ICSA 411: Week 3 Data Communication Contd

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ICSA 411 Week 3Data Communication (Contd)
Transmission Efficiency

• Elizabeth Lane Lawley, Instructor

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Protocols Preview

• ISOs OSI Seven-Layer Model
• While OSI model is increasingly out of favor in
  application development, it is still very useful
  in understanding networking in a conceptual
  context

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ISOs Open Systems Interconnection (OSI) Model

• 7 Application Layer
• 6 Presentation Layer
• 5 Session Layer
• 4 Transport Layer
• 3 Network Layer
• 2 Data Link Layer
• 1 Physical Layer

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Physical Layer

• Refers to transmission of unstructured bits over
  physical medium
• Deals with characteristics of and access to the
  physical medium

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Data Link Layer

• Provides for reliable transfer of information
  across physical link
• Includes
• transmission of blocks of data (frames)
• synchronization
• error control
• flow control

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Data Flow Simplex

• only transmit in one direction
• rarely used in data communications
• e.g., receiving signals from the radio station or
  CATV
• the sending station has only one transmitter the
  receiving station has only one receiver

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Simplex Illustration
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Data Flow Half Duplex

• data may travel in both directions, but only in
  one direction at a time
• provides non-simultaneous two-way communication
• computers use control signals to negotiate when
  to send and when to receive
• the time it takes to switch between sending and
  receiving is called turnaround time

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Half Duplex Illustration
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Data Flow Full Duplex

• complete two-way simultaneous transmission
• faster than half-duplex communication because no
  turnaround time is needed

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Full Duplex Illustration
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Asynchronous Synchronous Transmission

• Concerned with timing issues
• How does the receiver know when the bit period
  begins and ends?
• Small timing differences become more significant
  over time if no synchronization takes place
  between sender and receiver
• Synchronization occurs on the data link layer

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Asynchronous Transmission

• Serial communication
• Data transmitted 1 character at a time
• Character format is 1 start 1 stop bit, plus
  data of 5-8 bits
• Data may include parity bit

• Timing needed only within each character
• Resynchronization at each start bit
• Uses simple, cheap technology
• Wastes 20-30 of bandwidth

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Synchronous Transmission

• Parallel communication
• Large blocks of bits transmitted without
  start/stop codes
• Synchronized by separate clock signal or clocking
  data(e.g. Manchester encoding)

• Data framed by preamble/postamble bit patterns
• More efficient than asynchronous
• Overhead typically below 5
• Used at higher speeds than asynchronous

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Bit Stuffing

• Used to ensure that preamble/postamble patterns
  do not occur in transmitted data
• Typically, the pattern is 6 consecutive 1s framed
  by 0s
• Whenever five 1s are found in the data, sending
  station inserts a 0 after the fifth 1
• Whenever receiver detects five 1s followed by a
  0, the 0 is removed

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Bit Stuffing Example

• Suppose network layer gives 16 1s to the data
  link layer for transmission
• Data link layer sends the following start/stop
  flags and data to physical layer01111110 111110
  111110 111110 1 01111110
• Data link layer on receiver removes start/stop
  sequences and the stuffed bits
• Note Example shows only start/stop flags and
  user data

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Synchronization Choices

• Low-speed terminals and PCs commonly use
  asynchronous transmission
• inexpensive
• burst tendency of communication reduces impact
  of inefficiency
• Large systems and networks commonly use
  synchronous transmission
• overhead too expensive efficiency necessary
• error-checking more important

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Digital Interfaces

• The point at which one device connects to another
• Standards define what signals are sent, and how
• Some standards also define physical connector to
  be used

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Generic Communications Interface Illustration
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DTE and DCE
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RS-232C (EIA 232C)

• EIAs Recommended Standard (RS)
• Specifies mechanical, electrical, functional, and
  procedural aspects of the interface
• Used for connections between DTEs and voice-grade
  modems, and many other applications

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Mechanical Specifications

• 25-pin connector with a specific arrangement of
  leads
• DTE devices usually have male DB25 connectors
  while DCE devices have female
• In practice, fewer than 25 wires are generally
  used in applications

