CIT 513: Introduction to Computer Communications

greydullNetworking and Communications

Oct 30, 2013 (3 years and 11 months ago)

82 views

CIT 513: Introduction
to Computer
Communications

Mohammed A. Saleh

http://ifm.ac.tz/staff/msaleh/CIT513.html


1

WAN Technologies


Many

technologies

are

used

to

create

today's

wide

area

networks

(WANs)


Each

of

these

technologies

has

pros

and

cons,

making

some

of

them

well

suited

for

certain

environments

and

completely

impractical

in

others


Each

of

the

technologies

varies

in

terms

of

media,

speed,

availability,

and

cost



WANs

span

great

distances

compared

to

LANs

,

which

are

restricted

in

a

single

area


One

of

the

most

important

element

of

WAN

technologies

are

the

switching

methods
.


2

Switching Methods


In

order

for

systems

to

communicate

on

a

network,

there

has

to

be

a

communication

path

or

multiple

paths

between

which

the

data

can

travel


To

communicate

with

another

entity,

these

paths

move

the

information

from

one

location

to

another

and

back



This

is

the

function

of

switching


Switching

provides

communication

pathways

between

two

endpoints

and

manages

how

data

is

to

flow

between

these

endpoints


Two

of

the

more

common

switching

methods

used

today

include
:


Packet switching


Circuit switching



3

Packet Switching


Messages

are

broken

down

into

smaller

pieces

called

packets


Each

packet

is

assigned

source,

destination,

and

intermediate

node

addresses



Packets

are

required

to

have

this

information

because

they

do

not

always

use

the

same

path

or

route

to

get

to

their

intended

destination



The

biggest

advantage

of

packet

switching

is

independent

routing


Independent

routing

allows

for

a

better

use

of

available

bandwidth

by

letting

packets

travel

different

routes

to

avoid

high
-
traffic

areas


4

Cont …


It

also

allows

packets

to

take

an

alternate

route

if

a

particular

route

is

unavailable

for

some

reason
.


In

a

packet
-
switching

system,

when

packets

are

sent

onto

the

network,

the

sending

device

is

responsible

for

choosing

the

best

path

for

the

packet



This

path

might

change

in

transit,

and

it

is

possible

for

the

receiving

device

to

receive

the

packets

in

a

random

or

non

sequential

order


When

this

happens,

the

receiving

device

waits

until

all

the

data

packets

are

received,

and

then

it

reconstructs

them

according

to

their

built
-
in

sequence

numbers
.



5

Cont …


The

receiving

computer

reassembles

the

packets

into

the

original

message











Packet
-
switching


6

Cont …


Two

types

of

packet
-
switching

methods

are

used

on

networks
:


1.
Virtual
-
Circuit

Packet

Switching



A

logical

connection

is

established

between

the

source

and

the

destination

device


This

logical

connection

is

established

when

the

sending

device

initiates

a

conversation

with

the

receiving

device



The

logical

communication

path

between

the

two

devices

can

remain

active

for

as

long

as

the

two

devices

are

available

or

can

be

used

to

send

packets

once



After

the

sending

process

has

completed,

the

line

can

be

closed
.



7

Cont …

2.
Datagram

Packet

Switching



Datagram

packet

switching

does

not

establish

a

logical

connection

between

the

sending

and

transmitting

devices


The

packets

in

datagram

packet

switching

are

independently

sent



they

can

take

different

paths

through

the

network

to

reach

their

intended

destination



To

do

this,

each

packet

must

be

individually

addressed

to

determine

where

its

source

and

destination

are
.


This method ensures that packets take the easiest
possible routes to their destination and avoid high
-
traffic
areas.


8

Circuit Switching


It

requires

a

dedicated

physical

connection

between

the

sending

and

receiving

devices
.


Most

commonly

used

in

a

telephone

conversation

in

which

the

parties

involved

have

a

dedicated

link

between

them

for

the

duration

of

the

conversation
.


When

either

party

disconnects,

the

circuit

is

broken

and

the

data

path

is

lost



The

sending

system

establishes

a

physical

connection


The

data

is

transmitted

between

the

two,

and

when

the

transmission

is

complete,

the

channel

is

closed
.


