
• Dealing with errors

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Timing Problem Controlling
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Asynchronous and Synchronous Transmission

• Timing problems(rate, duration, spacing) require
  a mechanism to synchronize the transmitter and
  receiver
• Two solutions
• Asynchronous
• Synchronous

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Asynchronous

• Data transmitted one character at a time
• 5 to 8 bits
• Timing only needs maintaining within each
  character
• Resynchronize with each character

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Asynchronous (diagram)
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Asynchronous - Behavior

• In a steady stream, interval between characters
  is uniform (length of stop element)
• In idle state, receiver looks for transition 1 to
  0
• Then samples next seven intervals (char length)
• Then looks for next 1 to 0 for next char
• Simple
• Cheap
• Overhead of 2 or 3 bits per char (20)
• Good for data with large gaps (keyboard)

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Synchronous - Bit Level

• Block of data transmitted without start or stop
  bits
• Clocks must be synchronized
• Can use separate clock line
• Good over short distances
• Subject to impairments
• Embed clock signal in data
• Manchester encoding
• Carrier frequency (analog)

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Synchronous - Block Level

• Need to indicate start and end of block
• Use preamble and postamble
• e.g. series of SYN (hex 16) characters
• e.g. block of 11111111 patterns ending in
  11111110
• More efficient (lower overhead) than async

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Synchronous (diagram)
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Dealing with Error
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Types of Error

• An error occurs when a bit is altered between
  transmission and reception
• Single bit errors
• One bit altered
• Adjacent bits not affected
• White noise
• Burst errors
• Length B
• Contiguous sequence of B bits in which first last
  and any number of intermediate bits in error
• Impulse noise
• Fading in wireless
• Effect greater at higher data rates

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Error Detection Process
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Error Detection

• Additional bits added by transmitter for error
  detection code
• Parity
• Value of parity bit is such that character has
  even (even parity) or odd (odd parity) number of
  ones
• Even number of bit errors goes undetected

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Cyclic Redundancy Check

• For a block of k bits transmitter generates n bit
  sequence
• Transmit kn bits which is exactly divisible by
  some number
• Receive divides frame by that number
• If no remainder, assume no error
• For math, see Stallings chapter 6

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Error Correction

• Correction of detected errors usually requires
  data block to be retransmitted (see chapter 7)
• Not appropriate for wireless applications
• Bit error rate is high
• Lots of retransmissions
• Propagation delay can be long (satellite)
  compared with frame transmission time
• Would result in retransmission of frame in error
  plus many subsequent frames
• Need to correct errors on basis of bits received

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Error Correction Process Diagram
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Error Correction Process

• Each k bit block mapped to an n bit block (ngtk)
• Codeword
• Forward error correction (FEC) encoder
• Codeword sent
• Received bit string similar to transmitted but
  may contain errors
• Received code word passed to FEC decoder
• If no errors, original data block output
• Some error patterns can be detected and corrected
• Some error patterns can be detected but not
  corrected
• Some (rare) error patterns are not detected
• Results in incorrect data output from FEC

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Working of Error Correction

• Add redundancy to transmitted message
• Can deduce original in face of certain level of
  error rate
• E.g. block error correction code
• In general, add (n k ) bits to end of block
• Gives n bit block (codeword)
• All of original k bits included in codeword
• Some FEC map k bit input onto n bit codeword such
  that original k bits do not appear

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Device
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Line Configuration

• Topology
• Physical arrangement of stations on medium
• Point to point
• Multi point
• Computer and terminals, local area network
• Half duplex
• Only one station may transmit at a time
• Requires one data path
• Full duplex
• Simultaneous transmission and reception between
  two stations
• Requires two data paths (or echo canceling)

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Traditional Configurations
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Interfacing

• Data processing devices (or data terminal
  equipment, DTE) do not (usually) include data
  transmission facilities
• Need an interface called data circuit terminating
  equipment (DCE)
• e.g. modem, NIC
• DCE transmits bits on medium
• DCE communicates data and control info with DTE
• Done over interchange circuits
• Clear interface standards required

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Data Communications Interfacing
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Characteristics of Interface

• Mechanical
• Connection plugs
• Electrical
• Voltage, timing, encoding
• Functional
• Data, control, timing, grounding
• Procedural
• Sequence of events

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V.24/EIA-232-F

• ITU-T v.24
• Only specifies functional and procedural
• References other standards for electrical and
  mechanical
• EIA-232-F (USA)
• RS-232
• Mechanical ISO 2110
• Electrical v.28
• Functional v.24
• Procedural v.24

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Mechanical Specification
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Electrical Specification

