Comp1001-L10

clashjudiciousΗλεκτρονική - Συσκευές

8 Νοε 2013 (πριν από 3 χρόνια και 7 μήνες)

55 εμφανίσεις

Comp 1001: IT & Architecture
-

Joe Carthy

1

Computer Networks


A

computer

network

is

an

interconnected

collection

of

computers
.



The

goals

of

a

computer

network

include
:



Resource

sharing
:

programs

(O
.
S
.
,

applications),

data,

equipment

(printers,

disks)

are

available

to

all

users

of

the

network

regardless

of


location
.



High

availability
:

By

replicating

files

on

different

machines

and

having


spare

CPUs,

users

are

more

immune

from

hardware/software

failure
.



Less

cost
:

Small

machines

have

about

1
/
10

the

power

of

a


mainframe

but

1
/
1000

the

cost
.

A

network

of

such

machines

can

be


installed

at

low

cost
.

It

is

easy

to

increase

the

capacity

by

adding

new

machines
.



Communications

medium
:

Users

have

access

to

email

and

the

Internet




Comp 1001: IT & Architecture
-

Joe Carthy

2

Computer Networks


A

Network

may

be

classified

as
:



Local

Area

Network
:

LAN



Operates over small geographical area (1 to 2 kms) such as office
complex,hospital, university campus e.g. UCD network


High
-
speed networks: 10 to 100 Mbps are common but up to
1000MBps (Gigabit)



Wide

Area

Network
:

WAN



Operates

over

long

distances

-

national

or

global


e
.
g
.

a

Bank’s

national/international

network


Speed

depends

on

communication

medium

used


From

slow

56
Kbps

to

x

Mbps



Also


Tiny

Area

Network
:

TAN

-

e
.
g
.

network

of

a

few

PCs

in

the

home



Metropolitan

Area

Network
:

MAN
:

network

in

a

single

city






Comp 1001: IT & Architecture
-

Joe Carthy

3

Client
-

Server Model


Networks

provide

services

on

one

or

more

computers

for

all

users



The

machines

providing

a

service

are

called

servers
.




The

machines

(users)

using

the

service

are

called

clients
.




Several

servers

may

be

used

to

replicate

data/services

so

that

if

one

crashes,

users

can

still

access

their

data

and

the

network

services
.




Such

a

Client
-
Server

Model

is

decentralised




Example: A LAN with
file server(s)
. Users store files and access
software on the file server, but use their own PCs for processing.


Dedicated servers may also be used for email or for printing:
mail
servers
and
print servers
.



One advantage of client
-
server model is that users can avail of local
processing on their own machines as well as the shared services of the
network.





Comp 1001: IT & Architecture
-

Joe Carthy

4

Terminal
-
Mainframe Model


Centralised

Model



This

model

uses

a

central

mainframe

computer

to

do

ALL

processing



Users

use

terminals

to

access

mainframe

computer



Terminal

is

a

screen

and

keyboard

(can

use

a

PC)


Terminals

can

be

local

or

remote

(in

a

different

building/city/country)



Disadvantage
:

if

mainframe

crashes

or

needs

to

be

serviced

NO

work

can

be

done

-

single

point

of

failure



Advantages


Can

have

enormous

computing

power

to

run

applications

which

PCs

cannot

cope

with


Easier

to

maintain

and

secure

-

as

there

is

only

one

computer


Used

for



large

commercial

applications
:

e
.
g
.

banking,

insurance






Comp 1001: IT & Architecture
-

Joe Carthy

5

Terminal
-
Mainframe Model


Network

Topologies
.


Networks

can

be

interconnected

in

a

number

of

ways



The

method

used,

determines

the

shape

or

topology

of

the

network


Common

topologies

include
:



Bus
:

used

in

LANs



Ring
:

used

in

LANs



Star
:

used

in

terminal

mainframe

model


Single

point

of

failure


Tree
:

Land
-
line

phone

network


Comp 1001: IT & Architecture
-

Joe Carthy

6

Comp 1001: IT & Architecture
-

Joe Carthy

7

Network Configuration.


The

user

machines

in

a

network

are

called

hosts
.




The

hosts

are

connected

by

a

subnet




Subnet

carries

messages

between

hosts
.




The

subnet

is

made

up

of


1.
transmission

lines

(trunks,

channels,

circuits)

and



transmission

lines

carry

bits


2.
switching elements

(computers).


switching elements connect the transmission lines



Comp 1001: IT & Architecture
-

Joe Carthy

8

Broadcast Networks




In

this

system

a

message

is

broadcast

over

the

network



All

machines

have

the

possibility

of

receiving

the

message
.