The

primary

advantage

is
:


after

a

connection

is

established,

there

is

a

consistent

and

reliable

connection

between

the

sending

and

receiving

device


This

allows

for

transmissions

at

a

guaranteed

rate

of

transfer
.





9

Cont …


Circuit

switching

has

downsides
:


a

dedicated

communication

line

can

be

very

inefficient



After

the

physical

connection

is

established,

it

is

unavailable

to

any

other

sessions

until

the

transmission

is

complete


10

Cont …











Circuit
-
switched

network

11

Fiber Distributed Data Interface


This is a topology standard that uses fiber
-
optic cable
and token
-
passing media access.


Transmissions speeds of up to 100Mbps at distances of
more than 2 kilometers


FDDI combines the strengths of Token Ring, the speed
of Fast Ethernet, and the security of fiber
-
optic cable



Such advantages make FDDI a strong candidate for
creating network backbones and connecting private
LANs to create MANs and WANs.



Unlike the regular 802.5 network standard, FDDI uses a
dual
-
ring configuration


12

Cont …


The first, or primary, ring is used to transfer the data
around the network, and the secondary ring is used for
redundancy and fault tolerance


the secondary ring waits to take over if the primary ring fails



If the primary ring fails, the secondary ring kicks in automatically,
with no disruption to network users.



The other advantage is it using

fiber
-
optic cable as its
transmission media


resistance to EMI, the security offered by fiber, and the longer
distances available with fiber cable




13

Cont …


Strong points of FDDI


Fault
-
tolerant design


By using a dual
-
ring configuration, FDDI provides some fault
tolerance



If one cable fails, the other can be used to transmit the data
throughout the network


Speed



Use of multiple tokens Unlike the IEEE 802.5 standard, FDDI
uses multiple tokens, which increase the overall network speed.


Beaconing



FDDI uses beaconing as a built
-
in error
-
detection method,
making finding faults, such as cable breaks, a lot easier.



14

Cont …


Downsides of FDDI


High costs


The costs associated with FDDI and the devices and cable
needed to implement an FDDI solution are very costly; too
costly for many small organizations.



Implementation


FDDI setup and management can be very complex



Requires trained professionals with significant experience to
manage and maintain the cable and infrastructure.



15

Internet Access Technologies


It has become an integral part of modern business.


There are several ways to obtain Internet access


The type chosen will often depend on the cost as well
as what technologies are available in the area you are
located


We will also look at the terms related to the Internet


We will explore the some of the more common methods
of obtaining Internet access.


16

xDSL Internet Access


DSL is an Internet access method that uses a standard
phone line to provide high
-
speed Internet access



Most commonly associated with high
-
speed Internet
access


because it is less expensive


often used in homes and small businesses.



With DSL, a different frequency can be used for digital
and analog signals


which means that you can talk on the phone while you're
uploading data.


17

xDSL Internet Access


DSL is an Internet access method that uses a standard
phone line to provide high
-
speed Internet access



Most commonly associated with high
-
speed Internet
access


because it is less expensive


often used in homes and small businesses.



With DSL, a different frequency can be used for digital
and analog signals


which means that you can talk on the phone while you're
uploading data.


Use skype while uploading an email document


18

Variations of xDSL


Asymmetric DSL (ADSL)


The most common of the DSL varieties is ADSL


ADSL uses different channels on the line:


One channel is responsible for analog traffic


The second channel is used to provide upload access


The third channel is used for downloads


With ADSL, downloads are faster than uploads.


Symmetric DSL (SDSL)



Offers the same speeds for uploads and for downloads


Most suitable for business applications such as Web hosting,
intranets, and e
-
commerce


Not widely implemented in the home/small business
environment and cannot share a phone line.




19

Variations of xDSL


Integrated Services Digital Network DSL (IDSL)


This is a symmetric type of DSL that is commonly used in
environments where SDSL and ADSL are unavailable



IDSL does not support analog phones



Rate Adaptive DSL (RADSL)


This is a variation on ADSL that can modify its transmission
speeds based on the signal quality


RADSL supports line sharing



Very High Bit Rate DSL (VHDSL)


This is an asymmetric version of DSL and, as such, can share a
telephone line.


