Communications and Networks

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24 Οκτ 2013 (πριν από 3 χρόνια και 9 μήνες)

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Communications and Networks

Chapter 8 in Discovering
Computers Fundamentals

Quick Overview of Some of the
Main Points


Uses of Communications

Communication Channel/Transmission Media

Transmission Characteristics

Different type of lines

Communication Devices


Uses of Communications

Communication (in computer terms) is the
process by which one computer can transfer
data/information to another computer.

Telecommunications or Data

Read about the Uses of Computer
Communications DCF p.297

Communication is a combination of software
and hardware


At work, you might have a printer connected
to your computer. If you were in a network,
you could allow your co
workers access to
your printer. The

is the

you are sharing.

A network is collection of computers and
devices connected together via “some
method” to allow users to share resources.

Those methods are discussed in this chapter.


The internet is the world’s largest

When you request to see a website, the
web page (the electronic document) is
being sent to your computer through the

How is that information being sent? Via
telephone lines? Via satellites?

More on Network

VERY important to read the Networks
section on page 303

It explains the advantages of a network
very well!!!!!!!!!!


Know the difference between


peer lan network

Client/Server network


Internet peer
peer network



Ok, so you are going to have a network.
How are you going to physically connect

Make sure and read book on topologies
for more information, but here are the
three major ways:

Topologies of networks


The physical layout of the hardware

: consists of a single central cable to
which all devices are connected to

: cable to which devices are
connected to forms a closed ring

: all devices on the network are
connected to a central computer, thus
forming a star


Now that the computers are physically
connected, you have to make sure that
they all follow the same rules … or

A protocol is a set of rules and
procedures for exchanging information
among computers.

More on Protocols

This is a confusing topic because some computers
use many different types of protocols

Think of rules. We don’t all follow one set of rules.
We follow one set of rules for driving, and another set
of rules at the workplace, etc…

Computers follow different rules depending on what
they are trying to do and who they are trying to
communicate with.

I have two computers at home connected together in
a mini
network. They use a certain protocol in order
to communicate with each other. However, when they
communicate on the internet, they use another


Information travels on the internet in packets. Each
web page, for example is broken into many packets.
Each packet has the destination address, and is
labeled as 1 of 3, etc… The destination computer,
when it receives the packets reassembles them and
then displays the information. Both computers have
to follow a certain rule/protocol in order for it to work.
This protocol is TCP/IP.

TCP/IP is a widely used protocol on the internet. It
defines how to break the message into packets, how
to reassemble the message, what to do if some of the
packets get lost, etc…

A router is a device that routes these packets through
the network.

Token Ring and Ethernet

These are two other protocols discussed in the book.
These protocols determine how computers have
access to a LAN (local area network).

For example, on a network, a computer/device
cannot just send information at any given time. They
basically have to take turns. There are rules that
govern this and they all have to follow the same

If every computer on a network sends out its
information at the same time, you would probably
have too many collisions and the information
basically gets lost. The computers then have to
resend the information.

More on Token Ring

Token Ring is a protocol that is normally used
in a ring topology or star topology.

If you set up your computers in a ring, this is
probably the protocol you would use.

This protocol requires that a special signal or
token is passed from one device to another.
The device that has the token can transmit on
the network. There are few collisions since
they each take turns.

More on Ethernet

Ethernet is a protocol that is normally used in
a bus topology or star topology.

This protocol requires that the computers
“peak” onto the network and if it senses that
there is information being sent by someone
else, it waits.

More on protocols

Understand that there are many protocols out there,
depending on what you are trying to do.

HTTP is the protocol used for transferring web pages

SMTP is a protocol used for delivering email

Ethernet/Token Ring are protocols used to determine
when a computer

has access to the network

Once we figure out when the computers are going to
have access to the network,
we can use TCP/IP
protocol to manage the transmission of how that data
gets sent

from one computer to the other.

Your computer can follow TCP/IP AND ETHERNET
protocols … they are for two different purposes.


Understand what a firewall is!!!!!!!!

A firewall prevents outsiders into a

If you use DSL or Cable Modems, you
should have a firewall program installed
on your computer also … to prevent
people on the internet from accessing
your computer!!!

Internet vs. Intranet

This is the book!!!!!!!!!!!

An Intranet looks similar to what you
see on the internet but it is limited to
one particular location (a company).

A company’s intranet is a set of web
pages only accessible within the

Up Lines vs. Dedicated

Up lines

Telephone lines

Only has a connection while call is in use

Dedicated lines

Always a connection (24 hours a day)

Sometimes companies don’t want to buy their own
lines, so they LEASE them.

