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?