High Bit Rate DSL (HDSL)


This is a symmetric technology that offers identical transmission
rates in both directions.


HDSL does not allow line sharing with analog phones



20

Cont …


Why

are

there

are

so

many

DSL

variations?



Each

flavor

of

DSL

is

aimed

at

a

different

user,

business,

or

application
.


Businesses

with

high

bandwidth

needs

are

more

likely

to

choose

a

symmetric

form

of

DS
L


Budget
-
conscious

environments

such

as

home

offices

are

likely

to

opt

for

an

option

that

allows

phone

line

sharing

at

the

expense

of

bandwidth



21

Maximum Speeds of DSL


22

Cable Internet


This

is

an

always

on

Internet

access

method

that

is

available

in

areas

that

have

digital

cable

television


Attractive

to

many

small

businesses

and

home

office

users

because

it

is

both

inexpensive

and

reliable


Most

cable

providers

do

not

restrict

how

much

use

is

made

of

the

access



Connectivity

is

achieved

by

using

a

device

called

a

cable

modem



it

has

a

coaxial

connection

for

connecting

to

the

provider's

outlet

and

an

Unshielded

Twisted

Pair

(UTP)

connection

for

connecting

directly

to

a

system

or

to

a

hub,

switch

or

WAP
.


Providers

often

supply

a

cable

modem

free

of

charge


You

are

paying

for

the

rental

of

the

modem

in

a

monthly

service

fee


23

Cont …


Most

cable

modems

supply

a

10
Mbps

Ethernet

connection

for

the

home

LAN
.


You

wouldn't

expect

the

actual

Internet

connection

to

reach

these

speeds


The

actual

speed

of

the

connection

can

vary

somewhat

depending

on

the

utilization

of

the

shared

cable

line

in

your

area



In

day
-
to
-
day

application,

data

rates

range

from

1
.
5
Mbps

to

3
Mbps
.


One

of

the

biggest

disadvantages

of

cable

access


You

share

the

available

bandwidth

with

everyone

else

in

your

cable

area




24

Cont …


Most cable modems supply a 10Mbps Ethernet
connection for the home LAN.


You wouldn't expect the actual Internet connection to reach
these speeds


The actual speed of the connection can vary somewhat
depending on the utilization of the shared cable line in
your area



In day
-
to
-
day application, data rates range from 1.5Mbps to
3Mbps.


One of the biggest disadvantages of cable access


You share the available bandwidth with everyone else in your
cable area


As a result, during peak times, performance of a cable link
might be poorer than in low
-
use periods


In residential areas, busy times are evenings and weekends






25

Broadband Security Considerations


Whether

using

DSL

or

cable

Internet

access,

there

are

a

few

things

to

keep

in

mind



Each

of

these

technologies

offers

always

on

service



This

means

that

even

when

you

are

away

from

your

computer,

it

is

still

on

the

Internet



This

creates

a

security

risk


The

longer

you

are

online,

the

more

chance

someone

has

of

remotely

accessing

your

system
.



The

operating

systems

we

use

today

all

have

some

security

holes

through

which

some

people

are

waiting

to

exploit


Combining

OS

security

holes

with

an

always

on

Internet

technology

is

certainly

a

dangerous

mix


26

Cont …


DSL

and

cable

Internet

connections

have

to

be

protected

by

mechanisms

such

as

firewalls

to

protect

the

system


The

firewall

system

will

offer

features

such

as

packet

filtering


The

firewall

can

be

a

third
-
party

software

application

installed

on

the

system,

or

it

can

be

a

hardware

device



It

is

equally

important

to

ensure

that

the

operating

system

you

are

using

is

completely

up
-
to
-
date

in

terms

of

service

packs

and

security

updates


Today's

client

systems

typically

offer

automatic

update

features

that

will

alert

you

when

a

new

security

update

is

available
.


Following

a

few

safety

rules,

both

DSL

and

cable

Internet

can

provide

safe

Internet

access




27

Satellite Internet Access


DSL

and

cable

Internet

access

technologies

are

not

offered

everywhere


For

areas

where

cheaper

broadband

options

are

not

available



There

are

a

limited

number

Internet

options


One

of

the

primary

ones

is

Internet

via

satellite
.