• Digital signals
• Values interpreted as data or control, depending
  on circuit
• More than -3v is binary 1, more than 3v is
  binary 0 (NRZ-L)
• Signal rate lt 20kbps
• Distance lt15m
• For control, more than-3v is off, 3v is on

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Functional Specification

• Circuits grouped in categories
• Data
• Control
• Timing
• Ground
• One circuit in each direction
• Full duplex
• Two secondary data circuits
• Allow halt or flow control in half duplex
  operation

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Local and Remote Loopback
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Procedural Specification

• E.g. Asynchronous private line modem
• When turned on and ready, modem (DCE) asserts DCE
  ready
• When DTE ready to send data, it asserts Request
  to Send
• Also inhibits receive mode in half duplex
• Modem responds when ready by asserting Clear to
  send
• DTE sends data
• When data arrives, local modem asserts Receive
  Line Signal Detector and delivers data

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Dial Up Operation (1)
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Dial Up Operation (2)
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Dial Up Operation (3)
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Null Modem
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ISDN Physical Interface Diagram
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ISDN Physical Interface

• Connection between terminal equipment (c.f. DTE)
  and network terminating equipment (c.f. DCE)
• ISO 8877
• Cables terminate in matching connectors with 8
  contacts
• Transmit/receive carry both data and control

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ISDN Electrical Specification

• Balanced transmission
• Carried on two lines, e.g. twisted pair
• Signals as currents down one conductor and up the
  other
• Differential signaling
• Value depends on direction of voltage
• Tolerates more noise and generates less
• (Unbalanced, e.g. RS-232 uses single signal line
  and ground)
• Data encoding depends on data rate
• Basic rate 192kbps uses pseudoternary
• Primary rate uses alternative mark inversion
  (AMI) and B8ZS or HDB3

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Foreground Reading

• Stallings chapter 6
• Web pages from ITU-T on v. specification
• Web pages on ISDN

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Data Link Control
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Flow Control

• Ensuring the sending entity does not overwhelm
  the receiving entity
• Preventing buffer overflow
• Transmission time
• Time taken to emit all bits into medium
• Propagation time
• Time for a bit to traverse the link

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Model of Frame Transmission
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Stop and Wait

• Source transmits frame
• Destination receives frame and replies with
  acknowledgement
• Source waits for ACK before sending next frame
• Destination can stop flow by not send ACK
• Works well for a few large frames

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Fragmentation

• Large block of data may be split into small
  frames
• Limited buffer size
• Errors detected sooner (when whole frame
  received)
• On error, retransmission of smaller frames is
  needed
• Prevents one station occupying medium for long
  periods
• Stop and wait becomes inadequate

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Stop and Wait Link Utilization
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Sliding Windows Flow Control

• Allow multiple frames to be in transit
• Receiver has buffer W long
• Transmitter can send up to W frames without ACK
• Each frame is numbered
• ACK includes number of next frame expected
• Sequence number bounded by size of field (k)
• Frames are numbered modulo 2k

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Sliding Window Diagram
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Example Sliding Window
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Sliding Window Enhancements

• Receiver can acknowledge frames without
  permitting further transmission (Receive Not
  Ready)
• Must send a normal acknowledge to resume
• If duplex, use piggybacking
• If no data to send, use acknowledgement frame
• If data but no acknowledgement to send, send last
  acknowledgement number again, or have ACK valid
  flag (TCP)

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Error Detection

• Additional bits added by transmitter for error
  detection code
• Parity
• Value of parity bit is such that character has
  even (even parity) or odd (odd parity) number of
  ones
• Even number of bit errors goes undetected

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Cyclic Redundancy Check

• For a block of k bits transmitter generates n bit
  sequence
• Transmit kn bits which is exactly divisible by
  some number
• Receive divides frame by that number
• If no remainder, assume no error
• For math, see Stallings chapter 7

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Error Control

• Detection and correction of errors
• Lost frames
• Damaged frames
• Automatic repeat request
• Error detection
• Positive acknowledgment
• Retransmission after timeout
• Negative acknowledgement and retransmission

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Automatic Repeat Request (ARQ)

• Stop and wait
• Go back N
• Selective reject (selective retransmission)

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Stop and Wait

• Source transmits single frame
• Wait for ACK
• If received frame damaged, discard it
• Transmitter has timeout
• If no ACK within timeout, retransmit
• If ACK damaged,transmitter will not recognize it
• Transmitter will retransmit
• Receive gets two copies of frame
• Use ACK0 and ACK1

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Stop and Wait -Diagram
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Stop and Wait - Pros and Cons

• Simple
• Inefficient

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Go Back N (1)

• Based on sliding window
• If no error, ACK as usual with next frame
  expected
• Use window to control number of outstanding
  frames
• If error, reply with rejection
• Discard that frame and all future frames until
  error frame received correctly
• Transmitter must go back and retransmit that
  frame and all subsequent frames