Can use telephone lines or can use other lines
(like cable lines)

Cable Modem and DSL are dedicated

When you have dedicated lines you should think
of getting a firewall (more info in book)

Communication Devices

Types of hardware/equipment that transmits
data/information onto the transmission media.

You might have the phone line plugged into
your computer, but you need something to
send the data out on the phone line: modem



converts the computer digital signal to an
analog signal so it can travel over the phone lines.
Generally they transmit data between
28.8 Kbps
and 56 Kbps

Cable Modem


sends computer signal over cable
lines. They generally transmit data between
Kbps and 2Mbps

(2Mbps is approx. 2000Kbps).

Transfer Rates

Whenever we are referring to the speed
at which a line carries data (i.e., how
fast can we send info on the internet
from our computer at home) … we are
referring to transfer rate.

Transfer rate always uses the bps unit
(bits per second). Generally we refer to
it as Kbps (kilobits per second).

More on Transfer Rates

FYI: you don’t need to know how to do this,
but say you have a 56 Kbps internet
connection, how long does it take to
download a 1MB file??

Well, 1 megabyte = approx. 1 million bytes

1 million bytes = 8 million bits

56Kbps = approx. 56000 bits per second

8,000,000/56000 = approx. 153 seconds

153 seconds = approx 2.4 minutes!

More on The Telephone Network

Up lines

Uses standard telephone lines

Remember, telephone lines require
information in analog frequencies

Modem translates digital (computer signal)
on phone line into analog frequency so that
it can be sent on phone line

Even More on The Telephone


Uses standard telephone lines

However, it uses a different sending device (instead of a
regular modem it uses a DSL modem).

The DSL modem sends out the information digitally. Special
technique which allows us to use regular phone lines, in a
digital way, which allows the information to be sent faster.

ADSL is slower than SDSL.


Read about it in book!!!!!!!!!!! Uses telephone lines, and also
sends out info digitally. Uses and ISDN modem. This
technique is older and not as fast as DSL!

And Yet Even More on The
Telephone Network

Cable Lines

Does not use telephone lines

Uses cable lines

Comparable to DSL, though some say it’s faster

Read about T
carrier and ATM lines

Read about modems on page 9.28

Connecting Networks

The internet consists of many small
networks connected together.

A hub is a device that provides a central
point for cables in a network, see figure
on 9.30.

A router (as stated earlier) is a device
that receives packets on a network and
routes them to their destination!

Communication Channel

A communication channel is the path that
information takes when it is transferred from
one computer to another.

For example, if you were going from Houston
to Hawaii, your channel would be ALL the
different ways/roads you used to get there
(road to get to highway, highway to get to
airport, air to get to Hawaii, road to get to

Internet Communication Channel

If you were requesting information from

website, that
information might have to go through
telephone lines, then fiber optic cables,
then satellites, then back through phone
lines, in order to get to your house.
That is the communications channel
your information traveled on.

Transmission Media

BANDWIDTH is the width of the
communication channel. The higher the
bandwidth, the more data and information the
channel can transmit.

What can we use that will carry that
information/signal from one computer to
another? Cables? Satellites? What?

Transmission media consists of the materials
or techniques capable of carrying a signal.


Physical cables:

Pair Cable (phone cables)

Coaxial Cable (cable companies use this)

Optic Cable (Much of the internet backbone
uses this)

Most companies use Twisted
Pair cables in
their buildings. Cheap, and easy to install.

Most of the internet backbones use Fiber
Optic cable. Not much noise (interference),
fast, but expensive and hard to install.

Coaxial cable is in the middle.


Broadcast Radio

Cellular Radio


Expensive to install, and not as fast as some of
the other physical media, but useful when it’s too
difficult to install cables

Must have line
sight (read book for more info)

Satellites (Very expensive)

Recap on Transmission Media

So, back to our example. If you are trying to
view the

website, the
web page information must be sent through
the internet to your computer.

What path (communication channel) does it
take (i.e., what transmission media does it go

The information probably travels on twisted
pair cable out of the

computer, then onto some fiber optic cable,
maybe bounces off a couple of microwaves,
maybe back to fiber optic cable, then back
onto twisted pair cable and into your home.


There are many parts to a network:

Transmission media

physical vs. wireless

The cables or satellites that information travels through

Communications devices

equipment that send
out the info on the transmission media

Two different networks can use the same
transmission media (twisted pair cable), but
depending on the communications device,
software, techniques, the information can travel
faster on one network than the other.


how do we physically connect the


rules that the computers must all
follow in order to send the information. TCP/IP is a

The End