Each

machine

has

its

own

unique

address

and



typically

machine

will

only

"listen"

to

messages

that

are

sent

to

this

address


LANs

are

usually

broadcast

networks


Ethernet

is

commonest

example

and

is

used

in

UCD



Comp 1001: IT & Architecture
-

Joe Carthy

9

Point to Point Networks


Message

is transmitted from one computer (point) to another

(point) and so on, until the destination computer is reached.


This is analogous to the
postal system



Letter is transferred from post
-
office to post
-
office and finally to the

addressee.


WANs usually use point to point


Message is usually broken into smaller parts or
packets


Each packet is treated as a separate unit for delivery


Packets can take different routes through the network

o
e.g. if one point fails packets can be re
-
routed





Comp 1001: IT & Architecture
-

Joe Carthy

10

Data Transmission


How data is is physically transmitted


Medium
: carries the data e.g. copper wire, glass fibre




Shannon’s

Law



There is a
maximum

limit to the amount of data that can be
transmitted using any transmission medium
.



Data

is

transmitted

as

signals




The

number

of

signals

per

second

is

the

frequency

of

the

signal
.




This is measured in
Hertz

(Hz).


One cycle per second is 1
Hz
.


1,000 cycles per second is 1
KHz


One million cycles per second is 1

MHz
.


1,000 million cycles per second is 1

GHz





Comp 1001: IT & Architecture
-

Joe Carthy

11

Data Transmission


Bandwidth




Bandwidth of a medium is the maximum range of frequencies

that can be transmitted using that medium




determines the maximum amount

of data that can be
transmitted



A phone cable might have a bandwidth of .5 to 4 MHz



An optic fibre has a bandwidth of up to 10
8

MHz.




The higher the bandwidth the more data that can be

transmitted per second





Comp 1001: IT & Architecture
-

Joe Carthy

12

Transmission Media


Copper

Wires


Twisted Pair (TP)


Coaxial Cable (coax)
-

two types (thick and thin!)


10
-
Base
-
5; 10
-
Base
-
2 standards


Used with Ethernet


10 to 100 Mbps


Fibre Optic



Glass fibre


Up to 1000 Mbps


Wireless (x Kbps to x Mbps)


Radio


wireless LAN (
wi
-
fi
)

10
-
50 Mbps in UCD


Microwave


Infrared


Comp 1001: IT & Architecture
-

Joe Carthy

13

Twisted Pairs (tp)


They

are

used

in

telephone

network

for

the

local

loop

(connection

between

your

home

phone

and

the

local

telephone

exchange)
.



They carry electrical signals.


A tp consists of two insulated copper wires (1mm diameter) twisted to
reduce electrical interference.


Capacity: dependent on the distances involved but can be up to
several
Mbps

over a few Kms.


For example
ISDN

(Integrated Services Digital Network) lines offer
speeds from 64Kbps to over 1 Mbps


More recently (2003),
DSL

(Digital Subscriber Line) and in particular
ADSL

(Asymmetric DSL) lines are available to home users with speeds
of 1.5 to 6 Mbps.


ISDN and ADSL
both use digital

transmission and so must use a digital
line unlike the standard analog telephone line where a modem is used.
You must install an ISDN card or an ADSL card into your PC to use an
ISDN or ADSL line.


A
Network Interface Card

(
NIC
) is used to connect a PC to a LAN

Comp 1001: IT & Architecture
-

Joe Carthy

14

Twisted Pairs (tp)


TPs may be
shielded

(
stp
) or
unshielded

(
utp
) with the

shielded having extra insulation.


They are also classified into
Category
-
5 (CAT
-
5)

and

Category
-
6
(
CAT
-
6
).


CAT
-
5 can carry
10

or
100 Mbps

(10/100Mbps) over
short

distances e.g. up to 100 metres approx. The communications
standard used in this case is called
10/100
-
Base
-
T.


This is the type of cable that is often used in building to connect

PCs to a LAN.


Usually, the CAT
-
5 cable connects to either a
hub or switch

which is less than 100 metres from each PC. There may be a
hub for each floor/laboratory in a building.


CAT
-
6
cable

operates at 100/1000Mbps (
Gigabit

Ethernet) and
is typically used to interconnect hubs. It is more expensive than
CAT
-
5 cable.

Comp 1001: IT & Architecture
-

Joe Carthy

15

Three

Floor

Building

with

CAT
-
6

Cable

Backbone

connecting

LANs

on

each

floor




Comp 1001: IT & Architecture
-

Joe Carthy

16

Twisted Pairs (tp)


Large organisations frequently have a so
-
called "
backbone
"
network that interconnects separate LANs in different
buildings/rooms as in the diagram below.


