Fundamentals of Telecommunications Week 4: Equipment Basics

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Fundamentals of TelecommunicationsWeek 4
Equipment Basics

• Elizabeth Lane Lawley, Instructor

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Telephone Switching

• Process of connecting circuits to create a
  continuous path from sender to receiver
• Typically
• sending phone
• local loop wiring
• local telco office
• IXC (interexchange carrier)
• remote telco office
• remote local loop
• receiving phone

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Switching Mechanics

• Circuit switches
• Store-and-forward switches
• Digital switches (uses multiplexing)
• Network switches (stand-alone, not integrated
  into other equipment like computers or
  multiplexers)

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Leased vs. Dial-Up Lines

• Leased lines are a permanent circuit between two
  points
• They are wired around switching equipment, so
  that fewer errors can be introduced
• When not leased, the circuit can be a dial-up
  circuit again, passing through switching
  equipment rather than around

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PBX

• Private Branch eXchange
• Switch (commonly called switchboard) where all
  an organizations lines connect
• Benefits
• cost savings
• call management/tracking ability

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Fax

• Group 1-3 is analog Group 4 is digital
• Transmits with a recognizable tone, allowing
  voice/fax switches
• Scanner on sending side quantizes image to
  generate a bit stream
• Modem modulates bit stream into carrier signal
• Receiver demodulates, reconstructs image from
  pixel info, and prints

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Voice Mail

• Records voice data digitally, rather than on tape
• Can be stored on computers, forwarded easily as
  needed

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Client-Server Networks

• Hosts
• Mainframe
• Mini
• Micro

• Terminals
• Dumb terminals/asynch
• VT100 etc
• Smart terminals (synch)
• 3270 etc
• Micros emulating terminals

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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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Front End Processors

• Buffer data between slow terminals and fast
  processor
• Handles simple communication tasks
• Polling
• Code conversion
• Parallel/Serial conversion
• logging

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Multiplexing

• Several data sources share a common transmission
  medium, with each source having its own channel
• Line sharing saves transmission costs
• Higher data rates mean more cost-effective
  transmissions
• Most individual data sources require relatively
  low data rates

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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
• Bandwidth sum of inputs 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

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Synchronous TDM and PSTN

• 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, ...
• Data rate of 1.544Mbps
• Uses PCM to digitize voice transmission at
  8K/sec, frame length of 193bits

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SONET Synchronous Optical Network

• Specification for high-speed digital transfer via
  optical fiber
• Rates from 51.84Mbps to 13.2Gbps
• Uses Synchronous TDM

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Statistical Time Division Multiplexing

• requires digital signaling transmission
• data rate capacity required below the sum of
  connected capacity
• same concepts as synchronous TDM
• uses memory buffers to avoid loss of data
• widely used for remote communications with
  multiple terminals

• similar to medium-sharing done by LANs
• 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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Port Sharing Devices

• Concentrators
• Used when there are more terminals than ports in
  the front-end processor

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Line Splitters

• Allow one set of modems to handle multiple
  terminals communicating to host (via front-end
  processor)
• Polls each system for data to send across line
• Similar to multiplexing, but less complex