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Go Back N - Damaged Frame

• Receiver detects error in frame i
• Receiver sends rejection-i
• Transmitter gets rejection-i
• Transmitter retransmits frame i and all subsequent

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Go Back N - Lost Frame (1)

• Frame i lost
• Transmitter sends i1
• Receiver gets frame i1 out of sequence
• Receiver send reject i
• Transmitter goes back to frame i and retransmits

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Go Back N - Lost Frame (2)

• Frame i lost and no additional frame sent
• Receiver gets nothing and returns neither
  acknowledgement nor rejection
• Transmitter times out and sends acknowledgement
  frame with P bit set to 1
• Receiver interprets this as command which it
  acknowledges with the number of the next frame it
  expects (frame i )
• Transmitter then retransmits frame i

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Go Back N - Damaged Acknowledgement

• Receiver gets frame i and send acknowledgement
  (i1) which is lost
• Acknowledgements are cumulative, so next
  acknowledgement (in) may arrive before
  transmitter times out on frame i
• If transmitter times out, it sends
  acknowledgement with P bit set as before
• This can be repeated a number of times before a
  reset procedure is initiated

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Go Back N - Damaged Rejection

• As for lost frame (2)

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Go Back N - Diagram
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Selective Reject

• Also called selective retransmission
• Only rejected frames are retransmitted
• Subsequent frames are accepted by the receiver
  and buffered
• Minimizes retransmission
• Receiver must maintain large enough buffer
• More complex login in transmitter

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Selective Reject -Diagram
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High Level Data Link Control

• HDLC
• ISO 33009, ISO 4335

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HDLC Station Types

• Primary station
• Controls operation of link
• Frames issued are called commands
• Maintains separate logical link to each secondary
  station
• Secondary station
• Under control of primary station
• Frames issued called responses
• Combined station
• May issue commands and responses

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HDLC Link Configurations

• Unbalanced
• One primary and one or more secondary stations
• Supports full duplex and half duplex
• Balanced
• Two combined stations
• Supports full duplex and half duplex

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HDLC Transfer Modes (1)

• Normal Response Mode (NRM)
• Unbalanced configuration
• Primary initiates transfer to secondary
• Secondary may only transmit data in response to
  command from primary
• Used on multi-drop lines
• Host computer as primary
• Terminals as secondary

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HDLC Transfer Modes (2)

• Asynchronous Balanced Mode (ABM)
• Balanced configuration
• Either station may initiate transmission without
  receiving permission
• Most widely used
• No polling overhead

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HDLC Transfer Modes (3)

• Asynchronous Response Mode (ARM)
• Unbalanced configuration
• Secondary may initiate transmission without
  permission form primary
• Primary responsible for line
• rarely used

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Frame Structure

• Synchronous transmission
• All transmissions in frames
• Single frame format for all data and control
  exchanges

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Frame Structure
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Flag Fields

• Delimit frame at both ends
• 01111110
• May close one frame and open another
• Receiver hunts for flag sequence to synchronize
• Bit stuffing used to avoid confusion with data
  containing 01111110
• 0 inserted after every sequence of five 1s
• If receiver detects five 1s it checks next bit
• If 0, it is deleted
• If 1 and seventh bit is 0, accept as flag
• If sixth and seventh bits 1, sender is indicating
  abort

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Bit Stuffing

• Example with possible errors

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Address Field

• Identifies secondary station that sent or will
  receive frame
• Usually 8 bits long
• May be extended to multiples of 7 bits
• LSB of each octet indicates that it is the last
  octet (1) or not (0)
• All ones (11111111) is broadcast

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Control Field

• Different for different frame type
• Information - data to be transmitted to user
  (next layer up)
• Flow and error control piggybacked on information
  frames
• Supervisory - ARQ when piggyback not used
• Unnumbered - supplementary link control
• First one or two bits of control filed identify
  frame type
• Remaining bits explained later

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Control Field Diagram
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Poll/Final Bit

• Use depends on context
• Command frame
• P bit
• 1 to solicit (poll) response from peer
• Response frame
• F bit
• 1 indicates response to soliciting command

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Information Field

• Only in information and some unnumbered frames
• Must contain integral number of octets
• Variable length

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Frame Check Sequence Field

• FCS
• Error detection
• 16 bit CRC
• Optional 32 bit CRC

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HDLC Operation

• Exchange of information, supervisory and
  unnumbered frames
• Three phases
• Initialization
• Data transfer
• Disconnect

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Examples of Operation (1)
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Examples of Operation (2)
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Required Reading

• Stallings chapter 7
• Web sites on HDLC