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RS-232 DB-25 Connectors
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RS-232 DB-25 Pinouts
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RS-232 DB-9 Connectors

• Limited RS-232

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RS-422 DIN-8

• Found on Macs

DIN-8 Male
DIN-8 Female
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Electrical Specifications

• Specifies signaling between DTE and DCE
• Uses NRZ-L encoding
• Voltage lt -3V binary 1
• Voltage gt 3V binary 0
• Rated for lt20Kbps and lt15M
• greater distances and rates are theoretically
  possible, but not necessarily wise

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RS-232 Signals (Asynch)
Odd Parity
Even Parity
No Parity
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Functional Specifications

• Specifies the role of the individual circuits
• Data circuits in both directions allow
  full-duplex communication
• Timing signals allow for synchronous transmission
  (although asynchronous transmission is more
  common)
• See Table 5-5 on p. 115 for this spec

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Procedural Specifications

• Multiple procedures are specified
• Simple example exchange of asynchronous data on
  private line
• Provides means of attachment between computer and
  modem
• Specifies method of transmitting asynchronous
  data between devices
• Specifies method of cooperation for exchange of
  data between devices

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Limited Distance Modem Example (Point-to-Point)

• Only a few circuits are necessary
• Signal Ground (7)
• Transmitted Data (2)
• Received Data (3)
• Request to Send (4)
• Clear to Send (5)
• DCE Ready (6)
• Received Line Signal Detector (8)

• For exchange over PSTN, additional circuits
  necessary
• DTE Ready(20)
• Ring Indicator (22)

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Null Modem Cable

• Allows DTE to DTE direct communication

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EIA-232-D

• new version of RS-232-C adopted in 1987
• improvements in grounding shield, test and
  loop-back signals
• the prevalence of RS-232-C in use made it
  difficult for EIA-232-D to enter into the
  marketplace

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RS-449

• an EIA standard that improves on the capabilities
  of RS-232-C
• provides for a 37-pin connection, cable lengths
  up to 200 feet, and data transmission rates up to
  2 million bps
• equates with the functional and procedural
  portions of R-232-C
• the electrical and mechanical specifications are
  covered by RS-422 and RS-423

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Flow Control

• Necessary when data is being sent faster than it
  can be processed by receiver
• Computer to printer is typical setting
• Can also be from computer to computer, when a
  processing program is limited in capacity

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Stop-and-Wait Flow Control

• Simplest form
• Source may not send new frame until receiver
  acknowledges the frame already sent
• Very inefficient, especially when a single
  message is broken into separate frames, or when
  the data link is long enough for significant
  delays to be introduced

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Sliding-Window Flow Control

• Allows multiple frames to be in transit
• Receiver sends acknowledgement with sequence
  number of anticipated frame
• Sender maintains list of sequence numbers it can
  send, receiver maintains list of sequence numbers
  it can receive
• ACK (acknowledgement) supplemented with RNR
  (receiver not ready)

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Error Control Process

• All transmission media have potential for
  introduction of errors
• All data link layer protocols must provide method
  for controlling errors
• Error control process has two components
• Error detection
• Error correction

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Error Detection Parity Bits

• Bit added to each character to make all bits add
  up to an even number (even parity) or odd number
  (odd parity)
• Good for detecting single-bit errors only
• High overhead (one extra bit per 7-bit
  character12.5)

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Error Detection Cyclic Redundancy Check (CRC)

• Data in frame treated as a single binary number,
  divided by a unique prime binary, and remainder
  is attached to frame
• 17-bit divisor leaves 16-bit remainder, 33-bit
  divisor leaves 32-bit remainder
• For a CRC of length N, errors undetected are 2-N
• Overhead is low (1-3)

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Error Correction

• Two types of errors
• Lost frame
• Damaged frame
• Automatic Repeat reQuest (ARQ)
• Error detection
• Positive acknowledgment
• Retransmission after time-out
• Negative acknowledgment and retransmission

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Stop-and-Wait ARQ

• One frame received and handled at a time
• If frame is damaged, receiver discards it and
  sends no acknowledgment
• Sender uses timer to determine whether or not to
  retransmit
• Sender must keep a copy of transmitted frame
  until acknowledgment is received
• If acknowledgment is damaged, sender will know it
  because of numbering

