Data%20Transmission

1
Data Transmission

• The basics of media, signals, bits, carries, and
  modems
• (Part III)

2
Fundamental Measures Of A Digital Transmission
System

• Propagation delay
• Determined by physics
• Time required for signal travel across medium
• Delay propagation delay transmission delay
  queuing delay processing delay
• Throughput
• The number of bits per second that can be
  transmitted
• Related to underlying hardware bandwidth

3

Relationship Between Digital Throughput And
Bandwidth

• Given by Nyquists theorem
• where
• D is maximum data rate
• B is hardware bandwidth
• K is number of values used to encode data

4
Application Of Nyquists Theorem

• For RS-232
• K is 2 because RS-232 only uses two values, 15
  or -15 volts, to encode data bits
• D is
• For phase-shift encoding
• Suppose K is 8 (possible shifts)
• D is
• A signal can be periodic or aperiodic

5
Noises

• Noise interference by any other (undesired)
  signals
• Presence of noise can corrupt one or more bits
• One example of noise -- white noise, can not be
  removed
• Transmission impairments on digital transmission
  can lead to errors

6
Shannons Theorem

• Nyquists theorem
• assumes a noise-free system
• only works in theory
• Shannons theorem corrects for noise

7
Shannons Theorem (contd)

• Gives capacity in presence of noise
• where
• C is the effective channel capacity in bits per
  second
• B is hardware bandwidth
• S is the average power (signal)
• N is the noise
• S/N is signal-to-noise ratio an indicator of the
  signals quality

8
Application Of Shannons Theorem

• Conventional telephone system
• Engineered for voice
• Bandwidth is 3000 Hz
• signal-to-noise ratio is approximately 1000
• Effective capacity is
• Conclusion dialup modems have little hope of
  exceeding 30kbps

9
The Bottom Line

• Nyquists theorem means finding a way to encode
  more bits per cycle improves the data rate
• Shannons theorem means that no amount of clever
  engineering can overcome the fundamental physics
  limits of a real transmission system

10
Multiplexing

• Fundamental to networking
• Allow multiple channels/users share link capacity
• Multiplexing prevents interference
• Each destination receives only data sent by
  corresponding source

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Multiplexing Terminology

• Multiplexor
• device or mechanism
• accepts data from multiple sources
• sends data across shared channel
• Demultiplexor
• device or mechanism
• extract data from shared channel
• sends to correct destination

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Types Of Multiplexing

• Time Division Multiplexing (TDM)
• Only one item at a time on shared channel
• Item marked to identify source
• Each channel allowed to be carried during
  preassigned timeslots only
• Examples SONET/SDH, N-ISDN
• Pros fair, simple to implement
• Cons inefficient (i.e., empty slots when user
  has no data)

13
Types Of Multiplexing (contd)

• Frequency Division Multiplexing (FDM)
• Multiple items transmitted simultaneously
• Each channel is allocated a particular portion of
  the bandwidth (called bands).
• Example TV, radio
• All (modulated) signals are carried
  simultaneously (as a composite analog signal)

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Types Of Multiplexing (contd)

• Statistical Tine Division Multiplexing (STDM)
• Each timeslot is allocated on a demand basis
  (dynamically).
• Example ATM
• Pros improved performance
• Cons requires buffering when aggregate input
  load exceeds link capacity

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Transmission Schemes

• Baseband transmission
• Uses only low frequencies
• Encode data directly
• Broadband transmission
• Uses multiple carries
• Can use higher frequencies
• Achieves higher throughput
• Hardware more complex and expensive

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Scientific Principle Behind FDM

• Two or more signals that use different carrier
  frequencies can be transmitted over a single
  medium simultaneously without interference
• Note this is the same principle that allows a
  cable TV company to send multiple television
  signals across a single cable

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Wave Division Multiplexing

• Facts
• FDM can be used with any electromagnetic
  radiation
• Light is electromagnetic radiation
• When applied to light, FDM is called wave
  division multiplexing

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Summary

• Various transmission schemes and media available
• Electrical current over copper
• Light over glass
• Electromagnetic waves
• Digital encoding used for data
• Asynchronous communication
• Used for keyboards and serial ports
• RS-232 is standard
• Sender and receiver agree on baud rate

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Summary (contd)

• Modems
• Used for long-distance communication
• Available for copper, optical fiber, dialup
• Transmit modulated carrier
• Phase-shift modulation popular
• Classified of digital communication system
• Two measures of digital communication system
• Delay
• Throughput

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Summary (contd)

• Nyquists theorem
• Relates throughput to bandwidth
• Encourages engineers to use complex encoding
• Shannons theorem
• Adjust for noise
• Specifies limits on real transmission systems

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Summary (contd)

• Multiplexing
• Fundamental concept
• Used at many levels
• Applied in both hardware and software
• Three basic types
• Time-division multiplexing (TDM)
• Frequency-division multiplexing (FDM)
• Statistical time-division multiplexing (STDM)
• When applied to light, FDM is called
  wave-division multiplexing

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Reading Materials

• Chapter 5 Sections 5.8-5.11
• Chapter 6 Sections 6.6-6.11