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Introduction to Networking
Spring 2002 Computer Network
Applications
Analog Devices
Maintain an exact physical analog of (some form of) information.
Ex: Phonograph Cassette tape recorder Early telephone systems Radio Television
Spring 2002 Computer Network
Applications
Drawbacks of Analog Representation
Cannot handle all ranges of input Adds noise Signal loss Distorts the input
Spring 2002 Computer Network
Applications
Digital Representation
Uses numbers to record information Ex:
CDs Telephones Computers
Spring 2002 Computer Network
Applications
Using Digital to Recreate Analog
An Analog-to-Digital (A-to-D) converter transforms an analog signal into a sequence of numbers.
Conversely, a Digital-to-Analog (D-to-A) converter transforms a numerical value into an analog signal
A-to-D 1.41,3.14
Spring 2002 Computer Network
Applications
More about Digital Representation: Morse Code
The telegraph was used for communicating information;
It used two basic symbols for encoding information: “.” and “_”.
Ex of Morse codes: A ._ B _. . . C _._. E . T _
Spring 2002 Computer Network
Applications
Binary Encoding
Computers transmit & receive digital information over a network Bit (Binary Digit) 0 or 1 Byte = 8 bits
Actually, all electronic devices use bits to encode information.
Advantage: easy to distinguish between two (opposed) states;
Disadvantage: verbose
Spring 2002 Computer Network
Applications
Examples of Binary Coding
Binary representation of decimal numbers 2 10 8 1000 10 1010 15 1111
Binary representation of alphabet symbols (ASCII encoding) A 1000001 B 1000010
What about coding colors, for example red, green, yellow? How many bits are needed?
Spring 2002 Computer Network
Applications
Digital Representation of Information
Numbers & Text
Images
Sound
Video
Input Process Output
12 + 8 = 20000001100000001000----------------000010100
20
001000000000000000001000000000000010010110000011000011011011111111111100111111111111111110111111111111111100011111
8 9 20 7 8 19 5 6 15
000001000 000001001 000010100 .....
pitch,volume
time
0010101011111010101010010101010101111010001100101011011
0010101011111010101010010101010101111010001100101011011
0010101011111010101010010101010101111010001100101011011
0010101011111010101010010101010101111010001100101011011
0010101011111010101010010101010101111010001100101011011
Spring 2002 Computer Network
Applications
Information Coding
Modulation uses two devices: A modulator uses two signals:
A basic, regulate, agreed upon signal called carrier;
The signal denoting the information to be transmitted;
A demodulator which retrieves the info It measures how much the signal deviates from
the carrier.
Spring 2002 Computer Network
Applications
Modem
Device used to send/receive information via a network (usually consisting of phone lines);
Contains both a modulator and a demodulator; A dial-up modem dials a phone number and
answers a call.
A B
modem modem
Spring 2002 Computer Network
Applications
Sending Information on Wires
Electrical signals: reflect from the end of the wire network
wires require a terminator device; lose energy as they pass through a wire
to send a message along a long wire one needs a signal amplifier;
emit electromagnetic radiation that can interfere with signals in nearby wires limits the distance between adjacent wires.
Spring 2002 Computer Network
Applications
Detecting Errors
Electric and magnetic interference may disrupt wire signals and data can be damaged.
Parity bit: an extra bit added to every character so that the total
number of 1 bits is even. Ex: parity bit for A (1000001) is 0; and the parity bit
for E (1000101) is 1. When a character (or a byte) is received; the 1 bits are
counted; if the # is not even an error had occurred. Detects the error only in a limited number of cases.
Ex: if 10000010 is transmitted and 01111101 is received the error is not detected.
Spring 2002 Computer Network
Applications
Detecting Errors (cont.)
Checksum The sum of the bytes contained by a message
is appended at the end of the message; Ex: the checksum of the message ‘1 3 5’ is 9 At the message arrival, the receiving software
sums all the bytes except the last; If the sum != checksum, an error has
occurred. What if the checksum was damaged?
Spring 2002 Computer Network
Applications
Network Transmission Media
Electricity: Twisted pair, coaxial cable
Light: Fiber optics Radio waves
Coaxial cableExample:Cable TV
Shield
Radio or Micro WavesExample:Cellular phones
glass or plastic
Fiber Optic Cable
antenna
Ex. of twisted pair:Local phone lines
Spring 2002 Computer Network
Applications
Wiring
Telephone Twisted Pair Unshielded and susceptible to noise Not for higher data rates or long distances the rate of transfer is very low (4Mbps) Inexpensive
Coaxial Cable Central core with shield around it Shield insures radio frequency noise is not generated High data rates at long distances (140Mbps),
Spring 2002 Computer Network
Applications
Wiring (cont.)
Fiber Optic Light signals transmitted by light emitting diodes are
immune to electrical and magnetic noise Advantages:
Tremendous data transfer rates (10,000 Mbps) immune to electrical interference can reliably transmit signals over extremely long
distances highly resistant to corrosion impossible to tap undetected small in size
Disadvantages: very expensive difficult to install
Spring 2002 Computer Network
Applications
Fiber Optics
A thin fiber optic cable can carry as much data as 900 single copper wires, with minimal interference, and superior tensile strength.
Spring 2002 Computer Network
Applications
Radio Waves
Radio Waves Electromagnetic waves operating at radio
frequency; 100MBps Satellite
can transmit data over longer distances but propagation delay is significant (540 msec) (100Mbps)
Microwave high frequency radio signal sent through the
air
Spring 2002 Computer Network
Applications
Frequency Spectrum
All waves behave similarly Sound Radio Micro Light
Frequency differences Amount of data Distance Interference / Noise
ELF VLF LF MF HF VHF UHF Microwave Optical
100 1K 100K 1M 10M 100M 1G 10G Hertz
Navy/s
ubm
arine
s
TV: 22
0M -
500
MHz
AM:
550K
- 16
50 K
Hz
Public
Saf
ety:
150
M -
160
MHz
Public
Saf
ety:
460
M -
500
MHz
Cellula
r pho
nes:
800
MHz
Cordle
ss p
hone
s (s
ome)
: 90
0 M
Hz
PCS E
T: 2
GHz
TV: 54
M -
216
MHz
FM:
88M
- 10
8 M
Hz
Spring 2002 Computer Network
Applications
Transmission Speeds
Transmission Speed and Timetwisted pair1 mbps
coaxial10 mbps
fiber optic100 mbps
Item Bytes bits seconds
text 10,000 80,000 0.08 0.008 0.0008
image 500,000 4,000,000 4 0.4 0.04
video-10 sec 15,000,000 120,000,000 120 12 1.2
Raw transmission speeds over single wire (pair) with no compression and no overhead.
How about an image of 1,000,000 bits?