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Go-Back-N ARQ

• Uses sliding-window flow control
• When receiver detects error, it sends negative
  acknowledgment (REJ)
• Sender must begin transmitting again from
  rejected frame
• Transmitter must keep a copy of all transmitted
  frames

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Data Link Layer

• Specifies flow and error control for synchronous
  communication
• Data link module arranges data into frames,
  supplemented by control bits
• Receiver checks control bits, if data is intact,
  it strips them

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High-Level Data Link Control

• On transmitting side, HDLC receives data from an
  application, and delivers it to the receiver on
  the other side of the link
• On the receiving side, HDLC accepts the data and
  delivers it to the higher level application layer
• Both modules exchange control information,
  encoded into a frame

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HDLC Frame Structure

• Flag 01111110, at start and end
• Address secondary station (for multidrop
  configurations)
• Information the data to be transmitted
• Frame check sequence 16- or 32-bit CRC

• Control purpose or function of frame
• Information frames contain user data
• Supervisory frames flow/error control (ACK/ARQ)
• Unnumbered frames variety of control functions
  (see p.131)

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HDLC Operation

• Initialization S-frames specify mode and
  sequence numbers, U-frames acknowledge
• Data Transfer I-frames exchange user data,
  S-frames acknowledge and provide flow/error
  control
• Disconnect U-frames initiate and acknowledge

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Transmission Efficiency Multiplexing

• Several data sources share a common transmission
  medium simultaneously
• Line sharing saves transmission costs
• Higher data rates mean more cost-effective
  transmissions
• Takes advantage of the fact that most individual
  data sources require relatively low data rates

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Transmission Efficiency Data compression

• Reduces the size of data files to move more
  information with fewer bits
• Used for transmission and for storage
• Often combined with multiplexing to increase
  efficiency

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Alternate Approaches to Terminal Support

• Direct point-to-point links
• Multidrop line
• Multiplexer
• Integrated MUX function in host

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Direct Point-to-Point
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Multidrop Line
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Multiplexer
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Integrated MUX in Host
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Frequency Division Multiplexing

• Requires analog signaling transmission
• Total bandwidth sum of input bandwidths
  guardbands
• Modulates signals so that each occupies a
  different frequency band
• Standard for radio broadcasting, analog telephone
  network, and television (broadcast, cable,
  satellite)

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Synchronous Time-Division Multiplexing (TDM)

• Used in digital transmission
• Requires data rate of the medium to exceed data
  rate of signals to be transmitted
• Signals take turns over medium
• Slices of data are organized into frames
• Used in the modern digital telephone system
• US, Canada, Japan DS-0, DS-1 (T-1), DS-3 (T-3),
  ...
• Europe, elsewhere E-1, E3, ...

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Statistical Time Division Multiplexing

• Intelligent TDM
• Data rate capacity required is well below the sum
  of connected capacity
• Digital only, because it requires more complex
  framing of data
• Widely used for remote communications with
  multiple terminals

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Data Compression

• Works on the principle of eliminating redundancy
• Codes are substituted for compressed portions of
  data
• Lossless reconstituted data is identical to
  original (ZIP, GIF)
• Lossy reconsituted data is only perceptually
  equivalent (JPEG, MPEG)

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Run Length Encoding

• Replace string of anything with flag, character,
  and count

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Huffman Encoding

• Length of each character code based on
  statistical frequency in text
• Modified Group III Fax
• Encodes runs of black or white
• 4 million pixels to lt .5 million bits
• full page lt 1 minute _at_ 9.6kb/s

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Lempel-Ziv Encoding

• Used in V.42 bis, ZIP
• buffer strings at transmitter and receiver
• replace strings with pointer to location of
  previous occurrence
• algorithm creates a tree-based dictionary of
  character strings

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Lossy Algorithms (JPEG/MPEG)

• Scaling and color conversion (to YUV)
• Color subsampling (reduces hue info)
• Discrete cosine transformation
• Quantization
• Run-length encoding
• Huffman coding (lossless compression)
• Interframe compressions (MPEG only)