Satellite

access

provides

a

viable

Internet

access

solution

for

those

who

cannot

get

other

methods

of

broadband



Satellite

Internet

offers

an

always

on

connection

with

theoretical

speeds

advertised

anywhere

from

512
Kbps

upload

speeds

to

2048
Kbps

download

speeds


28

Cont …


One

of

the

primary

drawbacks

to

satellite

Internet

is

the

cost


Even

with

the

high

price

tag,

it

is

not

as

fast

as

DSL

or

cable

modem


Satellite

Internet

is

slower

and

more

costly

than

DSL

or

cable,

it

offers

some

very

attractive

features


Portability

-

wherever

you

go,

you

can

have

Internet

access
,

this

is

beneficial

for

remote

users

and

clients



Satellite

internet

access

is

also

considered

as

a

wireless

technology

where

atmospheric

conditions

can

significantly

affect

the

performance

of

satellite

Internet

access


29

Wireless Internet Access


It

is

provided

by

a

Wireless

Internet

Service

Provider

(WISP)


The

WISP

provides

public

wireless

Internet

access

known

as

hotspots



Hotspots

provide

Internet

access

for

mobile

network

devices

such

as



laptops,

handheld

computers,

and

cell

phones

in

airports,

coffee

shops,

conference

rooms,

and

so

on


A

hotspot

is

created

using

one

or

many

wireless

access

points

near

the

hotspot

location


As

of

today,

hotspots

are

not

everywhere,

but

finding

them

is

not

difficult



airports,

hotels,

and

coffee

shops

will

advertise

that

they

offer

Internet

access

for

customers

or

clients


30

Cont …


Establishing

a

connection

to

a

wireless

hotspot

is

a

straightforward

process
.


If

not

equipped

with

built
-
in

wireless

capability,

laptops

will

require

an

external

wireless

adapter

card



With

the

physical

requirements

of

the

wireless

card

taken

care

of,

connect

as

follows
:

1.
When

you

arrive

at

the

hotspot

site,

power

up

your

laptop
.

In

some

instances,

you

might

need

to

reboot

your

system

if

it

was

on

standby

to

clear

out

old

configuration

settings


2.
The

card

might

detect

the

network

automatically
.

If

this

is

the

case,

configuration

settings,

such

as

the

SSID,

will

be

automatically

detected,

and

the

wireless

Internet

will

be

available
.

If

Internet

access

is

free,

there

is

little

else

to

do
;

if

it

is

a

paid
-
for

service,

you

will

need

to

enter

a

method

of

payment
.

One

thing

to

remember

is

to

verify

that

you

are

using

encryption

for

secure

data

transfer
.

3.



31

Cont …

1.
When

you

arrive

at

the

hotspot

site,

power

up

your

laptop
.

In

some

instances,

you

might

need

to

reboot

your

system

if

it

was

on

standby

to

clear

out

old

configuration

settings


2.
The

card

might

detect

the

network

automatically
.

If

this

is

the

case,

configuration

settings,

such

as

the

SSID,

will

be

automatically

detected,

and

the

wireless

Internet

will

be

available
.

If

Internet

access

is

free,

there

is

little

else

to

do
;

if

it

is

a

paid
-
for

service,

you

will

need

to

enter

a

method

of

payment
.

One

thing

to

remember

is

to

verify

that

you

are

using

encryption

for

secure

data

transfer
.

3.
If

for

some

reason

the

wireless

settings

are

not

automatically

detected,

you

will

need

to

open

up

your

wireless

NICs

configuration

utility

and

manually

set

the

configurations
.

These

settings

can

include

setting

the

mode

to

infrastructure,

inputting

the

correct

SSID,

and

setting

the

level

of

encryption

used
.




32

Cont …


Some

companies

such

as

hotels

and

cafes

will

provide

wireless

Internet

access

by

connecting

a

wireless

router

to

a

DSL

or

cable

Internet

connection


The

router

becomes

the

wireless

access

point

to

which

the

users

connect



It

allows

clients

to

connect

to

the

Internet

through

the

broadband

connection



The

technology

is

based

on

the

802
.
11

standards,

typically

802
.
11
b/g,

and

client

systems

require

only

an

internal

or

external

wireless

adapter
.



33

Questions