Transmission Methods

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Chapter 5

• Transmission Methods

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Asynchronous Transmission

• Baud
• Signaling rate that data is sent through a
  channel
• Measured in bits per second (bps)
• Asynchronous
• Start/stop transmission of bits
• Receiver does not know when data is being sent
• Start bit
• Bit at beginning of a character and is always 0
• Stop bit
• Bit at end of a character and is always 1

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Asynchronous Transmission
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Synchronous Transmission

• Bits grouped and sent as data frame
• Eliminates start and stops bits
• Can be bit oriented or character oriented
• Bit oriented uses 8-bit pattern as a flag to
  start the frame
• Uses self-clocking to overcome propagation delay
• Sender and receiver must synchronize
• Special Character alerts receiver
• Synch character used to identify bit groups

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Synchronous Transmission (cont.)

• Internal clocking source
• Modem aligns transmitter and receiver
• Does not require start or stop bits to alert
  receiver
• Characters arrive in continuous stream of bits
  called data frames
• Bipolar encoding used to maintain clock
  synchronization over long periods of time

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Character Synchronization
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Duplex Operations

• Three types of channels for wired, cabled or
  radio frequency communications
• Simplex
• Half-duplex
• Full-duplex

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Simplex Half-Duplex

• Simplex
• Supports flow of data in one direction only
• Sender cannot receive
• Receiver cannot send
• Half-Duplex
• Data passes both directions but only not at same
  time
• Two wires are required for communication

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Half-Duplex Communication

• Data passes both directions but only one at a
  time
• Both devices can send and receive
• Not simultaneously
• Two wires are required for communication

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Full Duplex Communication

• Supports simultaneous bi-directional data flow
• Both devices send and receive simultaneously
• Uses four wires for communication
• Transmits over one pair of wires in one direction
• Receives data on other pair

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Four Wire Full Duplex Operation
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Frequency Bands

• Baseband
• Digital-only technology
• Information carried in digital form on a single
  channel (Ethernet LANs)
• Broadband
• Abundance of bandwidth
• Operates over wide band of frequencies
• Narrowband
• Carries voice information in specific frequency
  range
• Wideband
• Large capacity analog or digital circuits

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Broadband Media Services

• Digital broadband
• Pathway for streaming media services
• Users receive continuous real-time streams
• Competes with cable TV services
• Data and multimedia signal delivery at high rates

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Next Generation IP NetworkBroadband IP

• DSLAM
• DSL Multiplexer
• Broadband Access Servers at broadcast facilities
• Loop management devices to manage DSL services
• Video servers
• Act as content repositories for video streaming
• Video encoders used for MPEG content and IP
  multicast
• IP network security
• Authentication for user identification

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Multiplexing

• Single line supports multiple signals
• Simultaneous Transmission and Reception
• Advantages
• Efficient line use
• Lower line costs
• Improved reliability
• Lower DCE Costs

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Multiplexing Elements

• CSU/DSU
• Provides ports for physical connection
• Connects the local loop to the CO
• Buffer
• Buffers incoming data and provides data bit
  storage
• Frame Builder/Multiplexer
• Multiplexes information into an aggregate stream
• Line Interface
• Converts aggregate stream of data into a format
  suitable for line transmission

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Frequency Division Multiplexing

• Divides one channel into several channels or
  frequency bands
• Each band carries different signal
• Signals sent through multiplexer at same time
• Receiving end demultiplexes signal
• Frequency is filtered and separated
• Signal sent to its final destination

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Time Division Multiplexing

• Baseband technology
• Individual data/voice channels interwoven to
  single stream
• Framed bits and transmitted across communications
  channel
• Use dedicated data grade lines at constant
  frequency
• Splits circuit into time slots
• Assigns a channel to a repeating set of slots
• Also called Bandsplitting
• Divides the line and bandwidth
• Establishes smaller data rates

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Statistical Time Division Multiplexers

• Microprocessor offers buffering capability
• Line and Channel diagnostics
• Event reporting
• Dual line sharing
• Load balancing between lines
• Data switching

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Flow Control

• Statistical multiplexers use flow control
• Regulate flow of data through routers
• Ensures that no network segment becomes
  overloaded
• Flow control methods employed to stop and resume
  data flow

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Wave Division Multiplexing

• FDM technique used with fiber optic cable
• Divides light traveling through single fiber
  strand
• Optical channels referred to as lambda circuits
• Multiple channels transmitted over single fiber
• Each is sent at a different wavelength
• Each wavelength is a different color of light
• Infrared range is utilized to carry data

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WDM

• Laser produces light in range called a window
• Windows use wavelengths of 850, 1320, 1,400,
  1,550, 1,620 nanometers (nm)
• WDM supports 24 channels
• One wavelength can carry OC-3 for voice, another
  OC-12 for data

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Jitter Wander

• Jitter
• Difference between one timed signal and another
• Causes
• Changes in cable delay due to temperature changes
• Drift due to DC offsets in phase-locked loops of
  clock
• Random phase transients as synchronization
  distribution chain is reconfigured
• Frequency differences due to loss of
  synchronization at a node

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SDH Line Jitter

• Concern about video signal degradation
• SDH data stream is regenerated
• Optical or Radio link repeaters used
• Regeneration process introduces jitter
• Tends to build as more regenerator sections added
• Line jitter is avoided by using scrambling
• Scrambling creates a new data stream pattern
• Scrambling used at every regenerated section

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Signal Phase Variation
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Dense Wave Division Multiplexing

• DWDM Fiber optic transmission technique
• Employs wavelengths of light transmitting data
• Parallel by bit or serial by character
  transmission
• Supports eight or more wavelengths per channel
• Combines multiple optical signals so they can be
  grouped
• Multiple signal transports on single fiber
  increases capacity
• Incoming optical signals are assigned to specific
  frequencies
• Resulting signals are multiplexed out onto one
  fiber

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Dense Wave Division Multiplexing
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Quote of the Day

• Anyone who goes to a psychiatrist ought to have
  his head examined.
• Samuel Goldwyn