Transcript Chapter 1 - Introduction

Computer Networks and Internets with
Internet Applications, 4e
By Douglas E. Comer
Lecture PowerPoints
By Lami Kaya, [email protected]
© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.
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Chapter 5
Local Asynchronous Communication
© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.
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Topics Covered
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5.1 Introduction
5.2 The Need For Asynchronous Communication
5.3 Using Electric Current To Send Bits
5.4 Standards For Communication
5.5 Baud Rate, Framing, And Errors
5.6 Half And Full Duplex Asynchronous Communication
5.7 Limitations Of Real Hardware
5.8 Hardware Bandwidth And The Transmission Of Bits
5.9 The Effect Of Noise On Communication
5.10 Significance For Data Networking
© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.
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5.1 Introduction
• Digital devices, such as computers use bits to represent data
– Transmitting data across a NW means sending bits
• Physically, communication system can use
– electric current
– radio waves
– or light to transfer information
This hapter shows
• How bits can be encoded
• Discusses mechanism for sending characters
• Introduces channel measurements:
– bandwidth
– and delay
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5.2 The Need For Asynchronous
Communication
• Asynchronous means if a sender/receiver
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do not need to coordinate before data can be sent
a sender can wait arbitrarily long between transmissions
it can transmit whenever data becomes ready
a receiver must be ready to accept data whenever it arrives
• Useful when two devices operating at different speeds
• Electrical signal does not contain information for a receiver
to determine where individual bits begin/end
– To ensure meaningful exchange send
• Start bit before character
• One or more stop bits after character
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5.3 Using Electric Current To Send Bits
• How to encode bits?
– Each bit represented by a voltage, such as
• 0  negative voltage
• 1  positive voltage
• Or otherwise
– Figure 5.1 shows a waveform diagram
• visual representation of how an electrical signal varies over time
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5.4 Standards For Communication
Questions to be answered
• How long should sender hold a voltage on the wire for a single bit?
• What is the maximum rate at which HW can change the voltage?
• How can we be sure HW from different vendors work together?
– Specifications for communication systems are standardized by
• International Telecommunication Union (ITU)
• Electronic Industries Association (EIA)
• Institute for Electrical and Electronic Engineers (IEEE)
– Standard documents answer questions about a particular technology
• Timing of signals
• Electrical details of voltage and current
• One such popular standard produced by EIA is RS-232
– Serial transmission; bits travel one after another on the wire
© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.
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© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.
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5.5 Baud Rate, Framing, And Errors
• Baud rate
– number of changes in the signal per second
• Number of bits may be sent together as a signal/symbol
Bit rate = baud rate x number of bits per signal/symbol
bits per signal/second (bps)
• If sending/receiving HW are not configured to use the
same speed
– framing errors occur
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5.6 Half And Full Duplex Asynchronous
Communication
• Communication modes
– Simplex one-way
– Half-duplex; one-way at a time
– Full-duplex; two-way anytime
• Signals sent over a standard cable are used for
– Data transfer
– Control
• Connectors used
– Data Communication Equipment (DCE)
– Data Terminal Equipment (DTE)
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© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.
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5.7 Limitations Of Real Hardware
• How fast a HW can transmit bits across a wire?
• In practice, no electronic device can produce an exact
voltage or change from one voltage to another instantly
• No wire conducts electricity perfectly:
– Signal loses energy
• attenuation
– Noise/interference may exist
• errors
© 2007 Pearson Education Inc., Upper Saddle River, NJ. All rights reserved.
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5.8 Hardware Bandwidth And The
Transmission Of Bits
Each transmission system has a limited bandwidth (BW)
• BW is the maximum rate that HW can change a signal
• BW is measured in cycles per second or Hertz (Hz)
• BW limitations arise from physical properties and energy
• Every physical transmission sys has a limited BW
• Nyquist discovered (1920s) a relationship between the
BW and bit rate of a transmission system
– Maximum data rate that can be achieved for a BW of B is 2B.
– If system uses K possible values of voltages instead of two
Maximum data rate = 2 B log2 K bps
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5.9 The Effect Of Noise On
Communication
• Nyquist’s theorem provides and absolute (theoretical)
maximum bound that can not be achieved in practice
• A real communication system is subject to small amount
of background interference
– called noise
• Shannon extended Nyquist’s theorem (1948) that takes
into account noise in a transmission system
C = B log2 (1 + S/N) bps
• C: Capacity; effective limit on the channel capacity
• S: average signal power
• N: average noise power
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5.10 Significance For Data Networking (1)
Nyquist and Shannon theorems that have consequences
for engineers:
• Nyquist's work has provided an incentive to explore
complex ways to encode bits on signals:
– Nyquist's theorem encourages engineers to explore ways to
encode bits on a signal
– because a clever encoding allows more bits to be transmitted
• Shannon's Theorem informs engineers that no amount of
clever encoding can overcome the laws of physics
– that place a fundamental limit on the rate that can be achieved in
a real communication system
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5.10 Significance For Data Networking (2)
Shannon's Theorem helps explain how fast one can send data across a
voice telephone call:
• Ex: voice telephone system has a S/N ratio of approximately 30 dB
and a BW of approximately 3000 Hz
• The maximum number of bps is limited to:
C = 3000 log ( 1 + 1000 ) = approximately 30,000 bps
• faster speeds will only be possible if S/N ratio can been improved
• How can dial-up modems achieve higher throughput than Shannon
Theorem? one possibility compression
– Compression only works if the data has been encoded inefficiently
– Ex, when an 8-bit ASCII encoding is used to transfer email that contains
only upper and lower case English letters and digits
• only 62 of the 256 possible 8-bit values are actually used
• If the same data is encoded in 6-bit characters, 25% fewer bits are needed.
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