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Network Data
Organizational Communications and Technologies
Prithvi N. Rao
Carnegie Mellon University
Web: http://www.andrew.cmu.edu/course/90-702/
Reading
Data Communication Fundamentals
(Stallings and van Slyke) Chapter 5
TCP/IP and Other Protocol
Architectures (Stallings and van Slyke)
Chapter 12
Objectives
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Be familiar with basic data types
Recognize the difference between analog and digital
transmission
Be able to describe how computers handle
transmission errors occurring during transmission
Recognize the concept of bandwidth and how it
relates to the data transfer capacity of media
Introduction: Review of
Analog Signals
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Amplitude of a wave is measured in decibels
Phase of a wave (0 – 360 degrees) provides
information about the position of the wave
Frequency of the wave describes the number of
waves present over a given period of time. Measured
in cycles per second (hertz).
Introduction: Review of Digital
Encoding
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Sampling of analog signal is the basis for digital
encoding
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Manchester encoding
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Differential Manchester encoding
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Non Return to Zero
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Non Return to Zero Inverted
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Alternate Mark Inversion
Digital Versus Analog
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Digital technology provides benefits over analog for
data transmission
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Resilience in terms of immunity to external noise (crosstalk)
Faster and more flexible equipment can be used in digital
networks
Most ‘networks’ are digital or are moving in that direction
Asynchronous Character
Structure
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Handshaking permits the transmission of data in an
orderly fashion
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Framing data with special control bits indicating the
beginning and end of data
Using common timer or clock to determine when the data
starts and stops in a transmission
Asynchronous communication uses frames to indicate the
beginning and end of each piece of data that is transmitted.
Serial communication is an example.
Asynchronous Character
Structure
Start bit
7 or 8 Bit Character
LSB first
Parity
Stop bit
Synchronous Data Structure
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Handshaking permits the transmission of data in an
orderly fashion
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Framing data with special control bits indicating the
beginning and end of data
Using common timer or clock to determine when data starts
and stops in a transmission
Synchronous Data Structure

Synchronous communications uses a clock to
coordinate the movements of bits through the
network

No start and stop bits are required

Can be character (byte) oriented or bit oriented
Synchronous Data Structure
Flag
Cntl
Info
Variable Length
Information Field
Frame
Check
Sequence
Flag
Communication Strategies
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Simplex defines one way communication from sender
to receiver
Half Duplex defines bi-directional communication with
information traveling in only one way at a time
Full Duplex permits bi-directional communication
simultaneously
Error Handling
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Error detection is an important part consideration of
data transmission
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Parity checking
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Redundancy checking
Parity Checking
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Involves performing a basic calculation of the number
of digital 0’s and 1’s making up a transmission unit
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Parity calculated on even or odd number of 1’s
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Parity bit is set per frame (byte or character)
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Parity checking is found mostly in Asynchronous
communication
Parity Checking
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Both sender and receiver must agree agree on
whether to use odd or even parity
Example:
1 0 0 0 1 0 1 1 or 1 1 0 0 0 0 0 0
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Even number of 1s (4 or 2) so parity bit set to 1
Receiving computer checks for even parity seeing
parity bit set to 1
Cyclic Redundancy Checking
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Problem with parity checking is that two different
signals could both indicate the same parity
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More reliable is CRC or Cyclic Redundancy Check
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Check is performed by
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Totaling entire transmission
Divide by a constant prime number
Resulting remainder is the CRC validation
CRC Example
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Consider the following transmission unit
0 0 0 0 1 1 1 1 which adds up to 15 binary
Divide this number by 17 (constant prime number)
Remainder is 15 the CRC validation number
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Also called Frame or Block checking because it works
on the entire transmission not just the start and end
Parity Checking vs CRC
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CRC can be used with larger units of data (blocks or
frames)
CRC field is made part of the frame;inserted just
before the end of the frame delimiter
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Parity checking checks one byte at a time
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Parity checking can be ambiguous
Error Correction
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Process of recovery when error is detected
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Simple solution is retransmission
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Retransmission occurs if receiver does not send and
ACK signal
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Alternative for retransmission is sending a NACK
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Most protocols have some form of acknowledgement
Data Transfer Rates
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Rate at which signal can move from a 0 to a 1
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Speed of encoding process
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Amount of overhead involved in framing
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Level of error detection
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Amount of flow control or handshaking
Flow Control
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Required to control the speed of communication
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Required when receiver cannot accept rate of delivery of
data
Limits the speed of transmission
Receiver not ready tells sender to stop transmitting
Window manipulation can reduce amount of data
being transmitted
Capacity and Bandwidth
Network
Bandwidth
Source, storage
and output
Copper Wire
Plumbing
Pipe Diameter
Source, storage
and output
Small Pipe
Coaxial Cable
Large Pipe
Fiber Optic Cable
Larger Pipe
Summary
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Two basic types of transmission
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Analog
Digital
Flow control is based on
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Synchronous transmission relying on a clock
Asynchronous transmission indicating start and stop of data
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Parity and CRC are two methods for error checking
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Bandwidth is the effective capacity of media