Over short distances CAT
-
6 cable may be used for the backbone
but
optic fibre

is also often used as it can operate at higher
speeds over longer distances.


A
UPS

is also shown in the diagram.


UPS: Uninterruptable Power Supply
-

provides battery backup
power in event of power failure


A UPS can cost from as little as

200 to

x,000 depending on
amount of time it can keep system running


In life critical systems, backup generators are necessary to cope
in the event of power failure


Comp 1001: IT & Architecture
-

Joe Carthy

17

Types of Transmission


There

are

three

types

of

transmission
.



1
.

Simplex
:

information

is

transmitted

in

one

direction

only

and

the

roles

of

transmitter

and

receiver

are

fixed
.

This

form

is

not

used

for

conventional

data

transmission
.



2
.

Half

Duplex

(HDX)
:

transmission

is

allowed

in

both

directions

but

in

only

one

direction

at

a

time
.

Data

communication

systems

that

use

the

telephone

network

sometimes

transmit

in

HDX
.



3
.

Full

Duplex

(FDX)
:

sender/receiver

can

transmit

and

receive

from

each

other

at

the

same

time
.

In

order

to

transmit

in

FDX,

the

user

usually

has

private

direct

lines
.



Comp 1001: IT & Architecture
-

Joe Carthy

18

Types of Transmission

Comp 1001: IT & Architecture
-

Joe Carthy

19

Data Transmission: Multiplexing


With high bandwidth channels it is possible to share the channel so that a


number of users can use the channel at the same time
-

called
multiplexing
.



For example 4 users could each operate terminals at 1Mbps connected to
mainframe over a single 4Mbps line as shown in the figure below.



The 4 lines from the terminals are connected to a
multiplexer

which is
connected to another multiplexer by the 4Mbps line. The second multiplexer
de
-
multiplexes

the signal onto 4 separate lines for the computer.



Multiplexing is also used for voice transmission where optic fibres and
microwave cable can handle from 8000 to 16000 simultaneous
conversations.


There are a number of types of multiplexing.



Two common ones are
Time Division

Multiplexing (TDM) and
Frequency
Division

Multiplexing (FDM).


Comp 1001: IT & Architecture
-

Joe Carthy

20

Data Transmission


Multiplexing



Comp 1001: IT & Architecture
-

Joe Carthy

21

Data Transmission: Modulation


Analog

Transmission
:

Dominated

the

last

100

years

and

is

still


widely

used

in

phone

network
.


Network designers made use of telephone network which was aimed at voice

transmission. This is very poor for computer networking e.g. 2 computers

connected by a direct cable can achieve a data rate of 100 Mbps upwards

with very low error rate.



Using

phone

lines,

56

K
bps

is

the

maximum

transmission

speed

with

a


relatively

high

error

rate
.




Modems


Phone

lines

deal

with

frequencies

of

300

to

3000

Hz
.

A

computer

outputs

a

digital

serial

stream

of

bits

(
1
’s,

0
’s)
.



A

modem

is

a

device

that

accepts

such

a

bit

stream

and

converts

it

to


an

analog

signal,

using

modulation
.



It

also

performs

the

inverse

conversion
.




Thus

two

computers

can

be

connected

using

two

modems

and

phone

line
.


The

serial

interface

between

the

modem

and

computer

is

governed

by

the


RS
-
232

standard

(also

known

as

the

CCITT

V
24

standard)
.




Comp 1001: IT & Architecture
-

Joe Carthy

22

Data Transmission: Modulation


A

transmitting

modem

sends

a

continuous

signal

(tone)

in

the

range

1000

to


2000

Hz

to

the

receiving

modem
.




This

carrier

signal

is

modulated

to

an

analog

signal

as

in

the

figure

below
.




Its

amplitude,

frequency,

phase

or

a

combination

can

be

modulated

.





Comp 1001: IT & Architecture
-

Joe Carthy

23

Frequency Modulation


One

common

form

of

modulation

Frequency

Modulation

(FM)
.


In

FM,

the

carrier

signal

is

modulated

between

two

different

frequencies


(say

1200
Hz

and

2200
Hz)

without

affecting

the

amplitude

in

accordance


with

the

digital

signal

that

it

must

send
.





Comp 1001: IT & Architecture
-

Joe Carthy

24

Review


What is a network and why use one


Client Server and Terminal
-
Mainframe Models


LANs and WANs


Network Topology: Bus, Ring and Tree


Broadcast and Point
-
to
-
Point Networks


Data Transmission: Bandwidth and Shannon’s Law


Transmission Media: utp, coax, fibre, wireless


Hubs and Cabling


ISDN, ADSL, wi
-
fi


HDX, FDX, Multiplexing


Modulation