Network Data

1
Network Data

• Organizational Communications and Technologies
• Prithvi N. Rao
• Carnegie Mellon University
• Web http//www.andrew.cmu.edu/course/90-702/

2
Reading

• Data Communication Fundamentals (Stallings and
  van Slyke) Chapter 5
• TCP/IP and Other Protocol Architectures
  (Stallings and van Slyke) Chapter 12

3
Objectives

• 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

4
Introduction Review of Analog Signals

• 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).

5
Introduction Review of Digital Encoding

• Sampling of analog signal is the basis for
  digital encoding
• Manchester encoding
• Differential Manchester encoding
• Non Return to Zero
• Non Return to Zero Inverted
• Alternate Mark Inversion

6
Digital Versus Analog

• Digital technology provides benefits over analog
  for data transmission
• Resilience in terms of immunity to external noise
  (crosstalk)
• Faster and more flexible equipment can be used in
  digital networksMost networks are digital or
  are moving in that direction

7
Asynchronous Character Structure

• Handshaking permits the transmission of data in
  an orderly fashion
• 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.

8
Asynchronous Character Structure
7 or 8 Bit Character LSB first
Start bit
Parity
Stop bit
9
Synchronous Data Structure

• Handshaking permits the transmission of data in
  an orderly fashion
• 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

10
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

11
Synchronous Data Structure
Frame Check Sequence
Cntl Info
Variable Length Information Field
Flag
Flag
12
Communication Strategies

• 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

13
Error Handling

• Error detection is an important part
  consideration of data transmission
• Parity checking
• Redundancy checking

14
Parity Checking

• Involves performing a basic calculation of the
  number of digital 0s and 1s making up a
  transmission unit
• Parity calculated on even or odd number of 1s
• Parity bit is set per frame (byte or character)
• Parity checking is found mostly in Asynchronous
  communication

15
Parity Checking

• 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
• 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

16
Cyclic Redundancy Checking

• Problem with parity checking is that two
  different signals could both indicate the same
  parity
• More reliable is CRC or Cyclic Redundancy Check
• Check is performed by
• Totaling entire transmission
• Divide by a constant prime number
• Resulting remainder is the CRC validation

17
CRC Example

• Consider the following transmission unit0 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
• Also called Frame or Block checking because it
  works on the entire transmission not just the
  start and end

18
Parity Checking vs CRC

• CRC can be used with larger units of data (blocks
  or frames)
• CRC field is made part of the frameinserted just
  before the end of the frame delimiter
• Parity checking checks one byte at a time
• Parity checking can be ambiguous

19
Error Correction

• Process of recovery when error is detected
• Simple solution is retransmission
• Retransmission occurs if receiver does not send
  and ACK signal
• Alternative for retransmission is sending a NACK
• Most protocols have some form of acknowledgement

20
Data Transfer Rates

• Rate at which signal can move from a 0 to a 1
• Speed of encoding process
• Amount of overhead involved in framing
• Level of error detection
• Amount of flow control or handshaking

21
Flow Control

• Required to control the speed of communication
• 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

22
Capacity and Bandwidth
Network Plumbing
Bandwidth Pipe Diameter
Source, storage and output Source, storage and output
Copper Wire Small Pipe
Coaxial Cable Large Pipe
Fiber Optic Cable Larger Pipe
23
Summary

• Two basic types of transmission
• Analog
• Digital
• Flow control is based on
• Synchronous transmission relying on a clock
• Asynchronous transmission indicating start and
  stop of data
• Parity and CRC are two methods for error checking
• Bandwidth is the effective capacity of media